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2022-03-16: What If Charge is NOT Fundamental?

  • 09:52: ... can have right-handed chirality if their spin is clockwise relative to their momentum vector and left-handed chirality if it's ...

2022-02-10: The Nature of Space and Time AMA

  • 00:03: ... same answers and we did a recent episode on how it is the slowing of clocks as you descend into a gravitational field actually causes uh our ...

2022-01-27: How Does Gravity Escape A Black Hole?

  • 09:36: According to relativity, that means its clock slows.

2021-11-02: Is ACTION The Most Fundamental Property in Physics?

  • 06:47: ... relativity, clocks run at different rates depending on things like relative speed and ...

2021-10-13: New Results in Quantum Tunneling vs. The Speed of Light

  • 10:35: For a real, physical experiment, we need a clock that’s physically measurable.
  • 10:40: In 2020, a paper was published in the journal Nature that used the swiveling axis of a particle’s quantum spin as the clock hand.
  • 10:59: The rate of rotation can be used as an internal clock.
  • 11:38: They were just trying to verify whether using spins as an internal clock would work at all, and they were successful.
  • 11:50: ... for the faster-than-light Hartman effect, because the spin-based clocks that they were working with should still show the effect under FTL ...

2021-07-21: How Magnetism Shapes The Universe

  • 07:43: ... circularly polarized - and it can be left- or right-polarized, meaning clockwise or counterclockwise ...

2021-06-16: Can Space Be Infinitely Divided?

  • 03:14: ... distance measure has an uncertainty because   you can only clock the instant of the return to within one wave-cycle of the ...

2021-05-19: Breaking The Heisenberg Uncertainty Principle

  • 10:03: ... demonstrated this same principle in other systems like entangled atomic clocks, which may one day massively enhance the precision of our GPS ...

2021-03-09: How Does Gravity Affect Light?

  • 03:56: ... process that generates a photon can be thought of as a clock - be it an electric charge pulsing up and down a radio antenna, or an ...
  • 04:10: These are the ticks of a clock, and the frequency dictates the frequency and the wavelength of the photon produced by that motion.
  • 04:18: But from a great distance away, those clocks run slow, and so the frequency of light emerging from within a gravitational field is lower.
  • 04:37: ... horizon of the black hole, gravitational time dilation is so strong that clocks stop and the frequency of photons trying to escape is brought to ...
  • 06:15: Well, tricky - because there’s no change in clock speed in the horizontal direction - only the vertical.
  • 06:22: But we saw in our previous episode that a gradient in clock speed across the vertical extent of the object is enough to drag it downwards.
  • 10:19: ... because of two effects: your clock is ticking faster than clocks in the gravitational field, and space ...

2021-02-24: Does Time Cause Gravity?

  • 00:25: Clocks run slow in gravitational fields.
  • 01:05: We know that gravity must cause clocks to run slow on the basis of logical consistency.
  • 01:11: And we know that gravity DOES cause clocks to run slow based on many brilliant experiments.
  • 02:46: We can show this as a bunch of identical clocks.
  • 02:51: Clocks closer to the Earth take longer to tick for every tick on a distant clock.
  • 03:32: We can think of any object as being made of many tiny clocks.
  • 03:41: And each of those clocks has a velocity vector in time.
  • 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: ... have a 4-velocity, which is defined according to the ticking of your own clock - your proper time - which is zero for the timeless ...
  • 06:21: To us currency exchange looks favorable for space - a teapot gains a rapid plummet to its doom for an imperceptible slowing of its clock.
  • 07:29: It’s enough to imagine clocks that are infinitesimally separated and we still have our time gradient.

2021-02-17: Gravitational Wave Background Discovered?

  • 00:00: ... to that individual pulse so nature has graced us with these perfect clocks scattered across the galaxy but how do we use them to detect ...

2021-02-10: How Does Gravity Warp the Flow of Time?

  • 01:37: Mass and energy change the lengths of rulers and the speeds of clocks - and somehow those changes lead to objects being attracted to each other.
  • 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.
  • 03:48: Einstein’s thought laboratory - his gedankenlab - was filled with many incredible imaginary devices, but one of his favorites was the photon clock.
  • 04:07: The photon clock represents the simplest possible clock, and anything that we conclude for it also applies to any other clock.
  • 04:23: ... amount of time taken for one tick of the photon clock is the distance the photon travels divided by its speed - so twice ...
  • 04:40: They see the photon clock ticking, but the photon travels a longer path.
  • 05:09: Add an identical but stationary photon clock.
  • 05:12: It seems to tick more than once for a single tick of the moving clock.
  • 05:25: For an observer in the moving lab, it appears that the stationary clock is ticking slow.
  • 06:29: We have a photon clock in the lab and an identical one with the physicist.
  • 06:35: One tick of either clock is very short, which means that over that interval the lab moves only a tiny arc of the full circle.
  • 06:55: But after a full revolution, both observers ask each other how many ticks their clock ticked.
  • 07:01: And it turns out that the stationary clock did tick more - time slowed for the rotating case.
  • 07:27: In the twin paradox, the twin traveling to a nearby star and back has aged less even though both could see the other’s clock ticking slowly.
  • 07:42: ... point, so that she misses a bunch of the ticks of her brothers clock. ...
  • 08:15: ... lines of constant time for the moving clock tilt back and forth, and as that line tilts it fast-fowards over the ...
  • 08:31: The photon in the accelerating clock has to chase the upper mirror some, increasing the distance it needs to travel.
  • 11:14: So that photon clocks and matter do evolve more slowly in gravitational fields.

2021-01-19: Can We Break the Universe?

  • 01:09: ... relativity tells us that the clocks on the spaceship will appear to tick more slowly from the point of view ...
  • 01:35: That means it sees clocks back on the Earth ticking more slowly, and the Earth and the distance traveled being squished.
  • 02:08: ... observer on Earth thinks the astronaut’s clock ticked slow, but the astronaut thinks they traveled a shorter distance - ...
  • 03:31: According to the twin back on Earth, his sister’s clock has been ticking slower than his own.
  • 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?
  • 06:11: Both twins still perceive the other’s clock to be ticking slower - so which twin is older when they reunite?

2020-12-08: Why Do You Remember The Past But Not The Future?

  • 01:43: ... also have internal clocks that give a sense of time elapsed, but to understand why the ...
  • 07:56: Just rewind the clock to the moment before the asteroid’s very last interaction with the outside universe.

2020-10-13: Do the Past and Future Exist?

  • 02:08: Newton assumed that all particles, all observers, all points in space were ruled by a single, constantly ticking clock.
  • 02:16: ... universal clock meant it was possible to define a notion of “now” that everyone would ...

2020-07-28: What is a Theory of Everything: Livestream

  • 00:00: ... general in in the 20th century is that it was this really kind of like clockwork uh uh you know progression of sort of like strange observation uh ...

2020-06-22: Building Black Holes in a Lab

  • 01:29: ... study analog black holes - and by analog, I don’t mean old fashioned clockwork black holes - I mean analogies. Physical systems that aren’t black holes ...

2020-06-15: What Happens After the Universe Ends?

  • 03:28: Remember, the universes contain only light - no observers and no clocks.
  • 04:06: To really compare the sizes of two chunks of spacetime we need to grid them up with rulers and clocks.
  • 05:36: ... interval, and it’s equal to the amount of time that passes on the clock of the traveler - or the proper time of the ...
  • 05:48: So it turns out that we grid up the universe by the rate of ticking of the clocks of its travelers.
  • 05:55: But what if the universe has no clocks?
  • 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.
  • 06:20: As Roger Penrose puts it: in order for time, and hence space to be meaningful, a universe must be able to build a clock.
  • 06:26: A clock must see the spacetime grid - and to do that it must travel at sub-light speed.
  • 06:32: And in order to do that, the clock must have mass.
  • 06:36: ... if you have even a single electron in the universe you can build a clock and can tell the difference between the one light-second and the billion ...
  • 07:27: The photons and gravitons are massless - you can’t build clocks with them.

2020-05-04: How We Know The Universe is Ancient

  • 03:24: ... universe is expanding now, then in the past it was smaller. Rewind the clock according to the raw Friedman equations and there’s no alternative - the ...

2020-04-28: Space Time Livestream: Ask Matt Anything

  • 00:00: ... reading I'm building a concrete coffee table I'm working on this crazy clock puzzle that's one there going to be this cool wooden clock it's taking ...

2020-04-22: Will Wormholes Allow Fast Interstellar Travel?

  • 02:50: ... were accelerated into a circular path at near the speed of light, its clock would freeze compared to the other end. By entering the non-moving end ...

2020-04-07: How We Know The Earth Is Ancient

  • 06:07: ... the progress of evolution by mapping the fossil record to the geological clock. The ordering of the appearance of ancient species is found when we date ...

2020-03-31: What’s On The Other Side Of A Black Hole?

  • 01:53: ... So if they try to trace a path across the horizon in terms of their own clock, the moment of crossing never happens. It’s like Achilles chasing the ...

2020-01-27: Hacking the Nature of Reality

  • 02:16: ... search for the underlying clockwork of reality led to quantum field theory, in which all particles are ...

2020-01-20: Solving the Three Body Problem

  • 00:20: ... transformed our erratic-seeming cosmos into a perfectly tuned machine of clockwork predictability. Given the current positions and velocities of the bodies ...

2019-10-21: Is Time Travel Impossible?

  • 01:45: The spaceship’s clock slows down as it speeds up, and it stops completely at the speed of light.

2019-09-23: Is Pluto a Planet?

  • 03:19: That same motion almost perfectly reflected the clockwork predictions of Newtonian mechanics.

2019-07-18: Did Time Start at the Big Bang?

  • 04:20: So when I talk about rewinding the expansion, I mean running the clock backwards to track a shrinking scale factor.
  • 06:02: ... get that you can't think about the universe as having one big clock that Rewinds and then winks out of existence of the Big Bang or into ...

2019-07-15: The Quantum Internet

  • 10:57: ... distributed quantum computers, as well as achieve new levels of atomic clock synchronization and extreme precision in our interferometric ...

2019-05-01: The Real Science of the EHT Black Hole

  • 04:45: It can do this because the telescopes are synchronized with atomic clocks.
  • 10:01: ... the jet and we have the rotation direction of the black hole – roughly clockwise from our perspective, with an axis pretty close to our line of ...

2019-01-24: The Crisis in Cosmology

  • 15:22: The T in CPT symmetry isn't a literal rewinding of the clock.

2019-01-16: Our Antimatter, Mirrored, Time-Reversed Universe

  • 00:29: ... in the mirror for it to be parity symmetric he invites us to imagine a clock in a mirror ...
  • 00:42: ... and then he asks us to imagine building that same mirror clock in reality everything is constructed as though reflected - numbers get ...
  • 02:02: ... detector is placed to intercept those electrons and the clock ticks with every captured electron. In our reflected clock we need to ...
  • 03:02: ... what a switch to antimatter means how does this work in our Antion on a clock well antimatter atoms have negatively charged nuclei which means their ...
  • 10:26: ... to Feynman diagrams maybe that's why he was so into building antimatter clocks. So, yeah the universe is not symmetric under this simple version of T ...
  • 10:54: ... broken the CPT theorem looks safe Feynman's mirror reflected antimatter clock will work just fine but as well as ticking backwards every atom every ...

2018-12-20: Why String Theory is Wrong

  • 16:28: ... filming that episode in a mirror universe apparently balls were spinning clockwise when I said counter clockwise left became right and vice-versa on ...

2018-12-12: Quantum Physics in a Mirror Universe

  • 00:02: ... flipping the sign of the electric charge and time reversal sending the clock ticking backwards we'll come back to both of these they are going to ...

2018-07-04: Will A New Neutrino Change The Standard Model?

  • 03:50: Helicity can be right-handed, which means clockwise rotation, or left-handed or anticlockwise.

2018-05-23: Why Quantum Information is Never Destroyed

  • 02:33: If they allow us to rewind the clock and figure out a single unique history.

2018-05-09: How Gaia Changed Astronomy Forever

  • 05:53: ... current velocities and positions of the stars, we can actually wind the clocks backwards and forwards, to see where they came from and where they're ...

2018-04-18: Using Stars to See Gravitational Waves

  • 03:35: ... orbits just before merger, which for black holes and neutron stars clocks in at a few to maybe 1,000 orbits per second in the last ...
  • 05:41: ... resulting in a sequence of flashes more regular than an atomic clock. ...

2017-11-08: Zero-Point Energy Demystified

  • 06:59: Vacuum energy is real, and it's part of the fundamental clockwork of the universe.

2017-09-28: Are the Fundamental Constants Changing?

  • 09:26: We're also trying to develop atomic clocks accurate enough to track changes in Alpha in real time.
  • 10:50: ... of telescopes, more refined cosmological models, and better atomic clocks will also help scientists shave down those experimental errors little by ...

2017-08-16: Extraterrestrial Superstorms

  • 00:43: Meanwhile, the fastest winds ever measured, clocking 1,500 miles per hour, once raged in Neptune's Great Dark Spot.
  • 02:17: ... a clockwise-spinning surface, there's a pull, a Coriolis force, to the left, while on a ...
  • 02:34: The result is a raging vortex in the same direction as the Earth's rotation, clockwise in the southern hemisphere and counterclockwise in the north.
  • 03:42: However, that vortex rotates in the opposite direction, clockwise in the north and counterclockwise in the south.

2017-08-10: The One-Electron Universe

  • 04:43: ... actually goes backwards, just that if you reverse the ticking of the clock in the particles coordinate frame, its direction of motion appears ...
  • 05:15: T is time reversal, changing the direction of the coordinate clock.

2017-07-12: Solving the Impossible in Quantum Field Theory

  • 13:41: In Schrodinger's equation, all of the particles are tracked according to one universal master clock.
  • 13:48: ... approach, each particle is tracked according to its own proper time clock, which can vary in its tick speed depending on how fast the particle is ...

2017-07-07: Feynman's Infinite Quantum Paths

  • 04:58: In relativity, it's proportional to the proper time, so the time measured by the clock on a given trajectory.

2017-06-21: Anti-Matter and Quantum Relativity

  • 01:56: ... evolution of a particle's wave function according to one and only one clock, typically the clock in the reference frame of the ...

2017-03-29: How Time Becomes Space Inside a Black Hole

  • 04:32: It carries you with it and drives your personal clock forward as it does so.

2017-03-22: Superluminal Time Travel + Time Warp Challenge Answer

  • 01:54: ... in time at a constant rate for all observers, governed by one global clock. ...
  • 02:16: Einstein showed us that there is no universal clock.
  • 02:19: Instead, every space time traveler carries their own clock.
  • 02:22: The tick rate of your clock and your perception of simultaneity depends on your velocity.
  • 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.

2017-02-02: The Geometry of Causality

  • 01:31: An observer I leave behind with an amazing telescope, observes me traveling the entire original distance but will perceive my clock as having slowed.
  • 01:51: Everyone agrees on the number of ticks that occurred on everyone else's clock.
  • 02:00: ... time measured by a moving observer on their own clock is called proper time, but counting those clock ticks isn't the best way ...
  • 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.
  • 03:48: My world line is only through time, and the tick marks on the time axis correspond to my own proper time clock ticks.
  • 04:03: To me, their clocks tick slow.
  • 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].
  • 05:33: ... every future tick of my clock, a signal arrives from the left and the right, and I use that to build up ...
  • 05:44: Our traveler does the same thing, but from my point of view, their clock is slow, so I see them register signals at a different rate.
  • 07:33: ... told you that these contours show where clocks moving from the origin reach the same proper time count, but more ...

2017-01-19: The Phantom Singularity

  • 08:37: No clock ticks can't ever happen there.
  • 09:08: An object at the event horizon has to change its distance from the black hole to keep its clock ticking.
  • 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.

2017-01-11: The EM Drive: Fact or Fantasy?

  • 09:38: Joel, we're considering buying a lightly used EmDrive with your contribution, only 1,000 light years on the clock, apparently.

2016-09-29: Life on Europa?

  • 12:25: ... with not just inanimate objects running on behind-the-scenes clockwork, but with our behavior-- including our belief that we are free to choose ...

2016-06-01: Is Quantum Tunneling Faster than Light?

  • 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.

2016-02-17: Planet X Discovered?? + Challenge Winners!

  • 03:10: OK, time for the answer to our photon clock challenge question.
  • 03:14: ... asked you whether according to your perception, a clock, or a photon clock, traveling towards you at 50% of the speed of light, ...
  • 03:47: But it also affects clock rate.
  • 03:49: ... see, special relativity tells us that a clock that's moving relative to you will have a slower tick rate compared to a ...
  • 03:59: However, that's based on a sort of instantaneous comparison of the timelines of the two clocks.
  • 04:21: So a photon clock that stationary on the spacetime diagram moves straight up.
  • 04:30: ... a clock that first moves towards the observer and the stationary clock, ...
  • 04:48: To understand that, you need to draw lights like photon paths between the moving clock and the stationary observer.
  • 05:06: When the clock is approaching, its ticks are more clustered than when it's moving away.
  • 05:11: So the approaching clock ticks faster than the receding clock.
  • 05:14: ... this is just a complicated way of saying that the approaching clock sort of chases after its own photons, condensing the distance between ...
  • 05:25: ... the receding clock is backing away from the photons traveling in your direction, stretching ...
  • 05:38: Does the approaching clock appear to tick faster, slower, or at same rate compared to a stationary clock?
  • 05:48: One effect, time dilation, slows down the tick rate of the approaching clock.
  • 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.
  • 06:08: The relativistic Doppler effect will speed up an approaching clock by 1 over 1 minus v on c.
  • 06:20: The length of the observed second decreases by this factor for an approaching clock and increases for a receding clock.
  • 06:28: Our clock is moving at 50% the speed of light.
  • 06:34: The approaching clock appears to tick around 73% faster than your own stationary clock.
  • 06:40: While the receding clock appears to tick 42% slower than your clock.
  • 06:46: ... the time dilation, and not the Doppler effect, you get that the moving clock is always around 13% percent slower than your clock, whether approaching ...

2016-02-03: Will Mars or Venus Kill You First?

  • 08:54: ... imagine a clock that magically appeared at the moment of the Big Bang and somehow ...
  • 09:11: That clock would now read around 13.8 billion years.

2016-01-27: The Origin of Matter and Time

  • 00:37: This clock is a thing.
  • 01:43: You put something-- say this clock-- on this diagram.
  • 02:16: The slope tells you how much position is changing for each tick of the clock.
  • 03:31: But what does this look like if we replace our regular clock with a photon clock?
  • 03:37: Now remember, a photon clock marks time with a particle of light bouncing between two mirrors.
  • 03:42: Each back and forth bounce is one tick of the clock.
  • 03:50: Stationary, the world line of the photon clock looks like this.
  • 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.
  • 04:20: Regardless of the speed of that clock, the internal photons always do those 45 degree paths back and forth.
  • 04:28: On the timeline of the stationary clock, the ticks of the moving clock don't match up.
  • 04:34: The moving clock appears to tick at a slower rate.
  • 04:53: Now that means that in the frame of reference of the moving clock, it is stationary.
  • 04:58: And from that frame, the first clock appears to be moving.
  • 05:01: The whole space time diagram can be transformed to give the second clock's world line a constant location in space.
  • 05:20: So the second clock is still.
  • 05:22: But the first clock is moving.
  • 05:31: The now stationary frame sees the now moving frame as having a slower clock rate.
  • 06:13: First, let's think more carefully about what these clock ticks really are.
  • 06:42: In this analogy, those clock ticks become interactions between the internal parts of our atoms and nucleons.
  • 07:15: Just as with the photon clock, it's only the ensemble that can travel slower than light, or be still.
  • 10:21: What happens when an astronaut does a round trip at a large fraction of the speed of light, and returns to compare her clock to one left on Earth?
  • 10:29: From both perspectives, the other clock was moving, and so should have ticked slower.
  • 10:35: But which clock has the time lag when they get back together?
  • 11:08: In order to compare clocks, the astronaut has to turn around at the end of the journey and come home.
  • 11:33: So the answer is that the astronaut's clock, or the traveling twin, has experienced less time.
  • 12:07: ... in order to fit the whole album into the episode, you need to slow your clock by accelerating uniformly from rest to 99% of the speed of light by the ...
  • 12:17: The start of the song time should sync with the appearance of the photon clock.

2016-01-20: The Photon Clock Challenge

  • 00:12: ... a recent episode, we showed you how the ticking of a clock-- and in particular a photon clock-- slows down if that clock is moving ...
  • 00:21: The photon in the clock has further to travel from your stationary perspective.
  • 00:35: And the resulting slowing of clock ticks due to motion is the time dilation of Einstein's theory of special relativity.
  • 00:50: Imagine observing a clock moving toward you at 50% the speed of light.
  • 00:55: As the light from that clock reaches you, you see the hands of the clock tick.
  • 00:59: Eventually the clock reaches you, passes by, and continues to move away at the same speed.
  • 01:06: ... the clock was moving toward you, do you observe its hands to be ticking slower, ...
  • 01:22: ... the clock is moving toward you, do you see its hands ticking slower, faster, or at ...
  • 01:33: Submit your answers to the email on the screen, using the subject line Photon Clock Challenge Answer.

2016-01-13: When Time Breaks Down

  • 00:35: We have internal clocks, neural processors dedicated to the counting of seconds and hours.
  • 00:57: And a perfectly engineered clock will always tick at the same right.
  • 01:03: Any clock is just an arrangement of matter.
  • 01:28: But do the individual components of the clock feel the same flow of time?
  • 01:36: The most accurate clocks in the world are atomic clocks, which can drift by less than a billionth of a second each day.
  • 01:43: All of the atoms in such a clock feel the same length of a second.
  • 01:59: The faster an object moves relative to you, the slower its clock appears to tick.
  • 02:08: Its clock is frozen.
  • 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.
  • 03:11: Imagine a clock made from two mirrors and a photon bouncing between them.
  • 03:16: Every back and forth bounce of the photon results in a tick of the clock.
  • 03:26: The rate of ticks is consistent, time flows smoothly, until the clock starts moving relative to me.
  • 04:05: ... me, it appears as though the ticks take longer in the moving clock compared to an identical photon clock standing still right next to me, ...
  • 04:15: ... other guy who is riding along with the moving clock would see it ticking at the normal rate, and quite bizarrely, will see ...
  • 04:25: After all, my clock is moving relative to them.
  • 04:35: Now what if the clock is traveling at the speed of light?
  • 04:38: The apparent distance that the photon needs to travel to reach that top mirror becomes larger and larger as the clock speed increases.
  • 04:45: And that distance is infinite when the clock reaches the speed of light.
  • 04:49: From our point of view, the photon clock could never complete a tick because the photon could never reach that mirror.
  • 04:57: ... the way, similar arguments will show us that a photon clock in an accelerating reference frame-- say on a rocket ship in empty ...
  • 05:29: And so clocks must also tick slower the deeper they are in their gravitational field.
  • 05:39: So what does this odd example of the photon clock have to do with real time and real matter?
  • 05:44: Well, if our photon clock behaves this way, then so does the photon box.
  • 06:29: So as an atom races past you at high speed, you would see all its internal bits ticking slower, just like the photon clock.

2016-01-06: The True Nature of Matter and Mass

  • 07:55: Their clocks are frozen.

2015-12-16: The Higgs Mechanism Explained

  • 02:27: Their clocks should be frozen.
  • 02:35: ... of intrinsic quantum spin that we call chirality, and this can either be clockwise or counterclockwise relative to the direction of ...

2015-12-09: How to Build a Black Hole

  • 07:45: Clocks run noticeably slower.
  • 10:05: On our clock, the singularity forms infinitely far in the future.

2015-09-30: What Happens At The Edge Of The Universe?

  • 09:11: No monkeys were harmed in the making of "SpaceTime" and any events that can be consistently assigned to our clocks at PBS.

2015-09-23: Does Dark Matter BREAK Physics?

  • 08:02: ... the monkey were to calculate the clock time of an external observer as it fell, then that calculated time would ...
  • 09:54: ... falling through the event horizon, the monkey's clock, its universe, now contains events that happen at the horizon, including ...

2015-08-27: Watch THIS! (New Host + Challenge Winners)

  • 04:55: You don't know how clocks work.

2015-08-19: Do Events Inside Black Holes Happen?

  • 01:29: ... clocks in high-altitude orbit will get ahead of clocks on the ground by a few ...
  • 02:43: But everyone else insisted he never does, even after an infinite amount of time on any of our clocks.

2015-08-12: Challenge: Which Particle Wins This Race?

  • 00:22: ... means clocks run at the same rate everywhere, space and time are two separate things, ...
  • 04:13: ... if the particles depart simultaneously as measured by the clock at one end of the planet, which one arrives first according to the clock ...
  • 04:44: ... the radial geodesics, and work out who arrives first according to the clock on the other side of the ...

2015-08-05: What Physics Teachers Get Wrong About Tides!

  • 12:21: ... that you take a frame attached to Earth's center, with clocks-- that frame's not going to be inertial-- and then you work out what the ...
  • 13:22: ... a clock as if it runs-- pretending that it runs the same rate everywhere and ...

2015-07-29: General Relativity & Curved Spacetime Explained!

  • 01:33: But even in a world without gravity, we already know that clocks, rulers, and our eyes can all mislead us.
  • 03:22: Inertial frames, that means axes plus clocks, are the spacetime equivalent of the ant's xy grid.
  • 03:47: ... world lines will be geodesics, and their axis and clocks can serve as local inertia frames, provided that we think of them as ...
  • 05:59: Now if spacetime is flat, then clocks on the ground and on the roof should run at the same rate.
  • 06:15: The excess time is less than a second, but any discrepancy means that clocks are running at different rates.
  • 06:59: Remember, a frame consists of axes and clocks.
  • 07:02: And around Earth, spacetime curvature manifests itself in clocks much more than in rulers.

2015-07-15: Can You Trust Your Eyes in Spacetime?

  • 01:42: To record when and where events in this world happen I've got a clock to tell time, nice.
  • 01:55: My clock and my axis together make a frame of reference, which should also have y and z-axes, but I want to keep things visually simple.
  • 02:01: To represent this set up in a diagram, let's copy my x-axis onto a blackboard and add a vertical axis to show the time on my clock.
  • 02:09: ... axis is showing the distance ct that light travels per tick of my clock, which is interchangeable with clock ticks since I know the speed of ...
  • 02:28: Rather, they are events, each of which occurs somewhere along my axis in a one-dimensional physical space and at some moment according to my clock.
  • 02:42: All right, now, some weirdo in a red shirt carrying his own clock and x-axis approaches me from the left at constant speed.
  • 02:49: He passes me just as my clock reads zero.
  • 02:58: Anyway, say I plot the values of ct on my clock as the photon passes different marks on the x-axis.
  • 03:13: In the same amount of time on my clock, he passes fewer marks on my x-axis.
  • 07:58: ... changes in the monkey's position on my axis with respect to ticks of my clock-- that's ordinary velocity-- I'm going to track two hybrid quantities ...

2015-07-08: The Leap Second Explained

  • 00:41: ... 1967, we redefined the SI second again based on atomic clocks since they're more stable, but the atomic clock second was calibrated to ...
  • 01:16: The atomic clock second just inherited this discrepancy.
  • 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.
  • 01:34: Thus, 12:00 PM on our clocks is continually getting ahead of when the sun is directly over us.
  • 01:40: ... and grows to around one second and we insert leap seconds to force our clocks back into sync with the ...

2015-07-02: Can a Circle Be a Straight Line?

  • 09:40: ... since those discrepancies are measurable with atomic clocks, this has to be taken into account when you calibrate time systems, or ...

2015-06-24: The Calendar, Australia & White Christmas

  • 05:17: ... we'll just reckon time in atomic clock seconds or something like the Stargate system on "Star Trek," which, ...
  • 05:40: Before we sign off, I want to comment quickly on the leap second that we'll all be adding to our clocks this coming June 30.

2015-06-03: Is Gravity An Illusion?

  • 01:02: That's just some X-Y-Z axes to label points in space and a clock to track time.

2015-03-25: Cosmic Microwave Background Explained

  • 02:51: To answer that and to see why space used to be orange, we need to turn the clock back to about 400,000 years after the Big Bang, give or take.

2015-03-18: Can A Starfox Barrel Roll Work In Space?

  • 02:19: So say you want to do a barrel roll that's clockwise, as viewed from the rear.
  • 02:30: Now, on a planet, the clockwise and subsequent counter-clockwise torques required to do this are provided by the atmosphere.
  • 03:42: That flywheel is spinning clockwise, as seen from the rear of the ship, or counterclockwise as seen from the nose.

2015-02-25: How Do You Measure the Size of the Universe?

  • 05:03: One of those beams of raisin light would be just switching as the clock hits 13.8 billion years.
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