
20221214: How Can Matter Be BOTH Liquid AND Gas?
 14:26: ... various spinbased states from ferromagnets to quantum spin liquids to time crystals; come to think of it, let’s not review all the states of ...
 14:55: And while this stuff is rare on this mostly wet and gassy rock, supercritical fluids are surprisingly abundant in other parts of space time.
 15:09: If time travel were invented and you thought “Great!
 15:19: ... you already know it… but even in this current time period, if you’ve wanted to learn another language, then I’d like to ...
 18:29: These neutrinos would now have energies 10 billion times smaller than regular neutrinos, say from the Sun.
 19:30: The limit for the Milky Way’s black hole is around 100billion times brighter than the Sun.
 19:37: ... that a Milky Way quasar shining at the Eddington limit would be about 3 times brighter on our sky compared to the ...


20221208: How Are Quasiparticles Different From Particles?
 06:47: They have energy equal to the planck constant times their frequency.
 07:00: Compare that to electron holes, which are fermionlike in that you can only have one in a given spot at one time.
 09:23: We’ll do a deeper dive into superconductivity another time  today I just want to show you how this phenomenon is enabled by quasiparticles.
 10:50: At the same time, the negatively charged electrons in a metal lattice attract the positive nuclei.
 13:28: ... are many quasiparticles beyond the few I had time for today  for example, quasiparticles appear in lattices of quantum ...
 15:11: All part of the magnificent complexity emergent from simple fields spanning space time.


20221123: How To See Black Holes By Catching Neutrinos
 12:30: The future of neutrino astronomy is bright, and it’s hard to even guess what we’ll learn as we slowly build our neutrino map of space time.
 13:33: ... let users browse securely and privately on up to 10 devices at the same time. If you’d like to check it out, there’s a special Black Friday deal for ...
 15:05: ... human eye has a volume of around 6 cubic centermeters, which is 5,000 times smaller than your whole body, counting both eyes  so it’ll take 100 ...
 16:15: They’re likely to decay, but on a longer timescale than the other elements between the island and the current top of the periodic table.
 16:50: There’s a limit to how large a nucleus you can make this way  and that’s when the decay timescale is shorter than the rate of neutron bombardment.


20221116: Are there Undiscovered Elements Beyond The Periodic Table?
 01:38: ... number of outer shell or valence electrons, which increase by one every time you add a proton to the nucleus, until the shell fills and you start ...
 02:06: Over time three of these elements were discovered and the gaps were filled with scandium, gallium and germanium.
 03:43: But technetium is so unstable that by the time the Earth pulled itself together from the detritus of dead stars, all the technetium was long gone.
 14:19: ... eras of humanity are named by the materials we have mastered at the time  the Stone Age, the Bronze Age, the Iron Age, into the current Silicon ...
 14:33: For instance, Technetium is used all the time in medical imaging as a contrast agent, and in this case its short half life is actually an asset.
 15:46: ... new ground in humanity's journey to further and future horizons of space time. ...
 18:57: ... Cleveland points out that life may have had the potential to arise many times on early, except for the fact that once one incident of life got started ...


20221109: What If Humanity Is Among The First Spacefaring Civilizations?
 00:02: Half of the universe is filled with expansionist alien civilizations, and it’s only a matter of time before they’ll reach us.
 00:31: But even the current age of the universe is itself a blink of an eye compared to the amount of time yet to come.
 00:52: At the same time, when we look at the sky we see… stars.
 01:34: But in principle you can extend this principle to time.
 02:08: So perhaps there’s also an anthropic reason for us to find ourselves at the “beginning of time”.
 03:32: ... civilizations are evolving from primordial goop at a given cosmic time. ...
 04:50: Once a habitable star along with its habitable planets have formed, life needs time to evolve.
 04:57: And it needs lot of time.
 05:35: These events are all possible “hard steps”  extremely unlikely peryear, but will eventually happen after waiting a very long time.
 06:22: ... is simplistic  for example, some steps may be easy, but take a long time anyway, like due to the slow but steady buildup of biological ...
 06:59: Another important part of the story is: what’s the time limit for completing these steps?
 07:49: ... stars, also known as red dwarfs, can live for thousands of times longer than our sun, meaning they could have planets that remain ...
 10:02: There will be a billion billion times more civilisations born after us than before us.
 11:17: ... visible changes wherever they go, and that they stick around for a long time. ...
 11:45: ... to be able to expand through the cosmos close to the speed of light any time ...
 11:54: ... light speed, this delay will have basically no effect on cosmological timescales. ...
 13:34: ... to run a simulation of the emergence of intelligent civilizations over time, in which some civilizations become grabby and spread through the ...
 14:29: It gives us a bestguess waiting time of around half a billion years.
 14:33: Don’t expect to meet any aliens any time soon, even if they do fill half the universe.
 15:02: ... with aliens rushing in to grab this rare remaining patch of empty space time. ...
 16:41: ... it’s daunting to think about producing PBS Space Time for that long, your help makes it more possible that we’ll eventually be ...
 18:13: Benjamin Shropshire asks if anyone has ruled out hidden influences traveling from the measurement back in time to the creation of the entangled state.
 18:41: If there are multiple possible effects, then the cause gets to choose between them by picking from the time traveling signals.
 18:50: ... interpretation is equivalent to standard quantum field theory, with the timereversed signals corresponding to negative frequency ...
 20:25: This branch of my wavefunction only remembers not going to any of them, but I assume the other guy had a really great time.


20221026: Why Did Quantum Entanglement Win the Nobel Prize in Physics?
 03:28: For our quantum balls to know their own color the whole time, there would need to be extra information not contained in their wavefunction.
 06:05: Clauser talks about the time he proposed his idea for the experiment to Richard Feynman, who promptly kicked him out of his office.
 08:35: ... information about the eventual measurement direction the whole time and sort of conspire to look like standard quantum mechanics, even ...
 09:11: Aspect’s setup was very similar to Clauser's, with a beam of calcium atoms excited by light  this time a laser rather than an arc lamp.
 13:10: We’ll save the awesome details for another time.
 13:32: This is a rare episode of Space Time in which both Einstein and Feynman were wrong.
 14:14: All because a few scientists were willing to challenge the status quo, and seek the hidden secrets of space time.
 14:24: ... we get to comments two things: First it’s that time of year again where we ask you to take the annual PBS Digital ...
 14:31: The space time audience has always been amazing at filling out the survey and we’d love to continue the trend!
 15:33: So Alek, from all of us here at Space Time, we thank you for the light that you so clearly brought to science and to your community.
 16:12: ... all, we’re having a hard time keeping track of the Voyager’s faint signals, and the gravitational lens ...
 16:31: ... by the time these things launch it’ll be at least half a century since voyager, so ...
 19:41: 05TE informs us that if you recite the full Lagrangian equation three times in front of a mirror, the particle ghosts will appear.


20221019: The Equation That Explains (Nearly) Everything!
 03:54: ... will be equations describing how that system will move and change over time. ...
 06:57: ... this is like the energy of the photons as they change in space and time, and this is true for all the derivatives we are going to see here, they ...
 07:27: ... symbols which basically mean: first x, then y, then z, then do t for time. ...
 07:51: ... see them it means that whatever was written has to be repeated several times depending on the dimensions of spacetime, the number of charges, the ...
 12:39: ... we have another "h.c.", but this time hermitian conjugate tells us how antimatter picks up mass from the Higgs ...
 12:52: ... to the Higgs field. It tells us how that field changes in space and time and how it interacts with the massive bosons of the weak ...
 14:43: ... truly astonishing precision, and physicists are having a very difficult time finding situations where it fails. Because it will be a glitch in the ...
 15:13: ... we get to comments, for all the space time fans who are also history buffs, we’d like to let you know about a new ...
 15:40: ... Maelstroms of power and mystery shining out to us from across space and time, just like you Vivaan. We wish you a most luminous ...


20221012: The REAL Possibility of Mapping Alien Planets!
 01:25: ... because this requires exact measurement of the arrival time and phase of the electromagnetic wave  which gets harder the ...
 04:36: ... that location. As I mentioned, you'd catch something like a trillion times more light from the exoplanet, making it possible to ...
 06:05: ... spot. Unfortunately that spot is pretty far away. Like,10 times the distance of Pluto, or well over 500 times the ...
 07:45: ... and engineers who witness the launch we need a travel time of 2530 years. So our spacecraft need an average speed of ...
 10:07: ... around the focal column, mapping the surface one patch at a time. The ion thrusters come into play here also. On top of this, ...
 12:52: ... time, the image that the train of spacecraft sends back to Earth will get ...
 15:25: ... the show and let everyone in your immediate patch of Space Time know that it’s never aliens until it is at any rate. There’s ...
 15:45: ... strength of electromagnetism, which didn’t exist at very early times. About a trillionth of a second after the big bang the EM force ...
 19:23: ... between the other constants also be different during that time? Well yes! The fine structure constant is defined as the electron ...
 20:21: ... which means if you look far enough it says “LOL noobs”. Infinite times. ...


20220928: Why Is 1/137 One of the Greatest Unsolved Problems In Physics?
 02:55: ... number: the square of the charge of the electron, divided by four times pi, the permittivity of free space, Planck's constant and the speed of ...
 10:02: There’s no way for the alien civilization to recognize these numbers without knowing our units for distance, time, mass, electric charge, etc.
 11:20: ... is the translation factor between the dimensions of space and time in relativity; it’s also the relationship between mass and energy ...
 13:47: Back to Richard Feynman one last time.
 14:13: And perhaps that number was 1/137, the fine structure constant  whose value sets the rules of this particular space time.


20220921: Science of the James Webb Telescope Explained!
 01:09: What it takes to get time on the JWST, and how you can use it yourself.
 02:15: ... bigger  8m diameter to make it a worthy upgrade, which would give it 6 times the collecting area of ...
 04:34: JWST is also great at exploring galaxies of any epoch of cosmic time.
 07:06: ... will be used, the exact settings for those instruments, the exposure times, etc. ...
 10:48: We don’t have time to do a full tutorial right now, so I’m going to link some resources below.
 11:15: We’ve waited a long, long time for JWST, and its time to reap the rewards for our patience and the hard work of many brilliant and dedicated people.
 11:26: ... before seen, and some never imagined, from the furthest reaches of space time. ...
 11:50: Henson Shaving is the maker of the Al13 razor and even though I do have a beard, I do need a good shave from time to time.


20220914: Could the Higgs Boson Lead Us to Dark Matter?
 02:17: ... is just a way to represent the interactions of particles, plotting time versus space so we have two particles coming together, undergoing some ...
 03:06: But with enough particles and enough time, we should eventually see an interaction between a dark matter particle and a matter particle.
 03:51: ... this case the time and space axes are flipped, so now we’re looking at the annihilation of ...
 04:41: OK, let’s try rotating our Feynman diagram one more time.
 08:48: ... of momentum tells us that the product of velocity times mass of all particles going into a collision has to be the same as the ...
 11:59: This number tells you the fraction of times a Higgs decayed into particles that can’t be detected.
 13:20: But certainly a portal beyond the familiar physics of our luminous space time.


20220824: What Makes The Strong Force Strong?
 11:18: You see it all the time when a piece of neutral metal is attracted to a magnet which is itself also neutral.
 11:51: ... carry color charge, in fact they carry two charges at the same time  a positive and a negative of different colours  or more accurately, a ...
 12:49: Sometimes we even have to consider three possibilities like redantired + blueantiblue  2 times greenantigreen.
 15:09: As you can imagine, there’s a lot more to it  and so in future episodes we’ll dive deeper into the strongest force in all of space time.
 15:45: Links are in the description and tell them, politely, that Space Time sent you!
 16:59: Jeremiah Young asks whether relativistic time dilation occurs due to the thermal motion of particles.
 17:05: In other words, if heating something up means its particles are moving faster, does time slow down for those particles?
 18:57: ... Max Wyght asks if it's found that Quintessence evolves over time, would our estimates for the age of the universe be ...


20220817: What If Dark Energy is a New Quantum Field?
 00:36: ... alternative behaviors: the case where dark energy gets more intense over time rather than having a constant density expected of a vacuum energy. That ...
 06:25: ... and an unchanging omega of 1. But if dark energy has changed over time, both teams may have got the right answer and the discrepancy between ...
 09:13: ... of the particles of the field. The field strength can also change over time AND over space, so omega can change, and with it the behavior of dark ...
 10:08: ... where these are within a factor of a few of each other, at exactly the time when the acceleration begins? This could be a crazy coincidence, or ...
 10:55: ... a natural explanation for why dark energy kicked in at around the same time as stars and planets were able to form. This same “tracker” behavior ...
 13:04: ... scenarios break the rules in general relativity in the same way that time travel does. So, if the big rip is possible, so are time machines, and ...
 14:41: ... energy, or scalar quantum fields shift in a quintessencesaturated space time. ...
 15:05: ... going to skip comment responses this week and double up next time. But I did want to take a minute to thank those of you who support us on ...


20220803: What Happens Inside a Proton?
 04:17: ... photon. And there’s a set probability of that happening each time  it’s around 1/137. So every time you add another pair ...
 08:30: ... configurations that the field could pass through in the intervening time. No supercomputer could do that even given the entire life ...
 08:58: ... Actually, it also includes the impossible paths, but no time to explain that now. Every time you add the ...
 11:28: ... quantum field is a 3D pixelated lattice that evolves through time. As with the path integral, each time step results a ...
 12:52: ... it a very simple way. You can run your simulation multiple times for different lattice spacings to figure out that relationship. ...
 16:44: ... or at least each in a superposition of both up and down at the same time  and that means the actual direction gets randomly chosen ...


20220727: How Many States Of Matter Are There?
 00:05: We have solids, liquids and gasses, and plasmas, quarkgluon plasmas, nuclear matter, boseeinstein condensates, neutronium, time crystals, and sand.
 01:38: And then there are all those popsci media claims of “new state of matter discovered”  time crystals being a recent example.
 08:21: Time crystals are the latest and perhaps weirdest quantum state of matter.
 08:40: ... lowest energy state of a time crystal involves real motion, which makes them thermodynamically ...
 12:50: Just think of our universe as nested layers of states of matter, from the smallest to the largest scales of space time.


20220720: What If We Live in a Superdeterministic Universe?
 07:40: ... a local realist theory, every point in space and time  every event  can only be influenced by a spacetime event in its ...
 08:18: Let’s play out the Bell test on something called a spacetime diagram  with one dimension of space only on the x axis and time on the y.
 08:28: Alice and Bob start out together, acquire their entangled electrons, and then move sideways in space and up in time.
 14:30: ... you can choose whether or not to believe in a superdeterministic space time. ...
 16:56: For example, Planck units, or the energies or masses or decay timescales of common particles and elements.
 17:20: Dai Lixiuyuan and MrOvipare point out that we’d be able to tell what time "direction" the aliens use by their laws of thermodynamics.
 17:34: The second law of thermodynamics says that entropy must increase in the forward time direction.
 17:40: Their expression for the 2nd law should tell us what sign convention they use for time.


20220630: Could We Decode Alien Physics?
 02:25: ... one among Franklin’s many scientific interests. By Franklin’s time, we already knew that if you rub a glass rod with a piece of ...
 07:38: ... something called the cross product. Force equals charge times velocity cross magnetic field. This is a type of vector ...
 10:41: ... universe works just like our universe if you reversed the flow of time. ...
 11:52: ... need to figure out is how the aliens define the flow of time. Do they define the future as an increasing progression of positive ...
 13:42: ... It points to the existence of an underlying symmetry of space time. ...
 14:52: ... And the good news is that if they screw it up and accidentally bake timereversed antimatter brownies they don't blow up their planet. In fact those ...
 15:07: ... episodes extra hard during that time, and this gap will give us the time we need to make some extra amazing stuff. So today ...
 15:41: ... the Fermi paradox. We did an episode on this once upon a time. Knurlgnar24 reminds us that the thing that’s even ...


20220622: Is Interstellar Travel Impossible?
 02:19: ... humans to travel the stars, travel time needs to at least be of order a human lifetime, which means traveling at ...
 08:09: But we’re also traveling around 10,000 times faster than a reentering lowearthorbit satellite.
 08:19: The kinetic energy deposited by each particles is 1/2 times mass times velocity squared.
 08:26: ... speeds, the heat deposited onto our ship by the ISM is around a billion times lower compared to orbital ...
 14:02: ... stretching our species’ reach to distant tracts of interstellar space time. ...
 14:15: Making Space time would not be possible without the generous support of everyone who’s joined us on Patreon.
 17:09: ... the particlelike properties (which detector), but not both at the same time with the same ...


20220615: Can Wormholes Solve The Black Hole Information Paradox?
 00:26: ... from which followed our modern description of gravity, space and time in general relativity. Now, there’s an ...
 04:21: ... Von Neuman entropy of the radiation should increase over time as the internal information of the black hole ...
 05:20: ... von Neumann entropy of Hawking radiation should rise over time as more and more radiation is produced, but then at ...
 11:25: ... has been called the island rule. This was actually not the first time physicists had seen this equation. The island rule had already been ...
 13:07: ...  exploring all possible explanations of the result. In time one of these seems sure to land on the true path  a path that ...


20220601: What If Physics IS NOT Describing Reality?
 05:53: ... equation by asking how quantum information should evolve over time. ...
 10:33: ... “Are you a particle?” And “are you a wave?” at the same time. ...
 12:09: ... our only direct experience is with an entirely informational space time. ...
 16:52: ... that the model represents the pygmy mammoth that lived at the same time as its gigantic cousin. I always correct the misinformation ...


20220525: The Evolution of the Modern Milky Way Galaxy
 00:00: ... astronomer from Europe, but it was also his first time in the southern hemisphere observatory. He went ...
 01:09: ... everywhere. We see moments that appear frozen on the human timescale, but are really snapshots of the incredibly violent process of ...
 05:11: ... the evidence. Item 1: Stars that join the Milky Way at the same time, like during a merger, should have similar properties. ...
 09:42: ... and ultimately can be wrapped around the galaxy multiple times. In the end it disperses into the Milky Way’s ...
 10:34: ... and coming back down to punch through the disk three times. Most importantly, the massive core is still relatively ...
 13:54: ... foolish enough to stray into the Milky Way’s little patch of space time. ...
 14:08: ... This incredible tool allows you to search the entire Space Time catalog for any word or phrase and get links to the time code ...


20220518: What If the Galactic Habitable Zone LIMITS Intelligent Life?
 03:40: ... hydrogen and helium and became the gas giants. At the same time, the core of the collapsing protostar became hotter and ...
 08:01: ... next generation of stars had some metals, and so for the first time had the chance to build planets. These stars fell towards ...
 09:26: ... was probably good for driving evolution, but life needs time to recover. Overly frequent mass extinctions will result in ...
 10:04: ... gas converted itself into a disk of stars. It took some time for the emerging spiral disk to seed itself with enough heavy ...
 11:12: ... and the probability that life could emerge given the amount of time the system has been ...
 13:03: ... life is that other such civilizations just haven’t had time to make their presence known on the galactic scale. We’ve ...


20220504: Space DOES NOT Expand Everywhere
 02:38: ... also just slices out of 4D spacetimes  representing single instants in time. If you use general relativity to track how the FLRW metric changes over ...
 03:48: ... the sum of the squares of x, y & z, but here adding the dimension of time. And this thing is the scale factor, which represents the overall size of ...
 04:26: ... and its growth represents the increase in the scale factor over time. ...
 05:19: ... fact space is pulled inwards  pinched  and it’s completely static over time. But what about if it's embedded in a larger FLRW spacetime? Does the ...
 07:12: ... grid points instead of gridlines. But then we can add the dimension of time into our picture so that the expanding ring traces a sort of cone, and ...
 08:21: ... systems it contains. That tug of war did happen, but it happened a long time ...
 13:16: ... a glimpse of those receding galaxies  they will spend most of cosmic time far beyond the horizon, in which case we never would have even known ...
 15:31: ... electrons are actually the same electron bouncing back and forward in time. Wacky as they are, they tend to have legs these ideas. In his Nobel ...


20220427: How the Higgs Mechanism Give Things Mass
 00:46: ... This timing of FermiLab’s discovery is weirdly convenient. Over ...
 04:37: ... the U(1) and SU(2) symmetries so that they apply at the same time. We call this combined symmetry group U(1)xSU(2). The ...
 05:24: ... and their combination defines electric charge. Is it time to give up on this symmetry stuff ...
 16:02: ... to the matter particles  the fermions  but that’s for another time. But what about this new measurement of the W boson mass? ...
 17:09: ... episode ideas survey and if you haven't yet, there's still time. It’s just a short 8 question survey that you can use to ...


20220420: Does the Universe Create Itself?
 06:24: ... never does. Why? Well if the photon was traveling both paths at the same time, then the combination of phase shifts in the beamsplitters causes the ...
 12:05: ... “Observation” could well be code for “interaction”, in which every time two particles bump together and become entangled we have an act of ...
 12:27: ... need to keep asking the right questions of a participatory space time. ...
 14:02: ... could be complete nonsense. But I can say with certainty that PBS Space Time is generated by participation of its observers  that’s you guys  and ...
 14:50: ... center of the universe and the location of the big bang are points in time in the past. So, they’re fair as long as you explain what you mean by ...
 16:10: ... also have such low energy that it wouldn’t have any effect on any sane timescale. ...


20220330: Could The Universe Be Inside A Black Hole?
 01:30: ... singularity is the Big Bang, which we think of as a point in time at the beginning of the universe when all matter was compressed to ...
 02:46: ... point of infinite density in space, while the big bang singularity is a time of infinite density that included all of ...
 03:54: ... possible to define a geodesic at some point in space and time  say, the arc of a ball thrown through the air  and then you can trace ...
 04:24: ... all the way back to the Big Bang  it’s defined for all past and future times independently of the ...
 06:03: A timereversed black hole is easy enough.
 09:47: Just flip the timel axis and you have a white hole containing a bubble of expanding space that looks much like our universe.
 12:16: ... infinitum in a series of black holes, forming an infinitely nested space time ...


20220323: Where Is The Center of The Universe?
 02:20: In this picture the Big Bang isn’t something that happened at a single point in space, instead it happened everywhere at the same time.
 02:57: His general theory of relativity explains gravity as the warping of space and time due to the presence of mass and energy.
 03:03: Explains gravity as the warping of space and time due to the presence of mass and energy.
 06:16: So far we’ve ignored the dimension of time.
 06:29: So, very crudely, we can think of the radial direction as the dimension of time.
 06:34: ... in the math by the scale factor  and the scale factor increases as time ...
 06:47: But it’s fun to think of the center of the expanding hypersphere as being a location in time.
 07:33: Draw a line in that direction and at the same time reverse the flow of time to see what it intersects.
 08:27: ... because the line traced from you finger also ends up at the beginning of time. ...
 08:41: They’re the grid that defines the fabric of space time in general relativity, and correspond to the paths followed by light.
 10:03: The math of the FLWR metric and the Friedman equations tell us that as time approaches zero, the distance between any two points approaches zero.
 10:13: But at the same time there are infinite points.
 10:23: Well the size of the universe at t=0 is zero times infinity … which is neither zero nor infinity  it’s the point where the math breaks.
 12:32: ... around the same time, Richard Tolman made the same discovery, and so we have the ...
 12:47: ... gigantic scales, and that if you zoom out to many, many, many times larger than the observable universe, everything evens out ...
 14:27: ... the ball stack bounce), and then goes into the speed of light and even time ...
 18:13: Sorry we didn’t have time to dig deeper on this.
 18:16: Another time.


20220316: What If Charge is NOT Fundamental?
 03:29: ... to have roughly equal numbers of protons and neutrons, and at the same time allowed precise predictions of the outcome of collisions between ...
 08:40: But that’s a story for another time.
 13:57: You can check it out over on PBS Voices, link in our description, and let them know (politely) that Space Time sent you.


20220308: Is the Proxima System Our Best Hope For Another Earth?
 06:18: Such a short orbital period, combined with the star’s mass, gave them an orbital radius for the exoplanet of around 20 times smaller than the Earth’s.
 06:29: That sounds perilous, until you realize that Proxima’s energy output is nearly 600 times lower than the Sun’s.
 07:47: Proxima C is much bigger than it’s earthlike neighbour, at 7 times earth’s mass.
 09:34: That would keep the same side of the planet facing the star at all times.
 12:32: By the way, unlike the Sun which grows brighter over time, red dwarfs fade.
 17:58: ... the quantum weirdness can play  those are the dimensions of space and time. ...
 18:09: Even including relativity and the shifting nature of space into time and vice versa, you still have a consistent if mutable underlying grid.
 20:12: ... strings may have had sufficient density at extremely early times to leave an imprint on the CMB on the density fluctuations from the ...


20220223: Are Cosmic Strings Cracks in the Universe?
 07:33: ... 2D defects called domain walls, but that’s for another time. OK, so we’ve managed to freeze the quantum fields amidst the ...
 08:50: ... loops keep forming from the original giant cosmic strings. Over time, the size of the largest loops increases, while at the same ...
 10:09: ... might be sensitive enough. Then there’s the Pulsar Timing Array  as we’ve described previously, it detects gravitational ...


20220216: Is The Wave Function The Building Block of Reality?
 00:03: ... no big deal for particles to be in multiple different states at the same time, or to teleport between locations, or to influence each other faster than ...
 05:00: The behavior of the wave function is described by the Schrodinger equation, which tracks its evolution through space and over time.
 05:09: ... of each other. Another thing about the Schrodinger equation is that it’s timereversalsymmetric. Running it forwards generates future states, but running it backwards ...
 09:25: ... of two locations, where it is both “here” and “there” at the same time. Put these two facts together, and you will get a superposition of two ...
 10:48: ... then measuring how long it takes for the superposition to collapse. This time should be proportional to the size of the object. Experiments are ...
 12:55: ... shifting realities give rise to our solid, familiar, and singular space time. ...
 13:19: ... for collapsing our wave function in the direction of making more space time. ...
 14:42: Last time we tackled a question of utmost gravity. Literally. How does gravity itself escape the inescapable gravity of a black hole.
 16:00: ... R asks how we can know that a black hole’s mass has time to crush down to the singularity, given that time dilation slows the ...
 16:15: ... time dilation that we talk about approaching the black hole event horizon is ...
 17:42: ... at the moment it crosses the horizon from its perspective takes infinite time to reach us. So to lose an object beneath a growing black hole means ...
 18:37: ... refer you to that episode for more answers. dannymac63 likens Space Time to a black hole, in which it’s captivating and radiates information ...


20220210: The Nature of Space and Time AMA
 00:03: ... dennis of space time i hope you could hear me okay this is the second or third in our now ...


20220127: How Does Gravity Escape A Black Hole?
 00:35: ... force, but rather in terms of the curvature of the fabric of space and time. ...
 01:21: ... with his special theory of relativity, which describes how lengths and times and other properties depend on how fast you are moving relative to the ...
 02:08: ... waves from colliding neutron stars reach us at about the same time the corresponding electromagnetic radiation from the ...
 02:36: It would take 8 minutes for us to notice the sudden darkness, and the Earth would continue to orbit the nowempty patch of space for the same time.
 09:46: ... star continue to make their way out into the universe over infinite time. ...
 12:00: ... to exert its gravitational influence on exterior regions of space time. ...


20220119: How To Build The Universe in a Computer
 02:28: Finally, he’d allow time to tick forward  he’d move the bulbs according to their new velocities.
 02:56: ... the time of Isaac Newton it had been possible to write down equations describing ...
 03:06: And then to solve those equations to find their positions at any time in the future.
 03:53: In an NBody simulation Newton’s laws of motion and gravity are applied over a series of time steps.
 04:10: The predictions of these Nbody simulations can be as accurate as you like, as long as you make the time steps small enough.
 05:14: For a modern onemillion particle simulation of a star cluster, that’s a trillion computations per time step.
 12:01: ... inside our computers, and then peering into that simulated space time. ...


20220112: How To Simulate The Universe With DFT
 01:44: This is the timeindependent version.
 02:30: ... time independent Schrodinger equation isn’t the be all and end all of quantum ...
 03:34: That’s 26 times the data right?
 04:52: ... to the Schrodinger equation is like adding dice to this system every time we add a particle we increase the dimensionality of the ...
 06:37: ... in that space where the particles actually exist at a given point in time. ...
 17:23: We actually interviewed Stephenson on this show about the book back in the day, and at the time I was too polite to say  absolutely not possible.


20211229: How to Find ALIEN Dyson Spheres
 00:25: Time to update you on the hunt for galactic empires.
 01:00: ... radius of Earth’s orbit, we’d collect all of the Sun’s light  a billion times more than what we’d get just covering the planet in solar ...
 04:29: ... would need to be in order to be detected with the technology of the time. ...
 10:01: ... instruments to look for stars that were both too dim and too infrared, time and again they came up short. But the search has expanded massively. ...
 10:56: ... spheres is to look for strange variations in brightness and colour over time, as might be expected if a giant orbiting structure partially eclipses ...
 13:55: ... by younger species when they emerge in distant, future parts of space time. ...


20211220: What Happens If A Black Hole Hits Earth?
 00:12: ... probably nonsense. And I imagined that it was just a matter of time before a black hole finally found its way to the Earth. So that part ...
 01:49: ... dense. Countless black holes might have formed at the earliest of times, while still leaving plenty of matter left for ...
 04:01: ... about 10^18 kilograms of dark matter in the solar system at any given time. If that dark matter is tiny black holes then we might expect dozens, ...
 13:02: ... the crazy blackholespawning chaos that defined the birth of this space time. ...
 13:13: ... your thoughts into the comments and let them know, politely, that Space Time sent you. There’s a link in the ...
 13:52: ... they’re first released, you actually help spread the show to other Space Time subscribers and to new viewers. So if you’re already part of the early ...
 19:14: ... are technically quantum. This teaches us one important lesson: next time you clean up after your cat try not to get holographically projected ...


20211210: 2021 End of Year AMA!
 00:02: ... hi welcome to the end of year space time ama i'm meadow dowd as you may know um and i'm very excited to be ...


20211117: Are Black Holes Actually Fuzzballs?
 10:58: It would still cause massive gravitational lensing, time dilation, etc.
 11:28: It’s not that the interior is empty, but rather that space and time literally end at the surface.
 13:32: We can come back to those another time.
 13:43: Perhaps that’ll come when we find evidence of fuzziness surrounding those distant holes in the fabric of space time.
 14:10: ... the ball stack bounce), and then goes into the speed of light and even time ...
 14:35: The Space Time audience has always had a great showing on the annual survey and we’d love for that to continue this year.
 14:49: And we really do spend quite a bit of time analyzing and discussing the results.


20211110: What If Our Understanding of Gravity Is Wrong?
 00:36: They would need at least 5 times as much matter to provide the gravity needed to hold these galaxies together.
 01:02: ... stuff dark matter, and of course we’ve talked about dark matter many times on this channel  from the evidence for its existence to some of the ...
 01:18: But what if we’ve been thinking about this the wrong way all the time?
 02:52: ... which explains gravity as the curvature in the fabric of space and time rather than just as a classical ...
 10:29: ... pulled itself together under its own gravity at the earliest times. ...
 11:12: But if dark matter isn’t real, and regular matter controls gravity completely, then no structure should have been able to form at those early times.
 11:38: Their big change was that they allowed the scalar field to change its behavior over time.
 12:25: Modified gravity theories still can’t explain the Bullet Cluster  and I don’t have time to get into that and we’ve covered it before.
 14:01: ... or beyond general relativity by hidden gravitational modes of space time. ...
 15:32: Check out Vitals in the link in the description, and tell them that Space Time sent you!
 17:17: A few people pointed out an error  I said that the action reduces to an integral over proper time in general relativity.
 17:26: That was right  but I then went on to call this a “principle of least proper time” by analogy to the principle of least action.
 17:33: In fact, in general relativity objects in gravitational fields tend to maximize, not minimize their proper time.
 17:39: ... action  and the maximum is also a stationary point  of proper time and of the ...


20211102: Is ACTION The Most Fundamental Property in Physics?
 00:02: ... about time we discussed an obscure concept in physics that may be more fundamental ...
 01:22: ... you needed to do was know the exact vector forces on each body at each time, which lets you calculate its exact vector positions, velocities, and ...
 02:50: ... of kinetic and potential energy had the same minimizing property as does time for the path of light. He found that the ball will always follow the ...
 03:17: ... called this new quantity the Action. Formally, it’s the integral over time of the kinetic minus potential ...
 03:33: ... is zero, and that’s true at both the minima and maxima. Most of the time it’s the minimum, which is why most people still call this the Principle ...
 05:52: ... the force of gravity in terms of the bending of the fabric of space and time. One of Einstein’s motivations in developing general relativity was the ...
 06:36: ... is writing the Lagrangian as a function of something called proper time. ...
 06:47: ... things like relative speed and position in a gravitational field. Proper time is the time that an object will perceive in its own reference frame. If ...
 07:09: ... corrections, the Action simplifies into just the integral over proper time. Let's take a moment to think about that. Before Action was a somewhat ...
 08:06: ... figured out that light always travels the path that minimizes its travel time  the principle of least time. Well it turns out that this is just a ...
 08:39: ... the other side the individual particles make individual spots, but over time the spots make out this series of bands  an interference pattern. The ...
 09:56: ... quantum analog of the action existed that was related to the integrated time evolution of the wavefunction. And he realized that this quantity should ...
 10:26: ... come from adding up kinetic minus potential energy nor the proper time. Instead, it effectively calculates the phase shift that a particle picks ...
 12:16: ... configuration spacetime, where the shortest path minimizes proper time. In quantum mechanics configuration space could mean phase space  the ...
 13:46: ... the Action  pointing the surest way to the fundamental nature of space time. ...
 15:27: ... And honestly, I started out wondering the same and it took me a lot of time to get an inkling that there probably is something in it. I’m still not ...


20211020: Will Constructor Theory REWRITE Physics?
 00:42: Come up with a set of equations that predict how those numbers change over time  e.g.
 00:51: Based on some initial state  a starting set of these numbers predict how the system will evolve at all future times Step 4.
 06:14: For example, a wheel powered by falling water that also pumps that same water back up to the top while at the same time driving an electric generator.
 11:04: ...  of what is possible and what is impossible within this physical space time. ...
 11:25: ... military veteran and a special object they have from their time in service — a vial of lip gloss, a small stone, even a ...
 11:52: ... Ari Paul. Ari, without your support, the task of Space Time would be impossible, which in the language of constructor ...
 12:21: Last time we talked about the latest ideas on the weird world of quantum tunneling. Let’s see what you had to say.
 12:30: ... until some event causes a choice of position to be made. At that time the position is chosen from all possible positions, ...
 14:00: ... the barrier, and its wavefunction does seem to take time to traverse the barrier. It’s very, very difficult to define a ...


20211013: New Results in Quantum Tunneling vs. The Speed of Light
 00:55: Now we covered quantum tunneling a long long time ago  in fact it was the first video we did on quantum mechanics.
 04:22: For example, is the transition of the particle from one side of the barrier to the other instantaneous, or does it take some time?
 04:31: ... turns out it’s very hard to determine the socalled tunneling time because, in the fuzzy world of quantum mechanics, it’s hard to even ...
 04:43: One thing is clear at least  for a number of definitions of tunneling time, fasterthanlight movement, really does seem to be a thing.
 04:51: ... physicist Thomas Hartman in 1962, who found that for one definition, the time taken to tunnel can become independent of the thickness of the ...
 05:02: In other words, you can double the length of your barrier, and your particle will take the same amount of time to travel all the way through.
 05:45: It came down to the definition of tunneling time.
 05:56: It seems natural to define those times as whenever the center of the wavefunction passes the start and end points.
 06:12: ... like if you measured a train’s travel time through a tunnel by clicking a stopwatch as the center of the train ...
 06:24: You’ll get a shorter time than if you clicked for the same point at both tunnel ends.
 06:45: It’s hard to measure the travel time of a quantum train OR a quantum wavefunction because it’s hard to define the start and end points.
 07:20: So you can see how the question of tunneling time is a bit messy.
 07:45: ... previous work on tunneling time relied on the Schrodinger equation, which doesn’t incorporate ...
 08:40: And the thicker the barrier, the bigger the difference in arrival time.
 09:09: The study finds that the average travel time for tunneling particles is shorter than the average time the freeflying particles.
 10:16: ... as the theoretical calculations  how do we define the tunneling time? ...
 10:29: And even trickier, how do we define a tunneling time that we can actually measure?
 12:21: Because as we’ve discussed many times before, when it comes to the speed of light, the house always wins.
 13:06: ... the ball stack bounce), and then goes into the speed of light and even time ...


20211005: Why Magnetic Monopoles SHOULD Exist
 01:11: ... according to classical electromagnetism, it doesn’t matter how many times you slice it  you’ll never get isolated magnetic charges  what we call ...
 11:47: They should also be very massive  quadrillions of times the mass of the proton  and so should have quickly recollapsed the universe.
 13:13: ... not so surprising given that the LHC reaches energies about 100 billion times lower than is needed to produce the monopoles predicted by grand unified ...
 13:48: And so many of us remain obsessed with this elusive beast, and convinced of its inevitability according to the symmetries of space time.


20210921: How Electron Spin Makes Matter Possible
 14:26: ... mysteries. But that is a risk we’ll always take for you here on space time. ...
 16:27: ... So I came along only a little after, I imagine those were giddy exciting times! There was speculation that quasars could be swarms of neutron stars or ...
 18:44: ... finally, Steve Bogucki tells us that Space Time made him realize that he’s more interested in quantum physics than ...


20210915: Neutron Stars: The Most Extreme Objects in the Universe
 00:00: ... on this show  from the interiors of black holes to the time before the big bang. But today I want to take you on a journey that ...
 01:02: ... is to really explore the insides of these beasts. And it’s time we did so, because there we’ll find states of matter that exist ...
 01:42: ... Even the weakest neutron star fields are a billion times stronger than those of the earth or ...
 03:27: ... while our feet are in a foggy plasma with a density many million times greater then anything on Earth. And the ...
 06:38: ... outside a neutron star. Where the star is 50 billion times the density of earth, we might find a nucleus like Zinc80, ...
 07:06: ... it’s close to half a kilometer deep, and we’re at least a trillion times the density of matter on Earth. We are really relying on our ...
 08:15: ... localize it to being inside a given nucleus. By the time we reach the bottom of the crust, around a kilometer deep, ...
 09:34: ... even be the strongest material in the universe, a quintillion times stronger than ...
 11:14: ... the time we reach the bottom of the pasta layer, just above the neutron star ...
 12:23: ... we’re not sure if they exist in neutron stars. The only other time matter existed naturally in conditions like these was within a ...
 13:06: ... even more impossible than it already is. I think it’s time to power up our antigravity boosters and leave behind ...


20210907: First Detection of Light from Behind a Black Hole
 00:20: A few weeks ago a story made the rounds of popsci media proclaiming that for the first time light had been detected from behind a black hole.
 00:56: ... thought it would be worth doing a space time journal club on the Nature paper by Wilkins, Gallo, Costantini, Brandt ...
 02:36: We do this by watching how the spectrum of the quasar changes over time.
 03:13: ... largest black holes in the universe  anything more than a million or so times the mass of the ...
 06:35: ... the same time, the response of the gas on the far side of the quasar is slower than the ...
 08:28: In that case inner gas is moving faster like in the infall case, but now the red and blue sides should respond at roughly the same time.
 09:18: We’ll be able to watch millions of quasars all do their thing at the same time.
 09:23: OK, it’s time we got back to the discovery that started our little journey  the light that was detected from behind a black hole.
 11:54: ... how light reverberates around the most extreme regions of space time. ...
 12:10: It can be hard to find the time to sit down and read a whole book, but now you can in the blink of an eye.
 16:18: ... quips "If we figure out how to send messages backwards in time, I'll make sure I'm the first to know.” If I figure out how to send ...


20210818: How Vacuum Decay Would Destroy The Universe
 05:03: ... vacuum. This is where the field would prefer to spend its time given the choice. And then there’s another local minimum with a ...
 10:10: ... as decay is possible, it’s inevitable. At any instant in time there’s a tiny but real probability that a patch of ...
 12:07: ... I guess not. Good. So, just for now let’s enjoy whatever time we have left  perhaps mere billions of years  before ...


20210810: How to Communicate Across the Quantum Multiverse
 00:00: ... Hello There. I’m Matt from a different quantum timeline. I figured out the secret truth behind quantum mechanics and I’m sending ...
 03:30: ... how the wavefunction of a quantum system changes over space and time  and so it should completely determine the measurements we can make of ...
 08:55: ... enabling instant communication at any distance, and even backwards in time. Now Polchinski doesn’t actually tell us how to do this  he only proves ...
 09:36: ... to communicate between the worlds of the quantum multiverse. And this time he actually tells us how to do it, inventing what he calls the ...
 11:53: You’ve now successfully transmitted a single bit of information between quantum timelines.
 12:47: ... and we can instantly communicate across any distance and back in time, OR we can communicate across the branches of the quantum multiverse. ...
 13:40: ... If you check it out, remember to tell them, politely, that Space Time sent ...


20210803: How An Extreme New Star Could Change All Cosmology
 00:47: ... things that go bump in the night  astrophysical objects that vary over time. Astronomers using the ZTF caught a white dwarf that, at first glance, ...
 01:45: ... These are white dwarfs, the final fate of any star less than 8 or so times the mass of the ...
 03:19: ... the presence of gigantic magnetic fields. Fields around a billion times stronger than the earth or sun’s magnetic field. That’s at the top tier ...
 08:05: ... a little quantum mechanics, it was found that it must weigh in at 1.32 times the Sun’s mass. And that’s a lot, at least for a white dwarf. We’ve ...
 12:50: ... is a 1000 times denser still. That means it can support incredibly energetic electrons ...
 13:41: ... clue towards a better understanding of this generally weird space time. ...
 14:14: ... of countless white dwarfs that have merged since the beginning of time. With the help of your support we’ve deciphered their song, come to us ...
 15:05: ... last time we took a magnetic tour of the universe, exploring how magnetism shapes ...


20210721: How Magnetism Shapes The Universe
 03:38: Now I don’t want to spend too much time in the solar system  greater magnetic wonders lie beyond.
 04:12: That means the dipole field gets twisted up over time.
 05:34: That was measured for the first time only last year.
 12:59: ... such, astrophysicists are spending more and more time learning how to map and to model magnetic fields, to better understand ...
 18:33: And anyway, as many of you pointed out  we don’t really need wavefunction damping if we have the time variance authority pruning worlds for us.


20210713: Where Are The Worlds In Many Worlds?
 00:00: ... many worlds interpretation of quantum mechanics proposes that every time a quantum event gets decided, the universe splits so that every possible ...
 02:13: You can calculate the overlapping pattern at any point in time by calculating the motion of all ripples separately and then adding them together.
 02:58: At the risk of getting technical, the superposition principle holds for any linear system  and I’ll say more about that another time.
 04:38: To calculate this we use the Schrodinger equation, which tells us how the amplitude of the wavefunction changes over time and space.
 12:01: On a quantum scale, worlds  or wavefunction components  recombine all the time.
 12:26: But that’s for another time  and I’ll do my best to bring it to every future branch of our splitting quantum space time.


20210707: Electrons DO NOT Spin
 03:20: ... levels to split even further  for reasons that were, at the time, a complete mystery. One explanation that sort of works is to say ...
 09:51: Another thing to notice is that the cube can rotate any number of times, with any number of ribbons attached, and it never gets tangled.
 12:13: ... intrinsic angular momenta can only be observed as plus or minus a half times the reduced Planck constant, projected onto whichever ...
 13:49: ... time we talked about the connection between quantum entanglement and ...
 15:09: ... asks the following: If entropy only increased over time, which implies it was at its minimum at the Big Bang, does that ...
 15:31: ... we can’t really talk about the t=0 beginning of time, because that moment lost in our ignorance about quantum gravity and ...
 17:29: ... As with many of these things, the word has been corrupted over time and is now routinely ...


20210623: How Quantum Entanglement Creates Entropy
 00:27: ... of thermodynamics may be responsible for the arrow of time and is a key ingredient in solving the black hole ...
 02:13: ... may help us understand not just the 2nd law, but also the arrow of time and how the large scale world emerges from the quantum world in the ...
 13:09: ... And, as an extra trick, it also defines the arrow of time which itself points in the direction of increasing entropy ...
 17:55: ... asks one that I wish I’d had time to discuss in the episode. When using the Heisenberg ...
 18:54: ... all the gurgling and screaming that must wreath the earth at all times. Small ...


20210616: Can Space Be Infinitely Divided?
 00:00: ... many times can I half the distance between my hands? Assuming perfect ...
 00:40: ... no limit to the number of times you can half a number, but the same might not be true of ...
 02:21: ... way to give units of length  the square root of G times hbar over c^3. Where hbar is just the Planck constant divided by ...
 03:14: ... it, which reflects back to a detector that records the light travel time, which also gives you the distance because you know the speed ...
 06:39: ... Space is stretched by a factor equal to the effective mass times the gravitational constant divided by c^2. Let’s replace the ...
 07:10: ... also reduces the regular Heisenberg uncertainty, but at the same time this new uncertainty increases. You’re still ...
 08:37: ... the uncertainty in energy density in that volume. By the time we localize the electron to within one Planck length, ...
 10:55: ... we can’t sensibly define distances. We think that space AND time  spacetime  “go quantum” at that scale  but we just don’t ...


20210609: Are We Running Out of Space Above Earth?
 00:15: And while it made a lot of people very nervous to know a 22 ton rocket was going to fall out the sky, this sort of thing happens all the time.
 01:36: The quickest lowaltitude orbits circle the Earth 16 times a day.
 04:31: ... space junk because the amount of drag can change  in particular, during times of high solar activity when more energy is pumped into the atmosphere, ...
 06:03: Satellites in low earth orbit may need to be boosted a few times a year to prevent this inspiral.
 07:30: For example, we have the times that nations deliberately destroyed their own satellites.
 07:58: But the timing after the Chinese launch does make you wonder.
 09:45: ... estimated that the first satellite collision would probably happen some time in the next two to three decades, which was pretty spot on with the ...
 09:58: But the time to the next collision should be shorter  and indeed there have been several now, although thankfully none as cataclysmic as the first.
 10:53: I should add that StarLink will be in very low orbit, and so has a fast decay time  under 5 years.
 11:40: The danger is for objects spending any lengthy period of time in those zones.
 13:14: ... cosmic front yard, and continue our safe use of Earth’s orbital space time. ...
 13:28: This show would be much, much harder to do without the help of the Space Time community on Patreon.
 15:16: Two hundredths of a milligram is giant compared to the 1.6x10^35 meters of the Planck length or the 5x10^44 seconds of the Planck time.
 16:19: ... also a great excuse for why you didn’t finish your physics homework on time  no selfrespecting physics professor can confidently say that a planck ...


20210525: What If (Tiny) Black Holes Are Everywhere?
 05:39: The black hole loses energy one photon at a time, but the process seems smooth and continuous.
 08:48: This is the hypothetical time just when the universe was expanding exponentially quickly, and can be thought of as the bang in the big bang.
 12:30: ... should keep an open mind about the hypothesis while at the same time being extremely cautious about the pitfalls of confirmation ...
 14:50: In fact it was used for the first time in the most recent observing run.


20210519: Breaking The Heisenberg Uncertainty Principle
 01:36: Exact measurement of its energy means its location in time is blurred.
 03:31: ... example, how can a quantum object sometimes be a “wave” and some times be a “particle?” In a sense it is both, and in a sense it is neither  ...
 05:10: Those ripples are tiny by the time they reach Earth  they change lengths by something like 1 part in a billion trillion in the most powerful cases.
 10:17: ... possible to force ever more certain measurements of this uncertain space time. ...


20210511: How To Know If It's Aliens
 00:00: ... follow one rule on Space Time: It’s never aliens. But every rule has an exception and this rule is no ...
 03:17: ... magnetite  a common byproducts of microbial metabolism which at the time had no known abiotic source. This meteorite was special in other ways ...
 09:58: ... There was one idea that a pair of comets were in the radio beam at the time, but the Ohio State astronomers insist that those comets were not in the ...
 13:10: ... finally prove that we’re not the only living denizens in all of space time. ...
 14:48: ... conjecture states that the universe will always find a way to make time travel impossible  and it’s been speculated that quantum gravity ...


20210421: The NEW Warp Drive Possibilities
 00:15: He showed us that space and time can be warped  and so the warp drive was conceived.
 00:31: In 1915, Albert Einstein’s general theory of relativity revealed that the fabric of space and time is mutable and dynamic.
 01:27: Spacewarp drives, timewarp drives, and eventually just warp drives became an increasingly popular way of breaking Einstein’s rules.
 02:12: Today we’re going to pull them apart two new papers on warp drives and see if it’s really time to don our Star Trek cosplay and warp out of here.
 06:06: ... is that any fasterthanlight travel can be used to create closed time like curves, as we discussed ...
 12:10: And of course we still have the time travel issue  accelerating across the speed of light breaks causality and so probably can’t be a thing.
 12:47: And possibly also building a starship, to propel humanity into the galaxy on waves of warped space time.
 12:55: Space Time gets an enormous amount of help from our Patreon supporters.
 14:26: At nearly 17 times the mass of the muon, the tau should be even more sensitive to unknown particles.
 14:33: The problem is that the tau’s lifetime is around 10 million times shorter that the muon, which itself lives only for only microsecond or so.
 16:22: ... arrow isn’t moving because it’s stationary at each separate instant in time. ...
 16:40: Actually, in quantum mechanics, velocity isn’t just change in position with time.
 16:49: So a particle at an instant in time really does have an instantaneous velocity.
 17:47: So maybe Odin learned his quantum mechanics watching space time like the rest of us.


20210413: What If Dark Matter Is Just Black Holes?
 01:13: As we’ve discussed many times before, black holes are regions of gravitational field so intense that not even light can escape.
 02:28: We have, of course, been trying to find evidence for black hole dark matter for some time.
 03:31: When we look into the distance we’re actually looking back in time, so we can literally see star formation happening in the early universe.
 03:57: Also, if dark matter is produced as stars die, you’d expect its influence to increase over time.
 05:03: ... fluctuations would have been much stronger at earlier times and perhaps strong enough that the most massive of them would have ...
 05:43: The most massive black holes in the universe weigh in at millions to billions of times the mass of our Sun.
 06:30: I’m talking somewhere between a billion to a billion billion times more of them than there are stars in the universes..
 08:52: ... us to rule out MACHOs between roughly the moon’s mass to 10 or so times the mass of the Sun as a main contributor to dark ...
 10:20: ... similar constraints  if there were lots of black holes of several tens times the Sun’s mass then these binaries would long ago have been torn apart ...
 11:39: Whatever the case, dark matter is freaky stuff, fitting as the main material ingredient of our generally freaky space time.


20210407: Why the Muon g2 Results Are So Exciting!
 01:40: Let's take the one at a time.
 06:10: Over time, more and more complicated interactions were added.
 08:16: The muon is 200 times more massive than the electron.
 08:26: So the muon, is 40,000 times more likely to be perturbed in this way.
 08:31: It's 40,000 times more likely than the electron to encounter, say a virtual Higgs boson, or virtual proton or other hadrons.
 08:39: And it's 40,000 times more likely to encounter any completely unknown virtual particles that might be hiding out there.
 09:32: ... push closer to this level of confidence, and is designed to achieve four times the sensitivity of the Brookhaven ...
 11:05: The hallowed five sigma confidence will take time and many more muans.
 11:42: Perhaps this time, some of them will be right.
 11:56: And that dance, may just have revealed to us the next step on our path to a more complete understanding of our quantum space time.


20210323: Zeno's Paradox & The Quantum Zeno Effect
 00:24: Here’s how it goes: if you look at an arrow flying through the air at any instantaneous snapshot in time, the arrow doesn’t travel any distance.
 00:33: ... time is composed of an infinite number of these snapshots, and the arrow ...
 00:50: ... think of an instant as a vanishingly small interval of time, then during that time the moving arrow travels a vanishingly small ...
 01:07: That’s what a mathematician might tell you  but quantum mechanics also has a thing or two to say about extremely tiny distances and time intervals.
 12:27: So there you have why a watched quantum pot never boils. And it's because you did watch space time.
 12:55: ... you’ve enjoyed our recent series on time in general relativity and you want to dig deeper, you should check out ...
 13:04: ... the course you’ll explore everything from time in quantum mechanics to black hole entropy and learn what cuttingedge ...
 13:22: ... time we gave you an update on the crisis in cosmology  the tension between ...
 14:38: In might, for example, tell us that dark energy has changed over time  which might help us figure out what dark energy actually is.


20210316: The NEW Crisis in Cosmology
 00:22: ... And guess what  the tension is now even tenser. So is it time to rethink all of ...
 05:52: ... that the nature of dark energy has changed over time. The Planck team’s Hubble constant assumes that dark energy has had ...
 06:05: ... energy might be, But if dark energy has HAS changed over time it could explain the discrepancy AND indicate the dark ...
 08:03: ... measurements, for the nearest brightest stars, it’s 200 times more accurate than any previous ...
 10:40: ... flicker  but they flicker out of sync. There’s a time offset due to the fact that these different paths through the ...
 11:57: ... bars they give are still large, they’ll only get smaller over time. ...
 12:52: ... a real difference and also gets you access to the Space Time discord where you can nerdout 247. And today’s extra ...
 13:30: ... we explored the connection between gravity, light, and the flow of time. ...
 13:38: ... Timebucks asks how can we be traveling at the light speed in the time ...
 14:47: ... That's less clear, because we don't really know what time is. Nor space for that matter  or whether they're ...
 17:08: ... star, forest moons, ice planets  must be hard to recalibrate every time. Who knew George Lucas was such a jedi master of general ...


20210309: How Does Gravity Affect Light?
 01:38: ... as the force of gravity is mostly due to the way mass warps the flow of time. ...
 01:47: But the photon doesn’t experience the flow of time  it doesn’t even have any mass.
 02:43: By the time the beam reaches its target, the ship is moving a little faster than when it fired.
 03:50: And we get exactly the same prediction if we use the fact that time runs slow in gravitational fields.
 04:37: ... the event horizon of the black hole, gravitational time dilation is so strong that clocks stop and the frequency of photons ...
 05:19: This time you fire your laser across the deck perpendicular to the direction of acceleration.
 06:00: ... physical explanation for the prediction  the difference in the flow of time changes the frequency of outgoing ...
 06:10: Can this gravitational time dilation also explain the bending of a ray light traveling horizontally?
 06:31: To get technical: any massive object has a component of its 4dimensional spacetime velocity  its 4velocity in the time direction.
 06:47: But light is frozen in time from its point of view.
 06:51: It has no timecomponent to its velocity.
 06:53: It has, in fact, no time to lose.
 06:57: ... if we imagine light as a perfectly narrow ray, or even as a massless, timeless particle, none of our intuitive explanations say that it should be ...
 07:58: At any point in time, the expanding ripple can be thought of as an infinite number of sources of new circular ripples, or wavelets.
 08:47: ... closely packed  the fronts of those ripples don’t travel as far in the time it takes to make a new ...
 10:12: A photon passes by, and the amount of time it takes to cross that space is larger than if Earth wasn’t there.
 10:31: The photon has to travel further through a region of slowed time  and both conspire in the same direction to slow the apparent speed of light.
 11:02: But for you, tracking this from a distance, the effective speed of light decreases downwards, because time slows and space stretches.
 12:08: ... is due to a freefalling waterfall of space, rather than a gradient in time. ...
 12:32: Light is a wave and a particle; time slows or space flows in gravitational fields.


20210224: Does Time Cause Gravity?
 00:00: You are currently hurtling through time at the speed of light.
 00:10: And you’re standing in the wrong place at the time, you will rapidly accelerate to your doom.
 00:34: ... our recent episode, we saw why this gravitational time dilation is inevitable  it follows as surely as 1+1=2 if we accept the ...
 01:17: But I never explained WHY or HOW gravity causes the flow of time to slow down.
 01:28: Gravity does NOT warp the flow of time.
 01:30: It’s the other way around  the warping of time causes gravity.
 01:45: So how is it that time causes gravity?
 02:07: Everything is moving through the dimension of time.
 02:13: Let’s have just two dimensions of space and so we have space … for time.
 02:18: We show progression through time as the object moves up.
 02:21: You could say that it has a positive velocity through time, and zero velocity through space.
 02:33: We know that the presence of mass and energy warp spacetime  and the most intense part of that warping is in time  our gravitational time dilation.
 02:42: Things closer to the Earth move through time more slowly.
 02:55: Velocity through time increases away from the Earth.
 02:58: ... we move particles through time according to those velocities, we have this sense of time flowing in a ...
 03:16: It’s almost like Earth’s mass creates a drag on the flow of time around it.
 03:22: So what happens to an object sitting in this stream of time  parts further away from the Earth age faster, right?
 03:41: And each of those clocks has a velocity vector in time.
 03:49: In Einstein’s relativity you have to remember that time and space are not independent of each other.
 03:54: Objects don’t just have a velocity through space or through time  they have a velocity through spacetime.
 04:47: All individual 4velocities start out being purely in time, but the sum is rotated partially into space.
 05:03: ... it pays for that acceleration by losing velocity  Decelerating in the time ...
 05:31: But if we interpret time as a dimension like space, then a stationary mass really is moving at the fastest possible speed in the temporal direction.
 05:40: This is something we can come back to another time  for now let's go with it.
 05:44: The 4velocity of a massive object is pointed almost entirely in the time direction.
 05:50: On the other hand, light itself travels at the speed of light through space only, and not at all through time  a photon’s clock is frozen.
 05:57: ... might imagine it’s 4velocity is entirely rotated out of the time direction into space  although technically photons and other massless ...
 06:13: In this picture, a falling object trades some of its enormous velocity through time to pay for a small velocity through space.
 07:11: ... is always at multiple places at once, and so experiences the gradient of time ...
 07:29: It’s enough to imagine clocks that are infinitesimally separated and we still have our time gradient.
 07:45: They have no “velocity through time” to trade.
 07:48: But light DOES bend in a gravitational field  astronomers see it happening all the time in the effect we call gravitational lensing.
 08:08: ... our perspective in a couple of mindbendy ways to see how the flow of time determines the path of even timeless ...
 08:29: ... first you’re going to need to some time to think on everything I’ve just told you and let it settle, and we’re ...
 08:39: Last time we talked about the gravitational wave background  the ambient buzz of gravitational waves from the distant and ancient universe.
 08:46: Which, by the way, we may have detected using a pulsar timing array.
 09:10: ... Lora asks if I can elaborate on my comment “the time before the big bang”  which I mentioned in reference to a potential ...
 09:44: ... eternal inflation model, inflation may have lasted for a very very long time and still be continuing almost everywhere  but it ceased in isolated ...


20210217: Gravitational Wave Background Discovered?
 00:00: ... the entire universe must be thrumbing with these vibrations all the time and everywhere we call this underlying buzz the gravitational wave ...


20210210: How Does Gravity Warp the Flow of Time?
 00:00: ... will age approximately 1 second more than your head due to gravitational time dilation  and that’s assuming that your life is long and that you’re ...
 00:11: But that tiny difference in flow of time may be what keeps you stuck to this planet at all.
 01:30: General relativity, or “GR” explains the force of gravity as being due to curvature in space and time.
 02:22: Matter tells space AND time how to curve, and it’s the curvature of time that’s mostly responsible for telling matter how to move.
 02:30: There’s a deep connection between gravity and time  gravitational fields seem to slow the pace of time in what we call gravitational time dilation.
 02:43: ... ultimately, we’ll use what we learn to understand how curvature in time  this gradient of time dilation  can be thought of as the true source ...
 03:07: ... going to start out by me totally convincing you that time must run slow in a gravitational field  an effect we call gravitational ...
 03:17: But to do that I need to give you a quick refresher on regular old time dilation, which tells us moving clocks must appear to tick slowly.
 04:03: A counter ticks over every time the photon does a full cycle.
 04:14: And, in fact, to any matter  anything that can experience time, which in practice means anything with mass.
 04:23: ... amount of time taken for one tick of the photon clock is the distance the photon ...
 05:17: And this apparent slowing of time appears for everything in the moving lab.
 05:38: Time dilation due to motion is inevitable if we accept the axiom of the constancy of the speed of light.
 05:45: To get to gravitational time dilation all we need to do is add in the equivalence principle as our second axiom.
 05:52: It tells us that whatever we conclude about the passage of time in an accelerating frame must also be true in a gravitational field.
 06:46: Over that brief interval we know perfectly well what the time difference is between the two frames of reference.
 06:52: Both observers see the other’s time has slowed.
 07:01: And it turns out that the stationary clock did tick more  time slowed for the rotating case.
 07:14: The summary is this: two observers moving in straight lines to each other do perceive the other as timedilated  slowed.
 07:36: We can see that when we use a spacetime diagram to show how the traveler tracks the passage of time back on Earth.
 08:15: ... lines of constant time for the moving clock tilt back and forth, and as that line tilts it ...
 09:00: ... field must experience the same sense of weight AND the same time dilation that you would get from being spun in a circle at the right ...
 09:16: ... the speed of light and the equivalence of acceleration and gravity, then time must run slow in gravitational ...
 09:25: ... of blows me away that you can calculate the difference of the flow of time between an inertial and accelerating frame using pure special relativity ...
 09:53: ... the case of the twin paradox, gravitational time dilation gives the right relative time flows if you consider the ...
 10:08: Well, as deep as the distance back to Earth  which is why the time dilation in this case is so huge, even if the acceleration is mild.
 10:17: ... very different from our rotating space station then both gravitational time dilation and kinematic time dilation play separate ...
 10:38: Well no, it’s telling us that the source of the time dilation is fundamentally the same.
 10:45: We’ve reasoned our way to seeing that gravitational time dilation must be a thing if our axioms are right.
 10:58: What really is it about the gravitational field that’s causing time to tick slow?
 11:40: But ultimately, asking “why does gravity slow time” is a bit backwards.
 11:46: A better question may be “why does slowed time cause gravity”.
 11:56: You’re held in your chair right now by the curvature in time.
 12:04: And I’ll show you exactly why that’s true real soon, when we explore the tangled connections between time and gravity in a curved spacetime.


20210126: Is Dark Matter Made of Particles?
 00:00: By the time I finish this sentence, up to a billion billion dark matter particles may have streamed through your body like ghosts.
 01:27: But those possibilities are for another time  today we’re focusing on the bizarre physics of the dark sector.
 05:35: For a long time people thought the neutrino might be dark matter  being neutral and the most abundant known particle in the universe.
 07:09: There are some exceedingly clever experiments to do so  like we saw that time we visited FermiLab.
 08:35: ... in the insanely energetic early universe, and the leftovers from that time could still be throwing their weight around, so to ...
 10:33: ... existence constantly, borrowing energy from the crazy radiation of that time. ...
 10:57: We were left with a universe full of particleantiparticle pairs that would then just annihilate over time.
 12:32: We’ll talk more about those experiments another time.
 14:45: So by the time the base of the ladder even knows that anything at all has happened to the front of the ladder, it’s already inside the barn.
 14:59: ... acceleration, so shouldn’t acceleration be used to account for the time ...
 15:25: ... can we apply the gravitational time dilation to the traveling twin, as though they were standing on the ...
 15:42: It turns out that there’s no distinction between all the types of time dilation as far as the universe is concerned.


20210119: Can We Break the Universe?
 00:26: But from that can the inevitable conclusion that space and time themselves were relative  depended on the velocity of the observer.
 01:17: This is called time dilation.
 01:52: ... astronaut and an observer back on Earth about the relative passage of time and the distance traveled  but those aren’t paradoxes because both ...
 03:42: And during that time she sees her brother’s clock ticking slower.
 03:45: So if both perceive the other’s clocks ticking slow, who has experienced less time by the time they reunite?
 04:12: We have time on the yaxis and just one dimension of space on the x.
 04:15: Let’s say we’re in Earth’s reference frame so the Earth doesn’t move in space  just straight up, which means forward in time.
 04:22: The spaceship moves in both space and time  first away from the earth and then back towards it.
 04:33: ... line  all events corresponding to your notion of a given tick on the time ...
 04:57: ... the twin paradox because they allow us to track the apparent passage of time back on Earth from the point of view of the ...
 05:07: She counts every time a new years day happens on Earth according to her calculations.
 05:12: ... the spacetime diagram, that’s whenever one of these lines of constant time extending from January 1st on earth crosses the spaceship’s path  its ...
 05:28: In total, the traveler counts fewer Earth years because she misses some time in the middle corresponding to the turnaround point.
 05:36: Those years do happen back on earth, but they don’t correspond to any time point that happens to the astronaut.
 06:41: A line of constant time for the former is just a circle around the cylinder.
 07:12: And those future versions are older  by just enough to agree with the time dilation that the stationary twin would calculate.
 07:31: There’s a stationary frame of reference where lines of constant time are closed loops, while in every other frame they are endless helices.
 07:54: In the case of the closed universe, that closed topology DOES pick out a special frame  it’s the one with these closed constant time loops.
 08:46: We’ll also open and close the barn doors, timed so that the ladder has just enough time to pass through each.
 11:15: Remember that the line of constant time is a helix.
 11:40: They exist at the same point in space at different points in time.
 11:44: ...  safe from collision even if it wraps around the universe multiple times. ...
 12:41: ... the speed of light, causing them to wrap around the universe several times and ...
 15:40: This is the first time the flourescence has been directly observed, so it lends credence to the theory.
 16:05: ... don'thave time to go through all of them, so I'll just give Matteo Alessandro's ...


20210112: What Happens During a Quantum Jump?
 01:02: That proof required precision measurements that didn’t exist in Schrodinger’s time.
 06:11: But in time we figured out how to do exactly that.
 06:59: In the 1986 experiments, the electron in the trapped atom jumped between levels something like 100 million times per second.
 07:52: Then, after a period of time, the electron decays and fluorescence starts again.
 08:02: And the downward jumps when the electron decayed out of level 3 appeared to occur at completely random times.
 11:35: But more on that another time.
 11:44: For the longest time, physicists have shied away from asking which interpretation of quantum mechanics is correct.
 12:06: ... processes, or is driven by rigidly deterministic mechanic of space time. ...


20201222: Navigating with Quantum Entanglement
 04:32: At Earth’s surface it’s about 30 microTesla, which is about 100 times weaker than a fridge magnet.
 06:58: ... both up, both down, or a quantum superposition of both at the same time. ...
 07:10: ... And Earth's magnetic field isn't strong enough to influence spin in that time. ...
 07:27: They do that evenly in the absence of a magnetic field  75% of the time in the triplet state and 25% in the singlet.
 07:35: But even a weak magnetic field like the Earth's can affect the amount of time the radical pair spends in these states.
 07:41: ... that field has the correct orientation, the system will spend more time in the triplet state  with both electron spins aligned in the same ...
 08:26: The two molecules now share a pair of entangled electrons—they become a radical pair for a short period of time.
 08:36: But there’s the key  those byproducts are sensitive to the spin state of the valence electrons at the time of the reaction.
 11:40: ... to flock together to navigate the hidden lines of a geomagnetic space time. ...
 12:17: I couldn't give you a satisfactory answer at the time  and so to show our appreciation I'm not going to give you one now.
 13:37: ... observed as emission or absorption lines, the way the light decays over time, and other ...
 14:12: ... the neutron star to collapse into a black hole over an absurdly long timescale of 10^10^2070 or so ...
 14:28: And then those black holes evaporate into radiation on a comparitifly short timescale.
 15:29: Many of you pointed out how hard it is to wrap your heads around the sort of timescales we talked about in that episode.
 15:50: Then imagine that happening 10 times.
 15:57: Easy, start by imagining 10^31998 years and then do THAT 10 times.


20201215: The Supernova At The End of Time
 00:45: Just tiny pops scattered across the end of time, before an eternity of perfect boringness.
 01:52: ... supernova explosion  a last firework to celebrate the end of time. ...
 02:33: Well not quite the very beginning, but on the timescales of iron stars pretty close to it.
 04:15: Fowler’s finding solved a great mystery of the time.
 06:24: ... For the stellar core left behind by an ordinary star that should be 1.44 times the mass of the ...
 08:02: ... nucleus has been sitting neatly in its assigned column and row for many times the age of the ...
 10:41: By the time the iron star is fully formed, its Chandresekhar mass has dropped from around 1.44 to less than 1.2.
 10:48: times the Sun’s mass.
 11:37: I know this is a relief to many of you  the prospect of an anticlimactic end of time weighed on my mind also.
 11:45: ... to  Iron stars exploding in unimaginably distant future of space time. ...
 12:40: In other words, how does our psychological sense of the arrow of time arise when the laws of physics don't seem to care about the direction of time.
 14:24: And WE observers ride the wave of time in a particular direction.
 14:43: ... quote: even if time did flow backwards, from future to past, in each time step we would not ...
 15:01: ... that's it  if we think of time as just a stack of slices of the block universe, a prefered direction of ...
 15:25: ... the Copenhagen interpretation, you CANNOT reverse time in the double slit experiment, because after the measurement of particle ...
 15:38: ... the many worlds interpretation the wavefunction persists, so reversing time means reversing all outcomes of the experiment  all possible worlds, ...
 15:49: ... is a timereversed observer from a Many Worlds multiverse, and complains they actually can ...


20201208: Why Do You Remember The Past But Not The Future?
 00:00: The laws of physics don’t specify an arrow of time  they don’t distinguish the past from the future.
 00:06: The equations we use to describe how things evolve forward in time also perfectly describe their evolution backwards in time.
 00:12: ... and the conscious experience that emerges from it, see the arrow of time so ...
 00:29: INTRO In our last episode we gave one explanation for why the universe as a whole sees an arrow of time.
 00:39: The 2nd law of thermodynamics dictates that entropy must rise over time.
 00:44: Disorder tends to increase from one time step to the next.
 00:48: As long as we have a single timestamp in a highly ordered state, there’ll be a gradient of increasing entropy on either side of it.
 00:57: How does this cosmiclevel arrow of time get translated into our mental sense of time?
 01:03: To understand this, let’s think about where our sense of the passage of time arises in our brains.
 01:12: At any one instant in time, our mental experience holds an awareness of the previous instant, and the instant before that, fading into the past.
 01:26: On longer timescales when we think back we remember the events of the day, of the last month, or of our lives.
 01:33: Time is encoded in our mental experience in the form of memories that are timestamped, or at least timeordered, past to present.
 01:43: ... also have internal clocks that give a sense of time elapsed, but to understand why the thermodynamic arrow of time ...
 04:02: ... it isn’t destroyed by future violence, then fast forward many, many times the current age of the universe and the asteroid will decay into a mist ...
 04:11: ... too mysterious, but let’s look at this from the point of view of the timeless laws of physics and see if we can identify where the arrow of time ...
 04:36: Time increases upwards, while the horizontal axis is separation in space.
 05:53: For this lone proton, its formation and decay are identical events and it's fair to say this whole sequence has no arrow of time.
 06:13: Over time the rock changes  new clumps might hit the rock and become embedded, cosmic rays leave their mark, etc.
 06:28: The timereversed view of the asteroid looks nothing like the timeforward view.
 07:04: The reverse time direction seems unnatural, but what’s different in terms of the asteroid’s record, or memory of its formation?
 07:33: If the rock never interacts with any external influence and just decays over time, then in principle we could calculate that decay.
 07:50: But in the case of the asteroid, time symmetry is much more easily broken.
 08:36: ... the timereversed case we have an asteroid formed from an incredibly improbable ...
 09:14: ... the asteroid will loses those correlations one by one. This reversed time scenario doesn't actually break any laws of physics. It's just insanely ...
 09:25: So the key to understanding how our brains inherit the arrow of time lies in understanding the connection between entropy and correlation.
 09:41: ... elements are uncorrelated, but become more and more correlated over time as they interact with each other  for example, by sharing energy  ...
 10:06: Over time, connections and correlations were made as entropy grew.
 10:17: ... also tend to build correlations with their environments in a particular time ...
 10:29: ... not that physics prefers one time direction over the other  it’s just that a low entropy allows ...
 11:14: ... pure quantum states they’ll become more and more entangled in adjacent time steps, the arrow of time for a patch of the universe is defined by the ...
 11:34: ... as an ordering of memories  but that’s just one way the arrows of time plays out, tracing the gradient of increasing entropy and correlation ...
 11:52: ... recent episode we explored how our universe gains its directionality to time from entropy and the 2nd law of thermodynamics  or as Ryan Gallagher ...
 12:10: ... the universe were reversed  time ran backwards to the big bang, wouldn’t entropy appear to increase as ...
 13:18: Ryan Christopherson asks whether the random nature of quantum mechanics isn’t a larger hurdle to the reversibility of time than entropy.
 13:26: Yossi Sirote asks essentially the same question  doesn't the collapse of the wave function break time symmetry.
 13:33: ... wavefunction collapse is a random rather than deterministic event, then timereversal symmetry is broken the arrow if time is established by quantum ...
 14:01: And to finish here’s the time joke that a surprising number of people shared in the comments.
 14:05: Which is the observation that Time flies like an arrow, fruit flies like a banana.
 14:11: Toughen up Fluffy came in with something a bit different: a classic Kermit the Frog quote: "Time's fun when you're having flies."


20201118: The Arrow of Time and How to Reverse It
 00:00: ... wish you could travel backwards in time and do things differently? Good news: the laws of physics seem to say ...
 00:10: So why do we seem to be stuck in this inexorable flow towards the future? It's time to begin our journey towards really understanding time.
 00:24: ... don’t distinguish between the future and the past. Reverse the flow of time and their equations work exactly the same to describe a universe in ...
 00:54: ... are two distinct ways in which we perceive the unidirectionality of time. One is external: an egg might splatter when you drop it, but will never ...
 01:21: ... into the first part of this  How does entropy break the symmetry of time, and cause the future to differ from the past. We’ll follow with an ...
 01:38: ... mean when we say the laws of physics don’t care about the direction of time? Basically, if you reversed the motion of all particles in the universe  ...
 01:58: ... picture, where we have 2dimensions of space and one dimension of time, with time flowing ...
 02:08: ... block universe just sort of exists atemporally. The experience of time is had by looking at timeslices in a particular sequence, like a flip ...
 02:31: ... law perfectly describe how the electrons’ positions change over time. But if we flip this diagram on its head  reverse the flow of time, it ...
 02:53: ... the laws of physics don’t care about the direction of time. They just describe the relationship between time slices of our block ...
 03:12: ... must always increase or stay constant  entropy never decreases over time. So the 2nd law does dictate a direction for time  it breaks the ...
 03:47: Over time, energy tends to get shared out as evenly as possible due to, say, collisions.
 04:13: ... a situation where energy is not perfectly randomly spread out, then over time it’ll become more spread out. Entropy measures how randomly distributed ...
 04:50: ... how does this set the forward direction of time? Actually, it doesn’t. The 2nd law just tells us how a system is likely ...
 05:10: ... those particles random velocities and see what happens in the following time steps.Even though the velocities pointed in random directions, the ...
 05:43: ... this case, we defined the “up” time direction in the block universe as “towards the future”. But what does ...
 06:00: ... any lowentropy system and look at steps either before or after it in time, entropy is likely to be higher. Entropy actually doesn’t care about the ...
 06:19: ... side of the starting, lowentropy point, you perceive an asymmetry in time  particles expanding, entropy increasing on one side, or particles ...
 06:57: ... and then will be seen to increase  now on both sides of the point in time of that ...
 07:17: ... what does all of this mean for our universe and the universal arrow of time? Well it’s no accident I chose “expanding particles” to illustrate ...
 08:11: ... this is one argument for the direction of the arrow of time. If we think of time as just a series of instances  slices of the block ...
 09:25: ... we’re presented with a fun scenario. If entropy increases backwards in time before the big bang, does that mean time runs in reverse back then? ...
 09:39: ... first you need to explain one thing. How is the thermodynamic arrow of time  the flow of time defined by increasing entropy connected to our sense ...
 11:16: ... random numbers are then used by, like, physics and stuff for the rest of time. And our brains use those random numbers as part of their choicemaking ...


20201111: Can Free Will be Saved in a Deterministic Universe?
 00:19: So of course, "Space Time" needs to weigh in on the subject of physics and free will.
 00:36: In it, the past and future have a sort of eternal timeless existence from the point of view of some godlike observer outside both space and time.
 02:12: We'll talk about that connection or lack thereof another time.
 03:05: Two dimensions of space and one of time, the slices of the block universe represent the causal ordering of the universe.
 03:30: We'll ignore what happens at the beginning of time for today.
 03:34: ... networks of these threads may spend time as different types of matter and sometimes as brains, both as the crude ...
 05:30: You are freewilled if you or fundamentally unpredictable information can emerge from your brain's patch of space time.
 06:21: Think about a new thread of quantum information starting from nothing, let's say emerging from a packet of space time where no information enters.
 08:45: ... a very interesting area of research, which we may talk about another time, but the short story is that even for the simplest decision, the best ...
 11:12: So this nexus of quantum information processing may truly be an unpredictable black box decisionmaking machine, at least some of the time.
 13:20: I'm now gonna exercise my free will to not end this episode with me saying, "Space Time." (mysterious music continues) (intense music)


20201104: Electroweak Theory and the Origin of the Fundamental Forces
 03:33: ... only that, but experiments at the time indicated that, if they existed, these new ‘weak bosons’, W bosons, had ...
 05:01: Quantum mechanical equations of motion like the Schrodinger equation describe how the wavefunction evolves through space and time.
 09:16: ... symmetry  to not care about changes in some property  but at the same time, for the state of the system those equations describe to NOT have the ...
 13:32: ... time we talked about Singularity Theorems, and how they show that the ...
 13:45: ... next host of space time asks if physicists tend to be dubious about infinities, why are we then ...


20201027: How The Penrose Singularity Theorem Predicts The End of Space Time
 01:17: ... dense ball of matter would be surrounded by a surface where time froze. Beneath that “event horizon” all matter, ...
 06:38: ... it’s meaningless to continue to track the progression of space and time beyond one of these focal points. In other words, space and time ...
 07:05: ... Because they are the grid we use to map space and time, geodesic incompleteness means space and/or time end at these ...
 08:11: ... the shortest path to any new point that you reach, and at the same time you reached the end of north  maxed your northness  and ...
 08:49: ... defined everywhere. Penrose showed that space and time could have holes in it. These holes tend to be associated ...
 10:15: ... that geodesics converge towards each other looking backwards in time, but doesn’t mean they had to all meet. They could, for ...
 10:36: ... form a singularity, which meant they had to terminate. Time itself could not be traced beyond this point  which ...
 11:10: ... is probably the right time to tell you how the singularity theorems must be wrong  or ...
 12:51: ... infinitely into the future AND the past. With your help Space Time might do the same. Well, not into the past  hard to make ...
 13:38: ... and your friend do the same teleport, you’ll end up at different times depending on your relative velocities. Your velocities ...


20201020: Is The Future Predetermined By Quantum Mechanics?
 00:00:  Einstein's Special Theory of Relativity combines space and time into one dynamic unified entity, spacetime.
 00:07: But if time is connected to space, can the universe be anything but deterministic?
 00:16: ... Space Time" theme song) In the last episode, we saw that we could think of the ...
 01:27: The idea that all space and time exist from a sort of God's eye view from outside of both space and time.
 01:47: ... given that relativity is definitely a good description of space and time, what would it take for our future to remain ...
 08:15: Sometimes the quantum multiverse of ManyWorlds is depicted as this branching tree, in which the realities multiply as time progresses.
 09:35: Nonetheless, as your light cone moves forward in time you encounter these entanglement networks and you have to choose between them.
 10:50: It's that part of the global wave function that you are connected to via entanglement and that shares your timestamp of now.
 11:45: There are other interpretations that deserve mention but we're out of time.
 12:09: Of course, we still haven't got to why time needs to progress in one direction at all.
 12:15: Or how time even arises in the first place, but we'll have plenty more time for time another time, on "Space Time".
 12:32: At least I think it was last week, the whole time thing's very muddled in my head these days.
 12:54: ... that they would define as happening simultaneously at a given instant in time, their slice of now and compare those ...
 14:14: Is it the very short stretch of time, like one plank unit of time?
 14:34: But that experience almost certainly is emergent from the action of our neurons and those neurons take time to fire and communicate across the brain.
 14:41: So something that currently exists, our consciousness, is smeared out over at least a hundred milliseconds of time.
 14:59: ... for that to be a true zero time instant then it would need to be the instantaneous state of the brain, ...
 15:21: That means that at least a short chunk of the block universe has a real current existence, much larger than the plank time.
 15:28: And if that much time can exist then why not all of it?
 15:54: ... gives a nice Douglas Adams quote, "Time is an illusion, lunchtime doubly so." Grandpa's Place reminds us of ...
 16:09: Think of it as a corequisite study course to "PBS Space Time".


20201013: Do the Past and Future Exist?
 00:22: ... we’re starting a deep dive into the nature of time  and down that rabbit hole we’ll encounter the physical origin of time, ...
 02:27: Here’s a nice way to represent Newton’s picture of space and time.
 02:31: Let’s cut out one dimension of space  space then becomes a 2D slice at a given instant in time.
 02:38: Now let’s stack successive instants so that time becomes the 3rd dimension.
 02:52: We only experience a single slice at a time, and we usually think of the universe in that slice as the one that currently exists.
 03:01: But we can also imagine a perspective from outside both space AND time in which this entire structure just exists.
 03:12: From that perspective, time doesn’t have a particular direction, and there’s no preferred “present”.
 03:17: Time is just a dimension like space, and we only observe a flow of time if we play the slices in sequence like a flipbook.
 03:26: We sometimes use the term “block universe” or “block time” to refer to this view of all space and time just existing in this 4dimensional chunk.
 04:10: ... by the way  that perception of existing in a single instant in time is definitely an illusion  our awareness  in fact our conscious ...
 04:21: But that’s a topic for another, well, time.
 04:50: There’s a name for this idea that all of time, future and past, sort of just exists as this atemporal structure.
 05:38: ... had a single time axis that everyone agreed on, and everyone agreed what slice of the ...
 06:17: That means any distant event is already in the past by the time we see it.
 06:22: ... with a lightcone  signals from things inside that cone have had time to reach ...
 06:53: Which is the same as saying we have to wait until its light has had time to reach us.
 06:58: Our entire timeslice of our present is never observable in the present.
 07:16: That means as time passes you move straight up  fixed in space, only moving through time.
 07:22: As that happens, signals begin to reach you from the timeslice you’re trying to map.
 07:29: Successive shells of light from more distant parts of that timeslice arrive one after the other, expanding your view.
 08:02: At the same time, their entire perception of the spacetime grid is warped due to their motion.
 08:14: The upshot is that the moving observer builds an entirely different map of that time slice  they define the present differently.
 08:29: ... that’s the effect of Einstein’s special relativity  space and time tilt into each other, so that different observers will slice up block ...
 09:04: ... observable universe veers back and forth by a couple of centuries every time you switch ...
 09:33: At a given location in space, you can lay down all possible timeslices representing possible presents.
 10:03: Consider the timeslice of our present; we can imagine other observers on that time slice that we surely must “exist”.
 10:19: ... of the past to them. Just like it does for us. So now fill our present time slice with observers and their remains no part of the block universe ...
 12:35: ...  but soon we’ll dive back in, to see what these idea imply about time, determinism, and what it would mean to live in a block multiverse space ...


20201005: Venus May Have Life!
 00:11: In fact we may have detected the signature of alien life  Venusian life for the first time.
 01:20: But for the longest time Venus was thought to be among the LEAST habitable places in the solar system.
 01:28: Its atmosphere is 100 times the mass of Earth’s and mostly carbon dioxide.
 02:22: ... to the yellow colour of the clouds and dark patches that change over time. ...
 07:16: However the scientists calculate that almost all of the phosphene would be destroyed in that time.
 07:23: The abiotic production rate of phosphine is expected to be 10,000 to a million times too low to produce the amount of phosphine that was observed.
 12:02: However it may be time to refocus our solar system explorations.
 12:29: ... us a grander perspective on humanity’s place in a lifefilled space time. ...
 13:11: So head over to Terra, and make sure to (politely) tell them Space Time sent you!
 15:02: I go for every second Moore’s law doubling, which leaves my machines basically paralyzed about half the time.


20200928: Solving Quantum Cryptography
 05:01: This is laborious—it takes an insane amount of time to factor large numbers this way.
 10:33: But decoding errors becomes much, much harder as the codeword size increases—unless you know some things about the codeword ahead of time.
 11:53: To be secure against quantum attacks for the foreseeable future, McEliece would need an 8 Mb public key—8,000 times larger.
 14:11: ... to decrypt your embarrassing emails across the parallel quantum space time. ...
 14:30: ... faster or slower than chemical reactions  so what would be the timescale of evolution or lifespan of these ...
 14:43: ... researchers did not speculate on that, except to say that the reaction timescale has to be shorter than the timescale for these cosmic necklaces being ...
 15:10: Timescales may be driven by the timescale of the motion of plasma in the stars and by the size of these critters.
 15:17: that timescale is days, and size is kilometers.
 15:19: That would suggest slower timescales than the evolution of chemical life.
 16:01: There was Frederik Pohl's "The world at the end of time" has a Plasma creature living in a star at war with copies of itself.
 16:36: It takes time to do kick ass animations.


20200921: Could Life Evolve Inside Stars?
 01:13: ... by physicists Luis Anchordoqui and Eugene Chudnovsky, and today on Space Time Journal Club we’re going to see how legit this idea actually ...
 10:23: And who knows what other bizarre life forms may be waiting to be discovered, in distant, stranger parts of space time.
 10:32: ... Everyone. A few announcements before we get to comments It's that time of year again where PBS DIgital Studios and Space Time want to hear ...
 10:47: ... Space Time audience has always done a great job making their voices heard on the ...
 10:56: ... regardless going to keep making Space Time and keep making it better  and a big part of what makes that possible ...
 13:32: ... and inexplicable intuition for beauty guide us subtly, while at the same time remaining healthily skeptical even of our own ...
 15:20: Shirsendu Chatterjee points out that it’s finally time to give the address to the time traveler’s party that we hosted a year ago.
 15:26: At the time no one showed up, but that only tells us that time travel is false if we actually give the address as promised.
 15:34: Done  here’s the time and address.
 15:35: ... civilizations can read it  don’t want any of those lame steampunk time travelers showing ...
 15:47: ... guess that scientifically proves that time travel is impossible, or that we’re scientifically conclusively ...


20200908: The Truth About Beauty in Physics
 02:50: And general relativity is also more explanatory than Newton  it tells us that gravity results from the warping of space and time.
 04:17: ... a physical law is unchanged by some transformation  whether shifts in time, space, angle, or something more abstract like the phase of the ...
 06:45: ... is probably a good time for a quote from Einstein himself: “The supreme goal of all theory is to ...
 10:01: ... to unify Einsteinian gravity with the other force of nature known at the time  ...
 10:29: ... had a hard time accepting that such a beautiful theory could be wrong, and he modified ...
 13:02: ... leading to the most beautifully fundamental explanations of space time. ...
 16:13: Deep Time goes in both directions.
 16:15: I love the idea of future deep time.
 16:32: Yes, and this time we'll summon a God.


20200901: How Do We Know What Stars Are Made Of?
 04:12: ... photon coming from the core of the Sun will be or scattered so many times that what should be a 1second journey to the surface can take 10s of ...
 04:28: By the time a photon reaches the photosphere it has a 5050 chance of traveling the final 100km to space without bumping into anything.
 10:13: At around the same time as Ceclia Payne was doing all of this, other scientists were figuring out the rest of the mysteries of the stars.
 10:46: ... out for us  the giant balls of burning hydrogen scattered across space time. ...


20200824: Can Future Colliders Break the Standard Model?
 00:38: ... be nearly 4 times the size of the Large Hadron Collider, and it would be capable of ...
 02:57: ... was smaller but collided electrons with one another to get a thousand times higher luminosity than ...
 03:08: In colliderspeak, luminosity is a measure of the number of particle collisions across an area over a time period.
 05:48: The fact that the Higgs turns up at a “mere” 100 times the mass of the proton seemed to indicate that something beyond the standard model was needed.
 06:26: The Large Hadron Collider reaches energy a few times higher than the top of that range, so it should have seen such particles by now.
 09:03: If it goes ahead, it’ll hit energies of 100 TeV  8 times the current LHC energy.
 14:19: ... time we talked about the most destructive, parasitic, and generally unhealthy ...
 15:59: ... those who don't know  the Chandrashekar limit is 1.4 times the mass of the sun  it’s the maximum mass of a white dwarf before ...


20200817: How Stars Destroy Each Other
 00:02: Forget TMZ  Here on Space Time we have all the latest details on the dysfunctional, explosive relationships between the stars.
 01:22: ... into space and for a couple of weeks shining 10s of thousands of times ...
 08:23: The neutron star’s jets sweep it hundreds of times per second, slowly blasting away its gas.
 14:16: ... episodes get easier is that you’re getting smarter with all that Space Time you’re ...
 14:46: So if you’re comprehending Space Time does that make you Einstein?


20200810: Theory of Everything Controversies: Livestream
 00:00: ... this is the second in our very unusual twopart series here on space time our great quest to understand the universe to find the mechanics of ...


20200728: What is a Theory of Everything: Livestream
 00:00: ... everyone so this is a very special and rather unusual event space time has always been about digging towards the deepest layers of reality ...


20200720: The Boundary Between Black Holes & Neutron Stars
 01:14: One was a hefty 23 times the mass of our Sun  making it definitely a black hole, and pretty similar to other LIGO mergers.
 01:41: ... on Space Time Journal Club we’re going to try to figure it out  and we’ll do that by ...
 02:55: From the arrival times at the three observatories, the location of the event could be narrowed down to some small arcs on the sky.
 04:04: ... not entirely surprising  that event was 6 times further away than the 2017 neutron star merger, so would probably have ...
 05:20: That insane density gives the neutron star a surface gravity around 100 billion times stronger than the surface of the Earth.
 08:00: Those models have predicted maximum masses in the range 2 to 3 times the mass of the sun.
 09:33: So far we’ve never observed a black hole with masses lower than around 5 times that of the Sun.


20200708: Does Antimatter Explain Why There's Something Rather Than Nothing?
 00:22: ... and antimatter are always created in pairs, then in the beginning of time there should have been exactly the same amount of both. So where is all ...
 01:25: ... find an annihilation partner. These days there are around a billion times more photons than there are particles of matter  so we estimate that ...
 02:12: ... parity inversion, where the universe is reflected through a mirror; and time reversal, where all particles have their direction of motion and spins ...
 05:43: ...  for example, positrons  or antielectrons  are created all the time in nature  for example in the Sun, or in radioactive decay, or when ...
 10:56: ... allow the antiatoms to be dropped from the trap and freefall for some time. The CPT theorem states that the acceleration of an antiatom in Earth’s ...
 11:51: ... so. Apparently we have another one of these Space Time episodes where scientists busted ass to break physics and … didn’t. But ...
 12:37: ... have a wonderful next orbital revolution  from me and all the space time ...


20200630: Dissolving an Event Horizon
 00:30: Space and time switch roles, pathways open up to other universes, and in some cases time travel becomes possible.
 00:44: ... where the inward flow of space reaches the speed of light, and time freezes from the perspective of the outside ...
 03:04: The time machine is on the other side of that ring, by the way.
 04:20: ReisnerNordstrom black holes also have an inner horizon, interior to which space and time seem normalish.
 06:13: Once you have an extremal black hole by whatever method, it lasts for a very, very long time  if not forever.
 07:42: By the time the gas reaches the black hole it has lost much of the angular momentum it started with.
 13:27: ... small Big Bang corresponds not to just the very, very large late time of the previous universe, but actually to the “conformal infinity” of ...
 14:40: ... of photons  I THINK the idea is that when you rescale both space and time by the same factor, you also rescale the interaction probabilities to ...


20200615: What Happens After the Universe Ends?
 00:45: ... has decayed into faint radiation .... that infinite stretch of space and time is identically the SAME THING as the infinitesimal and instantaneous big ...
 02:27: A conformal scaling of spacetime means scaling both space and time.
 02:39: Light has time to travel across it once.
 02:42: Scale it up by around 30 quintillion times to describe a second universe that’s a billion light years across and lasts a billion years.
 02:51: Again, light crosses it once in that time.
 03:08: ... stay the same, and the rays pass close to each other the same number of times. ...
 03:22: Light takes 30 quintillion times longer to cross one than the other.
 03:33: And there’s the key point: light does not experience the flow of time.
 04:11: Surely, then, the big universe will have more length and time ticks.
 04:15: ... with only one dimension of space on the xaxis and one dimension of time on the y, and we choose our axes so that light travels at a 45 degree ...
 04:42: Let’s take two instantaneous events in this universe, separated in both space and time.
 04:48: The separation between them can be determined by the number of gridlines of space and time you pass on your journey.
 04:57: Lines representing constant distance or simultaneous times shift with the velocity of the observer.
 05:03: If I draw the line of constant time for all possible travelers passing by my position, I get these nested curves  hyperbolas.
 05:12: They show how time will tick for any constantvelocity observer passing through this point.
 05:23: ... define the separation between two events in spacetime is by the travel time of something taking the most direct path between them  a path of ...
 05:36: ... is the socalled spacetime interval, and it’s equal to the amount of time that passes on the clock of the traveler  or the proper time of the ...
 06:03: Light follows these tracks in between the time grid, and never, ever cross contours that mark even a single tick of a clock.
 06:17: Both space and time lose meaning for a photon.
 06:20: As Roger Penrose puts it: in order for time, and hence space to be meaningful, a universe must be able to build a clock.
 08:01: I’ll come back to why we might expect the mass granted by the Higgs field to change over time.
 08:08: Filled with only timeless radiation, it would possess no spacetime grid, so perhaps could be considered sizeless.
 09:13: Hence the concept of time is as meaningless as in the late universe.
 09:20: For the blacktie formal answer we’d need to delve into the math of the conformal equivalence of the beginning and end of time.
 09:34: ... transforming our grid of spacetime to fit infinite distance and time into the one map, while at the same time preserving the 45 degree path ...
 09:48: The edges of this map represent “conformal infinity”  where infinite space and time are compressed onto an edge.
 09:56: That’s for one dimension of space and one dimension of time.
 10:00: ... the edge becomes a 3D “hypersurface” in which infinite distance and time are compressed or “conformally rescaled” into a finite ...
 10:41: ... one important aspect of all of this is that in order for the ends of time to be stitched together by this sort of conformal rescaling, the ...
 11:39: If entropy can only rise over time, per the second law of thermodynamics, how did it get so low at the start?
 12:01: ... Big Bang is due to the tiny entropy in the gravitational field at the time. ...
 12:20: In CCC, all of the energy  and, importantly, the gravitational field  is smoothed out over infinite time between aeons.
 13:06: Wait infinite time and see if you find yourself in a big bang?
 14:49: ... orbital frequencies of colliding suppermassive black holes at the end of time to be propogated across the conformal infinity into the next ...
 18:05: ... supernova, and frozen by the heat death of the universe all at the same time? ...


20200608: Can Viruses Travel Between Planets?
 03:34: ... a long time it was believed that viruses evolved as escapees from DNAbased cells, ...
 13:13: ... notsofavourite, but still fascinating fellow denizen of space time. ...
 13:42: ... time we explored the possibility that the existence of hidden extra ...


20200527: Does Gravity Require Extra Dimensions?
 00:54: ... of gravity is vastly weaker than the other forces  I’m talking 10^24 times weaker than even the weak nuclear ...
 01:28: ... behavior of gravity in terms of the warping of the fabric of space and time. ...
 09:24: ... force of gravity between the masses which was something like 10 million times smaller than the force the lead balls felt due to the earth ...
 12:06: But there are other ingenious approaches that we can come back to another time.
 12:18: what were they again? space, space space, ... time.
 12:40: We should, like, all hang out some time  you know, after all this is over.
 13:29: ... the event horizon of a rotating black hole  regions like the Carter time machine and the infinite string of parallel ...
 15:44: ... impossibleseeming things like an infinite string of universes or a time machine indicate that the math may have led our physical understanding ...
 16:09: Instead of "You're in for a weird time.” I should have said "I'm in for a weird space!" because space and time switched places inside the blackhole.


20200518: Mapping the Multiverse
 03:11: Time flows up  for the most part  and one dimension of space is left or right.
 03:33: Your past light cone shows you the parts of the multiverse that you can see, because light has had time to reach your position from those parts.
 03:59: Space and time switch places.
 04:39: On the other side, space and time switch back to their proper order.
 05:24: In either case, you’re in for a weird time.
 08:06: And as some of you probably remember  faster than light travel means the possibility of time travel.
 08:13: There are trajectories in this torus that lead you back to your starting location  in both space AND time.
 08:24: This is the Carter time machine, after the aussie physicist Brandon Carter who did much of the early exploration of the Kerr spacetime.
 08:34: This time travel and naked singularity stuff is good reason to think this part of the mathematical structure of the Kerr black hole is NOT real.
 09:24: Well you sort of cross the same space, but not through the same time.
 09:34: You’d need to travel back in timeslashfaster than light to get back there.
 10:37: ... not the bizarro negative time traveling universe either  here, the laws of physics are the same as ...
 11:38: ... horizon but they’re causally disconnected, the difference being that time flows forward if you go in one direction and backwards if the ...
 11:50: So you have this messed up situation where the direction of the flow of time actually clashes at the inner horizon.
 12:06: ... there’s any stuff at all in the black hole then the forward flow of time carries a current of positive energy, while the backwards carries ...
 12:42: ... the whole thing collapses, shutting off any potential magical portals to time machines or new universes  probably before they ever ...
 13:44: Now you know everything you need in order to travel the multiverse and travel through time.
 14:05: ... time we talked about the greatest failed experiment in the history of physics ...
 16:01: I said infinity times zero is zero.
 16:07: ... it won't be arbitrarily close to you at some point in the rewind BEFORE time gets to ...
 16:36: The universe approaches infinite density in the limit as time approaches zero.


20200511: How Luminiferous Aether Led to Relativity
 08:34: ... angles to each other. The split beams bounce back and forth multiple times before being brought back together again. And just like in Thomas ...
 13:18: ... that revealed a much weirder, but still totally luminiferous space time. ...
 14:23: ... distance that they are today. Divide that age by 1000 and they're 1000 times closer ...
 15:01: But the singularity happens when you go from a tiny positive age to zero age. Infinity times zero is zero and that's your singularity.
 15:13: ... or B) what happened before 10^43 seconds  the socalled Planck time. At that point inflation seems to give our best picture, and I'm going to ...
 15:32: ... which are progressively smaller. Right  because we’re looking back in time at a younger, hence smaller universe. While we can’t see the tiniest of ...
 16:14: ... just that they were swimming against the expansion of space that whole time. ...


20200504: How We Know The Universe is Ancient
 00:17: ... above is fixed and unchanging. But the Earth was also thought to be timeless  until we learned to see billions of years of change in its geological ...
 00:55: ... the beginning of the universe. There aren’t even any photons from the time right after the Big Bang. So today we go deeper into deep time to ...
 03:24: ... could change in size  a result that Einstein himself dismissed at the time. In fact even before Hubble’s observation the Belgian physicist and ...
 06:06: ... there’s the fact that the expansion rate has changed over time. You can’t just assume that the galaxies were always moving away at their ...
 12:59: ... the release of that radiation and also how it had expanded before that time. There are other approaches to getting the necessary numbers to fuel the ...
 13:45: ... you can well imagine in these difficult times, every little bit of help is huge. There's no way we could be so ...
 14:21: So, last time we talked wormholes  one of the most awesome entities in general relativity that probably don't exist.
 15:09: ... of the wormhole. It's really a 2D slice out of 4D spacetime, with time and one dimension of space discarded. But the diagram has 3 dimensions, ...
 16:59: ... to test quantum theory. Miraculously emerging definitely alive every time. She's become a pretty hardcore Everetian always trying to convince me of ...


20200428: Space Time Livestream: Ask Matt Anything
 00:00: ... channel and you're used to seeing me I guess but usually there's this time lag like you're in my future I'm in your past this is the first time I ...


20200422: Will Wormholes Allow Fast Interstellar Travel?
 02:08: ... lay mostly dormant for another 20 years before it was resurrected  this time not to build particles, but in an attempt to break causality. John ...
 02:50: ... they also realized that this would allow time travel. For example, if one end of the wormhole were accelerated into a ...
 03:33: ... the fabric of only 2 dimensions of space at a single instant in time. The technical name is an embedding diagram  a 2D spatial sheet sliced ...
 03:45: ... keep time frozen for the moment and add back the 3rd spatial dimension. The ring ...
 04:08: ... if we unfreeze the wormhole  if we add back the dimension of time  that journey becomes impossible. Fuller and Wheeler proved that the ...
 04:50: ... the space and time coordinates are blended and mutated so that time doesn’t freeze at the ...
 05:43: ... how the wormhole evolves. The embedding diagram we saw was a particular timeslice of the Schwarzschild spacetime. If we instead take a slice a little ...
 06:25: If we include time slices in the past, we see the full evolution of the wormhole  it opens, it widens, shrinks, and then pinches off again.
 06:35: ... state where the wormhole is pinched off. But if you consider its entire timeline, it describes a wormhole that opens and then shuts ...
 11:03: ... this in his chronology protection conjecture  there can be no closed timelike curves  paths back to your own past  except in useless circumstances ...
 13:26: ... nature. Which we will of course discuss in an upcoming episode of space time. ...
 14:15: ... a regular episode. Instead we’re going to do the very first Space Time Livestream. It’ll be an AMA ask me anything style, at least you can ask ...


20200414: Was the Milky Way a Quasar?
 00:47: ... only is it home to an enormous black hole four million times the mass of the Sun, but it also swarms with smaller black holes, ...
 01:36: The most powerful AGNs are called quasars, and they can shine a thousand times brighter than their surrounding galaxy.
 08:16: Now A full quasar might devour many millions of times the mass of the Sun over one active period, which could last for several million years.
 11:41: ... the energy required to power these radio bubbles is several thousand times lower than that required for the Fermi Bubbles, evidence points to ...
 12:56: ... influence on the Milky Way  a notsoinactive galactic denizen of space time. ...
 14:47: ... couple of billion years, then the U235 dominates the changes at later times, leading to the classing concordia ...


20200407: How We Know The Earth Is Ancient
 00:00: ... billions of years like it’s no big deal. But how can we be sure about timescales so far beyond the capacity for human intuition? Our discovery of what we ...
 02:38: ... around the same time as Buffon was staring at warm lumps of iron, the Scottish geologist ...
 03:30: ... possibility of an ancient planet, and the idea of what we now call deep time. ...
 03:43: ... well: “the mind seemed to grow giddy by looking so far into the abyss of time.” Hutton didn’t propose a beginning for the Earth  instead he assumed an ...
 04:32: ... time and deep space. We’d known since Copernicus and Galileo that earth was ...
 05:50: ... of Species, he estimated a minimum age of the earth based on the erosion timescale for chalk formations in Southern England. His figure of 300 million ...
 06:36: ... within the atom – and yield new tools for probing vast stretches of time. ...
 07:17: ... of decay is expressed in terms of “halflife”  which is the amount of time for a given radioactive nucleus to have a 50% chance of decaying; or ...
 09:13: ... age of the earth is uraniumlead dating. Uranium decays on much longer timescales  710 million years for the U235 isotope and 4.5 billion years for ...
 09:33: ... atoms into their crystal structure when forming, while at the same time repelling lead. So any lead you find in those crystals came from uranium ...
 10:35: ... years old, based on his radiometric dating. This was around the same time that astronomers proved that Immanuel Kant’s Island Universes were ...
 11:02: ... to look beyond the Earth. We believe that the moon formed at the same time as the Earth  both coallescing after a giant planetary impact in the ...
 12:07: ... should add that science took its time getting to these large numbers for the age of the world. The Mayan ...
 12:35: ... that our ancient Earth cares  it’s revolved around the sun 4.5 billion times through deep space and out of deep time, and it’ll do the same again. ...


20200331: What’s On The Other Side Of A Black Hole?
 00:00: ... horizon  the ultimate point of no return as you approach a black hole  time and space themselves change their ...
 01:38: ... we need 3 dimensions of space instead of two, plus the dimension of time. And maps of the universe in this 4dimensional spacetime also have ...
 01:53: ... actually crosses the event horizon. That’s because at the event horizon, time appears to freeze from the point of view of a distant observer. And the ...
 03:14: ... out the converging lines of longitude. For black holes we instead fuse time with a something called a tortoise coordinate, after Zeno’s paradox. ...
 04:31: ... more popular for intergalactic travelers. On Penrose diagrams, space and time also bunch up at infinite distance so tha t the entire universe fits on ...
 06:15: ... we encounter a region that looks just like the black hole  but with time reversed. This is the white hole, and we’ve this before  but perhaps ...
 06:51: ... Schwarzschild black hole. One whose coordinates do not change over time, implying that it always ...
 07:32: ... a wormhole. We’ll come back to the detailed physics of wormholes another time  today we’re interested in what that journey can tell us about the ...
 07:57: ... hole to try to get to the other side. Within the black hole, space and time have switched roles. These lines represent steps towards the central ...
 09:55: ... Perhaps you emerge from the past “white hole” traveling forward in time, or from the future black hole but traveling backwards in time. Which ...
 11:37: ... Everyone. Welcome to the new Space Time studio aka my apartment because New York is in lockdown, but as you can ...
 14:15: ... the second law of thermodynamics  aka that entropy must increase over time. ...


20200324: How Black Holes Spin Space Time
 00:21: ... Schwarzschild’s eponymous metric describes the warping of space and time around a spherically symmetric mass. And if that mass is sufficiently ...
 03:27: ... interior for another episode. For a preview check out our episode on time machines. Yup, the math says you can visit your past within a Kerr black ...
 05:49: ... black hole you can execute a stable circular orbit as close as 3 times the radius of the event horizon  or 3 Schwarzschild radii. Any closer ...
 07:50: ... flow of space is represented in a particularly weird way  space and time switch places. In particular, the radial direction becomes timelike, so ...
 12:11: ... physicists, because they threaten several physicsbreaking phenomena  time travel, universehopping, and naked singularities. And we'll encounter ...


20200316: How Do Quantum States Manifest In The Classical World?
 00:04: ... episode of space time is brought to you from the information flowing through an impossibly ...
 01:48: ... decoherence seems to provide a partial answer  we talked about it last time  to observe a superposition there needs to be a knowable phase relation ...
 05:32: ... talk about this crazy result directly  in fact we already did it a long time ago. We’re here to talk about how entanglement is connected to ...
 13:47: ... their correlations, no matter how far separated in boring old space time. ...
 15:50: ... all, isn't the chance of finding yourself in the rare "survival" timeline the same between many worlds and copenhagen. Well, that depends on your ...
 16:29: ... probabilistic, then is it inevitable that there are exceptionally rare timelines where the universe gets to a state of low entropy to produce another Big ...
 17:40: ... more probable because more histories lead to that result. Most of the time you will naturally find yourself in one of the more common histories  ...


20200303: Does Quantum Immortality Save Schrödinger's Cat?
 00:00: To quote eminent scientist Tyler Durden: "On a long enough timeline, the survival rate for everyone drops to zero." Actually… not necessarily.
 00:39: ... to transition from simultaneously existing in many states at the same time, to having only one clear observable state at the moment of ...
 02:33: ... random 5050 chance of the radioactive decay over a certain period of time  that means the quantum wavefunction of the atom splits equally  the ...
 03:41: Let’s be specific  there are 100 polonium212 atoms, with halflife 300 microseconds  so each atom has a 50% chance of decaying in that time.
 04:21: ... and over, every 300 microseconds, we need to do it for nearly a million times the entire age of the universe in order for the physicist to be likely ...
 04:50: According to Copenhagen, all branches of the wavefunction besides “definitely dead” get cut off with ruthless efficiency almost all the time.
 05:37: The problem with this test is, of course, that 10^30 physicists need to die in other timelines for one to crawl out of the box feeling smug.
 05:52: Even in the rare survival timeline, everyone else will probably assume the experimental apparatus broke.
 06:04: ... to mortality is ultimately a sequence of quantum events  so there are timelines in which those incremental steps towards death never ...
 06:41: At least in THIS timeline.
 06:51: So the closer you are, the fewer Many Worlds timelines include your survival.
 06:57: ... is going to have to experience every single one of the bad future timelines. ...
 07:19: ... Worlds might be wrong  I say live as though this is your one quantum timeline While we’re discussing dubious methods for predicting survival times  I ...
 10:34: Anyway, if your name appears you’re one of the winners of the challenge question and you get your pick from the space time merch store.
 10:45: ... to make sure all correct answers got a prize in at least one quantum timeline  so if you didn’t see your name here, please congratulate the other you ...
 11:06: So let’s stick it to all those alternate realities, and make this the best of infinitely many diverging histories of space time.
 11:44: Check out the episode in the description below, and tell them, politely, that SPACE TIME sent you.
 12:55: ...  all it does is ensure that you won't see multiple outcomes at the same time  you don't see macroscopic quantum ...
 14:23: I'll expand on that another time.
 15:41: AKA, a space time nerd.


20200224: How Decoherence Splits The Quantum Multiverse
 00:55: If you want the full story, this would be a good time to pause and catch that episode.
 02:04: Wavefunctions evolve over time according to the Schrodinger equation, and that evolution tracks how the system’s properties might change.
 02:13: Another way to think about it is that the timedependent wavefunction maps all possible histories for the object.
 02:20: Over time the histories of a quantum system separate to represent every possible future the laws of physics allow.
 04:04: This time we'll use particles of light  photons as our quantum particle.
 05:59: In this case, the phases match perfectly when the wavefunction leaves the slits  peaks and troughs come out at the same time.
 14:38: ... itself contains so much more than our little, decohered slice of space time. ...


20200218: Does Consciousness Influence Quantum Mechanics?
 00:26: ... faster than light, or simultaneously experiencing multiple entire timelines ... that then talk to each ...
 07:38: So there’s the conflict  different observers say the wavefunction collapses at different times.
 11:13: This time you stand next to your friend and you perform the double slit experiment together.
 11:18: A single electron reaches the detector screen and you both learn its location at the same time.
 12:49: ... dive into the quantumclassical divide, on an upcoming episode of Space Time. ...
 13:12: This subject was actually suggested on the Space Time discord channel  so a big thanks to you lot for the great idea.
 13:31: You get access with the lowest $2 per month tier on Patreon, which has the added bonus of helping us keep the space time lights on.


20200211: Are Axions Dark Matter?
 03:02: ... found that no such field exists or if it is there then it’s a trillion times weaker than predicted by a CPviolating QCD. This discrepancy between ...
 05:16: ... isn’t a constant, but can change in value, both over space and over time. In other words, make theta a new type of field  a dynamic field rather ...
 12:47: ... age, than after those 13.5 billion years it would be around 10^10^50 times the size of our observable universe. And that sounds about right. He ...
 14:14: ... the age of the universe for inflation to make a volume 10^10^90 or 123 times our obserable universe. So Eathan is right ...
 14:39: ... must have had a beginning, so it hasn't yet lasted for infinite time  well also true, but there's no reason to imagine we are near that ...
 15:20: ... universe sized regions. The volume would want to be 10^10^123 (or 90) times our universe's volume, so the distance is the square root of that. Let's ...


20200203: Are there Infinite Versions of You?
 00:05: And does that mean that one of them didn't do that really embarrassing thing that one time?
 01:41: ... chance of that is 1/100^50 or 10^100  and that’s 100 billion billion times less likely than winning a raffle draw in which there are as many raffle ...
 03:33: That is, until the disrupting influence of external regions has time to reach that point.
 04:17: In fact, it’s got to happen infinite times and with infinite variations.
 05:14: OK, so refining our question: in an infinite universe, does every POSSIBLE thing happen infinite times?
 05:32: But if there are finite starting points then at least SOME of those starting configurations have to be repeated infinite times.
 07:24: ... in most cases we expect these values to repeat themselves infinite times in an infinite universe  eventually leading to an exactenough ...
 10:10: ... an infinite universe probably does duplicates its parts infinite times. ...
 14:20: RobTheImpure would like to know what is meant when we talk about "spaceless timeless particle scattering" in the context of the smatrix.
 14:48: But in smatrix theory and quantum field theory, time and space in the interaction region are fuzzy.
 15:08: ... to deflect the electron  in other words, the direction of the flow of time is irrelevant from the perspective of the virtual photon  what matters ...
 15:24: Another example is the schannel  tchannel duality, in which you get identical scattering results when space and time switch places.
 16:02: ... you hate it when you read a word a million times before you hearing it said out loud, but by then it's stuck in your head ...


20200127: Hacking the Nature of Reality
 03:50: ... these subnuclear particles together must be so strong that space and time should break down at those scales, and our even best field theory hacks ...
 06:04: At the time, nuclear scattering experiments were producing a startling variety of different particles.
 06:19: But at the time, prior to the discovery of quarks, no pointlike, elementary nuclear particles were known.
 07:36: ... the fact that antimatter can be treated as matter traveling backwards in time  that folds together large sets of Feynman diagram and helps us ignore ...
 12:56: ... of quantum field theory, it removes the very concepts of space and time. ...
 13:10: These only emerge later as a consequence of spaceless, timeless particle scattering.
 13:17: ... towards a better understanding of the quantum weirdness of space time. ...
 13:34: ... way to get smart people answering your questions is to join the Space Time discord ...
 15:56: ... powered by supermassive black holes a few tens of millions to 10 billion times the mass of the sun  and the small end of that range is indeed a ...


20200120: Solving the Three Body Problem
 00:00: ... famous for being impossible to solve. But actually it's been solved many times, and in ingenious ways. Some of those solutions are incredibly useful, ...
 00:20: ... be used in principle be used to calculate their locations at any distant time, future or past. I say “in principle” because the reality is not so ...
 01:24: ... positions and velocities of your gravitating bodies, plus a value for time. ...
 01:49: ... equations will then give you the state of the system at that time, no matter how far in the past or future. We call such a simple, ...
 05:25: ... break the motion of the system into many pieces, and solve them one at a time. A sufficiently small section of any gravitational trajectory can be ...
 06:09: ... method of solving differential equations one step at a time is called numerical integration, and when applied to the motion of many ...
 10:30: ... is that the state of the system seems to get randomly shuffled over time. The motion is actually perfectly deterministic  defined between one ...
 12:55: ... purposes  with enough precision to work just fine. Good to know next time you’re in a chaotic orbit, trying to astronavigate around two other ...
 15:41: ... a huge fan of PBS Space Time and, if you like them, you’ll like ours as well. In fact, I’d like to ...


20200113: How To Capture Black Holes
 00:00: ... closer to reality, at least according to the papers in today’s Space Time Journal ...
 00:24: ... holes. That was stunning enough, but the real promise lay ahead. Every time we learn to observe the universe in a new way we discover new things. ...
 02:16: ... of searing gas that surround some of these monsters? Well today on Space Time Journal Club we’ll be looking at a pair of 2019 papers that talk about ...
 02:50: ... galaxy contains a supermassive black hole of millions to billions of times the mass of the Sun. Recently we’ve also learned that the galactic ...
 06:53: ... they find one of these traps, and they’ll remain stuck there for some time. ...
 07:17: ... black holes can end up in the same migration trap and merge one at a time, ultimately reaching enormous sizes. This is one of the calculations of ...
 11:50: Awesome, sure, but what else would you expect from this supremely badass and frankly, totally metal Space Time.


20200106: How To Detect a Neutrino
 00:35: ... 𝘰𝘶𝘵) ♪ ♪ (𝘴𝘱𝘢𝘤𝘦𝘺 𝘱𝘪𝘯𝘨 𝘳𝘦𝘷𝘦𝘳𝘣𝘦𝘳𝘢𝘵𝘦𝘴) ♪ ♪ (𝘧𝘢𝘥𝘦 𝘰𝘶𝘵) ♪ (title) "SPACE TIME" (title) "SPACE TIME" ♪ (𝘫𝘰𝘺𝘧𝘶𝘭 / 𝘢𝘯𝘵𝘪𝘤𝘪𝘱𝘢𝘵𝘰𝘳𝘺 𝘴𝘺𝘯𝘵𝘩 𝘨𝘶𝘪𝘵𝘢𝘳 𝘱𝘶𝘭𝘴𝘦𝘴) ♪ ...
 01:46: ♪ ♪ ♪ (𝘢𝘥𝘥 𝘥𝘦𝘦𝘱𝘦𝘳 𝘴𝘺𝘯𝘵𝘩) ♪ But all neutrinos have the bizarre quality that they oscillate between these types over time.
 01:49: ♪ ♪ But all neutrinos have the bizarre quality that they oscillate between these types over time.
 04:46: ... ♪ ♪ between certain pairs of properties, in this case, energy and time. ...
 05:20: ♪ ♪ That greatly limits the amount of time they can exist, ♪ ♪ and so limits the distance that they can travel.
 09:19: ♪ ♪ DR. DON: You know, you're welcome to come anytime, ♪ ♪ we love Space Time.
 09:36: ♪ ♪ It's honestly one of my goto resources when I'm researching ♪ ♪ a new episode of a Space Time.
 09:42: ... 𝘴𝘭𝘰𝘸𝘭𝘺 𝘧𝘢𝘥𝘦 𝘰𝘶𝘵) ♪ ♪ (𝘦𝘵𝘩𝘦𝘳𝘦𝘢𝘭 𝘣𝘦𝘭𝘭𝘴 𝘴𝘭𝘰𝘸𝘭𝘺 𝘧𝘢𝘥𝘦 𝘰𝘶𝘵) ♪ (title) "SPACE TIME" (text): "Matt is currently lost in the Fermilab tunnels, so comments ...


20191217: Do Black Holes Create New Universes?
 00:24: Physicists have been struggling for some time to figure out why our universe is so comfy.
 03:54: There are various proposals for how such a bounce might happen  all of which are massively speculative, and perhaps we’ll cover another time.
 07:16: ... will cause black holes to spontaneously appear  and given infinite time these will eventually outnumber those produced by stars or stellar black ...
 07:56: ... know that our physics can really be extrapolated to the insanely long timescales required for these quantum fluctuations to ...
 10:08: Lee Smolin calculates that optimized cutoff at around 2 times the mass of the Sun.
 12:19: Or, you know, our universe's momma might be a black hole, and we live in an endlessly evolving, proliferating space time.
 16:20: Then the internet's going to cease to exist some time in the next 30 years guys.


20191209: The Doomsday Argument
 00:08: We might hope for a trillion times that if we colonize the galaxy.
 03:48: Weinberg calculated that the density of dark energy should most typically be observed to be around 510 times the density of matter.
 03:57: That was in the early to mid 90s, right before dark energy was actually discovered and found to be around three times that of matter.
 07:44: That’s the scenario in which humanity only has about as much time left as has already past.
 08:01: In fact we can use statistics to estimate the probability of our species lasting a certain amount of time.
 12:12: Best answers get a pick from the Space Time merch store.
 12:29: Which means you’ll be around for next week’s episode of space time.
 12:34: ... difference  ensuring we'll be able to keep making and expanding Space Time until doomsday  whether it's soon or ...
 13:11: ... any level  even $2  also gets you access to our new Space Time discord channel, where at this very moment brilliant and curious ...
 15:57: These are basically flat mappings of a donut. And there are weirder, multiply connected finite space times that are also flat.
 16:56: Last time in an attempt to be "funny" I dissed english weather.
 17:01: Shaun Humphreys kindly took the time to pen an extensive treatise on why the British climate is really optimal for humans.


20191202: Is The Universe Finite?
 00:22: Every time you walk out the door, light from the Big Bang strikes your face, enters your eyes.
 01:02: ... on Space Time Journal Club we’ll delve into the Nature Astronomy paper that just ...
 12:01: ... and perhaps verifying the positively curved, finite geometry of space time. ...
 12:12: If you want to dive deeply into understanding the building blocks of space time then you need to study quantum theory.
 12:56: But before we get to that, I want to invite you to check out the Space Time discord, which you can access with the lowest $2 patreon tier.
 13:17: ... do all PhD physicists casually talk about complex stuff like on Space Time videos, or do most employed physicists just shut up and ...
 16:02: ... universe is mostly empty Singapore Breaking News likes to play space time loudly so mom thinks they're getting more brainy but in the background ...


20191118: Can You Observe a Typical Universe?
 02:35: ... the weak anthropic principle states that we must live in a place and time in the universe capable of supporting observers  in our case, a ...
 06:16: ... which means extreme high entropy states must be the norm  in the full timeline of our universe, but probably also across the multiverse, if it ...
 09:32: ... density fluctuations to evolve and expand for the right amount of time, so that life may always be in big ...
 13:03: ... given your privileged status as a typical conscious observer of space time. ...
 14:03: So from your students and everyone here at Space Time we wish you well.
 14:56: ... the universe needs to last a reasonable amount of time, have stable regions and energy sources for those structures to form, and ...


20191111: Does Life Need a Multiverse to Exist?
 02:50: We must find ourselves at a place and time in the universe capable of producing observers.
 12:58: And we will test it  next time, and so perhaps make a little more sense of our extremely finetuned patch of space time.
 15:34: ... got inside the first eukarote cell  actually did happen multiple times for different types of cells, and therefore doesn't qualify as a great ...


20191104: Why We Might Be Alone in the Universe
 12:21: We skipped comment responses last episode, so today we're covering two episodes  loop quantum gravity and time travel.
 14:22: ... that you can travel around to end up back where you started  in both time and ...
 14:32: Some One asks if a circle in 2 spatial dimensions would allow for closed time like curves.
 14:44: Time travel is easy in flatland.
 14:57: At the beginning of the time travel episode I invited future time travelers to show up on set. None of you did.
 15:16: Surely as time travelers you could have reminded me in the loop quantum gravity comments the week before.
 15:23: Or maybe Stephen Hawking's chronology projection conjecture prohibits anyone from remembering to post addresses to time traveler parties.


20191021: Is Time Travel Impossible?
 00:00: If time travel turns out to be possible, I hereby invite future time travelers to join me on set I’ll post the address one year from today.
 00:19: ... Time travel stories are cool because both the past and future are somehow ...
 00:34: Time marches on, inexorably and only forward.
 00:41: ... and general theories of relativity changed the way we think about time forever, and believe it or not, their raw equations permit time ...
 00:53: So let’s review the possibilities, and decide how possible time travel really is.
 00:58: ... first approach to time travel uses only special relativity, which describes how intervals of ...
 01:08: A fastmoving spaceship appears to experience a slower rate of time compared to someone waiting back on Earth.
 01:15: Do a trip around the galaxy at close to the speed of light and very little time might pass from the perspective of the traveler.
 01:27: However, that’s a oneway trip in time, and is really just traveling in the same temporal direction at different rates.
 01:34: So the original Planet of the Apes style time travel is possible.
 01:38: But it’d be nicer to be able to go back in time.
 01:52: And at faster speeds, time should actually tick backwards.
 02:42: That means it can only travel backwards in time, not forwards.
 02:51: If we could control tachyons then perhaps at least we could send information back in time.
 03:29: But GR describes a warping of space AND time.
 04:00: This has the obvious benefit of allowing you to teleport between distant points in space, but also between distant points in time.
 04:14: ... to the speed of light or drop it into a deep gravity well – its rate of time flow will slow relative to the other end of the ...
 04:24: They will be offset in time: one portal permanently stuck in the past of the other by some set interval.
 04:41: ... wormholes appearing and vanishing on the tiniest scales of space and time. ...
 04:53: But for now, to build a useful time machine a wormhole has to be large enough to fit through and it has to be stable.
 05:11: In order to keep our wormhole time machine operational, its throat needs to be kept open.
 06:05: Some other time travel options also involve using negative energy densities  for example the Alcubierre warp drive, which we already covered.
 06:14: In short – if you have exotic matter you can probably time travel.
 06:51: We can’t completely rule out wormhole or warp drive time machines based just on the energy conditions.
 06:58: And as it turns out, there may be other ways to build time machines without either negative or imaginary masses.
 07:26: This generates sublightspeed paths through spacetime that form closed loops, ending up back where they started in both space and time.
 07:36: We call such paths closed timelike curves.
 08:06: So it turns out that it’s not so hard to find solutions in general relativity with closed timelike curves.
 08:29: So to build this time machine we just need construct an entire universe – which allows us to travel back in time only within that universe.
 08:38: Dragging the fabric of space in a circle can give us our timeloops in very special, and frankly useless cases.
 08:51: The maelstrom of spinning spacetime may generate closed timelike curves deep down below the event horizon.
 08:59: ... that’s fun: you can travel back to your own past, but never to the time before you fell into the black hole, which is probably the only thing ...
 09:08: Unless it’s an Interstellarstyle black hole . . . general relativity doesn’t directly refute black hole time machine libraries.
 09:16: So it seems we have lots of ways to send things back in time, but it all seem useless for actually making time machines for one reason or another.
 09:25: But the weird thing is that we don’t know of one consistent, fundamental law in physics that prohibits true time travel.
 09:34: And yet most physicists still think it’s impossible because time travel threatens the commonsense chain of cause and effect.
 09:45: Break causality and you can create paradoxes– timetravel to kill your grandfather and you would ever be born to time travel in the first place.
 10:04: It states that the laws of physics will always prevent time travel or allow it only when doesn’t cause paradoxes.
 10:12: In other words, the universe has to make sense, timetravel or no.
 10:17: One way for a closed timelike curve to exist without causing a paradox is expressed in the Novikov SelfConsistency Principle.
 10:26: Igor Novikov suggested that closed timelike curves are fine as long as they’re selfconsistent.
 10:33: As long as the backwards timetraveling configuration of matter always leads to exactly to the same forwardtraveling configuration.
 11:04: ... if you travel back in time and prevent yourself being born – no problem  your photo doesn’t slowly ...
 11:13: Or timetravel could be genuinely impossible.
 11:17: ... vacuum may be unstable in the infinitely iterating loops of a closed timelike ...
 11:47: Approximate theories can make bad predictions – like the possibility of time travel.
 11:53: One final argument that time travel is impossible is that we don’t see time travelers.
 11:59: Stephen Hawking put this to rigorous test when he organized a cocktail party for time travelers, only advertising the event after it ended.
 12:16: For now we seem doomed to time travel only forwards, and very slowly at that.
 12:21: We remain firmly in the grip of that one dimension that we can never halt nor reverse it's pace: time.


20191015: Loop Quantum Gravity Explained
 00:00: It’s time we talked about loop quantum gravity.
 01:26: But there are more fundamental conflicts: namely, background independence and the problem of time.
 01:34: ... I’ll mention the problem of time briefly in a little bit, but the real focus is going to be on this ...
 01:58: Like actors on a stage, where the actors are particles and wavefunctions and fields and the stage is the coordinates of space and time.
 03:33: ... classical physics, we have variables like position, time, momentum, energy  mathematical expressions that represent the ...
 03:45: Some of these  say, position and time  also form our background coordinate system.
 04:30: ... on a spatial coordinate system and the change in that location over time. ...
 04:51: In quantum mechanics, time is treated completely separately to other variables  there is no “time wavefunction” or “time operator”.
 05:00: This is completely at odds with general relativity, in which time is treated as just another dimension.
 05:06: This is the “problem of time” that I mentioned, and it’s strongly connected to background independence.
 06:20: It then gives a sort of equation of motion for how these metrics evolve through time.
 11:49: And connected to this, LQG doesn’t solve the problem of time.
 13:02: This was tested in 2009 by looking for differences in the arrival time of light from a gamma ray burst nearly a billion light years away.
 13:40: One way or another, we live in a seriously loopy space time.
 14:09: Now, last time we talked about Black Hole Harmonics, let's see what you had to say.
 14:30: ... with a scenario for two actual black hole pairs to colliding at the same time in the same general ...
 14:53: If those paths were different lengths we'd see the same signal separated by a small time delay.
 16:16: ... few of you asked a really onpoint question: if the fabric of space and time can be stretched and if can have waves, that means it must have a sort ...


20191007: Black Hole Harmonics
 00:22: Today on space time journal club we’ll explore two papers that claim to have detected black hole harmonics.
 03:12: In the latter cases we can describe a vibrating string as a series of standing sine waves of different frequencies, all happening at the same time.
 04:19: A harmonic oscillation that decays over time is called a quasinormal mode.
 04:33: They leak away over time, but while present they warp the shape of the event horizon.
 08:09: ... first one LIGO reported: GW150914 – a pair of black holes, each 30 or so times the mass of the sun, spiraling into each other one and a half billion ...
 10:19: ... theorem by checking whether the frequency of oscillations and the time for the decay of those oscillations agrees perfectly with Einstein’s ...
 12:29: ... two pairs of binary black holes should merge near each other at the same time. ...


20190930: How Many Universes Are There?
 03:13: That means the number of bubbles forming at each point in time should be proportional to the volume of the inflating universe at that time.
 04:25: By the time we get to one full second we’ve multiplied by 10^78 10^32 times, so our volume is 10^78 to the power of 10^32, or around 10^10^34.
 05:43: ... they probably have the same number of dimensions as ours – 3 space, 1 time – but their contents and physics could be very ...
 09:16: There are vastly more universes born one second after ours than were born at the same time as ours.
 09:24: Now, imagine that there’s a set amount of time for the first intelligent life to form in any one of these universes.
 09:37: The real timescale is going to be much more fuzzy than this, but it also doesn’t really matter for this argument.
 09:54: The same happens in the exact same second for universes that formed at the same time as ours.
 10:45: ... inflating multiverse are in the youngest universes that have had time to form intelligent ...
 13:15: Well we probably shouldn’t just accept it – but at the same time, current lack of proof doesn’t mean something is wrong.
 13:28: But here’s something that’s not hard to get – Space Time swag.
 14:56: Last time we visited that old argument  Is Pluto a planet?
 16:42: If Earth formed at 100 times its current distance from the sun it wouldn't have cleared its orbit either, and so wouldn't be considered a planet.
 18:04: ... maybe we need to be ready to change the definition again  perhaps this time with more input from planetary ...


20190923: Is Pluto a Planet?
 03:10: Uranus had been spotted many times throughout history, but was only identified as a planet after William Herschel recorded its movement in 1781.
 14:01: First up, we're launching a Space Time discord  It'll be the perfect place to ponder and discuss the fundamental nature of the universe, 24/7.
 15:15: So, last time we looked at the harsh realities of what it would take to terraform Mars.
 16:27: ... Venus are 1) its day is several months long, and that's a long time for any photosynthesizing life to survive without the sun, and 2) it has ...


20190916: Could We Terraform Mars?
 05:34: ... amount of CO2 in the atmosphere – which is a factor of around 100 times too low to make a ...
 08:31: Several thousand times the total annual energy consumption of the entire Earth.
 15:23: However we do it, Mars will surely be our first step, our proof of concept if we choose that destiny  if we choose to terraform space time.
 19:15: ... Europe, and most of Asia have been at bottom of the world all this time without realizing ...


20190903: Is Earth's Magnetic Field Reversing?
 00:11: I mean, it has before … many, many times.
 01:52: Many times.
 02:09: ... out Earth’s field has completely flipped direction 183 times over the past 84 million years – so a little more than once per half a ...
 02:36: We may be no more “due” than we were at any other time in the past half million years.
 12:34: For now at least we remain protected from the worst ravages of solar storms, and of our dangerously irradiated space time.
 13:15: But, until then, Space Time and PBS Digital Studios want to hear from all of YOU.
 13:28: Space Time fans have always been amazing at taking the time and making their voices heard on the survey and we’d love for that to continue.


20190826: How To Become an Astrophysicist + Challenge Question!
 00:00: ... school had massive ups and downs and I thought of quitting plenty of times It was amazing I was using the Hubble Space Telescope to help unravel ...
 03:35: ... of modern physics It doesn't really matter what you major in but by the time you've taken all of that physics you probably qualify for a physics ...
 08:00: ... as I can or better Find those mentors it's going to be a challenging time But it will Forge you into a scientist One piece of advice regarding ...
 10:46: ... more universes form in the following second twice as many a Thousand times as many less more to answer this you'll need to assume a rate for ...
 11:51: ... Submit answers within two weeks of release of this episode to PBS space time at gmail.com with the subject line Eternal inflation challenge and check ...


20190819: What Happened Before the Big Bang?
 00:13: That picture of the Big Bang is very solid, down to about a trillionth of a second after the supposed beginning of time.
 00:43: These days, the best accepted description of the time before the Big Bang is given by inflation theory.
 01:07: ... if you haven't yet, because today we're going to peer further back in time and explore a stunning implication of inflationary ...
 02:50: I mentioned last time that quantum fields can hold energy without actually having particles.
 05:13: ... region of the inflating universe would approach this minimum at the same time Inflation would shut down smoothly and the universe would be reheated ...
 10:14: Good material for the eternally expanding future library of PBS Space Time.
 10:33: ... time start at the Big Bang" and "What caused the Big Bang, the real physics ...
 11:35: It was around 3000 Kelvin at that time.
 12:50: Dominic H quips "Did time start at the Big Bang? Let me guess depends on your definitions of "Did", "Time", "Start" and "Big Bang" " Ah...


20190806: What Caused the Big Bang?
 01:14: We'll come back to those another time.
 01:56: But physicists are a skeptical bunch and most of the time they don't just make up stories and start believing them without good reason.
 02:23: Let's dig into each of these one at a time.
 05:22: ... for our universe  then that energy gets more and more spread out over time. ...
 06:05: Now, a field doesn't just jump to the lowest energy state, it makes its way there by changing the field strength one step at a time.
 07:26: ... field has a high field strength due to the extreme temperatures of that time. As the universe cools the field loses strength and ...
 11:40: ... Microwave Background, is if lots of these bubbles collide and then have time to ...
 13:08: Space Time.


20190725: Deciphering The Vast Scale of the Universe
 00:49: ... countless galaxies to the cosmic horizon and almost to the beginning of time. ...
 03:52: ... were the first “standard candles.” If you time how many days or weeks it took a given Cepheid to fluctuate and you can ...
 04:32: Or around 20 times the entire diameter of the Milky Way disc.
 05:45: FYI this is pretty much my favorite place in the world and I spend as much time here as I can.
 06:04: And unlike regular spaceships, this dome can fly at many times the speed of light.
 06:23: ... of billions of stars of our Milky Way galaxy at a few hundred billion times the speed of ...
 06:50: As we zoom out, now at several trillion times the speed of light, our local group of galaxies comes into view.
 07:32: ... we accelerate to one hundred quadrillion times the speed of light we see the extent of our modern mapping of the ...
 08:55: If we continue exploring – if we keep looking deeper and further – who knows what we’ll find in the expanding horizons of space time?


20190718: Did Time Start at the Big Bang?
 01:02: And before the Big Bang singularity, well, they say that there was no 'before' because time and space simply didn't exist.
 01:46: ... Only a few hundred thousand years after the hypothetical beginning of time. ...
 01:56: ... space and matter in the universe was once crunched at least a thousand times closer together There's also the relative abundance of simple elements ...
 03:27: ... That's the distance between any two points in space at some moment in time Relative to their distance at some other reference moments that ...
 04:32: Do that enough times and any two points, no matter how far apart they were, will end up as close together as you'd like.
 04:38: ... it enough times and the universe could end up as hot and dense as you like But it'll ...
 05:15: ... there weren't spatial dimensions for it to happen in At the same time we say the Big Bang happened Everywhere at once because even the tiniest ...
 06:02: ... or into existence if you're going forward No, you have to think about time in the way Einstein Intended there is no universal clock time is ...
 07:22: ... geodesics end at the Big Bang singularity and their timelines end with them Or they start depending on how you want to think about it ...
 08:14: ... so I'm taking my time to explain something I already told you is wrong But it's important ...
 08:52: ... that all space was compacted into a single point and that this is where Time started. Ok. So what are the ...
 09:30: and the timelines they embody through the Big Bang and out the other side If so, what do we find there?
 11:12: ... extreme quantum fluctuation could initiate a new Big Bang given infinite time or The same amount of time could lead to all particles randomly ...
 11:36: ... about that from quantum fluctuations from nothing  Stephen Hawking's timeless interpretation of internal inflation that draws on the holographic ...


20190715: The Quantum Internet
 04:09: It simply states that: “you cannot take a quantum state and copy it perfectly and end up with two copies of the same state existing at the same time”.
 04:18: ... quantum state to single quantum state  both forwards and backwards in time. ...
 09:18: But storing delicate quantum states for any length of time is hard work – especially if you don’t want insanely expensive supercooled devices.
 11:21: ... us to take advantage of the incredible properties of our quantum space time. ...
 15:03: Battlemechs, submarines, spaceships, DeLorean time machines  nuclear is probably the easiest option for the badass vehicles of the future.


20190701: Thorium and the Future of Nuclear Energy
 02:49: ... of the fastmoving neutrons created by its own fission But it's many times more fissile if those neutrons are first slowed to become socalled ...
 17:49: ... measure that spin But this definitely doesn't extend more than 10 or so times the black hole event horizon Oppie asks why NASA isn't dedicating more ...


20190620: The Quasar from The Beginning of Time
 00:04: ... built this land has provided another window: it allows us to observe a time when the universe was still cooling from the fire of its own ...
 02:48: It is enormous. I still remember the first time I came to a telescope like this.
 04:20: ... to match and correct the warping of incoming light. To do this in real time, Gemini creates its own artificial guide star by shooting lasers to ...
 05:12: ... stretched the wavelength of that light so that it was infrared by the time it reached the earth and this spectrograph. Their redshift tells us how ...
 07:10: ... bring us closer to understanding this mysterious, this magnificent space time. ...
 07:38: We can now see ripples in the fabric of space time itself.


20190617: How Black Holes Kill Galaxies
 00:08: ... that Black Holes could have masses of Millions or even Billions of times that of the Sun It came as a bit of a shock they were discovered as the ...
 05:09: how does a galaxy know to stop growing when its 1000 times larger than the Central Black Hole?
 05:24: ... to have almost no active star formation and haven't had any for long time The shortlived hot massive stars that give Spiral galaxies, like the ...
 07:21: ... growth to that of star formation they grow together and peak at the same time around 10 Billion years ago right before star formation begins to die ...
 11:24: ... equations you can learn more at Brilliant.org/spacetime Last time we talked about all the cool elements that get made when neutron stars ...
 12:39: ... picked up a silly mistake I said that the halflife is the average decay time of a radioactive element It's ...
 12:49: It's the amount of time for half of the given sample to decay.
 14:13: ... cat loves the sound of my voice and moves closer the the screen every time Actually that's just because Mittens can sense my allergy and like all ...


20190606: The Alchemy of Neutron Star Collisions
 02:47: ... after being created in a neutron star collision now the average decay time or halflife differs between different radioactive isotopes some decay ...
 13:02: ... And so electrons were more spread out the density was 100^3 times lower than at recombination and so the mean free path of photons was a ...


20190516: The Cosmic Dark Ages
 00:04: ... trillion stars fill the observable universe with light. But there was a time before the first star ...
 00:15: A time we call the cosmic dark ages.
 00:24: ... object as it was billions of years ago. In this way we can peer back in time and literally see the past in motion. In fact we’re able to see some of ...
 01:34: ... the first atoms formed. The universe became transparent for the first time, and we see the light freed at that moment as the cosmic microwave ...
 04:46: ... of this? We do see a handful of primitive galaxies shining out from this time, including one from right near the beginning of reionization when the ...
 07:19: ... born the first supermassive black holes with millions, even billions of times the Sun’s mass – inescapable spheres the size of solar systems. And in ...
 08:36: ... has itself ionized a bubble spanning several million light years. By the time the quasar’s light reaches the edge of that bubble, the universe has ...
 09:21: ... were unlucky enough to be redshifted to the doomed wavelength at that time would be ...
 11:17: ... ionization bubble can tell us how much neutral hydrogen was left at the time this quasar was shining. It lets us track the progress of reionization ...
 12:16: ... the only habitable epoch in the past and future history of space time. ...
 13:07: ... time we talked about some quantum encryption  ways to secure messages using ...


20190509: Why Quantum Computing Requires Quantum Cryptography
 09:06: That’s because Werner, like Niels, can only pick a random basis each time on which to project the photons.
 12:02: This time Albert creates a set of entangled particle pairs and transmits one half of those pairs to Niels.
 13:50: Your browsing history may one day depend on it, and I assume that history isn’t just old episodes of Space Time.
 16:16: That's probably around 5 times larger than the event horizon, in the case of this quasar.


20190501: The Real Science of the EHT Black Hole
 00:04: How do you take a picture of a black hole and what have we learned from seeing one for the very first time?
 00:14: By now I expect you’ve seen this picture many times.
 01:00: It has an estimated mass of several billion times that of the Sun, which gives it an event horizon larger than the solar system.
 06:42: For a nonrotating black hole the sphere should be at 1.5 times the Schwarzschild radius.
 07:05: That disk terminates at a few times the Schwarzschild radius – again depending on black hole spin.
 10:54: ... Einstein’s wildest prediction and the strangest object in all of space time. ...


20190424: No Dark Matter = Proof of Dark Matter?
 00:03: ... of the bullet cluster was mapped using gravitational lensing this time the warping of life of galaxies behind that cluster it turns out that ...


20190410: The Holographic Universe Explained
 00:00: We live in a universe with 3 dimensions of space and one of time.
 00:51: ... the past several months on Space Time, we’ve seen those close clues, and we’ve built a the foundations needed ...
 13:11: Now stack many maps to represent slices in time.
 17:33: Cuallito notes that it's looking more and more like Roger Penrose might literally be a timelord.


20190403: The Edge of an Infinite Universe
 00:37: For example we have the “observable universe” – that patch that we can see, and beyond which light has not yet had time to reach us.
 01:17: We visited these cosmic horizons in one of the early Space Time episodes.
 04:32: Regular coordinates of space and time are useless there – they blow up to infinities.
 04:38: Physicists found mathematical ways to fuse space and time into new coordinates that suppressed the infinities.
 05:19: ... diagrams, for Brandon Carter who came up with them around the same time. ...
 05:40: As a quick review: start with a graph of space versus time – a spacetime diagram – then compactify.
 05:49: ... horizontalish contours are our old time ticks  moments of constant time across the universe, while the ...
 06:03: The contours bunch up towards the boundaries so that every step on the map covers more and more space and time.
 09:38: But this time we’re not mapping space versus time – we’ll just map two dimensions of hyperbolic space.
 11:40: So this disk can represent an infinite antide Sitter universe with 2 spatial dimensions at a single instant in time.
 12:34: In fact let’s add the dimension of time to our hyperbolic projection.
 12:38: Stack a bunch of Poincare disks – each representing an instant in time.
 12:53: On the other hand the surface of the disk has only one dimension of space – the circumference – and the same oneD of time – 1+1.
 16:25: ... has a very clear range of validity  it's valid in systems that are time symmetric  systems where the global properties of the spacetime don't ...
 16:45: However our universe on its largest scales is not time symmetric  it's expanding, so the past looks very different to the future.
 17:58: ... working normally, so there would be some minutes to hours of bad times as our molecules start to betray ...
 18:09: To make matters worse, the Earth would be falling apart at the same time.


20190328: Could the Universe End by Tearing Apart Every Atom?
 00:25: ... what if the energy density in each patch of space increases over time in that case the acceleration itself would be accelerating but that ...
 05:51: ... expansion but in that case the dark energy is decreasing over time. ...
 06:20: ... increases. The result would be that the acceleration increases over time offhand, that doesn't sound so much crazier than regular old dark energy ...
 07:02: ... event horizon'. Now, if the expansion is accelerating, then, over time the distance between patches of lightspeed space gets smaller and that ...
 11:35: ... Time to get our affairs in order. But more likely is that W is exactly 1, ...
 12:11: ... energy conditions of general relativity that prohibit negative mass and time ...


20190320: Is Dark Energy Getting Stronger?
 00:41: Today we’re doing a Space Time Journal Club to reveal another clue.
 01:51: It was revolutionary and radical at the time, but a few Nobel prizes later and dark energy is now textbook cosmology.
 03:19: ... of dark energy, dark matter, and everything else at the earliest of times when the CMB was released long before the first stars ...
 03:36: ... to calculate how the universe should have evolved from those early times, and how fast it should be expanding ...
 04:33: If we have lots of supernovae at different distances when we have a set of rulers spanning both time and space.
 04:50: Our rulers span much of cosmic time, and together they give an expansion history.
 06:03: We miss the first 25%, and we’ve found too few supernovae over the first 50% of cosmic time.
 13:07: ... out much stronger dropped off rapidly, or even that it oscillates over time. All very ...
 14:27: One way or another I guess it’d be nice to know whether dark energy will one day rip to shreds the subatomic fabric of space time.
 16:50: We didn't have time to fit more in this episde, but there's a ton of cool stuff happening.


20190313: Will You Travel to Space?
 01:28: Now in classic eccentric billionaire style, Branson spends much of his time on his private island in the British Virgin Islands.
 01:43: It's a long story for another time.
 03:49: ... difference to the motivations of Musk & Bezos, who have talked many times about the importance of an interplanetary humanity – they want to ...
 09:52: Maybe 25 years, earlier if you get rich. Richard Branson is 68 and has been waiting a long time for this.
 12:37: It's a hell of a time to be alive, watching humanity's first tentative steps off the Earth, and into the fringes of space time.


20190306: The Impossibility of Perpetual Motion Machines
 02:12: ... wheel pushes masses outwards on one side, but that same at the same time increases the separation between those masses, so that moment of inertia ...
 03:07: ... review: the 2nd law states that entropy can never decrease over time – in other words, energy will tend to spread itself out as evenly as ...
 11:29: You do get your pick from the Space Time store.
 12:11: “ da Vinci was ahead of his time in some ways – he knew perpetual motion was bunk even before the laws of thermodynamics came along.
 12:21: Had only da Vinci known that his own perpetual motion machines could’ve score him such cool swag from Space Time.
 12:43: ... example you can check out the Destination Pluto, which follow the timeline of the New Horizons mission from its inception to the date of its close ...
 14:34: People tend to talk about recombination as having happened instantaneously and at the same time everywhere.


20190220: Secrets of the Cosmic Microwave Background
 00:08: ... when the hot dense early Universe became transparent for the first time It sounds like random static But that buzz contains an incredible wealth ...
 02:05: ... some places the overdensities were so large that matter only just had time to float at the centre before being frozen in place by Recombination no ...
 07:42: ... a standard ruler Spots of this size represent fluctuations that had time to collapse exactly once which means their size had to be equal to the ...
 11:14: ... that were small enough to oscillate at least once during this brief time should be enhanced Their peaks on the power spectrum should be raised ...
 12:34: ... like random minuscule fluctuations in this faint noisy buzz So next time you hear the static of an untuned TV or radio remember that in that ...


20190207: Sound Waves from the Beginning of Time
 00:29: [PBS Space Time intro] The field of cosmology and the study of the universe on its larger scales was once the least precise in all of astrophysics.
 02:48: There exists no lines of sight to anything during that time, and we'll come back to that later.
 06:20: Well, the exact distance that sound could travel over the age of the universe at that time.
 11:57: These rings give us a standard ruler on the sky, spanning all of cosmic time.
 12:26: It appears unchanging over time, just as we'd expect if dark energy is the energy of the vacuum itself.
 14:49: ... of poorly thoughtout "theories" that they get bombarded with all the time. ...
 15:16: Ergo, time will prove negative mass' existence.
 15:26: ... fallacy to draw analogies between revolutionary ideas that took a long time to prove or discover and fringe ideas that may or may not ever be ...
 17:28: [PBS Space Time outro]


20190130: Perpetual Motion From Negative Mass?
 00:13: Because you could use negativemass apples to build warp drives, travel in time, and construct a perpetual motion machine.
 00:52: And if you can travel faster than light you can also travel backwards in time.
 04:00: ... time to get into why this is the case, or what the spin of a field even ...
 05:02: Even weirder, a positive mass should attract a negative mass while at the same time being repelled by it.
 14:07: 6 randomly selected correctslashcreative answers will receive their choice of any piece space time swag from our brand new merchandise store.
 14:21: Winning perpetual motion machines will be revealed in an upcoming episode of space time.


20190124: The Crisis in Cosmology
 01:03: ...and at the same time that universe was revealed to be expanding.
 10:00: ...given we figured them out from data at the opposite ends of time.
 15:12: ...the simultaneous reversal of charge, parity, and time.
 15:17: First up: a few of you asked questions about time reversal, so I want to clarify.
 16:14: And I guess you're referring to the idea that timereversed energy has its sign flipped.
 16:20: So, the simple answer is no, because the T in CPT isn't a true time reversal.
 16:26: But in the case of a true time reversal, the answer is, essentially, yes.
 16:31: And a negative mass particle, moving backwards in time,...
 16:34: ...is mathematically the same as a positive mass particle moving forward in time That notion makes sense in the math,...
 16:57: Well, a true time reversal that included the interior of a black hole...
 17:27: Both the T of CPT and true time reversal symmetry.
 17:50: ...times the speed of dark...


20190116: Our Antimatter, Mirrored, TimeReversed Universe
 00:03: ... is a perfect mirror opposite in charge handedness and the direction of time but one by one these symmetries were found to be broken threatening to ...
 02:02: ... deeper symmetry cpt symmetry the combined flipping of charge parity and time and this symmetry lies at the foundations of quantum field theory ...
 03:02: ... are two ways to do this mixing yielding two types of mutual Kaon one time let's call it KS he is shortlived and has what we call an even CP state ...
 08:43: ... symmetry holds then T symmetry must also be violated why because that time reversal operation needs to bring us from a broken CP reflected universe ...
 10:20: Matter that was going forward in time looks like parity flipped antimatter going backwards in time.
 10:26: ... interpretation of antimatter as time reverse matter was first proposed by Ernst Stueckelberg in 1941 but is ...
 10:54: ... you get back to where you started. If CP is violated then this simple time reversal is also violated and we see this violation in the asymmetry ...
 13:52: ... the comments to that episode or catch up on last week's comments next time  in fact, I really want to address a few points made by FieldStrength ...


20190109: Are Dark Matter And Dark Energy The Same?
 00:55: Today on Space Time Journal Club, let’s pick apart J.S.
 01:31: So we conclude that galaxies, and for that matter the universe, has 510 times as much matter as we can actually see.
 05:56: At the same time, the repulsive force between a negative and positive mass should repel the positive and attract the negative mass.
 06:39: ... mass in the galaxy attracts a halo of negative mass, but at the same time that positive mass is repelled inwards by the surrounding ...
 09:31: ... for the bit of the sine wave where the universe is expanding from zero time – the big bang  slowing down towards the first peak and then collapsing ...
 10:00: You can build traversable wormholes, Alcubierre warp fields, timemachines, antiunicorns.
 12:52: I’m also happy to be corrected on anything I said, and I’ll address that in the comment responses next time.
 12:59: ... mass, antigravitational, positive pressure, antideSitter space time. ...


20181220: Why String Theory is Wrong
 02:35: He was playing around with the newfangled general relativity in five dimensions, 4 space and 1 time, because why not.
 07:54: They can even wind around it, perhaps multiple times in either direction, before forming a closed loop.
 08:01: ... number of times a string winds around this compactified dimension is called its winding ...
 08:55: ... least, they give exactly the same physics, either winding number times radius, or mode number divided by radius can be used to define the ...
 11:49: ... independent reasons that we don't have time to get into, 11 is also the magic number for super gravity dimensions. ...
 15:37: ... are spatial coordinates flipped, charges are also reversed and in fact time is reversed ...
 16:28: ... when you reverse parity charges and time do you get a universe that behaves like, ours one full of mirror ...


20181212: Quantum Physics in a Mirror Universe
 00:02: ... in the case of those other symmetries I mentioned the shift in position time or phase can be by any amount small or large these are continuous ...


20181206: Did Life on Earth Come from Space?
 00:37: ... solar ultraviolet radiation in space can reach a hundred million times the UV intensity of sea level this will typically completely destroy any ...


20181121: 'Oumuamua Is Not Aliens
 00:07: To repeat the "Space Time" maxim, it's never aliens, until it is.
 00:29: ... seen similar things countless times before, lonely rocks, remnants of the formation of our solar system ...
 01:10: It caused a justifiable stir at the time.
 01:15: ... Oumuamua has been making the news again, and this time it's for a new paper by Shmuel Biali and Avi Loeb that investigates the ...
 01:34: Today on "Space Time Journal Club," I want to do two things.
 08:31: ... likely, interstellar space needs to contain something like 10 to 100 times the number of Oumuamualike space rocks than we ...
 12:53: But I do know that Oumuamua is one of the most fascinating discoveries in astronomy in recent times.
 12:59: Whether or not it's aliens, it's likely our first encounter with the natural or unnatural denizens from interstellar space time.
 16:04: It appeared to say that the weak force is 1,024 times stronger than gravity.
 16:17: It should have been 10 to the power of 24 times stronger.


20181114: Supersymmetric Particle Found?
 02:27: We'll come back to all of this in detail another time.
 04:05: The highest energy cosmic rays can have energies around a billion times that of the LHC.
 08:19: ... chance that two tau neutrinos could have been seen in the amount of time ANITA has been ...
 09:49: It may also be that there were some gigantic bursts of regular neutrinos at the time of the observed events.
 10:05: In fact, one of the two events may have been associated with a distant supernova that was observed around the same time and location.
 10:30: And remember, there were two events at different times.
 11:04: Actually, given the amount of time IceCube has been in operation, it probably should have.
 12:00: Perhaps we will verify the existence of the stau and with it confirm the supersymmetric nature of space time.
 14:32: ... for the 2D world sheet, which has one dimension of space and one of time, this is the shape traced out by a string moving through ...


20181107: Why String Theory is Right
 03:29: When a point particle is moving through space and time it traces a line.
 03:34: On a spacetime diagram, time versus one dimension of space, this is called its world line.


20181031: Are Virtual Particles A New Layer of Reality?
 00:29: And every time two particles interact, an infinite number of virtual particles mediate infinite versions of that one interaction.
 05:36: And they can even travel faster than light or backwards in time.
 10:17: It doesn't change over time.
 16:15: A lot of that time, life was evolving towards the point that it could become intelligent.


20181025: Will We Ever Find Alien Life?
 01:11: A series of very recent surveys of our galaxy reveal none of the above while, at the same time, proving the abundance of potentially habitable worlds.
 04:38: As we like to say on Space Time, it's never aliens until it is.
 11:14: Over time, one whack job or impulse controlchallenged individual will push the metaphorical big red button.
 12:40: The alternative may be eternal quiet across the lifeless reaches of space time.


20181018: What are the Strings in String Theory?
 05:57: Constructive interference only happens if the wavelength of the wave fits a neat number of times along the length of the string.
 10:13: In fact, the theory only works in precisely nine spatial dimensions, plus one for time, plus one for M theory, which we'll come back to.
 11:44: Our universe of 3D space and 1D time is like flatland on this 5D object called a 5brane.
 13:17: And you can decide for yourself whether you accept the fundamental stringy nature of space time.
 13:24: Thanks to 23andMe for supporting PBS Digital Studios and "Space Time".
 13:35: ... month of October is Family History Month, which is a great time to explore and learn more about your own family and ancestry, a ...
 14:03: Roman R. asks whether computation at an event horizon would experience massive time dilation relative to an outside observer.
 14:33: The slow readout by Hawking radiation is equivalent to the time dilation issue.


20181010: Computing a Universe Simulation
 00:44: ... notion of existence or nonexistence like if the tiniest chunks of space time or chunks of quantum field or elements in the abstract space of quantum ...
 01:48: ... so long as it's underlying mechanics are rulesbased evolution over time. ...
 03:48: ... the same time, the actual information content in matter and radiation is probably more ...
 05:35: The mass of a 100 kilometer radius black hole would be 30 times that of our sun.
 06:05: You'd need a black hole a few million times the mass of the sun and 10 million kilometers in radius.
 06:33: Every instant in time must progress to the next instant.
 08:39: So it could have performed around 10 to the power of 120 operations in that time.
 09:33: Protons will have started to decay by the time we simulate last Monday.
 10:39: But hey, you could simulate 10 to the power of 70 universes in that time.
 11:57: And these insights may also lead to real paradigm shifts, perhaps ultimately revealing the fundamentally informational nature of space time.
 13:22: Gamma rays should be barely slowed down at all, far less than the measured twoish seconds time delay.
 14:48: Seriously, Hollywood screenwriters really need to hire Space Timers as consultants to get their terminology right and pay us lots of money.


20181003: How to Detect Extra Dimensions
 00:43: ... on "Space Time Journal Club," I want to tell you about one in particular described in a ...
 01:11: These superdense remnants of dead stars churned the fabric of space and time in their death spiral.
 02:17: Add one dimension of time to give us 4D spacetime, which we'll also refer to as 3plus1dimensional spacetime.
 04:44: ... in terms of strength, gravity is vastly weaker, 10 to the power of 32 times weaker, than even the weak nuclear ...
 11:06: ... way, comparison of the electromagnetic and gravitational wave arrival times also allowed us to verify that gravity really does travel pretty much ...
 15:35: Whining won't get you anywhere, except this time.
 15:39: This time, it really worked the tree.
 15:40: Brilliance, wit, and/or whining that's what it takes to make it on "Space Time."


20180920: Quantum Gravity and the Hardest Problem in Physics
 00:24: [MUSIC PLAYING] The first few decades of the 20th century was a time of miracles for physics.
 00:34: First, Einstein's relativity utterly changed the way we think about space, time, motion, and gravity.
 01:12: ... today, on "Space Time," I'm going to begin our discussion of the great quest for this union, the ...
 01:45: In it, the presence of mass and energy warp the fabric of space and time.
 01:56: General relativity incorporates the earlier special relativity, which describes how our perceptions of space and time also depend on motion.
 02:06: ... the earlier ideas of Isaac Newton, in which space and time are treated as separate and universal, special and general relativity ...
 02:41: That math started with the Schrodinger equation, which tracks these probability waves through space and time.
 02:49: But the Schrodinger equation treats space and time as fundamentally separate in the oldfashioned Newtonian way.
 03:06: Nowadays, modern quantum field theories fully incorporate the melding of space and time predicted by special relativity.
 03:13: And yet they still don't directly incorporate the warping of space and time predicted by general relativity.
 04:30: In fact, it's very possible to shoehorn the curved geometry of general relativity into the way quantum field theory deals with space and time.
 04:38: ... you have strong gravitational effects on the smaller scales of space and time, like the central singularity of the black hole or at the instant of the ...
 06:37: The uncertainty principle also defines the precision tradeoff between time and energy.
 06:41: So this same argument can be used to suggest a fragmentation of time.
 06:46: Try to measure any time period shorter than 10 to the power of negative 43 seconds, the Planck time, and boom black hole.
 08:19: But in the resulting math, the new quantum fields still lie on top of a smooth, continuous grid of space and time.
 11:18: Space and time simply cannot behave in the familiar way below the Planck scale.
 14:56: But I dropped that "observable" part a couple of times later on.


20180912: How Much Information is in the Universe?
 00:41: ... informational at its most fundamental level, and its evolution through time can be thought of as a ...
 02:48: ... length is the smallest meaningful measure of distance, at around 1.6 times 10 to the negative 35 ...
 03:31: Well, the radius of the universe is something like 47 billion light years, which is a few times 10 to the power of 61 Planck lengths.
 06:17: Neutrinos and photons formed in the big bang are probably a billion times more abundant than protons.
 07:02: As I mentioned last time, black holes contain most of the entropy in the universe.
 09:14: It would be the end of space time.
 10:41: ... it among other things to give you a bit of a behind the scenes on "Space Time" stuff, especially graphics, as well as the extra details on challenge ...
 11:19: ... off the simulation, I'll see you next week for a new episode of "Space Time." So we're a bit behind on comment responses due to my ...
 11:51: Yeah, for a show that's about a 13.8billionyearold universe, we do try to keep it on human time scales, at least most of the time.
 13:24: Spencer Twiddy is amused at my use of the word "prompt" for the time scale of evaporation by Hawking radiation.
 13:31: Yeah, that's the weird thing about time scales of the end of the universe.
 13:35: They're so ridiculously long that even other ridiculously long time scales look short in comparison.
 13:50: ... really useful to be able to ignore insignificant amounts of space, or time, or energy in the presence of much larger ...
 13:59: ... the fact that the hypothetical proton decay time is 10 to the power 40 years, Vinay K asks if a proton created in, say, ...
 14:27: ... that has a constant probability of decaying in a given amount of time. ...
 14:35: So the halflife is the amount of time it takes for something to have a 50% chance of decaying at some point during that time.
 14:43: The actual amount of time it takes to decay is random, and there's a constant probability of it happening at every set interval.
 15:12: ... halving of the number of protons will happen 10 times by 10 to the power of 41 years, so the fraction of protons left will be ...


20180905: The Black Hole Entropy Enigma
 07:50: So entropy can be defined as the information hidden in a system's macroscopic configuration times the Boltzmann constant.


20180830: Is There Life on Mars?
 00:11: ... a little robot named "Opportunity." 15 years after its mission began, 57 times longer than it signed up for, "Opportunity" has gone ...
 05:55: This evidence for ancient flowing water wasn't definitive at the time.
 07:19: It was dated to around 4 billion years old from a time when we now believe Mars had liquid water on the surface.


20180823: How Will the Universe End?
 00:50: That 100 trillion years is 10,000 times the current age of the universe.
 01:05: In fact, our universe will spend almost all of its infinite time in darkness, slowly crawling towards maximum entropy and ultimate heat death.
 05:33: ... like 10 to the power of 18 years when the universe is a million times older than the age of the last stars' ...
 05:49: Around 10 times longer still, and the entire megagalaxy will either have dispersed or fallen into the massive black hole at the galactic center.
 07:38: But all black holes evaporate over time via Hawking radiation, something we've discussed in detail.
 07:51: The small black holes, say, around 10 times the mass of the sun, completely evaporate in around 10 to the power of 67 years.
 09:29: In that case, there will be structure in the universe for a very, very long time.
 09:44: Over infinite time, nothing is truly stable.
 12:34: We'll explore these extreme futures of spacetime time in the near future of "Space Time." Before we get to comments, two things.


20180815: Quantum Theory's Most Incredible Prediction
 00:43: We know this because the predictions of quantum field theory stand up to experimental test time and time again.
 00:59: We've talked about QFT many times before, starting with the very first quantum field theory, quantum electrodynamics.
 05:02: So that electron diagram you did in middle school, it's time to kill that idea just like you kill your tamagotchi.
 09:40: ... was an amazing result for the time, but a lot of time has passed since then and physicists were not content ...
 10:11: Over time, physicists have included more and more corrections refining the prediction of the G factor to increasing precision.
 12:31: We have to conclude that we are getting closer and closer to the truth in our search for theories to explain the underlying mechanics of space time.
 16:08: Sebastian [INAUDIBLE] cautioned him to enjoy this one, the second time it feels nowhere near as good, speaking from experience.


20180801: How Close To The Sun Can Humanity Get?
 00:00: [MUSIC PLAYING] The sun an entity worshiped as a god throughout time and across cultures.
 01:00: 100 times Earth's diameter.
 07:34: That's seven times closer than any humanmade object has ever come.
 07:38: ... 88 days because of its eccentric orbit, Parker will get roughly 10 times closer to the sun than the closest ...
 08:41: This time if our luck holds, we'll come close to touching our home star to unlock the mysteries of our closest stellar neighbor in space time.
 09:09: And Space Time fans have always been a huge help.


20180725: Reversing Entropy with Maxwell's Demon
 01:59: Let's explore this using the analogy we used last time, the Go board.
 05:50: ... time the demon sees a high speed particle approaching from the right, it ...
 11:17: ... second law, the limits of information processing, and even the arrow of time. ...
 11:30: ... get tangled up in all of that in an upcoming episode of "Space Time." Last time, we delved into the true nature of entropy and the cause of ...
 11:49: ... that all microstates are equally probable over long periods of time. ...
 12:44: ... system is so overwhelmingly small, that it never happens on any sane time ...
 12:55: On the other hand, if the universe lasts for infinite time, then principal entropy drops of all sizes should eventually happen.


20180718: The Misunderstood Nature of Entropy
 00:07: ... law of thermodynamics have been credited with defying the arrow of time, predicting the ultimate heat death of the universe, and providing the ...
 01:13: ... in deepest humiliation." We've looked at entropy in the past, but it's time to go much deeper to unravel the great ...
 02:28: Around a half century after Carnot, Rudolf Clausius was inspired to quantify this tendency of heat energies to decay over time, enter entropy.
 05:18: So if you look at the system at some random point in time, it'll be in a completely random microstate chosen from all possible microstates.
 05:48: ... are nearly 2 times 10 to the power of 107 ways to arrange the pieces and almost all of them ...
 06:09: That microstate is a factor of 2 times 10 to the power of 107 less likely than one of the many smoothly mixed microstates.
 07:47: ... of the number of microstates consistent with the current macrostate times the Boltzmann ...
 08:59: ... in equilibrium must increase in entropy, simply because at any future time, it's current microstate will most likely be one of the more common types ...
 10:19: It seems to add the arrow of time.
 10:22: ... Newtonian or quantum mechanical, don't care about the direction of time, and yet, the second law of thermodynamics clearly distinguishes between ...
 10:40: It's almost like the concept of time is emergent and statistical, just like entropy.
 10:45: ... brilliant macrostate that is you, until I see you next week on "Space Time." The field of statistical mechanics has given us some of the most ...
 11:43: Brilliant, Math and Science Done Right, is proud to support "Space Time." To learn more about Brilliant, go to brilliant.org/spacetime.


20180711: Quantum Invariance & The Origin of The Standard Model
 03:40: The function is an oscillation in quantum possibility, moving through space and time.
 05:09: ... time, we'll shift the phase by different amounts at different locations, while ...
 11:02: We should be amazed that we can learn that language and through it, comprehend the underlying nature of space time.
 11:10: Last time on Space Time Journal Club, we looked at a new result potentially detecting a particle beyond the standard model, the sterile neutrino.


20180704: Will A New Neutrino Change The Standard Model?
 00:18: ... on "Space Time" Journal Club, we'll look at a paper that reports a compelling hint of a ...
 06:21: That would require regular lefthanded neutrinos to spend at least a bit of their time as sterile righthanded neutrinos.
 06:41: ... order to evolve that way, neutrinos must experience the flow of time, which means they can't be moving at the speed of light, which means they ...
 07:44: So the MiniBooNE experiment starts with muon neutrinos, and some of these transform into electron neutrinos by the time they hit the vat.
 10:33: Either way, we'll have peered just a little deeper into the fundamental building blocks of space time.
 11:15: With perfect knowledge of the current universe, it should be possible to perfectly trace the universe backwards and forwards in time.
 12:22: So Earth is around 2,000 times more massive than the entire asteroid belt.


20180627: How Asteroid Mining Will Save Earth
 00:45: His net worth was three times greater than that of our richest tech billionaires, inflation adjusted.
 00:52: And the source of that fortune timely exploitation of a vast, thenuntapped natural resource, oil.
 09:46: The first asteroidmining missions are slated for the 2020s, but the timetable is very loose.
 10:26: ... begin of expanding humanity's reach into more distant regions of space time. ...
 10:42: Today's episode of "Space Time" was sponsored by Audible.


20180620: The Black Hole Information Paradox
 00:44: It gradually leaks away through Hawking radiation over unthinkably long time scales.
 01:43: With perfect knowledge of the current universe, it should be possible to perfectly trace the universe backwards and forwards in time.
 04:26: ... unwarranted extrapolation from an untrustworthy approximation." But over time, the importance of the contradiction became ...
 06:14: For the outside universe, everything that ever fell into the black hole remains frozen in time and smeared flat over that horizon.
 07:57: ... because the interior of the black hole doesn't even exist on the same timeline as the external universe, it's arguable that those copies don't even ...
 08:11: ... position and momentum, that can't both be perfectly measured at the same time. ...
 11:44: Einstein's general theory of relativity reveals them to be regions of frozen time and cascading space.
 12:36: Brilliant, math and science done right, is proud to support "Space Time".
 14:23: They can have any mass, can travel faster than light, and can even travel backwards in time.
 15:08: Yeah, that was due to time dilation.
 15:10: ... you're trying to throw together a quick graphic and forget that "Space Time" viewers notice ...
 15:20: In these frames of reference, sometimes 21 months takes 8 minutes due to "ran out of time" dilation.


20180613: What Survives Inside A Black Hole?
 00:34: At that horizon, time is frozen and the fabric of space itself cascades inwards at the speed of light.
 08:52: In Einstein's general relativity, a spinning mass drags the fabric of space time around with it in a phenomenon known as frame dragging.
 11:03: We will delve into all of that in an upcoming episode of "Space Time".


20180523: Why Quantum Information is Never Destroyed
 00:32: They are mathematical rules that dictate how systems evolve in time.
 00:59: But determinism in the forward time direction does not guarantee that the same laws can perfectly predict the past.
 01:07: And yet, this sort of deterministic symmetry, timereversal symmetry, is essential for information itself to be conserved.
 01:43: For example, if the equations stay the same from one point in time to the next, then energy is conserved.
 02:05: For example, we could reverse all electric charges, or we could flip the xaxis by looking in a mirror, or we could make time run backwards.
 02:14: Timereversal symmetry isn't covered by Noether's theorem, but it's still tied to a conservation law.
 02:23: ... say that a system is timereversal symmetric if its equations of motion allow us to perfectly predict the ...
 02:38: ... entire universe would be timereversal symmetric if knowing the exact state of every particle in the universe ...
 02:50: That would mean that the exact configuration of the universe at any point in time defines the exact configuration at any other point.
 02:57: ... one thing, this timereversal symmetry means complete information about the configuration at all prior ...
 03:20: But this sort of future determinism doesn't have to be timereversal symmetric.
 04:36: It ensures conservation of information and timereversal symmetry because of an even more fundamental rule the conservation of probability.
 04:51: The time dependent Schrodinger equation describes the time evolution of this thing called the wave function.
 05:30: In that sense, it's deterministic and timereversal symmetric and so conserves information.
 05:51: ... the Schrodinger equation guarantees time reversibility and so the conservation of information, as do the more ...
 06:34: And as time goes on, a particle's properties will continue to have possible values.
 06:42: If this is true, and it must be, we say that the time evolution of the wave function is unitary.
 06:56: ... statement about how probability works, but the condition also ensures timereversal symmetry and conservation of ...
 07:49: ... you can trace a quantum state indefinitely forwards and backwards in time. ...
 09:08: So this interpretation is neither deterministic nor timereversal symmetric.
 09:14: ... manyworlds or the de BroglieBohm pilot wave theory preserve this time ...
 09:43: There is one situation where timereversibility appears to be broken regardless of your favorite interpretation of quantum mechanics.
 10:02: In an upcoming episode, we'll see whether quantum information really can be deleted from the otherwise perfect memory of space time.
 10:42: "Space Time" is going to go quiet for the next couple of weeks to get our house in order.
 11:36: The key is that on the scale of these systems, the universe is time translation symmetric.


20180516: Noether's Theorem and The Symmetries of Reality
 03:47: On the other hand, the gravitational field across the whole stretch of road doesn't change from one point in time to the next.
 03:54: The system is symmetric to time translations.
 04:01: Noether's theorem reveals that this time translation symmetry gives us energy conservation.
 04:35: See, Einstein's description of gravity reveals the dimensions of space and time to be dynamic and changeable.
 04:43: If the very nature of space can change over time, then continuous time symmetry is broken.
 05:00: The law of conservation of energy is fundamental in Newtonian mechanics, in which space and time are unvarying and eternal.
 05:14: It only applies for parts of the universe where we can approximate space as unchanging over time.
 06:16: This is a rather abstract quantity that measures the effort involved in moving between two states over some time interval.
 06:24: It's a generalization of Fermat's principle, which states that light will always take the path between two points that minimizes the travel time.
 07:22: ... can apply it to not just the symmetries in the dimensions of space and time, but also to more abstract ...
 08:46: Yet, she gained little public recognition in her time and is still only known to the more eager students of math and physics.
 09:27: ... workings of the universe through the continuous symmetries of "Space Time." Thank you to Brilliant for supporting PBS Digital ...
 10:07: Fabrice, your continuously timesymmetric contributions are fundamental to the conservation of "Space Time." So thank you.
 10:31: ... in that it will measure light curves, so it changes in brightness over time for billions of objects across the entire ...
 11:16: ... general, it'll do better in what we call the time domain by scanning the entire sky every few nights, and it'll keep ...
 11:58: But its extreme precision is possible because it comes back to every field many times over the five years of its operation.
 12:05: Each time, stars will have moved slightly, but they'll have moved in extremely predictable ways.
 12:47: ... people, on "Space Time," we only ask that you start with a passing familiarity with quantum ...


20180509: How Gaia Changed Astronomy Forever
 01:10: That's 1,000 to 2,000 times smaller than the resolution of the Hubble Space Telescope.
 01:28: An atlas that we can wind both forwards and backwards in time.
 03:17: ... the first time, we have a complete census of the stellar population far beyond the ...
 08:02: Gaia even helps us with the pulsar timing array, a galactic scale gravitational wave observatory which we spoke about recently.
 08:40: This is our Milky Way, and we just became much more familiar with our galactic home in space time.
 09:23: They start out red, but heat up over time.
 10:29: On the other hand, a star that weighs twice as much as the sun, burns about 25 times brighter and burns out 10 times faster.


20180502: The Star at the End of Time
 01:55: ... to the power of 26 watts or around the energy equivalent of 20 million times the Earth's entire nuclear arsenal every ...
 02:41: ... We observe that a red dwarf with 10% of the Sun's mass is about 1,000 times fainter than the ...
 02:56: That means it's burning through its fuel 1,000 times less quickly.
 03:58: A red dwarf with 10% the Sun's mass has just as much fuel to burn as the Sun does, yet it burns it 1,000 times slower.
 04:07: That means it should live 1,000 times longer so 10 trillion years instead of the Sun's 10 billion years.
 04:20: Just like the Sun, the cores of red dwarf stars shrink and heat up over time.
 07:19: ... an awfully long time at many times the current age of the universe, Red dwarfs will surely be ...
 08:38: ... in the warmth of the last stars to burn in the darkening end of space time. ...


20180425: Black Hole Swarms
 00:24: The stars are so densely packed that the night sky would be 500 times brighter than our own.
 00:30: A supermassive black hole four million times the mass of our sun lurks in the center.
 01:07: ... this episode of Space Time Journal Club, we're going to delve into the recent nature paper, Hailey, ...
 03:28: Now, this process takes a really long time for a stellarmass black hole.
 05:44: Xray binaries likely spend most of the time in a quieter phase, with the gas just trickling slowly from the companion star.
 07:35: Now, that's tens of thousands of times the black hole density anywhere else in the galaxy so yeah, it's a swarm of black holes.
 08:13: So, next time you see the Milky Way in the night sky, find the bright patch, just to the edge of the constellation of Sagittarius.
 09:06: ... sign up at curiositystream.com/spacetime and use the promo code, space time, during the signup ...
 10:38: Two gravitational waves crossing paths will add together at any one point in space and time.
 11:16: ... rogue wolf notes, that stellar gravitational wave detectors, like pulsar timing arrays, are a bit like using the rustling of leaves and grass to see the ...


20180418: Using Stars to See Gravitational Waves
 00:07: Now that gravitational waves are definitely a thing, it's time to think about some of the crazy things we can figure out with them.
 00:58: In three cases, both members of the black hole binary pair were well over 20 times the mass of the sun.
 02:45: For the first time, the event behind a gravitational wave signal was also seen in light.
 04:26: That's roughly 10 times the distance from the Earth to the moon.
 05:34: We'd need a network of perfect timing devices scattered across the galaxy.
 05:59: The international pulsar timing array is a massive effort spanning many universities and radio observatories around the world.
 06:13: It looks for shifts in the time of arrival of their signals.
 06:35: More time and more data will hopefully yield a detection.
 10:07: We wanted to invite you to join the full live call next time.


20180411: The Physics of Life (ft. It's Okay to be Smart & PBS Eons!)
 02:54: Not only is life stunningly complex, but that complexity increases over extremely long time scales, in fact, over eons.
 03:29: The Earth's biosphere, at least, becomes less boring over time.


20180404: The Unruh Effect
 00:06: Every time you accelerate, put your foot on the gas, quicken your step, get out of your chair, you generate an event horizon behind you.
 00:23: ... MUSIC] Around the same time that Stephen Hawking was demonstrating the existence of the black hole ...
 01:34: A spacetime diagram has two axes, time and, well, space, with time on the vertical axis.
 01:40: We can show an object's path through space and time using world lines on the diagram.
 02:11: It's just determined by the unit of space and time that we choose.
 02:47: ... our observer moves forward in time, as long as they don't travel faster than light, their past light cone ...
 07:41: Every time an Unruh particle hits the detector, it would click.


20180328: The AndromedaMilky Way Collision
 01:04: At three billion years, it'll be two and 1/2 times bigger.
 02:41: Time the pulsation rate, and you know how luminous the star is.
 03:49: For a long time, we had no idea about Andromeda's transverse velocity.
 06:56: The average distance between stars is around 100 billion times greater than the average size of a star.
 08:37: ... sometimes think how lucky we are to live in the time before our collision with Andromeda, a time when we have such a clear ...
 09:06: Will those astronomers ever figure out that there are countless other island universes stretching across a much vaster space time?
 11:28: Instead of Tshirts, we're sending some stacks of Space Time stickers.


20180321: Scientists Have Detected the First Stars
 00:57: Today, we're doing a Space Time Journal Club to explain this discovery.
 02:51: [MUSIC PLAYING] The TLDR is that there should have been this brief period of time when the universe was eating up 21 centimeter photons from the CMB.
 04:03: That period represents the time between the birth of the very first stars to the onset of very active black hole growth.
 05:15: The only thing colder than this ambient hydrogen at the time, was dark matter.
 05:48: More time and more data will help sort this out.
 08:27: So we chose six correct answers to receive "Space Time" tshirts.
 08:46: If you didn't win this time, there's a link in the description so you can grab your own tshirt any way.
 08:51: That way when you do win next time, you can get our upcoming tshirt, which will be even cooler, if that's possible.
 08:58: Nice way to show your appreciation for PBS "Space Time."


20180315: Hawking Radiation
 00:00: [MUSIC PLAYING] He was perhaps the greatest genius of our time.
 00:30: This is "Space Time." And it's time for Hawking radiation.
 00:50: ... of extreme density like the dead core of a massive star, space and time could be dragged inwards to create a hole in the universe, a boundary in ...
 02:40: ... let's take a deep dive into the quantum field theory of curved space time to glimpse the true nature of Hawking ...
 03:39: A negative frequency can be thought of as a mode that travels backwards in time and can be interpreted as corresponding to antimatter.
 05:37: By the time this trajectory has found its way back out into flat space again, those fluctuations look like real particles.
 06:21: Say that three times fast.


20180307: Should Space be Privatized?
 00:11: This time people watched with anticipation as SpaceX launched its new Falcon Heavy rocket.
 01:44: Ronald Reagan signed the Commercial Space Launch Act of 1984, which for the first time made commercial launches legal.
 07:03: Massive human effort is wasted every time a large program is scrapped, often for poorly informed political reasons or as publicity stunts.
 09:06: ... may ensure the benefit of all in our next steps in exploring outer space time. ...
 11:36: But by the time a red dwarf finishes using its fuel, the entire star is made of helium.
 12:04: ... stars consume their fuel thousands of times more slowly than the sun, which means they can live hundreds of times ...


20180228: The Trebuchet Challenge
 00:31: Physics steals words all the time.
 01:43: ... accelerated by the same force over the same path, the quantity half mass times velocity squared has to be ...
 04:08: The counterweight travels through a short arc, while the end of the opposing arm travels through a much longer arc in the same amount of time.
 04:58: There's no time for all of that.
 06:16: You raise the counterweight to the same height as last time and let it fall.
 06:19: This time, you release the projectile a little earlier, so it takes a more vertical trajectory.
 06:52: This time we'll use numbers.
 08:12: We'll choose six correct entries to receive "Space Time" Tshirts.
 08:16: That way, next time you besiege a fortress, you can do it under the banner of "Space Time."


20180221: The Death of the Sun
 00:12: The sun has inflated to around 1 1/2 times its original size, and shines brighter than ever before.
 00:34: It's time to watch the death of the sun.
 03:21: ... inflates to around the size of Venus' orbit, shining with a few thousand times its current ...
 04:57: The entire star dims and shrinks to around 10 times the current radius.
 05:31: This time it's outer layers will reach Earth's orbit, and probably then some.
 08:47: ... to look to the greater galaxy for a new home across far reaches of space time. ...
 10:40: Does this mean that the combined system of the Earth plus you gains mass every time you jump, because you gain potential energy?


20180214: What is Energy?
 01:15: He realized that the sum of mass times velocity squared for a system of particles bouncing around on a flat surface is conserved.
 02:47: ... example, du Chatelet's gravitational potential energy, mass times the gravitational acceleration times height, is just a statement about ...
 03:07: ... as a fun time exercise for the student, see if you can show that the square of the ...
 03:26: ... as long as the downward gravitational acceleration doesn't change over time, the ball should lose speed on its way up at the same rate it gained ...
 04:24: The key is that the field doesn't change over time.
 09:45: See, the law of conservation of energy arises because of symmetry, in particular time translational symmetry.
 09:53: Energy is conserved if the physics of a system, for example, the nature of a force field, stays the same over time.
 10:29: What if the universe as a whole is not time symmetric, for example, in the case of an expanding universe?
 10:53: ... context of Einstein's general theory of relativity due to the potential time evolution of ...
 12:19: In our recent Space Time journal club, we talked about the discovery of the amazing Chronos, the planet eating star.


20180131: Kronos: Devourer Of Worlds
 00:24: It's also what we'll be discussing on today's "Space Time" Journal Club.
 07:36: ... be sure to keep you updated on future episodes of "Space Time." We recently discussed the gradual brightening of the sun and the ...
 09:34: TheRealMirCat points out that if Earth is our only home by the time all of this happens, we deserve to burn.


20180124: The End of the Habitable Zone
 01:46: But over time that hydrogen has been diluted.
 02:30: So over time, the sun's core shrinks and heats up, brightening the entire star.
 02:54: Now that's way too small to have any effect on human time scales.
 02:58: But it's huge on geological time scales.
 03:54: This dangerous feedback cycle dominates on the time scales of the glaciation cycle and of anthropogenic climate change.
 08:29: But simple prokaryotic life, the very first life to emerge, will hold out until CO2 flatlines, perhaps in less than 2 billion years time.
 09:47: ... expanding to consume the only home we've ever known in all of space time. ...
 10:29: In no time at all you'll be coding gravity simulations in Python and calculating the radiation emitted by black holes.
 10:49: Last week we talked about how slicing a piece out of space time caused different observers to disagree on the nature of the vacuum.
 11:09: Actually, all quantum fields are affected by the presence of a space time horizon.
 13:22: I think it's up to "Space Time" standards and you guys will enjoy it.
 13:34: But come on, "Space Time." I know you went back on "Space Time," Gabe?


20180117: Horizon Radiation
 00:37: Different observers might disagree about speeds, lengths, or times, but the laws of physics should be the same for everyone.
 01:17: Generally speaking, a horizon is a boundary in space time from beyond which no influence can pass.
 01:29: As weird as the space times with horizons may be, the statement from before holds.
 02:03: ... inaccurate explanations of Hawking radiation around, and here at Space Time, we want to be as simple as we can, but no ...
 05:28: Sound waves can be described in terms of variation over time or variation over frequency.
 07:51: They behave like simple harmonic oscillators, so their value over time is like a simple sine wave.
 08:51: ... operator that can raise or lower the number of particles, one at a time, by changing the number of particles or oscillations in each momentum ...
 11:31: In some cases, changing the boundaries of space time actually reduces the number of particles, for example, in the Casimir effect.
 11:53: So this year, keep your eye on the horizon the event horizon and the strange things it does to the quantum contents of space time.


20180110: What Do Stars Sound Like?
 04:45: At the same time, gas moves vertically in and out during the same oscillation, reaching velocities of 0.1 meters per second.
 09:09: Because the science of asteroseismology can now translate the messages of stars twinkling at us from across space time.
 09:53: In no time at all, you will be coding gravity simulations in Python and calculating the radiation emitted by black holes.
 11:53: ... light year away, that jet would have diverged to something like 10 times the size of our solar system by the time it reaches ...


20171222: Space Time VR
 00:16: ... which we explore the electromagnetic spectrum, the speed of light, and time ...
 00:36: It's quite pretty to look at, so if you can get your hands on a Gear VR, download Space Time VR at the Oculus store, and let me know what you think.


20171220: Extinction by GammaRay Burst
 01:17: ... cosmic catastrophes, but let's start with the one that could happen any time a supernova or gammaray burst frying our ...
 01:33: At five or six different points in time over the past half billion years, a large fraction of species simply vanished from the fossil record.
 02:41: Now, some won't die that way, but any star more than around eight times the Sun's mass will.
 07:27: The exposed inner star shines several times hotter and hundreds of thousands of times brighter than the Sun.
 09:04: Maybe in a few 250millionyear orbits, a stellar time bomb will wander into our vicinity.
 10:09: 23andMe can help connect you with your family and remind you of what you have in common, which can be particularly important this time of year.


20171213: The Origin of 'Oumuamua, Our First Interstellar Visitor
 00:12: This is the first time we've ever seen an asteroid like object that came to us from interstellar space.
 00:19: Today, on Space Time Journal Club, we'll see what mysteries it can unlock.
 01:27: But by now, we've had time to study it with many of the world's great telescopes.
 01:31: For this Space Time Journal Club, we'll be discussing what we've learned about Oumuamua since its discovery.
 06:09: A third possible origin for Oumuamua is that it's been traveling for a very very long time.
 08:30: LSST will be able to see objects around 14 times fainter than Pan Starrs.
 09:00: In the meantime, it joins its countless interstellar cousins, orphaned planetary debris, stretching across the reaches of space time.
 09:46: And use the promo code Space Time during the login process.
 09:50: [INAUDIBLE] Leonard asks whether a particle can have momentum higher than its mass times the speed of light.
 10:05: The equation for momentum, P = M times V, only works at low speeds approaching the speed of light.
 11:14: [INAUDIBLE] 777 asks, if you try to do a Fourier transform of PBS Space Time, do you get PBS infinite series?
 11:23: Actually, I think you need an infinite series of PBS Space Time to get a Space Time localization of PBS infinite series.


20171206: Understanding the Uncertainty Principle with Quantum Fourier Series
 00:44: We've discussed it in earlier videos on quantum mechanics, but it's time we looked a little deeper.
 02:46: So intensity changing over time.
 03:36: ... instead of representing a sound wave in terms of intensity changing with time, you can also represent it in terms of its frequency components, each ...
 03:48: When you switch between a time and a frequency representation, you're doing a Fourier transform.
 04:02: In the physics of sound, time and frequency have a special relationship because any sound wave can be represented in terms of one or the other.
 04:35: The tighter you want to make that time window for the wave packet, the more frequency components you need to use.
 04:56: A blip of sound that exists for only one instant in time?
 05:01: Is it even possible to make an instantaneous spike at one point in time out of a bunch of sine waves that themselves extend infinitely through time?
 05:12: ... to get a spike at one point in time, you need to use infinitely many different frequency sine waves, each of ...
 05:22: So then if we make a sound that is perfectly located in time, it doesn't have a frequency, or it has all frequencies.
 05:30: ... the same time, a sound wave with a perfectly known frequency is a simple traveling sine ...
 06:20: To keep things simple, we're just going to consider a wave function that doesn't vary in time.
 06:33: So position, rather than time, becomes the first of our Fourier pair.
 07:08: ... or momentum, just as a traveling sound wave can be expressed in terms of time or ...
 07:41: And of course, this means that position and momentum have the same kind of uncertainty relation that time and frequency had in the sound wave.
 11:01: ... you will soon understand as some of the weirdest behaviors of space time. ...
 13:13: ... kill time during warp journeys by scanning light curves of distant stars for the ...


20171129: Citizen Science + ZeroPoint Challenge Answer
 05:55: OK, time for the answers to the recent zeropoint challenge question.
 06:28: ... able to effectively apply 200,000 of their millions of setae at any one time, and each seta can support 200 micronewtons of ...
 08:54: ... emerge when the plates were separated by around one micrometer, 100 times smaller than our 0.1 millimeter ...
 09:45: Plus, of course, the original Space Time Tshirts.
 09:58: See you all next week for a fresh new episode of Space Time.


20171122: Suicide Space Robots
 03:26: But this time, the alien atmosphere proved the savior, rather than the destroyer of robots.
 04:12: But at that point, the little rover had lasted 25 times its original mission plan.
 06:55: To unlock the secrets of the comet, we've resorted to a time honored approach to scientific inquiry smash the object and see what happens.
 09:54: ... little silicon lives to blaze the very first paths into outer space time. ...
 10:22: ... you of what you have in common, which can be particularly important this time of ...
 11:20: I guess we shouldn't be surprised at the insane depth and specialization of the knowledge of Space Time viewers.
 13:10: We actually did a Space Time journal club on that paper.


20171108: ZeroPoint Energy Demystified
 02:27: The universe tends towards disorder, and so a highly unusual arrangement will decay over time.
 08:05: ... adult geckos are able to efficiently apply 200,000 setae ends at any one time to a surface, and each center produces sufficient Van der Waals force to ...


20171102: The Vacuum Catastrophe
 01:55: ... state of any fuel oscillation must be half of the tiny Planck constant times the frequency of that ...
 09:33: ... for a time, it was thought that neutrinos might actually be dark matter until it was ...
 09:48: And at the same time, neutrinos don't interact electromagnetically, so they're, in that sense, dark.


20171025: The Missing Mass Mystery
 00:49: However, we've known for some time that around 95% of the energy content of the universe is in dark matter and dark energy.
 01:59: Its gravity holds galaxies together and governed to the growth of largescale structure in our universe throughout cosmic time.
 03:23: ... of helium and deuterium today tells us that there should have been 10 times as much hydrogen to start with than we actually see today in galaxies ...
 03:55: It carries with it a map of the structure of the cosmos from those early times.
 07:40: At the same time, this stuff is expected to be extremely lowdensity, only around 10 times that of intergalactic space.
 10:51: The stuff of countless future solar systems is still riding the cosmic web, falling in from the darkest reaches of space time.
 12:09: The energy is borrowed from the energy of the vacuum for the minuscule time allowed by the uncertainty principle.
 13:13: Waste of time.


20171019: The Nature of Nothing
 00:07: This episode of "Space Time" is about nothing, because it turns out that nothing is one of the most interesting somethings in all of physics.
 03:25: Inside a perfect vacuum, all of the field at all locations should be in the vacuum state, exactly zero energy at all times.
 03:45: Well, it's also impossible to simultaneously perfectly define time and energy.
 03:51: ... more tightly we try to define the time window for the behavior of a quantum oscillator, the less certain we can ...
 04:00: On extremely short time scales, a quantum field exists as a blur of many energy states.
 05:01: For example, in QFT, virtual particles can have any mass and any speed, including speeds faster than light, and can even travel backwards in time.
 05:41: And the higher the energy of the particle, the less time it can exist.
 05:55: And so virtual photons can exist for any amount of time, long enough to carry the electromagnetic force to any distance.
 07:26: According to the best existing theory of the time, those orbitals should have had exactly the same energy.
 15:53: Some of the greatest minds of our time think about nothing all day long.


20171011: Absolute Cold
 06:56: We'll need another episode to explore the quantum nature of nothing as we peer deeper into the coldest, darkest, and emptiest patches of Space Time.
 07:20: It's also a great place to study up on some of the concepts we cover in Space Time.
 07:53: We never would've guessed we'd reach this point when we started making Space Time early in 2015.
 08:09: Of course, we have to give a Space Time tshirt out 1 millionth subscriber.
 08:20: And for everyone else, how about we keep making Space Time for as long as humanly possible?
 08:26: ... week in Space Time Journal Club, we talked about the new observation of a potential pair of ...
 09:25: The density of stars in the Milky Way core is around 100 times that of the Milky Way disk.


20171004: When Quasars Collide STJC
 00:29: We may be about to find out, because astronomers report spotting a pair of them in a close binary orbit for the very first time.
 00:38: ... MUSIC] Today on Space Time Journal Club, we're going to dig into a paper that reports the detection ...
 00:58: This is extremely cool, because we knew for a long time that such tight binaries must exist.
 03:43: In order to measure such a small separation at such a large distance, we need resolution around 100 times better than the Hubble Space Telescope.
 05:46: Here, we can see two bright spots far from the black holes, presumably from a burst of jet activity some time ago.
 07:35: Each time they do that they lose a bit of orbital energy or angular momentum, causing them to fall deeper into the gravitational well.
 07:51: However, by the time the black holes are only a few lightyears apart, there shouldn't be any stars left in between them.
 09:19: ... with a galaxysized gravitational wave observatory called a pulsar timing ...
 09:31: But more on that another time.


20170928: Are the Fundamental Constants Changing?
 00:20: There is a hint of evidence that the fundamental constants that govern our universe may evolve over time, and even from one location to another.
 00:30: [MUSIC PLAYING] The laws of physics are the relationships we observe between space and time, and the fields and particles that occupy it.
 01:33: They predict that the fundamental constants may not be constant at all, and instead, may vary over time and space.
 01:44: We'll come back to it another time.
 02:54: ... speed of light defines the relationship between space and time, so is it even meaningful to talk about it changing independently to its ...
 07:05: Fine structure splitting in those absorption lines can then be used to track changes in Alpha through cosmic time.
 09:26: We're also trying to develop atomic clocks accurate enough to track changes in Alpha in real time.
 11:04: We may one day find that our sacred laws of physics and their underlying constants aren't so constant after all beyond our little patch of space time.
 12:23: ... may vary, but you guys really help with the stability of "Space Time." And today, I want to give a big shout out to Richard Sinegor, who's ...
 13:10: It would send a spacecraft to at least 550 times the Earth's orbital radius, out beyond the edge of the solar system.
 13:21: That light could be magnified up to 100 million times.
 14:40: Next time [INAUDIBLE] gets dragged out of war by a Thargoid, I'll try to snap some nice pics of exoplanets over its shoulder before I'm vaporized.


20170920: The Future of Space Telescopes
 01:41: Our sun is about 10 billion times brighter than Earth.
 02:24: ... chronographs can allow detection of objects from 100,000 to a million times fainter than the central star, but no where near the factor of 10 ...
 06:25: ... focus, and you have an image of the distant source that's 100 or 1,000 times better in resolution than the Hubble Space ...
 09:57: It may be possible to launch multiple such telescopes that have several times the light collecting power of the Hubble Space Telescope.
 10:22: As we launch our new observatories, our vision will be keener, allowing us to peer more sharply and to ever greater depths into space time.


20170913: Neutron Stars Collide in New LIGO Signal?
 00:15: ... only a few weeks ago, a new rumor emerged, that LIGO had for the first time spotted gravitational waves from the collision of a pair of neutron ...
 02:06: A remnant core between 1.4 and around 3 times the mass of our sun instead ends up as a neutron star.
 02:39: ... stars can rotate up to thousands of times per second and have enormous magnetic fields that result in jets of near ...
 04:00: See, black holes only form in the deaths of the most massive stars, those over approximately 20 times the Sun's mass.
 04:49: In fact, a typical neutron star merger needs to be about 10 times closer to us than a typical black whole merger for LIGO to be able to see it.
 05:10: We can see black hole merges across 1,000 times more universe compared to neutron star mergers.
 08:55: It definitely happens in supernova explosions, which for a long time were thought to be the primary source of heavy elements.
 11:05: ... may have just spotted for the first time a long theorized astrophysical catastrophe, one that may have birthed a ...
 11:49: "Gravitational Waves Rewinding Time" includes some fascinating behindthescenes footage at the observatories.


20170830: White Holes
 01:27: In fact, it's a time reversed black hole.
 01:31: ... black hole is defined as a region of inward flowing space time with a one way boundary called the event horizon, from inside of which ...
 01:41: That makes a white hole a region of outward flowing space time.
 02:31: ... particular case a single point of mass in an otherwise empty space time. ...
 03:03: We've talked quite a bit about the bizarre behavior of space and especially time at and below the event horizon of a black hole.
 03:16: The time that happens inside a black hole is not part of the past or future history of the outside universe.
 03:34: Once you fall into the black hole, the Schwarzschild metric tells us that space and time switch their roles.
 03:45: It now occupies an inevitable future time.
 04:08: If we follow the Schwarzschild metric back in time, we find something very strange.
 04:42: To refresh your memory, in a Penrose diagram, the x and yaxes are redefined from space and time to merge space and time into new coordinates.
 04:53: They compactify space time so that time bunches up towards the edges, and the borders correspond to infinite past and future.
 05:04: Also, lines of constant distance and time curve, so that light paths always travel on 45 degree paths.
 05:39: ... traveling at that 45 degree angle takes infinite time to escape the event horizon, and the region beyond that line represents ...
 05:50: There the dimensions of space and time switch roles.
 05:54: The once vertical contours of space are now timelike and flow inexorably towards the future singularity.
 06:21: Map into the past, and we see a time reflected version of our future black hole.
 06:27: Everything about it is time reversed.
 06:32: ... within is timelike, but instead of flowing towards the singularity, it flows away, and the ...
 08:18: Light has to traverse infinite time to reach our location.
 08:49: It really is just a black hole, but viewed backwards in time.
 08:52: Yet general relativity is time reversal symmetric.
 08:56: Something that can happen forwards in time should also be able to happen in reverse.
 09:29: This law defines the direction of the flow of time.
 09:32: To reverse time, you need to break the law.
 09:46: ... an incredibly rare entropy dip could lead to an effective reversal of time and a white hole could ...
 09:58: However, it would immediately explode in a burst of energy as soon as entropy and time resumed their normal flow upwards and forwards.
 10:32: It's an expanding outpouring of space time containing a vast amount of energy, and the bang itself can never be entered.
 10:49: It happened everywhere at the same time.
 11:22: More on that another time.
 11:32: The past white hole was revealed when we traced the eternal black hole backwards in time.
 11:38: In fact, what we did was to maximally extend space time.
 11:57: The mathematics of the Schwarzschild metric describes an entirely independent region of space time parallel to our own.
 12:19: In the not too distant future, we'll investigate the reality of this mysterious parallel patch of space time.
 12:55: LegalZoom is offering "Space Time" viewers 15% off their next purchase from LegalZoom.


20170824: First Detection of Life
 07:03: Programs like Breakthrough Starshot, as discussed in this video, promise the first closeup observations of a new world in several decades time.
 09:03: ... with the clarity granted by being in space, will enable us for the first time to perform Sagan's 1990 experiment on an Earthlike alien ...
 11:06: LegalZoom is offering Space Time viewers 15% off their next purchase from LegalZoom.


20170816: Extraterrestrial Superstorms
 00:34: ... is Jupiter's Great Red Spot, stretching an incredible two to three times the diameter of the planet ...
 08:41: During that time, Juno snaps a picture every 60 seconds, fast enough to catch at least six angles of a given feature.
 09:23: With all the little plumes and eddies, it is the first time we've seen so many storms within storms, especially in the Great Red Spot.
 10:48: ... discoveries on its interactive website and by staying tuned to "Space Time." Thanks to LegalZoom for sponsoring this ...
 11:23: LegalZoom is offering "Space Time" viewers 15% of their next purchase from LegalZoom.
 12:11: ... counterpart is the same particle bouncing forwards and backwards in time. ...
 12:25: Keith Gaughan wonders if the imbalance between matter and antimatter is due to the time in the history of the universe that we're observing.
 12:31: Well, actually, the fun thing about the one electron proposition is that it doesn't matter when in time you are.
 12:41: To prove this to yourself, try drawing a spacetime diagram, time on the yaxis and space on the xaxis.
 12:54: Make each turning point at a different time so the zigzags are uneven.
 13:06: It doesn't matter what times you choose.


20170810: The OneElectron Universe
 00:01: ... is really the one same electron traveling forwards and backwards in time. ...
 00:58: And that electron traverses time in both directions.
 01:01: ... bounces in time, eventually traversing the entire past and future history of the universe ...
 01:17: When the electron is moving backwards in time, it's a positron, the antimatter counterpart of the electron.
 01:29: However, Richard Feynman did take at least one aspect very seriously the mathematical equivalence of antimatter as timereversed matter.
 02:34: It exists as a line traced by its passage through space and time, rather than as a pointlike particle at one instant in time.
 02:41: The pointlike electron is just a segment of that worldline if we take a slice through space time at one instant in time.
 03:03: If an electron can reverse its course in time, then its worldline looks like a zigzag.
 03:25: That one electron zigzagging back and forth 10 to the power of 80 times looks like all of the electrons in the universe.
 04:36: But reversing a particle's motion is mathematically the same as watching it in reverse time.
 04:43: ... doesn't mean that time actually goes backwards, just that if you reverse the ticking of the ...
 05:15: T is time reversal, changing the direction of the coordinate clock.
 05:27: ... if you make all of these changes at the same time flip the charge, invert the parity, reverse time a particle should end ...
 05:37: But if you just flip the charge and in parity so do a CP transformation you still have to reverse time again to get back where you started.
 05:46: So that means CP transformations leave an object timereversed.
 06:16: In the sense of these fundamental symmetries, antimatter is timereversed matter.
 06:22: ... already saw how expressing antimatter as timereversed matter is extremely useful in simplifying quantum field theory ...
 06:49: That virtual particle in the middle may be an electron traveling forwards or backwards in time.
 07:03: We can think of the annihilation of an electron and positron as just the electron being deflected back in time.
 07:11: Similarly, the creation of a particle pair is the electron being scattered in time.
 07:18: ... countless scattering events, some of which change its course through time. ...
 07:44: The biggest is that we should see equal numbers of electrons and positrons at any time.
 07:50: After all, when that first electron makes it to the end of time, it needs to travel back again as a positron in order to have any more electrons.
 08:45: ... from itself by countless passages across the cosmos and across all of time. ...
 10:19: If you see your name below, you're a lucky winner, and we have a space time tshirt for you.


20170802: Dark Flow
 09:40: ... dark flow does turn out to be real, we may have detected for the first time the influence of a neighboring region of the greater universe, beyond ...
 11:41: ... the state of the other, as though that influence propagates back in time along the path of the measured ...


20170726: The Secrets of Feynman Diagrams
 02:19: In Feynman diagrams, we depict the electron as an arrow pointing forwards in time, while the positron is an arrow pointing backwards in time.
 02:28: We'll soon see the power of representing antimatter as timereversed matter.
 02:36: Time direction is irrelevant for the photon.
 02:40: Throw these on a plot of space versus time, and we have a Feynman diagram a useless one.
 03:31: ... like this with time increasing upwards, this vertex represents an initial electron that ...
 06:44: These particles aren't even limited by the speed of light or the direction of time, which leads to all sorts of fun.
 08:01: For any particle besides the going and outgoing on shell particles, any energy, speed, and even direction in time is possible.
 08:28: We can think of the differences just being the photon traveling forward in time in one case and backwards in the other.
 09:09: That includes paths backwards in time.
 09:12: Mathematically, a timereversed electron looks exactly like a positron like this.
 10:14: Now, the interpretation of antimatter as timereversed matter is one that some, including Richard Feynman, took quite seriously.


20170719: The Real Star Wars
 00:23: [THEME MUSIC] The decades directly following World War II were a time of optimism in many ways.
 06:45: ... highly elliptical orbits that allowed them to spend most of their time above a particular location on the surface, ...


20170712: Solving the Impossible in Quantum Field Theory
 03:04: This is a good time to introduce our first Feynman diagram.
 03:18: In a Feynman diagram, one direction is the time in this case, up.
 11:46: In future episodes, we'll talk more about what is now the most complete description we have for the smaller scales of space time.
 12:44: ... reminder to current or wouldbe Patreon patrons we made some "Space Time" eclipse glasses super handy for not going blind watching the great ...
 13:48: ... Feynman's approach, each particle is tracked according to its own proper time clock, which can vary in its tick speed depending on how fast the ...
 13:57: Derivation of Schrodinger from Feynman requires approximating all of the separate proper time coordinates to give a single time coordinate.
 14:52: ... would require superluminal speeds to reach their destination at the same time as the straight line ...


20170707: Feynman's Infinite Quantum Paths
 03:51: ... trick was to slice the time taken for the journey into small intervals and at each time step allow ...
 04:51: It's proportional to both the transfer between kinetic and potential energy over a path and the travel time.
 04:58: In relativity, it's proportional to the proper time, so the time measured by the clock on a given trajectory.
 05:06: For the largescale classical universe, minimizing proper time lets you derive all equations of motion.
 08:16: That means it treats space and time symmetrically and so works very naturally with Einstein's theory of special relativity.
 08:23: On the other hand, the Schrodinger equation gave time a special role, so it doesn't work with relativity at all.
 11:28: In part, by describing antimatter as regular matter traveling backwards in time.
 11:35: Find out how on the next episode of "Space Time." Thanks a ton to all of our supporters on Patreon.
 12:06: On August 21, for the first time in 40 years, the United States will experience a total solar eclipse.
 12:14: But astrophysicists agree that the most stylish way to view this event is with the official "Space Time" eclipse glasses.


20170628: The First Quantum Field Theory
 01:18: ... modes in fundamental fields that exist at all points in space and time through the ...
 01:49: We covered the Dirac equation last time and you really should watch that episode first if you haven't already.
 02:22: ... any point in time, every point on a vibrating string is displaced by some distance from its ...
 09:02: Yet, in particle interactions, particles are created and destroyed all the time.
 12:15: And we're going to need another episode of Space Time.
 14:46: ... talk more about how the special relativistic infinities were fixed next time, using a process called renormalization, but this trick doesn't work for ...
 15:11: And I guess it's possible there are quantum timelines where I missed it on the first attempt but it wasn't this one.


20170621: AntiMatter and Quantum Relativity
 00:16: Einstein's theory of special and general relativity had changed forever the way we think about motion, space, and time.
 01:21: ... how these matter waves, represented as wave functions, change over time, and allowed physicists to predict the evolution of quantum systems, such ...
 01:47: In relativity, the dimensions of space and time are intrinsically connected and they float into each other as frames of reference change.
 02:06: Relativity tells us that the passage of time depends on velocity.
 07:14: The only time we can actually interact with an electron is when one has a positive energy, which would leave it sitting on top of the sea.
 10:18: But there will be time for all of that in the future.
 12:00: My new plan is to level up my skill in time for the solar eclipse in August.
 14:54: Pradhyumn asks if I can make a video recommending some good books on space and time?
 16:04: We will try to keep our accuracy to within a factor of a thousand next time.


20170607: Supervoids vs Colliding Universes!
 00:38: That's what all the media hype is saying, which means, it's time for another Space Time Journal Club to sort it out.
 04:13: But by the time the photon is on its way out, the expansion of the universe has actually stretched out the cluster, weakening its gravitational pull.
 09:22: This can happen spontaneously anywhere in the greater inflating space time, resulting in bubble universes.
 09:37: But regardless, in an infinitely inflating space time, collisions between bubble universes are eventually expected.
 10:38: But if not, perhaps once upon a time, we really did collide with an entirely separate bubble of space time.


20170531: The Fate of the First Stars
 04:20: OK, physics time the cores of stars are under extreme pressure due to the gravitational crush of their great mass.
 04:58: A star 10 times the mass of the sun shines around 10,000 times brighter.
 05:04: Now, burning through 10 times the fuel at 10,000 times the rate, compared to the sun, means its life is 1,000 times shorter only 10 million years.
 05:15: ... population three stars would have had masses of at least several times that of the sun, while the largest would have been as much as 1,000 or ...
 07:37: Pressure and temperature have time to equalize across the cloud before it breaks apart.
 09:15: ... of the socalled supermassive black holes, with millions to billions of times the mass of the sun, that we find lurking in the centers of ...
 10:12: When we look out into the universe, as far as our telescopes can see, we do see primitive looking galaxies shining out from the earliest of times.
 10:40: They may have raged for only a cosmic instant at the beginning of time.


20170517: Martian Evolution
 00:42: Unless we manage to selfdestruct in the next little while, humans will be around for a long time.
 00:58: But if those challenges are met, we may reach a time when many generations of humans have lived and died on the red planet.
 04:41: Over time, people are likely to develop a resistance to lowG wasting, after which strength may go down.
 07:26: Unless humans stay underground or in wellprotected shelters pretty much all of the time, they will see an increase in DNA damage from these.
 09:21: They'll find Earth's nearly three times higher gravity incredibly uncomfortable.
 10:31: ... way planet to planet, then star to star across the reaches of space time. ...


20170510: The Great American Eclipse
 00:17: Get the timing right, and the sky will blacken.
 02:13: This time that path crosses the United States.
 02:17: The fun starts at 9:04 AM Pacific time, when the edge of the moon's shadow first reaches the West Coast.
 05:27: These are called shadow bands, and they've perplexed scientists for a long time.
 05:54: It's the one time you can stare safely straight towards the sun.
 05:58: ... probably the first time in your life, you see the chromosphere, red from a specific electron ...
 06:52: Totality will reach Depot Bay at 10:16 AM Pacific time and last just under two minutes.
 07:13: ... into the Atlantic at Cape Romain, South Carolina, at 2:49 PM Eastern time, an hour and a half after it first made ...
 08:54: Without it, I may never have ended up doing a show like "Space Time." Thanks to legalzoom for sponsoring this episode.
 09:25: legalzoom is offering Space Time viewers 15% off their next purchase from legalzoom.


20170503: Are We Living in an Ancestor Simulation? ft. Neil deGrasse Tyson
 05:19: ... 100 trillion to 100 quadrillion binary operations for every second of time that the brain ...
 06:52: In other words, it will be capable of simulating the entire mental lives of all humans in history a million times over every single second.
 12:11: Just for now, I strongly recommend that we proceed as though we are real life observers, part of the original space time.
 12:46: You make "Space Time" possible.
 14:53: But it's important to remember that the laws of physics are time reversible.
 15:17: That perfect time reversal would include the reverse of every particle interaction that happened in the original expansion.
 16:08: An example of that would be the perfect time reversal of an expansion.


20170426: Are You a Boltzmann Brain?
 02:32: The air molecules are moving randomly, and over time they pass through all possible arrangements that they could have.
 03:18: Entropy increases because particle positions and velocities get randomized over time.
 03:37: For example, tiny, localized dips in entropy happen all the time, when you get a chance convergence of a few particles in one corner of the room.
 04:12: But, given infinite time, any nonimpossible arrangement will happen.
 04:17: Imagine an infinitely large room a universe that's in perfect thermal equilibrium for infinite time.
 05:37: The universe will spend almost all of its time in that highentropy state.
 05:57: What other time could we possibly observe?
 06:18: Similarly, we must have appeared at a time and in a universe capable of producing biospheres.
 10:06: We're actually going to jump into that rabbit hole next time, and it'll be with a very special guest.
 10:12: ... fluctuations in the infinite chaos of a max maximally entropic space time. ...


20170419: The Oh My God Particle
 06:58: Empty space isn't really empty, it's full of lowenergy microwave photons leftover from the heat glow of the very earliest of times.
 07:11: ... rays with energies over 5 times 10 to the power of 19 electron volts, about 8 joules, can't travel far ...
 07:27: For years, it was thought that no cosmic ray could exceed it, except that the OMG particle was six times more energetic.
 09:30: Studying cosmic rays may crack open the mysteries of both the largest and the smallest scales of space time.


20170410: Here's Why I Love PBS
 00:00: ... guys, this is Matt from "Space Time." As a lot of you know, there is a proposal to massively cut the funding ...


20170405: Telescopes on the Moon
 09:03: ... lookout tower, granting the clearest views of all the rest of space time. ...
 10:30: Last week on "Space Time," we talked about time space, the way time and space, which rolls, beneath the event horizon of a black hole.
 10:41: Feinstein 100 asks whether energy could be extracted from the flowing space time inside a black hole.
 10:48: Well, not really from the flow of space time actually inside the black hole.
 10:59: Quasars do this all the time to power their insane luminosities.
 11:03: Hydroelectric power plants on earth do it with the flow of space time that we experience as Earth's gravitational field.
 12:19: But you also give nice compliments, so I'm sure they forgave you for calling out their wrongness all the time.
 12:26: ... angular components in the line element, AntiTwister over at the Space Time subreddit had some interesting ...
 12:34: He points out correctly that it's only the radial component that becomes timelike.


20170329: How Time Becomes Space Inside a Black Hole
 00:06: ... on Space Time, we're going to talk about timespace, or the strange switching in the ...
 00:20: [MUSIC PLAYING] What does this bizarre statement, space and time switching roles, even mean?
 00:35: Or does it correspond to real timeywhimy weirdness?
 00:49: First we'll think about what the flow of time looks like without black holes or even spacetime curvature.
 01:10: I'm going to show you the math one more time.
 01:22: Different observers may report that two events are separated by different distances delta x and by different amounts of time delta t.
 02:05: In flat space time, that negative sign in front of the delta t drives that forward evolution.
 02:11: This makes t, the timelike coordinate, while x is the spacelike coordinates.
 02:18: For causality to be maintained, the timelike coordinate must always increase.
 02:29: In our episode on superluminal time travel, we saw that in flat space, this means traveling faster than light, which is, of course, impossible.
 02:44: We're going to see how this changes the behavior of time in very strange ways.
 03:26: And time and space nicely separated.
 03:41: But as long as you are outside the event horizon, time behaves itself mostly.
 04:45: It becomes timelike.
 04:50: Meanwhile the coordinate previously known as time, t, lost its negative sign and become spacelike.
 05:02: But what does all of this timespace switching actually look like?
 05:07: Let's fall into the black hole one more time, now graphically instead of mathematically.
 05:40: Ahead along our time axis and at right angles to all of our space axes.
 06:18: Your future light cone and your time axis begin to blur together with the inward radial axis of the black hole.
 06:26: At this point, it's time we switch diagrams.
 06:33: It deals with the extreme stretching of space and time by compact defined lines of constant space or time close to its boundaries.
 06:44: But an important thing to remember is that the lines of constant space and time are curved so that light cones remain upright.
 08:05: As we fall with the faster than light flow of space time, we overtake light that is outward pointing.
 09:02: Those directions, those spatial freedoms, are now described by what was once the time coordinate.
 09:09: But it's no longer timelike.
 09:15: ... so isn't actually traveling in time, even though there's a sense of past events in one direction the ...
 09:53: However the old radial dimension isn't spacelike, it's timelike.
 10:12: In the same way that all world lines move towards the future in the outside universe, time is laid radially.
 10:20: And r is timelike, unidirectional.
 10:24: The singularity becomes a future time, not a central place.
 10:47: ... dimensional flip does give us some fascinating insight into how time and space blend together in what is perhaps the strangest place in all ...
 11:13: I think Space Time viewers would really enjoy Cowboy Bebop, a Scifi noir that follows the adventures of a gang of space bounty hunters.
 11:23: Crunchyroll is offering Space Time viewers a 30 day free trial if you go to Crunchyroll.com/spacetime.
 11:34: ... guys, in our recent Space Time journal club, we talked about a paper describing a recipe for making ...
 11:47: A few of you are unsure of what constitutes a time crystal.
 12:12: Really though, the term time crystal is just used to refer to anything that has a pattern of internal states that repeats over time.
 12:36: Firstly yeah, these time crystals oscillate at an integer multiple of the electromagnetic field frequency.
 12:43: So the time crystal oscillation and the EM field oscillation are in resonance.
 12:49: For every one, two, three, et cetera cycles of the time crystal, the EM field gives a little push.
 12:55: ... integer factor, because if the EM field were pushing halfway through the time crystal period, it would be pushing in the wrong ...
 13:04: It's like when you're pushing a swing, you don't need to push every time to keep it going.
 13:09: But you do need to push at the right time or you'll slow it down.
 13:20: So in Frank Wilczek's initial idea, his hypothetical time crystals didn't require any input energy to keep them oscillating.
 13:28: For Wilczek's time crystals, the oscillating state is an equilibrium state.
 13:46: It's now been proved mathematically that time crystals can exist in equilibrium.
 14:35: It's worth commenting here that our goal on Space Time is a little bit different to most science media.
 14:50: That's especially true of Space Time Journal Club.


20170322: Superluminal Time Travel + Time Warp Challenge Answer
 00:02: Traveling faster than light and traveling backwards in time are the same thing.
 01:32: We've spent some time talking about how these contours define the flow of causality.
 01:40: In addition, today's episode is going to add to the recent time warp challenge question.
 01:48: In the olden days, before Albert showed us the way, time was thought of as universal.
 01:54: ... simultaneously in a state of now, and that all points move forward in time at a constant rate for all observers, governed by one global ...
 02:08: In the olden days, the same time axis of a space time diagram would apply to everyone, but no longer.
 02:19: Instead, every space time traveler carries their own clock.
 02:34: Everyone draws their space time diagram time axis parallel to their direction of motion, because that's their experience of stillness.
 02:42: The tick marks on that time axis also depend on velocity and represent the speed of everyone's personal clock in their proper time.
 03:07: Connect the ticks of all possible time axes, and you get these nested hyperbolae.
 03:17: A straight line journey to any location on one of these contours seems to take the same amount of proper time for every traveler.
 03:37: ... spacetime interval for flat space with a negative sign in front of the time part, then changes in your space interval have to be negative, as long ...
 04:10: That uphill journey is equivalent to time travel.
 04:53: It just hangs out at x equals 0 and rolls upward in time.
 05:45: Do they perceive you as traveling in time?
 05:56: To see what the Annihilator sees, let's transform the space time diagram to their perspective.
 06:05: Their time axis is their own world line, and their space axis is symmetrically reflected around the path of light.
 06:15: These are the spacetime intervals as calculated from the zero point in space and time, the beginning of the race.
 06:55: The Paradox still appears to be traveling forward in time with respect to the Annihilator, even though it's traveling faster than light.
 07:46: ... that looks a bit like time travel, but a physicist on the Annihilator would still infer that the ...
 07:58: However, there are perspectives where time travel seems real.
 08:03: Let's look at the perspective of a different space time traveler, one traveling at very near the speed of light.
 08:09: ... we see that the Paradox really does appear to travel backwards in time according to this new time ...
 08:30: To do that, we first need to outrace photons that were admitted at the space time point that we want to perceive.
 08:48: Let's fill in the space time diagram with all four quadrants.
 09:30: We constructed this time traveling path using two different reference frames.
 09:53: Superluminal paths aren't real worldlines Real worldlines don't point backwards in time under Lorentz transformations.
 10:23: Reverse the flow of time, and you reverse the flow of you.
 10:29: Even our fantasies of time travel are just another pattern emerging from our oneway trajectory through the temporal part of spacetime.
 10:39: Thanks to everyone who submitted answers to the timewarp challenge.
 10:58: Also, it's that time of year again, time for the annual PBS Digital Studios survey.


20170315: Time Crystals!
 00:06: What exactly are time crystals?
 00:09: Are they the bling inside your time turner, the flux in your flux capacitor?
 00:15: In today's edition of "Space Time Journal Club," we find out.
 00:18: [MUSIC PLAYING] In "Space Time Journal Club," we review new scientific papers that are making waves.
 00:41: ... physical review letters, entitled "Discreet Time Crystals: Rigidity, Criticality, and Realizations." This paper proposed ...
 00:58: But first up, what on earth are time crystals?
 01:09: He suggested a type of matter that exhibits a sort of fundamental oscillation over time.
 01:14: So some property of the material goes through a repeating cycle, but how does that make it a time crystal?
 01:30: Time crystals repeat some internal state with constant separations in time.
 01:35: The name time crystal is somewhat out there, but Wilczek wasn't the first to use it in reference to a regularly repeating system.
 01:44: That may have been Arthur Winfree in his "The Geometry of Biological Time," where it's used to describe periodic biological systems.
 02:01: Wilczek came up with a simple model in which charged particles in a superconducting ring break what we call continuous time translational symmetry.
 02:29: In regular matter in equilibrium, statistical properties stay the same over time.
 02:34: ... imaginary system broke this time translational symmetry, because there are global statistical differences ...
 02:47: Lots of things change over time.
 03:15: This would break time translational symmetry, which makes most physicists nervous.
 03:23: ... of the University of Tokyo showed from theoretical arguments that time translational symmetry can't be broken by a quantum system in ...
 03:35: That sounds bad for time crystals, but that's where this new paper by Yao et al.
 03:51: ... Yao, also at UC Berkeley, and his team proposed a way to make time crystals by using some sort of external input of energy to force the ...
 05:54: The analogous phase diagram for time crystals plots interaction strength between atoms versus imperfection in the spinflip driving signal.
 06:05: This triangle at the bottom is where time crystals live.
 06:08: ... become too messy and the interaction strength is too weak, then the time crystal effectively melts into regular time symmetric matter, in which ...
 06:43: ... Yao and team laid out a practical approach to building time crystals, in August 2016 two teams have synthesized them in the lab in ...
 07:15: A time crystal within a space crystal.
 07:27: The diamond [INAUDIBLE] three times the microwave period.
 07:36: ... both fit the predicted phase diagram, their time asymmetry melting when subjected to too much perturbation or too little ...
 07:52: That means time crystals, at least by Yao et al.'s definition of them, can exist.
 08:07: ... should also add that while their systems do break continuous time translational symmetry, they have a different type of symmetry ...
 08:17: That means if you shift forwards or backwards in time in steps of exactly their period, they will return to the same state.
 08:24: Scientists are using the term discrete time crystals to describe such systems.
 08:30: Time crystals could have their first application in quantum computing.
 09:00: Time crystals with their resilient spinflip cycle could be the next step in building stable quantum memory.
 09:08: Time crystals could also help bridge the gap between quantum mechanics and general relativity.
 09:15: Before this year, time stood out as a major symmetry that hadn't been broken.
 09:21: And unlike in relativity, quantum mechanics treats space and time very differently to each other.
 09:29: ... that we've seen matter settle into discrete lattices in time just like in regular crystals, perhaps it's a first step in a quantum ...
 10:07: ... Nature of Matter by Professor David Ball may not cover time crystals, but it does go into some amazing detail about some other weird ...


20170308: The Race to a Habitable Exoplanet  Time Warp Challenge
 00:02: If you travel faster than light, you can travel backwards in time.
 00:06: In fact, breaking the cosmic speed limit doesn't just allow time to be reversed.
 00:13: Any FTL journey will appear to someone, somewhere in spacetime, as time travel.
 00:18: In fact any FTL ship set on the right trajectory can find its way back in time to a point before its journey began.
 00:37: And in the process, you'll prove to yourself that FTL travel is time travel.
 01:17: If two events happen in spacetime, observers with different velocities will report different separations between them, in both space and time.
 01:27: We can combine those space and time intervals into the spacetime interval.
 01:59: But why does this have to mean traveling back in time?
 03:40: When you transform the diagram to the perspective of the Annihilator, the paradox should suddenly appear to act like a time machine.
 04:21: ... way back to the beginning of the race, the beginning in both space and time at the moment the Annihilator is ...
 04:34: Show that the time travel implied by FTL isn't just an illusion.
 04:51: A selection of correct submissions will receive a Space Time Tshirt and a habitable exoplanet.
 04:58: Email responses to pbsspacetime@gmail.com, and make sure you use the subject line time warp challenge, because we filter by subject line.
 05:08: See you next week for a new episode of Space Time.


20170301: The Treasures of Trappist1
 01:52: The gravitational influence on each other allows us to figure out their masses, between 0.4 and 1.4 times Earth's mass.
 07:42: ... about half the magnitude of Io's on TRAPPIST1h, they may be up to 100 times as bad on ...
 08:38: ... giant boost in the number of possible homes for life out there in space time. ...
 09:14: LegalZoom is offering Space Time viewers 15% off their next purchase from LegalZoom.
 09:40: After all, EELT will have nearly three times the collecting area of GMT.


20170222: The Eye of Sauron Reveals a Forming Solar System!
 00:17: We still see his terrible flaming eye gazing down at us, biding his time.
 03:10: ... dust, revealing whatever planets managed to form from the debris in that time. ...
 03:57: By the time it's at least 10% of the Earth's mass, it will have cleared its orbit of other planetesimals, and will have rounded out into a planet.
 06:19: It's bright enough to be a few times Jupiter's mass.
 08:31: And that would make sense if they formed at the same time.
 09:39: But one does not simply cease questioning on space time.
 10:08: ... sign up at curiositystream.com/spacetime, and use the promo code Space Time during the signup ...


20170215: Telescopes of Tomorrow
 00:45: It's time for the next generation of great observatories.
 00:57: With a diameter of 6.5 meters, compared to Hubble's 2.4, it has over five times Hubble's collecting area.
 01:57: Longer wavelengths of light scatter less easily than shorter wavelengths, and so have an easier time escaping these dustpacked stellar nurseries.
 02:17: Webb's larger mirror allows to detect objects 16 times fainter than Hubble.
 02:36: Light from these earliest of galaxies has been traveling through our expanding universe since near the beginning of time.
 05:02: Together, they provide an effective aperture, 24.5 meters in diameter, and a collecting area over 80 times Hubble's and nearly 15 times Webb's.
 06:17: If we build GMT's giant mirror in space, it would produce images 10 times sharper than Hubble's.
 06:23: But on the ground, that resolution is normally limited to about 10 times worse than Hubble's.
 06:53: ... order to track this turbulence in real time, GMT will shine six powerful sodium lasers 90 kilometers into the upper ...
 07:05: ... mirrors will deform up to hundreds of times per second to keep the guide stars, along with everything else in the ...
 07:48: The Large Synoptic Survey Telescope's primary mirror spans 8.4 meters 3 and 1/2 times larger than Hubble's.
 08:15: In a way, LSST focuses more on the dimension of time rather than space.
 08:34: LSST is hundreds of times faster.
 09:41: ... even expect to find unknown unknowns in the deepest reaches of space time. ...
 11:10: So last time, I showed you how you can visualize the effects of special relativity on spacetime using geometry.
 11:30: Normally, I just claim that that's how we say it in Australia, but I don't think that will fly this time.
 12:02: So you have time, space, and spacetime interval.


20170202: The Geometry of Causality
 00:11: When time is relative, paradoxes threaten.
 00:43: ... space and time switch roles, but to truly understand that bizarre statement, we need to ...
 00:54: Today, we're going to look at the amazing geometric structure that time, or more accurately causality, imprints on the fabric of spacetime.
 01:16: Special relativity tells us that our experience of both distance and time are, well, relative.
 01:41: ... combination of this length contraction and time dilation allows both moving and stationary observers to agree on how ...
 02:00: ... time measured by a moving observer on their own clock is called proper time, ...
 02:35: But we want that intuition because, more than proper time, the spacetime interval defines the flow of causality.
 02:42: In relativity, 3D space and 1D time become a 4D entity called spacetime.
 02:49: To preserve our sanity, we represent this on a spacetime diagram plotting time and only one dimension of space.
 03:06: If I don't move through space, I still travel forward in time at a speed of exactly one second per second according to my proper time clock.
 03:16: Motion at a constant velocity appears as a sloped line, and the time axis is scaled so that the speed of light is a 45 degree line.
 03:48: My world line is only through time, and the tick marks on the time axis correspond to my own proper time clock ticks.
 04:06: They time their journey on these slow clocks, so I perceive them traveling for longer.
 04:20: Drawing a connecting line at the tick of every traveler's proper time clock gives a set of nested hyperbola, but these aren't just [INAUDIBLE].
 04:29: ... are kind of the contours defining the gradient of causality down which time flows, and etched into spacetime by the equations of special ...
 04:40: To understand why, we need to see how these proper time contours appear to other spacetime travelers.
 04:58: To transform the diagram, we need to figure out what they see as their space and time axes.
 05:04: Time is easy.
 05:06: They see themselves as stationary, so their time axis is just their own constant velocity world line.
 05:15: ... events at different distances, but that all occur simultaneously at time t equals ...
 05:28: To observe those points, I just wait around until their light had time to reach me.
 05:53: ... the same time, they're moving away from the signals coming from the left and towards ...
 06:18: Even just doing this graphically, we see that the traveler's xaxis is rotated by the same angle as their time axis.
 07:03: This transformation allows you to calculate how properties, like distance, time, velocity, even mass and energy, shift between reference frames.
 07:33: ... contours show where clocks moving from the origin reach the same proper time count, but more generally, each represents a single value for the ...
 08:20: ... that an event very close to the origin in both space and time can be separated from that origin by the same spacetime interval as an ...
 08:35: ... seems to demand that, but remember, it takes the same amount of proper time to travel from the origin to a nearby nearfuture event compared to a ...
 09:08: ... the spacetime interval, it becomes increasingly negative in the forward time direction, so we can represent this as a valley dropping away from me ...
 09:18: I naturally slide through time by the steepest path, straight down.
 09:59: To reverse the direction of your changing spacetime interval is to reverse the direction of causality, to travel backwards in time.
 10:08: ... in which faster than light travel is the only way to flip your space time ...
 11:18: "Space Time" is possible only through your support.
 12:33: We'll get back to this another time.
 12:58: Joan Eunice asks whether there's a spot near a quasar where a stable orbit could be created, and what would time dilation be like there?
 13:12: It's three times the Schwarzschild shield radius for a nonrotating black hole.
 13:17: Below that, accreting material spirals into the black hole very quickly, and yeah, time dilation would be significant there.
 13:24: We actually do see the effect of time dilation in some of the xray light coming from right near the black hole.
 13:30: ... atoms, orbiting at around 10 times the Schwarzschild shield radius, undergo an extremely energetic electron ...
 13:51: That gravitational redshift is the same thing as gravitational time dilation.
 14:47: These giant black holes have been growing since the dawn of time by creating gas and by merging with other black holes.
 14:54: ... of an insanely large first generation of stars, perhaps thousands of times the mass of the ...
 15:05: But by now, some of those have grown to billions of times the mass of the sun.
 15:43: No, watching "Space Time" doesn't count as extra credit, but it can't hurt.


20170125: Why Quasars are so Awesome
 01:05: They have a supermassive black hole, millions to billions of times the mass of the sun.
 02:38: That timing allowed astronomers to identify a tiny starlike point of bluish light as the source of the radio emission.
 03:59: Take a black hole of millions to billions of times the mass of the sun.
 06:55: ... bright quasar, the accretion disk falls into a region less than 100,000 times smaller than a single pixel on the Hubble Space ...
 09:32: Although it's far away, its light comes to us from a time long after the peak of the quasar epoch.
 09:39: It's a late relic from a more violent time.
 10:35: ... Carroll's Mysteries of Modern Physics Time builds up to some excellent lectures on the nature of time in ...
 10:47: With the Great Courses Plus, you can watch as many different lectures as you like anywhere, any time, without any tests or exams.


20170119: The Phantom Singularity
 02:39: An example of a frame dependent singularity that might be familiar to space time viewers is the event horizon of the black hole.
 02:54: It's possible to pass their time zones infinitely quickly, but only because of your choice of spherical coordinates.
 03:26: In fact, it's happened many times before.
 05:07: Hey, this is "Space Time." We can deal.
 05:17: The Schwarzschild metric allows us to compare two points or events in space time around a massive object from the perspective of different observers.
 05:26: For example, a short space time path of some object, so it's world line, might move an object a distance delta r over a short timestep delta t.
 05:44: That delta s squared thing is the space time interval, and it's a strange and interesting quantity.
 05:50: Every inertial, so nonaccelerating observer, will agree on the same space time interval for every pair of events and for every world line.
 06:07: ... require faster than light motion, then the square root of the space time interval is equal to the amount of time that the object itself feels ...
 06:21: We call that the object's proper time.
 06:30: In fact, it's 2 times the gravitational constant times the mass.
 08:12: The entire space time interval for a nonmoving point at the event horizon is 0.
 08:18: But remember, for sublight speed world lines, the space time interval tells us the rate of flow of proper time.
 08:26: So does that mean time doesn't pass for an object hovering at the event horizon?
 08:33: Time certainly doesn't pass at the event horizon.
 08:40: ... the prohibition against objects experiencing time at the event horizon is actually a prohibition against objects spending ...
 08:51: No temporal thing, nothing that normally experiences the passage of time, can have a space time interval of 0.
 08:59: At the event horizon the only way to get a nonzero space time interval is to have a nonzero delta r.
 09:17: And once inside, inward spatial movement continues to be the only way to fuel the ticking of an object's proper time clock.
 09:30: There is one thing that can have a space time interval of 0, light.
 09:36: Actually, anything capable of traveling at light speed can only have a space time interval of 0.
 10:13: ... the fact that even an outgoing light ray takes infinite time to move any distance, so using boring old time and distance, delta t and ...
 10:36: There are ways to construct our space time axes so this singularity just evaporates.
 10:41: For example, EddingtonFinkelstein Tortoise Coordinates that compactify with the stretching of space time to cancel out the infinities.
 11:45: There, the causal roles of space and time switch places, and the central singularity becomes not so much a location in space but an inevitable future.
 12:03: Standby to explore what happens when you switch the causal rolls of time versus space to space time.
 12:24: And as always, a big thank you to our Patreon supporters for really making space time a lot easier to do.
 12:53: ... week, we inaugurated the "Space Time" Journal Club by looking at Harold White et al's paper on an apparently ...
 13:28: A scientist who decided to look into this has to divert grant money, lab space, personnel, and most critically a lot of their own time and energy.
 15:09: But we don't pee our pants every time.
 17:22: ... Scatterty reminds us, the first rule of "Space Time" Journal Club is we talk over our thoughts and remain open to all ...


20170111: The EM Drive: Fact or Fantasy?
 00:00: [MUSIC PLAYING] One of Space Time's most requested topics is the controversial EmDrive.
 00:21: Today, we inaugurate the Space Time Journal Club by taking a very careful look at this result.
 01:23: ... for edition one of the Space Time Journal Club, we're jumping into Harold "Sonny" White, et al., 2016 ...
 04:59: ... a typical commercial nuclear power plant or the power consumption of a time traveling ...
 09:14: This is Space Time Journal Club, which means it's now open for discussion.
 09:22: Back to our regularly scheduled, slightly less speculative science next week on Space Time.
 10:13: ... principle, yes, as long as the timing between the antennae can be kept precise enough, so that when you bring ...
 13:59: ... books, I'll be stealing every single one of my ideas from old Space Time ...


20170104: How to See Black Holes + Kugelblitz Challenge Answer
 00:35: There's been no reasonable doubt about the reality of black holes for some time.
 01:43: At around 15 times the mass of the sun, the dark object in this system can't be anything but a black hole.
 06:27: The surface of our star is represented by its starting radius at t equals zero, but as time moves forward, the radius shrinks as the star collapses.
 06:41: On the Penrose diagram, that horizon extends both forwards and backwards in time.


20161221: Have They Seen Us?
 07:49: But if such radio waves travel to us from the earliest of times, then they become stretched out as they travel through an expanding universe.
 08:42: Loeb and Zaldarriga's numbers assume pointing SKA at a target star system for an entire month and adding up all of the radio emission over that time.
 09:30: That means compensating for the difference in exposure or integration time with the sheer size of the telescope.
 10:04: ... aliens would need a radio telescope trillions of times the SKA's surface area and equivalent to a dish around three times the ...
 10:31: ... along with James Bedford, point out, that one month integration time requires a very consistent narrow frequency signal, which may not be ...
 11:02: Our SKA will cost a few billion dollars by the time it's done in 2030.
 12:35: If anyone else inhabits this very local region of the galaxy, then they may have been aware of us for some time now.
 14:25: If time dilation approaches infinity, do you reach the singularity before the black hole evaporates?
 14:32: So a distant immortal observer, with a ridiculously good telescope, will detect photons from the falling monkey at all future times.
 17:18: Cornerrecord asks about that thing when you're in a black hole and time becomes space and space becomes time?


20161214: Escape The Kugelblitz Challenge
 00:14: ... compactifies our representation of the dimensions of space and time, allowing us to fit onto the one diagram the infinitely stretched ...
 02:30: Space and time switch places, and the singularity soon forms, with all space within the black hole flowing towards it faster than the speed of light.
 03:18: For anything in this region, there isn't enough time to clear the impending event horizon, even traveling at the speed of light.
 07:03: They've heard that you watched PBS Space Time, and so they want your opinion on the plans.
 07:29: ... within two weeks of release of this episode for a chance to win a Space Time tshirt and, obviously, to save the ...
 07:44: So assuming there's no kugelblitz apocalypse, I'll see you next week for a new episode of Space Time.


20161208: What Happens at the Event Horizon?
 00:06: At the event horizon of the black hole, space and time are fundamentally changed.
 00:22: Its time you learned it.
 00:38: ... is lost to the universe forever while we see falling objects freeze as time stands still at the ...
 02:19: ... graphing time versus just one dimension in space, we can look at the limits of our ...
 02:30: With the right choice of space and time units, the speed of light becomes a diagonal line on the spacetime diagram.
 03:04: It lives at x=0 on the space axis, but exists through all the times on the graph.
 03:16: ... mass of the black hole stretches space and time so that light rays appear to crawl out of the vicinity of the event ...
 04:25: It also curves the lines of constant time and constant space in what we call a conformal transformation so that light always follows a 45 degree path.
 04:48: ... one dimension of space and red horizontalish lines are fixed moments in time. ...
 05:07: They're extremely finely separated at the edges so that any tiny stretch on the graph represents vast distances and/or times.
 05:24: ... us hugs the edge of the diagram and crosses an enormous number of time and space steps, only reaching us in our very distant ...
 06:16: Weirdly, the lines of constant position and constant time switch.
 06:28: It becomes unidirectional, flowing inexorably downwards, just as time flowed inexorably forward in the outside universe.
 08:07: It will, nonetheless, have experienced far less time than us when it emerges into flat spacetime in our far future.
 09:35: ... the monkey isn't actually above the horizon for infinite time, it only appears that way to us because as long as we're outside the ...
 15:11: ... that the particle riding the wave does have a definite position at all times and that position defines its future ...
 18:25: ... was unable to pull the math together in time for the fated Solvay Conference, and so derived the simpler description ...


20161130: Pilot Wave Theory and Quantum Realism
 00:55: ... claim stuff like things are both waves and particles at the same time, the act of observation defines reality, cats are both alive and dead, or ...
 01:58: But at the same time, it's considered one of the least orthodox.
 02:03: Because orthodoxy equals radicalism plus time.
 03:11: ... object could show both wave and particlelike behavior at the same time without being fundamentally ...
 03:59: This wave guides the motion of a real pointlike particle that has a definite location at all times.
 04:13: That's the equation at the heart of all quantum mechanics that tells the wave function how to change across space and time.
 13:45: And the problem is that the universe at this time wasn't dense enough and was expanding too quickly for strange matter to form in any great abundance.
 14:07: ... expected to be more stable the larger they are, and so would grow over time. ...


20161116: Strange Stars
 02:16: The remaining neutron star is millions of Kelvin in temperature, and may be spinning thousands of times per second.
 06:29: If the density is high enough, the conditions at the core may be so extreme that they resemble the time even before the Quack Epoch.
 09:52: ... about it long and hard and we decided that we want to keep making Space Time forever and ever and ever and ever, and to keep making it better along ...
 10:18: It'll really help us keep pushing Space Time forward.


20161109: Did Dark Energy Just Disappear?
 04:32: ... basically means this: if you repeated this experiment many, many times, about 0.27% of the time the uncertainties, so the messiness in the data, ...
 05:03: But given that many thousands of different experiments are being run by professional scientists at any one time, false 3sigma results do happen.
 12:55: With The Great Courses Plus, you can watch as many different lectures as you want, any time, anywhere without tests or exams.


20161102: Quantum Vortices and Superconductivity + Drake Equation Challenge Answers
 03:24: ... in that region  that's "N (sub) ast" for "astrophysical factor"  times the probability that a technological civilization will form on any given ...
 08:43: For everyone else, thanks for playing, and you can always pick up your own Space Time tshirt: link in the description.
 08:50: See you guys next week on Space Time!


20161026: The Many Worlds of the Quantum Multiverse
 05:33: ... find that the cat is alive, it's because we're part of an entire quantum timeline in which the radioactive decay and subsequent poisoning never ...
 05:45: But there's an equally valid timeline in which it did, and another version of us experiencing that.
 06:20: ... of particle trajectories, of histories, merges into the single timeline of the observer's ...
 06:37: Those alternative histories continue, and we find ourselves in just one of those timelines.
 07:07: ... invites the idea that reality splits into different branches every time quantum states diverge into different possibilities for example, at ...
 07:21: ... would lead to an unthinkably large number of alternate timelines or worlds that contain all possible realizations of this universe since ...
 07:57: It's just that Copenhagen merges them into a single timeline with its wave function collapsed.
 10:19: Any given timeline is a predictable chain of cause and effect.
 11:12: This is Dianna from Physics Girl right here on Space Time.
 12:30: So keep watching PBS Space Time to make sure that Matt follows up with that challenge.


20161019: The First Humans on Mars
 00:24: NASA's new budget mandates a 25year timescale for putting astronauts in orbit around Mars.
 00:36: SpaceX plans a spacecraft capable of transporting 100 people at a time to Mars, not to visit, but to settle.
 05:40: Mars's gravity is 0.38 times that of Earth.
 07:39: Mars will be fun, but we're really holding out for those Venusian cloud cities here at Space Time.
 07:46: We want to thank Curiosity Stream for supporting this episode of Space Time.
 09:52: A star's core needs to be more massive than around three times the mass of the sun in order to collapse into a black hole.
 10:49: However, they may also have been from PBH's from an even earlier time.


20161012: Black Holes from the Dawn of Time
 01:00: Once upon a time, the entire universe had the density of a stellar corpse.
 02:29: Yet if we rewind time, those fluctuations must have been much stronger.
 03:39: PBHs could have been formed at a few grams to tens of thousands of times the mass of the sun, depending on which formation model you go with.
 05:07: For one thing, if these little knots of warped space time are everywhere, then they should produce obvious gravitational lensing.
 06:17: ... kilograms, or a much smaller number of really big PBHs around 20 to 100 times the Sun's ...


20161005: Are We Alone? Galactic Civilization Challenge
 05:53: Submit your answers with full work to pbsspacetime@gmail.com within two weeks of release of this video for a chance to win a Space Time tshirt.
 06:09: See you next week for a new episode of Space Time.


20160929: Life on Europa?
 03:36: Sounds unpleasant, but the regions around these vents are teeming with life 10 to 100,000 times the density of organisms compared to the sea floor.
 08:35: ... be great and all, except it adds a lot of extra weight and development time to the mission, and it also makes it pretty hard for NASA to scout out ...
 09:31: ... we'll see you next week on "Space Time." Last week we talked about the weirdness of quantum entanglement and the ...
 10:29: ... prediction is that the second particle is aligned one way 50% of the time and the other way 50% of the ...


20160921: Quantum Entanglement and the Great BohrEinstein Debate
 00:55: ... the time they grow up and go to college to study physics, the notion of object ...
 04:01: ... be transmitted instantly across any distance, and even back in time, violating locality and possibly violating ...
 04:31: The argument between Bohr and Einstein felt a bit philosophical at the time.
 10:15: The universe seems to conspire to avoid the paradox of information traveling faster than light, or backwards in time.
 11:11: ... get back to these in an upcoming episode of "Space Time." Last minute announcement tomorrow night, Thursday, the 22nd, is PBS ...
 12:13: Humans are not on track to extreme cultural enlightenment over time scale.


20160914: SelfReplicating Robots and Galactic Domination
 02:46: It's time to generalize.
 06:34: But the exponential nature of the process means that the entire galaxy would be covered in these things in that amount of time.


20160907: Is There a Fifth Fundamental Force? + Quantum Eraser Answer
 04:03: ... I asked you to tell me why it's impossible to send any real data back in time using the delayed choice quantum eraser experiment, and so cheat on the ...
 06:17: That information can't travel backwards in time or faster than light.
 06:59: But there's still time for a minirant about the role of consciousness in quantum mechanics.
 07:32: Admittedly, this decoherence appears to affect the wave function at times before the apparent cause of the decoherence.


20160824: Should We Build a Dyson Sphere?
 06:06: Once complete, the Dyson swarm would harvest a good fraction of the sun's energy, so trillions of times the current energy output of the planet.
 08:37: However, there's enough mass in the solar system to run a type 3 civilization's Kugelblitz swarm for many times the current age of the universe.
 10:05: And we don't go in for that hokey stuff here on "Space Time." Thanks to "The Great Courses plus" for sponsoring this episode.
 10:39: With "The Great Courses plus," you can watch as many different lectures as you want, any time, anywhere, without any tests or exams.
 12:11: ... we see exactly the right time offset between a hit on the screen and a hit at one of the detectors, ...
 12:38: ... figure out which photons correspond to which detectors until the arrival times at the screen are compared to the arrival times at the other ...


20160817: Quantum Eraser Lottery Challenge
 00:11: In fact, the delayed choice quantum eraser seems to show that information can travel backwards in time.
 00:19: ... challenge question, I'm going to propose a way to use it to create a timetraveling communication device and use it to cheat on the ...
 00:45: Photons are fired one at a time through the two slits.
 02:25: ... regarding whether the observer knows the path appears to travel back in time, even if was only a tiny fraction of a second in the experiment that was ...
 02:44: ... between two points, but can also send information back in time. ...
 04:02: ... the screen, which could theoretically be very distant in both space and time. ...
 04:27: So maybe we bounce them between Earth and the moon, like, 8,000 times.
 05:27: We'll choose five correct answers to receive "Space Time" tshirts.
 05:40: See next week for a fresh new episode of "Space Time." [MUSIC PLAYING]


20160810: How the Quantum Eraser Rewrites the Past
 02:52: Instead, particles land in simple clumps, one for each slit, as though they were traveling as simple particles the whole time.
 03:20: Better pretend like you are particles that whole time.
 09:50: Perhaps the evolving tapestry of entanglement in all its impossible complexity is what really defines reality in this space time.
 10:00: This episode of "Space Time" is supported by Audible.com.
 10:03: Right now, Audible is offering "Space Time" viewers a 30day trial period.
 11:08: And they're at least several times the mass of the sun.
 12:52: Let's deal with one end of the world at a time, people.


20160803: Can We Survive the Destruction of the Earth? ft. Neal Stephenson
 01:52: It's time to us what we need to do in order to survive the next end of the world.
 03:28: But in the case of a big asteroid impact or a nearby supernova, it could come with so little warning that we might not have time to do much about it.
 03:39: So lead time is important.
 03:57: How much lead time would we really need to build this thing, assuming we started right now?
 04:11: In the book that I wrote, in "Seveneves," two years is the span of time that people have in which to do something.
 04:32: On the other hand, given that we know that some of these catastrophes will happen eventually, in a way we've already been given the lead time.
 06:25: None are going to hit us any time soon.
 10:46: See you next time on "Space Time." Last week we talked about the spectacular weirdness of the single particle doubleslit experiments.
 11:35: ... function conceal what we call hidden variables, that may change over time and space according to the wave function, but that at any one instant ...
 12:49: ... given that this is 100 times smaller than the Planck length for anything within a human's mass, ...


20160727: The Quantum Experiment that Broke Reality
 03:11: The interference pattern is seen even if you fire those photons one at a time.
 10:12: ... to the many worlds interpretation, which we will right here on "Space Time." Thanks to The Great Courses Plus for sponsoring this ...
 10:57: With The Great Courses Plus, you can watch as many different lectures as you want any time, anywhere, without tests or exams.
 12:43: For example, for every one orbit of Jupiter's moon Io, its moon Europa orbits twice and Ganymede four times.
 12:57: These integer ratios maximize the amount of time that the planets spend in closest proximity.


20160720: The Future of Gravitational Waves
 00:43: These oscillations echoed the final 1/10 of a second of the end spiral and merger of a pair of black holes, each around 30 times the mass of the Sun.
 00:54: This incredibly important observation was hailed at the time as representing the dawn of gravitational wave astronomy.
 01:14: This time, they're a bit smaller, at 14 and eight solar masses.
 02:12: ... LIGO's arm lengths of about 1/1000, the diameter of a proton and a few times smaller than the more powerful September ...
 03:30: However, they hadn't had time to give due care to the newer signal so they decided to keep quiet about it until they were sure sure.
 05:10: ... out the direction that the wave came from, which is determined by the time difference in the signal between the two ...
 05:56: You had the halflife so the average time for the decay of a polonium212 nucleus.
 06:03: You needed to figure out how many times the alpha particle would encounter the walls of the nucleus in this time.
 08:06: And I'll see you next time for a brand new episode of "Space Time." [THEME MUSIC]


20160706: Juno to Reveal Jupiter's Violent Past
 00:37: It may only have 1,000th of the sun's mass, but it weighs more than all the other planets combined, two and 1/2 times more.
 01:22: ... grew to at least 10 and maybe over 40 times the mass of the Earth before it had enough gravity to start holding on ...
 02:02: It's 20,000 times the strength of Earth's field, giving Jupiter the brightest auroras in the solar system.
 02:15: That's 5.2 times Earth's average distance from the sun.
 06:43: Another clue is that other real exoplanetary systems tend to have super Earths, rocky planets several times the mass of our own planet.
 10:03: We'll keep you up to date right here on Space Time.
 11:54: ... to complete an infinite number of tasks, closing the remaining gap every time he reaches the tortoise's last ...
 12:06: ... it doesn't need to take an infinite amount of time to complete an infinite number of tasks, as long as the time taken for ...
 12:17: ... example, if the time taken to catch the tortoise is, say, half a minute at first, and then ...


20160629: Nuclear Physics Challenge
 01:15: It has a half life of 0.3 microseconds, meaning half of any sample of polonium 212 will have decayed to lead in that time.
 02:00: And every time it approaches one wall, there's a chance it will tunnel through.
 02:19: A good first step might be to figure out how many times the alpha particle encounters the wall in that 0.3 microseconds.
 03:43: Submit your answers with full work to PBSSpacetime@gmail.com within two weeks, so by July 13, for a chance to win a "Space Time" tshirt.
 04:06: And I'll see you next week for a new episode of "Space Time."


20160622: Planck's Constant and The Origin of Quantum Mechanics
 00:44: Run as fast as you like, by the time you reach the tortoise's initial position, it'll have moved forward.
 00:53: To overtake a tortoise, you need to travel to its previous position infinite times.
 07:38: He set this minimum energy to be the frequency of a particle's vibration times a very, very small number, a number that had yet to be measured.
 09:36: ... gain or lose energy by absorbing or emitting one particle of light at a time. ...
 09:52: ... a quantum of energy equal to the now familiar frequency of the wave times the Planck ...
 10:47: And combined with a small handful of other fundamental constants, it governs the behavior of everything in this space time.
 11:23: With "The Great Courses Plus," you can watch as many different lectures as you want any time, anywhere, without any tests for exams.
 11:30: Help support "Space Time" and start your onemonth trial by going to thegreatcoursesp lus.com/spacetime.
 14:52: Gravitational waves are ripples in the fabric of space time, so they have to go where the space time goes.
 14:58: Dylan T tells us that he met his wife by suggesting that they Space Time and Chill.


20160615: The Strange Universe of Gravitational Lensing
 00:22: We have hardware to build internal models of our environment in which space is a simple 3D grid, static with time.
 01:14: In the real universe, both space and time can be curved.
 01:21: Here's our playlist on curved spacetime, time, if you want to go deep into this idea.
 02:43: "The New York Times" article is wonderful.
 05:42: By measuring the time delay between fluctuations for those different paths, we can actually measure the path lengths.
 06:25: As those stars sweep in front of the quasars in a vortex, its different parts change in magnification to different degrees and at different times.
 09:13: Next time you look up at the sky, remember, your eye is following strange curved paths.


20160608: New Fundamental Particle Discovered?? + Challenge Winners!
 02:32: Those happen all the time.
 04:39: OK Time for the solution to the dark energy challenge question.
 05:02: We're going to talk about everything space time.
 05:05: So space and time, really anything physics or astrophysics.
 06:51: But the response takes time.
 09:03: For the main question, I asked you to figure out how many times the universe doubled in size after dark energy first started to show its influence.
 09:12: And how many times it would double in size in the future before matter no longer has any significant influence on expansion.
 11:21: So you need to double that 0.36 about 1.5 times to get to the current scale factor of 1.
 12:40: If your name appears on screen below me, you got this right, and were randomly selected to receive a space time Tshirt.
 13:29: We're come back to both of these ideas in future episodes of "Space Time." [MUSIC PLAYING]


20160601: Is Quantum Tunneling Faster than Light?
 00:57: That distribution, and the way it changes over time, is coded in the object's wave function.
 02:23: See an object's de Broglie wavelength depends on its momentum, so mass times velocity.
 05:27: It's actually extremely hard to test this because we can't make clocks accurate enough to time such a ridiculously quick event.
 06:11: In the absence of quantum tunneling, that barrier should reflect its photon 100% of the time.
 06:29: About 99% of the time the photon is reflected.
 06:32: But 1% of the time it will resolve itself beyond the barrier and it will continue on its path.
 07:39: At that point, you can get an incredibly precise measurement of any differences in photon travel time.
 08:30: But even without a barrier, this location fuzziness leads to uncertainty in the arrival time of the photon.
 08:39: ... unimpeded photon could arrive at the earlier time of the tunneling photon, because its wave packet includes that in its ...
 09:39: Stay tuned for the implications of this on both quantum and cosmic scales of space time.
 09:46: This episode of Space Time is supported by Audible.com.
 09:49: Right now, Audible is offering Space Time viewers a 30 day trial period.


20160525: Is an Ice Age Coming?
 01:04: It's 10,000 to 15,000year warm patches are separated by glacial periods that last several times as long as.
 05:16: Each year's layer carries bubbles of the Earth's atmosphere from that time.
 06:01: Temperature goes up and down on the roughly 40,000year time scale of changing obliquity.
 06:26: Every time Earth's orbit becomes more circular, the planet warms and the glaciers go away.
 11:08: So we've probably at least offset the next glaciation, although it wasn't coming any time soon, anyway.
 11:57: See you next week for more cold, hard facts on Space Time.
 14:51: Conservation of energy is one such law that work in flat space time.


20160518: Antigravity and the True Nature of Dark Energy
 01:56: It doesn't change over time.
 04:18: In general relativity, energy slash mass and pressure both curve space time.
 05:17: Instead, the overall effect of pressure on the curvature of space time is a purely relativistic effect.
 11:35: This law is a property of a Newtonian universe, in which space and time are fixed static dimensions.


20160511: The Cosmic Conspiracy of Dark Energy Challenge Question
 00:22: But before we get there, it's time for some homework.
 00:34: We live in unusual times.
 02:25: If that constant truly stays constant over time, then it represents an energy of the vacuum.
 02:47: At the same time, the total amount of regular matter in any expanding region remains constant.
 04:16: ... the size of a grain of sand, it has doubled in size approximately 100 times to get to its current ...
 04:36: It'll go on to double in size approximately infinity times in the future.
 04:42: OK, so let's think about time not in billions of years but in the number of doublings.
 06:07: I'll choose three correct answers at random for both the main and the extra credit questions to receive Space Time tshirts.
 06:48: See you next week for a fresh new episode of Space Time.


20160504: Will Starshot's Insterstellar Journey Succeed?
 07:13: With a 20 plus year travel time and 4.4 years for the pics to be beamed back to Earth, that gets us the data in 45 to 50 years.
 08:35: ... first up close alien snapshots in around 50 years right here on "Space Time." In the last episode, we talked about how dark energy causes this ...
 09:54: ... a constant cosmological constant and so has an unchanging density with time, then no, space time doesn't get ripped ...
 10:15: But if the density of dark matter varies with time, then there are a range of possibilities.
 10:20: The one where space time rips itself apart at a fundamental level, the socalled Big Rip, happens when the density of dark energy increases.


20160427: What Does Dark Energy Really Do?
 01:43: ... tells us the total amount of expansion that happened during that time. ...
 01:53: But how do we figure out how much time the light traveled for?
 02:53: ... so far away that the universe will have expanded significantly in the time it took their light to reach us so billions of light years ...
 04:28: ... mean the universe expanded a lot during the corresponding light travel time. ...
 07:01: See, this observation actually came before the discovery of the inexplicable geometric flatness of the universe that we talked about last time.
 08:46: Typically, the Hubble Parameter becomes lower over time.
 08:56: And so we'll have a constant doubling time.
 09:38: ... that, we'll need another episode of "Space Time." In the last episode, we talked about how a general relativistic ...
 10:16: If positively curved, that would mean a finite but very, very large greater universe with a volume at least 250 times that of our observable universe.
 12:22: People think silly things all the time.


20160420: Why the Universe Needs Dark Energy
 00:08: ... of space vastly beyond our capacity to physically explore and at a time billions of years in the ...
 02:53: But the spatial geometry of the universe at a fixed instant in time can be flat or curved.
 03:12: Spatial snapshots of the cosmos at an instant in time will be curved like the surface of a sphere except the 3D surface of a 4D hypersphere.
 04:13: Zero spatial curvature, at any given time.
 04:56: ... exactly its escape velocity, one that will slow to a stop over infinite time, should be flat, with the both left and right sides coming out to ...
 08:25: In fact, the universe reached that tipping point pretty recently, on cosmic time scales.
 11:58: And, besides, we get every take right the first time.


20160413: Will the Universe Expand Forever?
 02:45: ... second, which is the escape velocity on Earth's surface, then, by the time it almost stops moving, it will be so far away that Earth's ...
 03:53: ... expansion rate of the universe to slowly grind to a halt over infinite time. ...
 04:59: Now the Friedmann equation tells us how a evolves over time.
 05:09: Now, if you guys have taken some calculus, that's the time derivative of a.
 07:26: ... a long time until the late '90s it was believed that the answer to whether the ...
 09:46: It's webby time.
 12:06: The Sun has orbited the Milky Way around 18 times since it formed.


20160406: We Are Star Stuff
 00:31: Space, time, energy, mass.
 01:30: So the tapestry of our universe is woven across the dimensions of space and time and complexity.
 05:36: ... the largest stars, any bigger than around eight times the sun's mass, reach the ends of their lives, they become super giants, ...
 10:58: ... I should say that the curvature of space at a single, constant moment in time has to be one of these ...
 11:10: So constant time spatial curvature, which is different to the curvature of spacetime.
 11:17: The time part is certainly curved, which leads to the expansion of space, even if the space part itself can be flat.
 13:09: So that's something like 10 to the power of 20 times faster than light.
 13:21: Guess what we're talking about next week on "Space Time?" The end of the universe.


20160330: Pulsar Starquakes Make Fast Radio Bursts? + Challenge Winners!
 01:33: In fact, checking through archival data, it was found to have repeated itself multiple times.
 03:18: At this time, the universe was full of plasma, atomic nuclei, and free electrons.
 04:21: ... the other common charged particles hanging around the universe at this time, have much smaller scattering crosssections than electrons ...
 05:57: 46.6 billion light years in radius now, translates to 42.3 million light years then, which is a volume of 2.7 times 10 to the 71 cubic meters.
 07:07: ... of electrons in that column segment, which is just electron density, times the scattering crosssection of the ...
 08:29: If you see your name scrolling below, we've chosen your correct answer to receive the "PBS Space Time" Tshirt.
 08:57: See you all next week for brand new episode of "Space Time." [MUSIC PLAYING]


20160323: How Cosmic Inflation Flattened the Universe
 00:29: ... simple model tell us that something strange must have happened in early times, an insane growth spurt that we call ...
 00:55: ... edge to the other, that those most distant points should never have had time to communicate with each ...
 02:26: They're defined by how fast sound waves could have traveled by the time the CMB was created.
 05:00: ... for a very short period of time, blow it up much faster than the speed of light so that most of it ...
 06:08: For this to work, that inflationary expansion had to throw neighboring regions of space apart at many times faster than the speed of light.
 06:57: ... he added this as a way to allow his theory to describe a static space time, a universe that's neither expanding nor ...
 09:58: Time may not have begun with the Big Bang.
 10:02: ... we'll rewind to before the beginning of the universe very soon on "Space Time." In a recent episode, we told you why space things are the shape they ...
 10:39: So even though the gas originally had motion in many directions, over time, it sweeps into a single bog flow.


20160316: Why is the Earth Round and the Milky Way Flat?
 02:15: Here it's fine to think about gravity Newtonianly as a force rather than as an Einsteinian warping of space time.
 06:36: That's 10 times lower than its compressive strength.
 10:34: Symmetries really do shape the universe on all the scales of space time.
 12:16: And at the moment of recombination, when the CMB was emitted, it was emitted by all of the observable universe and beyond at the same time.
 12:26: This patch of space, the Milky Way, the Earth, has been bombarded with cosmic background radiation for all of cosmic time.
 12:38: But as the universe got older, radiations from further and further away had time to get to us.


20160309: Cosmic Microwave Background Challenge
 02:10: ... the time it reaches us, right now, the universe has expanded so that the galaxies ...
 03:46: Email your answers to pbsspacetime@gmail.com within two weeks for a chance to win a PBS Space Time tshirt.
 04:47: Happy physicing, and see you next week for a full new episode of Space Time.


20160302: What’s Wrong With the Big Bang Theory?
 00:10: The Big Bang Theory suggests that once the entire universe was compacted into an infinitely small speck at the beginning of time.
 00:48: ... power of minus 32 of a single second after the hypothetical beginning of time. ...
 02:14: For a very brief period soon after the beginning of time, these forces are combined.
 03:30: We need to produce energies a trillion times larger than is possible with the Large Hadron Collider.
 03:46: We do think that we can describe gravity and the shape of space time at these densities and temperatures.
 04:31: ... of the resolutions might be [CLEARS THROAT] string theory another time. ...
 04:57: But there are some clues still visible at later times.
 05:15: But it's still 1,000 times smaller than the modern universe.
 06:03: If you have a cup of coffee, and drop in some cold milk, it will all smooth out and become the same temperature after a bit of time.
 06:11: ... relativity, when the CMB was released, there just hadn't been enough time for this mixing to have ...
 06:23: ... most distant patch in that direction, there needs to have been enough time for something to travel between those points to diffuse and even out ...
 06:50: A photon emitted on one side of that grain wouldn't have time to get to the other side, not even in 400,000 years.
 07:30: ... way around this problem is to somehow have the universe, once upon a time, be small enough so it could easily get all nicely mixed together, and ...
 09:25: Those boundaries are being chipped away at all the time.
 09:44: We'll get back to that on future episodes of Space Time.
 10:25: ... the FriedmannLemaîtreRobertsonWalker metric, which describes a space time in which all matter is perfectly smoothly ...
 10:38: That works on the largest scales in which galaxies and galaxy clusters are a speckled foam on top of a much vaster space time.


20160224: Why the Big Bang Definitely Happened
 00:03: Let's see how far back in time we can push our certainty, and let's see what questions still lie beyond the limit of our understanding.
 00:23: And I'm talking about both scientific and religious dogmas of the time.
 00:55: I'm not going to spend a lot of time describing the theory.
 02:09: And so we know that once upon a time, it had to be much smaller.
 02:15: ... just a tiny part of the universe, and run the laws of physics forward in time to predict the ...
 02:36: ... was once compacted into an infinitesimal point, a singularity at time t equals 0, the hypothetical instant of the Big ...
 03:05: So we know that at some point in our rewind, pure general relativity will give us the wrong predictions for the behavior of space time.
 03:23: ... times after that point, our understanding is good enough to make some pretty ...
 03:51: ... moment when the universe was around 400,000 years old and about 1,000 times smaller than it is today, it hit a critical temperature of 3,000 degrees ...
 04:20: It's still traveling today, carrying with it an image of that early time.
 04:49: So at least that far back in time, the Big Bang theory is right.
 05:36: When we look to vast distances, we're also looking back in time and we see the very first galaxies soon after they collapsed from these blobs.
 07:03: ... this time, nuclear fusion raged across the cosmos, baking some of the existing ...
 08:01: Because we've recreated the conditions of the universe at this time.
 08:29: But there are clues to that earliest of times.
 08:40: ... rewind to the very beginning of space time on the next episode of "Space Time." A couple of weeks ago, the LIGO ...
 09:41: ... out that it's kind of weird that Advanced LIGO was turned on just in time to catch the gravitational waves from the merger of black ...
 11:04: As we mentioned in our earlier video right after the first detection, the rumor at the time was that the detection was in the engineering data.
 11:14: ... runs before the official turnon with the sensitivity improving each time as more and more of the upgrades came ...
 11:58: ... grant us stunning insights into the fundamental nature of space time. ...


20160217: Planet X Discovered?? + Challenge Winners!
 00:22: [MUSIC PLAYING] Caltech planet hunters Mike Brown and Konstantin Batygin have been the trail of something big out there for some time.
 01:31: ... solution, a very, very distant giant planet, with a mass well over 10 times that of the Earth, and a stretched out eccentric orbit with a period, a ...
 01:57: That's 700 times the Earth's average overall radius, placing it far outside the Kuiper Belt.
 02:51: But did we really find Planet X this time?
 02:55: ... quote Mike Brown, "This is different because this time we're right." Existing telescopes should be able to see this thing, even ...
 03:10: OK, time for the answer to our photon clock challenge question.
 03:59: However, that's based on a sort of instantaneous comparison of the timelines of the two clocks.
 04:06: To determine the observed tick rate, you also have to factor in the travel time of light.
 04:25: By definition, ticks correspond to intervals on our time axis.
 04:42: That's regular time dilation.
 04:57: Where those paths intersect the vertical world line tells you the observed time interval between the ticks.
 05:25: ... traveling in your direction, stretching out the distance, and hence the time, between tick ...
 05:48: One effect, time dilation, slows down the tick rate of the approaching clock.
 05:53: At the same time, the relativistic Doppler effect speeds it up.
 05:58: Time dilation will slow a moving clock down by this, 1 over the square root of 1 minus velocity squared over c squared.
 06:46: ... you think of just the time dilation, and not the Doppler effect, you get that the moving clock is ...
 06:56: By the way, this is an effect that we see out there in the universe all the time.
 07:06: And they pulsate much more quickly than they should because our sense of their time is compressed.
 07:12: ... the same time, supernovae that are moving away from us due to the expansion of the ...


20160211: LIGO's First Detection of Gravitational Waves!
 00:05: Gravitational waves have been directly detected for the very first time.
 00:45: The new, advanced LIGO is around 10 times more sensitive.
 01:13: And when here at "Space Time" we realized this was probably the real deal, we were too.
 01:30: This is a great time for you to watch it if you haven't already.
 05:37: However, physicists will be all over this data, looking for holes in the theory for some time.
 07:18: ... keep you up to date in future episodes of "Space Time." In a recent episode, we heated up the debate on the question of ...


20160203: Will Mars or Venus Kill You First?
 06:42: So let's say you've taken the much easier threemonth flight to Venus, half the time of the Mars trip.
 06:56: It's a pleasant 0.9 times Earth's gravity, and the atmospheric pressure is around that of Earth's surface.
 07:26: Your skin will start to blister and dissolve at the same time.
 08:31: So we'll be around for many more episodes of "Space Time." In the last episode, we finished our series on the origin of matter and time.
 08:43: Jona Storm asks, if there's no such thing as universal time, how can we say the universe is 13.8 billion years old?
 08:54: ... then somehow found its way to planet Earth without having spent a lot of time traveling close to the speed of light, with respect to ...


20160127: The Origin of Matter and Time
 00:00: [MUSIC PLAYING] Einstein's theory of special relativity has shown us mass and time are not the concrete things we imagine them to be.
 00:16: In this episode, we're going to rebuild our understanding and explore the origin of matter and time.
 00:46: And a location in time, typically right now.
 00:55: A thing's mass, and a thing's experience of time.
 01:05: Today, we're going to bring together these ideas to explore what matter, time, and things really are.
 01:13: A while ago, we introduced the space time diagram.
 01:17: It's just a graph of position in space just one special dimension for simplicity versus position in time.
 01:24: In this picture, a thing ends up tracing a path through time and space.
 01:31: In fact, thinking in four dimensional space time, a thing is its world line.
 01:52: But it will move up at a nice steady space in time.
 01:57: Time marches on.
 02:01: Now, it moves both in space and time, because position is changing.
 02:11: Constant speed equals constant change in position x with time t.
 02:46: For example, we could make the time divisions 1 second, and the space divisions 300,000 kilometers, because that's how far light travels each second.
 03:10: They're separated more by time than space.
 03:15: And we call them time light paths.
 03:23: There's not enough time for anything to travel that much space.
 03:37: Now remember, a photon clock marks time with a particle of light bouncing between two mirrors.
 03:46: Now we'll get back to why this is a good measure of the flow of time in a minute.
 03:54: The clock travels smoothly straight upward in time.
 04:08: A second photon clock with a constant speed with respect to the first, travels a steeper time light path.
 04:28: On the timeline of the stationary clock, the ticks of the moving clock don't match up.
 04:36: This is the same result that we saw in the episode on time dilation.
 05:01: The whole space time diagram can be transformed to give the second clock's world line a constant location in space.
 05:17: Our space and time axes shift.
 05:41: So what this means is that there's no single preferred vertical time axis, or indeed, horizontal space axis.
 05:48: We can draw that time axis along any constant velocity timelike path, and just Lorentz transform to get a valid perception of space time.
 05:57: This means that the flow of time is not a universal thing.
 06:06: But there's no global rate of time flow that everyone can agree on.
 06:10: What defines that local time flow?
 06:40: Or be it on time scale shorter than the plank time.
 07:04: And on our space time diagram, our object becomes an impossibly complex ensemble of light speed world lines confined in equally complex ways.
 07:28: We're extrapolating the validity of space time diagrams, and these tiny lifelike segments into the quantum realm.
 07:45: ... give us a sense of stillness, a sense of thingness, and a sense of time. ...
 07:59: That time manifests as the rate of change of its internal machinery.
 08:09: Now here's something that seems to be a more concrete reality than the flow of time.
 08:18: One of them a point on the space time diagram can influence another if a signal can travel between the two.
 08:24: Those causal timelike paths can be thought of as a series of lightlike segments.
 08:42: Time traces that ordered sequence, and looks different from different perspectives.
 08:50: ... this picture, time and mass and matter become emergent properties of the causal propagation ...
 09:01: But what defines the direction of the flow of time?
 09:09: Great questions for future episodes of "Space Time." For our recent episode on when time breaks down, you guys had some amazing questions.
 09:18: Kovacs asks, how can it be that if an elementary particle doesn't experience time, that they can still decay?
 09:26: ... between states, like the electron's chirality flip, is experiencing time, which goes handinhand with them having ...
 09:58: So when I say that elementary particles don't feel time, that's what I'm talking about.
 10:04: ... basic vibrations of their quantum fields the time that the electron or quark feels is felt by the composite particle, ...
 10:14: So a lot of you independently realized that the time dilation of special relativity seems to generate a paradox.
 10:35: But which clock has the time lag when they get back together?
 11:16: And special relativity only describes the relative effects on time and space due to a constant relative motion.
 11:28: [INAUDIBLE] tells us that accelerating reference frame feels a slower passage of time.
 11:33: So the answer is that the astronaut's clock, or the traveling twin, has experienced less time.
 11:40: Ectoplasm2369 asks whether you'd feel time dilation in a warp drive.
 11:47: So for the Alcubierre warp metric, there's actually no time dilation either due to motion or acceleration.
 11:55: Your timeline remains synced to the timeline of your point of origin.
 11:59: Bruno JML would like to know in what reference frame Pink Floyd's "Dark Side of the Moon" syncs to when time breaks down.
 12:17: The start of the song time should sync with the appearance of the photon clock.


20160120: The Photon Clock Challenge
 00:35: And the resulting slowing of clock ticks due to motion is the time dilation of Einstein's theory of special relativity.
 00:44: Time dilation plays a key role in today's challenge question.
 01:44: ... entries from the correct submissions to receive brandnew PBS Space Time Tshirts, which are also available for anyone to preorder at a link ...
 02:03: We'll see you next week for a fresh new episode of Space Time.


20160113: When Time Breaks Down
 00:09: ... and explore the connection between matter, motion, and the nature of time. ...
 00:31: Our perception of time depends on how much attention we pay to it.
 00:47: Our often flawed perception of time comes from watching patterns in our brains evolve.
 01:24: The sum total of this motion results in a smooth, consistent evolution of time.
 01:28: But do the individual components of the clock feel the same flow of time?
 01:50: This is where our conception of time starts to break down.
 01:56: See, the flow of time depends fundamentally on motion.
 02:04: A particle moving at the speed of light experiences no time.
 02:09: Those electrons and quarks bounce around at such high speeds inside the atom that they experience time very differently to the atom itself.
 02:17: And in a sense, the most elementary particles are intrinsically timeless.
 02:22: The familiar smooth flow of time only emerges as these particles are bundled into what we think of as matter.
 02:29: Time happens for the atom in a way that it doesn't for the atom's parts.
 02:34: ... I want to show you why time depends on motion, which in turn will show us why light speed particles ...
 03:02: We're going to use a very close cousin to the photon box to explore time a thought experiment of Einstein's that we'll call the photon clock.
 03:26: The rate of ticks is consistent, time flows smoothly, until the clock starts moving relative to me.
 04:28: This effect is the time dilation of Einstein's special relativity.
 04:55: Time is frozen.
 05:35: This is the gravitational time dilation of general relativity.
 05:39: So what does this odd example of the photon clock have to do with real time and real matter?
 07:09: Time passes more slowly.
 07:11: Move fast enough, and time barely passes at all.
 07:24: But now it looks like this same bundling of light speed particles can also given matter time.
 07:30: Atoms feel time in their internal evolution similar to our own perception of the changing patterns in our brains.
 07:47: Is time even a real dimension?
 07:51: We'll have time for all of this on a future episode of Space Time.


20160106: The True Nature of Matter and Mass
 00:02: NARRATOR: Einstein showed us that matter, mass, and the flow of time are intrinsically connected, but opened the question, are they even real?
 00:40: The answer to this will take us to a much deeper question what is the origin of matter and time?
 07:49: What about time?
 07:51: A single photon experiences no time, nor does any massless particle.
 07:57: But our photon box has mass, so it must experience time.
 08:02: When and where does this time arise?
 08:10: Do they get time when they bounce off the wall?
 08:13: Does the ensemble of photons somehow feel time that individual photons do not?
 08:18: ... these questions when we delve deeper into the mystery of matter and time in the next episode of "Space Time." In the last episode of "Space ...
 09:19: ... thought of as the electron being both right and lefthanded at the same time, because the interchange happens on time scales shorter than the Planck ...


20151216: The Higgs Mechanism Explained
 01:06: We'll get into it in more detail another time.
 02:19: As we'll see in the next couple of episodes, this masslessness means that particles should travel only at the speed of light and experience no time.
 02:29: But these particles are distinctly not timeless.
 02:51: The electron evolves, meaning it does experience time, so it must have mass.
 05:34: ... giving and taking away the weak hypercharge on infinitesimally short time ...
 05:53: ... all invented so that electrons can be left and righthanded at the same time? ...
 07:33: ... now, we'll be delving deeper to the mysteries of matter and time in the next episode of "Space Time." In the last episode, we told you ...


20151209: How to Build a Black Hole
 00:21: ... talked about them as general relativistic entities, as space time regions whose boundary curvature effectively removes the interior from ...
 00:56: Yet, to actually form a black hole, Einstein's descriptions of mass energy and space time are not enough.
 02:51: ... like this, we need to think not in regular 3D space or even 4D space time but, rather, in six dimensional quantum phase ...
 03:34: The Pauli exclusion principle basically just says that two things can't occupy the same place at the same time.
 06:04: ... tells us that particular pairs of quantities, position and momentum or time and energy, must, when taken together, contain a minimum degree of ...
 07:41: The space time curvature at the neutron star's surface is pretty extreme.
 07:58: And yet, below the star's surface, their lurks the potential event horizon, the surface of infinite time dilation.
 08:26: It's three times the mass of the sun.
 08:45: Space time is radically altered inside the star with all geodesics, space time paths, turning inward, towards the center.
 09:46: But on our timeline, nothing ever happens beyond the event horizon again.
 09:58: The material of the star and all of events that happened to it are no longer part of the timeline of the external universe.
 10:43: ... for black holes and for the universe, in another episode of "Space Time." In a previous episode, we talked about the Alcubierre ...
 11:07: Now, "The Good Stuff" guys talk some smack about the lack of beards here on "Space Time." And sure, they have some pretty luxuriant flavor savers.
 13:05: NASA, this is "Space Time." Tell Mr. Willis to stand down.


20151125: 100 Years of Relativity + Challenge Winners!
 00:36: ... beautiful theory is an essential part of what inspires us here at "Space Time." General relativity's profound description of space and time, of matter ...
 01:08: It inspires us at "Space Time" to try to share those mysteries with you.
 01:24: On December 9, we'll delve deeper than ever into the weirdness of black holes, after which we'll start exploring the nature of matter and time.
 03:41: ... the one relating change in position, average acceleration, and time, with a starting velocity of zero in the frame of Apophis in ...
 06:55: ... tech, you could successfully redirect Apophis given that sevenyear lead time. ...
 07:47: And be sure to join us in two weeks because we're going to learn how to build a black hole on the next episode of "Space Time." [THEME MUSIC]


20151118: 5 Ways to Stop a Killer Asteroid
 02:47: But small probabilities have a way of adding up to a certainty, over time.
 03:28: So we've got some time.
 05:17: Now unfortunately, this doesn't work for planet killers, which are at least a million times more massive.
 08:31: ... hit in the meantime, I'll see you on the next episode of "Space Time." In a recent episode we talked about the origin of life and the Fermi ...


20151111: Challenge: Can you save Earth from a Killer Asteroid?
 00:27: It is currently predicted that it will miss both times, but here's a hypothetical.


20151105: Why Haven't We Found Alien Life?
 00:17: We've asked this question before here on "Space Time," and if you haven't seen that episode you should check it out.
 02:42: ... of the biology here because we can say a lot just based on how much time it took to get through each step on the path to building technological ...
 04:40: ... nature has provided us with a perfect time capsule for studying the very early Earth, zircons super hard silicate ...
 05:51: But either way, it looks like Earth became a slimeball teeming with life in a crazy short amount of time.
 08:29: No, multicellular life evolved independently dozens of times.
 08:34: ... just took a really long time for those single cells to become complex enough to form large ...
 09:17: Maybe it's just time.
 10:20: Well, there's something to work towards on the next episode of "Space Time." Last week we talked about Miguel Alcubierre's warp drive.
 10:37: We cram a lot of details into each episode of "Space Time" and sometimes things like this do slip through, so thank you for catching it.


20151028: Is The Alcubierre Warp Drive Possible?
 00:17: Star Trek warp drives zip around the galaxy at hundreds of times the speed of light.
 00:21: But traveling at the real cosmic speed limit of 1 times the speed of light would make for some pretty dull scifi.
 03:59: Any FTL device can, in principle, be used to make a time machine.
 04:30: Does anyone else get the idea that Stephen Hawking really doesn't want us to build time machines?
 07:09: ... make it so on the next episode of "Space Time." Last week, we talked about gravitational waves, and whether the advanced ...
 08:57: Now, this is a stunning result. But as we say on "Space Time," it's never aliens.


20151022: Have Gravitational Waves Been Discovered?!?
 00:45: There's the slowing of time in gravitational fields.
 00:58: We love Einstein because he's been proven right so many times.
 05:16: Bounce them off mirrors back and forth 400 times before bringing the beams back together.
 05:33: ... paths and lengthen the other, and then vice versa, oscillating with time. ...
 06:14: Well, a gwave leaves a very distinct signature, first contracting one arm while stretching the other, and then oscillating over time.
 07:18: After seeing nothing for a long time, LIGO shut down so it could level up into advanced LIGO.
 07:24: ... pretty insane engineering upgrades make it 10 times more sensitive, which actually means it sees 1,000 times more volume of ...
 08:24: So every time this has happened in the past, the team has been told at the very end, sorry, just a drill.
 09:24: ... hear any more, we'll definitely tell you on a future episode of "Space Time." Last week, we talked about real spaceship options for getting to the ...
 11:05: ... to explore the Earth, born too early to explore the galaxy, born just in time to watch PBS "Space ...


20151015: 5 REAL Possibilities for Interstellar Travel
 01:31: The key is finding the right balance between speed of the starship and the amount of time it would take us to develop the tech to build it.
 03:58: Still, this tech is perhaps the most achievable in the shortest time.
 05:13: We've only been able to do this with small numbers of antiprotons at a time, not the kilograms we'd need to get to the stars.
 05:37: That's something like 50 times more energy per kilogram of fuel than the best fusion options.
 05:52: ... may even be possible to push 0.8C, which would be nice, because then time dilation really kicks in, bringing travel time down to 3.3 years from ...
 07:56: ... light focused in a small enough region would bend the fabric of space time enough to produce a singularity, the Kugelblitz, German for ball ...
 08:24: Such a black hole would radiate nearly 160 petawatts, which is roughly the equivalent of 10,000 times the world power consumption.
 09:17: Honestly, if we had to colonize in the absolute shortest possible time, then it's nukes, like the Orion project.
 09:55: ... explore the galaxy with near light speeds dilating apparent travel time down to human ...
 10:49: We hit warp seven next time on Space Time.


20151007: The Speed of Light is NOT About Light
 02:46: It would imply that space and time and matter don't exist.
 03:56: This tells us that the electromagnetic force holds clues to the fundamental interplay between space, time, and velocity.
 04:06: ... between reference frames the transformation that represents space and time in our ...
 04:28: An example is the Galilean transformation, which basically says that velocities add together and space and time don't depend on velocity.
 05:17: ... who realized that the Lorentz transformation tells us how space and time are connected and that it also predicts the speed of ...
 05:39: This transformation is so profound that it is inevitable based on a few simple statements about the nature of space and time.
 09:12: In fact, the very existence of mass and space and time tells us that the universal speed limit is finite.
 09:19: ... the Lorentz transformation gives us the special theory of relativity time dilation, length contraction, and, of course, mass to energy ...
 09:36: ... conclusions once we have the basic relationship between space and time as described by the Lorentz transformation and we accept Einstein's ...
 09:59: Time dilation and length contraction are infinite.
 10:02: There is no time and space, no cause or effect, because all locations and times communicate with each other instantly.
 10:29: ... so I can see you back here on the next episode of "SpaceTime." Last time on "SpaceTime," we talked about the edge of the universe and ...
 11:17: RedomaxRedomax asks what you would see if you traveled 18 times the distance to the particle horizon to come back to where you started.
 11:26: ... that number, 18 times the particle horizon, only applies if the universe has positive ...
 11:39: ... it won't then you'd get back to your starting point a long, long, long time ...
 11:48: If you travelled at the speed of light, it would take around 750 billion years, or 55 times the current age of the universe.


20150930: What Happens At The Edge Of The Universe?
 00:20: The universe defines all of space and time that exists.
 02:26: Even light takes time to make any journey.
 02:29: So we have to factor in the time interval, especially when space is changing.
 06:46: ... of the universe, you'd need to travel an absolute minimum of 18 times the distance to the particle horizon to get back to where you started, ...


20150923: Does Dark Matter BREAK Physics?
 01:09: And galaxy clusters do this all the time, turning the background universe into a funhouse mirror of stretched out and duplicated galaxies.
 04:35: But they ultimately have a hard time getting all of the observed effects.
 06:46: ... to the familiar standard model particles, but that are hundreds of times more ...
 07:36: ... dark matter particles on the next episode of "SpaceTime." Last time on "SpaceTime," we talked about black ...
 08:02: ... the monkey were to calculate the clock time of an external observer as it fell, then that calculated time would ...
 08:16: It would encompass all future time.
 08:21: The time interval that encompasses all future everything approaches zero, or at least the Planck time.
 08:39: ... monkey to the outside universe can be received at arbitrarily distance times in the future, only signals within its past light cone can catch up to ...
 08:48: The monkey may see some time dilation effects from the local part of its universe.
 09:26: And so in its reference frame, the rate of Hawking radiation is not time dilated.
 10:03: The monkey's horizon crossing corresponds to a time when the black hole exists.
 10:30: And I'll see you next time.


20150827: Watch THIS! (New Host + Challenge Winners)
 00:41: ... out an expression based on those proportionality constants for how much time it takes to do a complete ...
 01:03: Namely, how much time it takes to go and come back.
 01:05: Cut that in half, and you know how much time it takes to reach the other side.
 03:01: Oh, so you're the new host of Space Time?
 05:15: Guys, it's time for me to go.
 05:23: And it's unfortunately time for me to go.
 05:37: So I'll see you guys in a couple weeks for a fresh episode of Space Time.


20150819: Do Events Inside Black Holes Happen?
 01:23: This is called gravitational time dilation and the same thing happens around Earth, just to a lesser degree.
 02:43: But everyone else insisted he never does, even after an infinite amount of time on any of our clocks.
 03:01: ... those events just don't occur, even if we wait an infinite amount of time. ...
 04:13: ... event that makes it into my movie of the history of the world out at time ...
 08:02: Because of time dilation, we would detect any laser pulse that the monkey sends with a lower frequency, i.e.
 08:10: ... just before the monkey freezes from our perspective, the time dilation is so severe that any light he emits gets redshifted to ...
 11:46: ... the challenge questions, this is officially my final episode of "Space Time." Our last full episode dealt with misconceptions about what causes ocean ...


20150812: Challenge: Which Particle Wins This Race?
 00:02: Hey Space Timers.
 00:03: It's time for another Tshirt challenge question.
 00:09: This time around, you're going to need some math, and you'll need to be familiar with high school level physics.
 00:22: ... means clocks run at the same rate everywhere, space and time are two separate things, and gravity is an actual force that masses ...
 01:08: You can also work out how much time it would take to go halfway around the globe.
 01:37: At the same time that the orbiting particle passes this point, let's release the second particle from rest from exactly the same height.
 02:50: ... situations is actually the key to figuring out the travel time of the second particle without using ...
 03:28: Email your answers to pbsspacetime@gmail.com before 5:00 PM New York City time on the date that you see on the screen.
 05:27: As I told you, it's a harder challenge this time.
 05:29: But you also have more time, and both the Newtonian and Einsteinian versions of the question are pretty fun exercises.
 05:45: We'll announce the solution in two weeks' time.
 05:47: But I will see you guys next week for my final episode of Space Time.


20150805: What Physics Teachers Get Wrong About Tides!
 00:00: [MUSIC PLAYING] As we've seen before on "Space Time," gravity affects the motion of all objects identically.
 04:14: ... can't lift something by pulling up on it with a force that's 10 million times smaller than its Earth ...
 05:26: Remember, the radially inward acceleration caused by Earth's own gravity on objects is 10 million times bigger.
 09:35: ... was a lot of overlap between our two talks that I wasn't aware of at the time that we aired our ...
 12:04: A lot of you wanted clarification on how curvature of time is what's responsible for, say, circular orbits around the Earth being geodesics.
 12:14: You can't really break it up into separate space and time curvatures.
 12:39: ... force are just the pieces of the geodesic equation that involve "time" components when projected into that ...
 14:56: And I guess I got lots of time because I'm only going into eighth grade.
 15:00: You got lots of time, Iwon't tellmyname and you're asking all the right questions.


20150729: General Relativity & Curved Spacetime Explained!
 01:26: Remember, humans experience the world and talk about the world dynamically, as things moving through space over time.
 05:04: OK, so Einstein's gravityfree curved space time sounds like it's self consistent.
 05:21: ... but there's one experimental fact that I can use to show you that space time must be curved, just based on what we've seen in this series of episodes ...
 06:15: The excess time is less than a second, but any discrepancy means that clocks are running at different rates.
 06:27: Thus, the very existence of gravitational time dilation, regardless of its degree, requires that spacetime be curved.
 06:36: ... fact, to the extent that we can speak about space and time separately at all, most of the everyday effects on earth that Newton ...
 07:07: ... even though it's hard to visualize, it's curved time that makes the free fall orbits of satellites looks spatially circular ...


20150722: SPECIAL ANNOUNCEMENT + Flat Spacetime Geometry Comments
 00:03: Last week, part 2 of our general relativity series went a little deeper into some aspects of flat space time geometry.
 00:24: ... sad to say that I'm stepping down as the writer and host of "PBS Space Time." This fall, I'll be starting fulltime work at the US National Science ...
 00:31: And while I'd love to keep doing "Space Time," I just won't have the time to do it right.
 00:36: "Space Time" will continue.
 01:14: Without question, "Space Time" has one of the most curious, most cerebral, bestbehaved audiences on YouTube.
 01:38: Let's do some space time questions.
 02:03: But as John Buluba and [INAUDIBLE] pointed out, after you view it a second time, it starts to make more sense.
 02:23: It can save you time if you look at the answers that I've already given that person.
 03:05: ... a space time picture the flat space time picture tells you is that in a world ...
 03:27: And they are distinguishable from noninertial observers in a geometric way in space time.
 03:33: ... my French, and also corrected an error that we had made in the space time diagrams that we showed at Minute 504 in the ...
 03:42: Some of the dots and we had placed in the two red guy and blue guy space time diagrams were placed inconsistently.
 04:32: ... notice that all the space time diagrams that I drew in the last episode were from the point of view of ...
 05:41: ... have another week, now, before we dive into Part 3 curved space time, and, finally, an explanation of Einstein's selfconsistent view of what ...
 05:51: See you next week on "Space Time."


20150715: Can You Trust Your Eyes in Spacetime?
 00:08: So today, on "Space Time," it's spacetime.
 01:42: To record when and where events in this world happen I've got a clock to tell time, nice.
 02:01: To represent this set up in a diagram, let's copy my xaxis onto a blackboard and add a vertical axis to show the time on my clock.
 02:16: I know that seems like an awkward way to record time, but you'll see in a minute why it's convenient.
 03:13: In the same amount of time on my clock, he passes fewer marks on my xaxis.
 03:46: It coincides with my time axis.
 04:34: ... spacetime diagrams are great for visualizing cool phenomena like time dilation, or length contraction, or disagreements between observers ...
 07:16: ... a more standard drawing of motion over time, like what you might see in a physics 101 class, tangent vectors to ...
 07:44: Instead, we want to represent some aspect of dynamical motion over time through space as a static geometric object.
 07:58: ... the monkey's position on my axis relative to the monkey's clock and the time on my clock relative to the monkey's ...


20150708: The Leap Second Explained
 00:52: ... the 18th and 19th centuries using the old definition of a second to keep time. ...
 01:28: That means right now, we're measuring the day and keeping actual time on our clocks using slightly different concepts of a second.
 01:54: Now, over time, Earth's rotation will keep slowing and leap seconds will become more frequent.


20150708: Curvature Demonstrated + Comments
 00:13: So today, I'm going to step back and take some time to address your questions, hopefully in a way that clears things up.
 02:37: ... it up into 10,000 steps, then 100,000 steps, then a million steps, every time, taking very, very, very small vectors and smaller and smaller and ...
 03:08: ... time, I want you to pretend the curve is already drawn, and the ant is going ...
 06:34: ... after that, we'll be putting the notions of curvature and flat space time together to discuss what curved space time is, what the curves and ...
 06:46: All of that will be coming up in the forthcoming episodes of Space Time.


20150702: Can a Circle Be a Straight Line?
 03:26: Mathematicians realized a long time ago that this definition generalizes very nicely and it's also very useful.
 07:51: To prepare for that, you should watch our episode "Are Space and Time an Illusion?" Watch it like 10 times.
 09:25: Indigo said that tracking time in the future might become a challenge if you have to consider relativistic effects.
 09:33: GPS breaks if time dilation isn't taken into account.
 09:36: Time also runs at different rates at different locations on Earth that have different elevation.
 09:40: ... with atomic clocks, this has to be taken into account when you calibrate time systems, or for instance, when you measure the rate at which the Earth ...


20150624: The Calendar, Australia & White Christmas
 00:16: ... parts of the planet will receive more direct sunlight at different times of ...
 04:17: If you look on time scales of tens of thousands or hundreds of thousands of years, Earth's orbit is a lot crazier than you would expect.
 05:17: ... we'll just reckon time in atomic clock seconds or something like the Stargate system on "Star ...
 05:27: If we're a spacefaring species by then, then locking time exclusively to Earth's seasons might be weird.
 05:50: That's our episode, Space Timers.
 06:04: ... usual, I'll report and respond on the next episode of "Space Time." Last week we talked about different possible ways to signal aliens, ...


20150617: How to Signal Aliens
 00:58: ... the total area equivalent to a large city pointed right at Earth full time, our leakage would be hard to detect, let alone ...
 02:23: Nothing natural pulses with stellar brightness for a nanosecond at a time, so laser pulses would look distinctly artificial to our Vulcan friends.
 03:13: You have to keep transmitting for a very long time to increase the odds that someone will see the signal or there's not much point.
 06:07: As always, I'll report back on the conversation on the next episode of Space Time.
 06:55: ... change our concept of the geometry of the world, including the fact that time has a geometric ...
 08:42: ... that are relatively close to the origin of your frame in both space and time. ...


20150610: What Happens to a Helium Balloon in Freefall?
 00:00: [SOUND EFFECTS PLAYING] [MUSIC PLAYING] Hey, Space Timers.
 01:31: And I'll see you next week with the answer to the challenge and a fresh episode of Space Time.


20150603: Is Gravity An Illusion?
 01:02: That's just some XYZ axes to label points in space and a clock to track time.
 01:57: ... other words, the net force on an object will equal that object's mass times its acceleration only if you're measuring that acceleration using an ...
 09:55: We'll tackle that another time.
 09:57: For now, just reflect on Einstein's inspired thinking and how he got there, maybe next time you get in a car or a train.
 10:03: We'll reconvene next time our accelerated paths cross in curved spacetime.
 11:26: I'm a trained astrophysicist and I have a pretty tough time just getting some facts straight for the show.


20150527: Habitable Exoplanets Debunked!
 05:52: So does that mean identifying the only potentially habitable worlds is a waste of time?
 06:41: ... and we'll see what you guys have to say on the next episode of "Space Time." Last week, we talked about e equals mc squared and issued a challenge ...
 06:50: If everyone on Earth picked up a hammer at the same time, by how much would Earth's mass increase due to the excess gravitational potential energy?
 07:26: ... the first five that came in on or after our earliest submission time were Charles Eubanks, who actually got the answer from his wife Carolee, ...
 08:15: This is PBS "Space Time," and we have standards.
 10:26: ... ironing out the details, but it is highly likely that at some time in the future, the SI unit will reflect the physics that I articulated ...


20150520: The Real Meaning of E=mc²
 03:08: This all gets a little bit more complicated in general relativity, but we'll deal with that another time.
 04:53: ... amount of extra light energy that we're not allowing to escape this time. ...
 06:33: They're made of particles called quarks, whose combine mass is about 2,000 to 3,000 times smaller than a proton's or neutron's mass.
 06:48: Every time he says "gluons" in that video, just substitute "quark potential energy," and you'll have a roughly correct picture of what's going on.
 08:06: So without realizing it, you've really been measuring the cumulative energy content of objects every time you've ever used a scale.
 08:58: ... Submit your answers no earlier than 5:00 PM New York City local time on this ...
 09:11: ... also have correct explanations to count, on the next episode of "Space Time." Last week we talked about NASA ...


20150513: 9 NASA Technologies Shaping YOUR Future
 00:21: Problem say you're an astronaut working outside the ISS using a wrench for hours at a time while wearing a bulky spacesuit.
 03:58: Now a really big problem astronauts in micro gravity lose bone density 10 times faster than people on earth with osteoporosis do.
 05:27: ... timing the return time of the laser pulse echos and using image rendering ...
 07:03: Not directly time and space things, something else, but there will be a challenge question posed at the end of next week's episode, so please tune in.


20150506: Should the First Mars Mission Be All Women?
 00:02: 2013 marked the first time that women made up 50% of the NASA astronaut candidate class.
 00:18: Look, it's no secret that we here at "Space Time" love the idea of cloud cities on Venus.
 01:29: ... of them were in orbit six months or less, which is a lot less time than the eight months of zero G in each direction that you'd have on a ...
 04:10: Now, this argument has been made many times over the years, including by some NASA employees.
 06:44: Suppose it turns out that by the time we're ready for the Mars mission we can manage radiation but cannot manage the vision issues.
 07:23: ... report on the evolving discussion on the next episode of "Space Time." Last week we asked what the most realistic artificial gravity in scifi ...
 07:34: We have limited time.
 10:25: The more people we get watching, the better we can make "Space Time."


20150429: What's the Most Realistic Artificial Gravity in SciFi?
 04:35: For example, any time you stand up from a chair, your head has an upward speed of about 1 meter per second.
 06:20: ... about 80% of Earth's radius, a halo insulation would need to rotate 19 times a day to produce 1 g or about 0.015 ...
 07:35: ... 1.3 RPMs, that would give you Coriolis forces that are 10 times smaller than the ones you see in "2001." So that's mostly unnoticeable ...
 08:30: So I'll report any blanks that you all fill in for us on the next episode of "Space Time." Last week, we asked whether space and time are an illusion.
 08:58: We had some originally, but had to cut them for time.
 09:20: Why exactly am I concluding that space and time are illusions?
 09:39: Temporal order is not a universal fact and neither are other "familiar" aspects of time.
 09:43: So is time an illusion?
 09:53: ... present at both events, which corresponds to a zero or negative space time interval between those ...
 10:08: ... when we disagree about sequence, we're really agreeing that the space time interval between them is positive and that neither event could have ...
 10:15: I did not address the direction of time in this episode.


20150422: Are Space and Time An Illusion?
 00:00: [INTRO MUSIC] Today's episode is about space, time, and the nature of reality.
 00:09: ... name is Gabe, and this time, it really is "Space Time." [THEME MUSIC] If you pay attention, this ...
 00:31: Give up your intuitions about how time and space work.
 00:46: Spacetime refers to whichever external reality underlies our collective experiences of the space between things and the time between events.
 00:55: Why can't space and time just be a reality?
 01:08: Fact those observers don't agree about how much time passes between events.
 02:25: Or phrased another way, if time and space as we usually conceive of them aren't part of objective reality, then what is?
 02:52: ... relative motion will measure different distances and different elapsed times between the same two events, they always agree about the spacetime ...
 03:34: ... In other words, even though we can't agree about past, present, future, time, or distance, we all appear to agree about ...
 03:53: Normally we think that time is responsible for causality.
 04:25: So he proposed the following radical idea maybe reality is not a three dimensional space that evolves in time.
 04:40: No time.
 05:08: In contrast, our experiences and measurements of time and space don't correspond to anything, per se.


20150415: Could NASA Start the Zombie Apocalypse?
 04:29: I will report the best ones on the next episode of "Space Time." Last week's episode was about farting your way to the moon.
 05:45: ... me to a paper by an MIT physicist indicating that in a curved space time, you may actually be able to do this actually have reactionless was ...


20150408: Could You Fart Your Way to the Moon?
 04:54: That's 500 times more than a day's worth of gas.
 04:59: About 500 to 700 times higher recoil velocity, a little over 77 meters per hour, or enough to get you back to the ISS in under eight minutes.
 05:58: ... always, I'll report any findings on the next episode of "Space Time." Last week I considered whether the moon in Majora's Mask might harbor a ...
 07:15: It turns out that time to disassembly versus time to impact is a pretty tricky issue.
 07:35: But it would still take ten thousand trillion trillion times the current age of the universe to do so.
 07:57: Keep it up, and I'll see you guys next time.


20150401: Is the Moon in Majora’s Mask a Black Hole?
 03:25: For instance, the tidal force from Earth's moon on you right now it's about 10 million times smaller than Earth's pull on you.
 04:00: ... force when hovering just above the planet's surface would be 200,000 times smaller than the moon's current measly tidal force on ...
 04:22: ... Termina's moon to be wait for it between a billion and 100 trillion times denser than Earth's moon, depending on whether you think the tidal force ...
 04:48: So a trillion times the density of Earth's moon that's ridiculous.
 05:23: ... that's a quadrillion times more massive than MatPat's estimate, but still millions of times less ...
 06:39: For example, gravity would be billions of times stronger on Termina's moon than on Earth.
 07:00: ... you come up with or conclusions you find on the next episode of "Space Time." Last week we talked about why the cosmic microwave background once made ...
 08:14: Tharks asks whether given enough time, the CMB will eventually redshift into the FM radio band.
 08:49: With each passing moment of time, any observer sitting anywhere will see photons that were emitted from progressively more distant locations.
 09:27: But once I do, if they check out, I'll report the answers here on "Space Time." [MUSIC PLAYING]


20150325: Cosmic Microwave Background Explained
 03:58: With no more free electrons to redirect the light, the universe became, for the very first time, transparent.
 05:24: I'll tackle as many as I can on the next episode of "Space Time." Last week, I challenged you to stabilize a gyrodriven Star Fox barrel roll.


20150318: Can A Starfox Barrel Roll Work In Space?
 06:04: In fact, automotive engineers are trying to make flywheels the braking mechanism in cars to avoid losing energy to friction every time you saw.
 07:00: ... out people who submit correct answers on the next episode of "Space Time." Last week, we talked about what might destroy planet Earth, and you guys ...
 09:05: But even if Ceres is an asteroid, it's only about three and a half times as massive as 4 Vesta, still not enough energy to destroy the Earth.


20150311: What Will Destroy Planet Earth?
 01:24: Almost certainly no, but even if there was, humanity has neither the time nor inclination to make one quadrillion of anything.
 02:15: But asteroids orbit the sun about 35 times slower than that.
 06:42: We'll see what you all came up with on the next episode of "Space Time".
 06:46: Last time, we talked about occupying Venus, and you guys had a lot to say.


20150304: Should We Colonize Venus Instead of Mars?
 00:11: [MUSIC PLAYING] Going to Mars has been a fixture in our collective cultural consciousness for a very long time.
 01:45: It's closer to the sun, which means about four times more available solar power then you have on Mars.
 02:15: In Earth orbit, astronauts lose bone mass at about 10 times the rate of someone with advanced osteoporosis.
 06:07: If we start a grassroots movement, I'll let you know on the next episode of "Space Time".
 07:19: And to Brandon Spears, sure, we could always use help here at "Space Time".


20150225: How Do You Measure the Size of the Universe?
 01:23: That's the maximum amount of time light has had to travel to us.
 03:39: So it spends more time in the expanding space, in the rising dough.
 04:21: Remember, as I said a long, long time ago, we first need to get the universe's age.
 04:54: ... beams would travel through the expanding space and reach us at different times. ...
 05:32: ... interesting threads from that conversation on the next episode of "Space Time." Last week, we asked whether it's irrational to believe in ...


20150218: Is It Irrational to Believe in Aliens?
 03:40: ... not only enough places for intelligent life to arise, but also enough time for at some of that life to spread around the ...
 04:11: Our more advanced future selves might need even less time to do this.
 06:53: I'll report any interesting findings on the next episode of "Space Time." Last week we asked, what planet is Super Mario World?


20150211: What Planet Is Super Mario World?
 00:10: My name is Gabe, and this is "Space Time." [MUSIC PLAYING] So how can we understand Mario's crazy jumping ability?
 01:19: ... reach during a jump from the surface of that planet, and the amount of time it takes you to reach that ...
 01:29: g equals twice the height divided by the square of the rise time.
 01:37: ... the formula to measure g on "Super Mario World." All we need to do is time one of Mario's jumps and measure the height of that ...
 01:45: Time, we can measure with a simple stopwatch.
 02:22: Timing the jump is trickier, though, because it happened so fast.
 02:25: ... ended up timing 15 successive jumps, dividing that by 15 to get the upanddown time for ...
 02:51: That means "Super Mario World" has about eight times the surface gravity of Earth.
 03:33: Remember, gravity is eight times stronger on Super Mario World than it is on Earth, yet Mario jumps much higher than we can on Earth.
 04:00: On a planet with eight times Earth's surface gravity, your blood is eight times as heavy.
 04:46: Even on Jupiter, g is only 2 and 1/2 times or so Earth's value.
 04:59: ... the surface gravity, and you find those with values that are many times Earth g's are thought to be gas ...
 05:55: I'll report any interesting discoveries on the next episode of "Space Time." And hey, if you like space stuff, please subscribe.
