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2022-12-14: How Can Matter Be BOTH Liquid AND Gas?

  • 14:26: ... superconductors and superfluids; nuclear matter of various types; photonic matter; various spin-based states from ferromagnets to quantum spin ...

2022-12-08: How Are Quasiparticles Different From Particles?

  • 01:44: ... state - say, by thermal vibrations or in the case of solar cells by a photon, at which point the electron is free to move from atom to atom - for ...
  • 06:25: ... - so this makes the phonon a quantum of a sound wave, similar to how a photon is a quantum of light - of an electromagnetic ...
  • 08:29: ... or perhaps quasi-positrons, and we have phonons, which are analogous to photons. ...
  • 08:40: ... a nucleus and electrons bound to that nucleus by the exchange of virtual photons. ...
  • 10:31: In fact, it becomes possible for the phonons to take on another property analogous to the photon - it becomes the carrier of a force.
  • 10:44: Normally we think of electrons as repelling each other via the electromagnetic force - mediated by photons.
  • 12:37: Each electron is spin half, making them fermions, but two electrons have spin 1 - like a photon and that's for reasons we can’t get into.
  • 12:47: But this means they act like photons in that many Cooper pairs can occupy the same quantum state.
  • 14:20: After all, the elementary particles like electrons, photons, and quarks are just excitations in the elementary quantum fields.

2022-11-23: How To See Black Holes By Catching Neutrinos

  • 01:16: ... - so particles of matter rather than force-carrying bosons like the photons of regular astronomy, and neutrino's fermion type is lepton, so they're ...

2022-11-09: What If Humanity Is Among The First Spacefaring Civilizations?

  • 19:21: ... RNG could be correlated with the event that produced the photons, leading to the violation of the Bell inequality even if the photon never ...

2022-10-26: Why Did Quantum Entanglement Win the Nobel Prize in Physics?

  • 06:33: ... would then drop down again, with the lost  energy carried away by photons. ...
  • 06:42: ... zero quantum spin, and which also resulted in  the creation of two photons. ...
  • 06:54: ... in this transition, in order to conserve angular momentum the pair of photons needed to have a total spin of zero, which translates to them ...
  • 07:18: Hidden variable theories, on the other hand,  allow the polarization to be set at the moment the photons are created.
  • 07:25: By measuring these polarizations by passing both photons through polarizers, Clauser and Freedman could perform a Bell test.
  • 08:22: Their orientation was already decided  when the entangled photons were produced.
  • 08:27: So what if that orientation has some influence on the polarization direction of the photons at the moment of their creation?
  • 08:35: ... the photons might carry hidden information about the eventual measurement ...
  • 08:47: To close this loophole it would be necessary to somehow set the measurement direction after the photons were produced.
  • 08:55: That sounds incredibly difficult, because in case you didn’t know photons move pretty fast.
  • 09:22: ... you have to rotate it, but it’s kinda hard to do that faster than a photon can travel across your optical ...
  • 09:54: ... means our entangled photons could be sent to different polarizers depending on an electrical ...
  • 10:10: ... of this means that the photons can’t know how they’re going to be measured at the moment of their ...

2022-10-19: The Equation That Explains (Nearly) Everything!

  • 06:57: ... we have the photon field which is usually represented with a capital A, this is the field ...
  • 07:51: ... need their kinetic energy in every possible direction. Except if two photons come close they'll just pass through each other, but if two say gluons ...

2022-10-12: The REAL Possibility of Mapping Alien Planets!

  • 06:05: ... momentum of the wind, solar sails catch the momentum of light - of photons from   the Sun. More traditional propulsion methods that ...

2022-09-28: Why Is 1/137 One of the Greatest Unsolved Problems In Physics?

  • 01:43: ... process results in the emission of photons  of specific energies that we observe as spectral lines - sharp peaks in ...
  • 04:05: ... the repulsive energy between two electrons is 137 smaller than a photon with wavelength equal to the  distance between the ...
  • 06:02: ... of alpha is the base probability that an electron will emit or absorb a photon, or in the case of two electrons interacting by,  say Feynman ...

2022-09-21: Science of the James Webb Telescope Explained!

  • 04:20: Energetic ultraviolet photons reach us as infrared, stretched and worn out by the journey.

2022-09-14: Could the Higgs Boson Lead Us to Dark Matter?

  • 04:06: ... particles somewhere in space could annihilate to produce gamma ray photons, which could be picked up by telescopes like the Alpha Magnetic ...
  • 06:19: ... also exclude photons, because not interacting with light is the first defining characteristic ...

2022-08-24: What Makes The Strong Force Strong?

  • 06:07: We can think of each charged particle as generating a constant buzz of virtual photons around it, forming what we think of as its EM field.
  • 11:25: This is possible because the mediating particle of electromagnetism, the photon, is itself electrically neutral.
  • 11:32: That means photons can interact with objects without affecting their electric charge, and thus neutral objects can interact with magnetic fields.
  • 11:40: ... that's where the similarities end, because gluons are not neutral like photons. ...

2022-08-17: What If Dark Energy is a New Quantum Field?

  • 09:05: Alternatively, it can be thought of as a fifth energetic component of the universe on top of baryons, dark matter, neutrinos, and photons.

2022-08-03: What Happens Inside a Proton?

  • 01:31: ... electrons and any other charged particle   via photons. We’re going to come back  to a full description of QCD very ...
  • 03:12: ... For example there are various ways the first electron could emit a photon which is absorbed   by the second, or vice versa. Or it could ...
  • 04:17: ... field - emitting and absorbing a virtual   photon. And there’s a set probability of  that happening each time - it’s ...
  • 05:31: ... virtual gluon   of the gluon field rather than virtual  photons of the electromagnetic field.   We can draw Feynman diagrams ...

2022-07-20: What If We Live in a Superdeterministic Universe?

  • 00:26: Photons passing through two slits at once, electrons being spin up and down, cats being both alive and dead.
  • 12:35: ... “random” color of individual photons of that light was used in place of a random number generator to decide ...
  • 12:46: Now, this experiment used the polarization direction of photons rather than the spin direction of electrons, but it’s the same deal.

2022-06-01: What If Physics IS NOT Describing Reality?

  • 10:02: ... “delayed-choice” experiment. This   experiment causes a photon to behave like a wave  or a particle depending on the question ...
  • 10:33: ... up or down, but not left or right,   the wavefunction of the photon they considered  could tell you either which path the photon ...
  • 16:52: ... matter is shrinking because there’s no  stretching of the traveling photons. And finally,   in a universe where galaxies shrink you’d ...

2022-04-27: How the Higgs Mechanism Give Things Mass

  • 02:50: ... electromagnetism, and oscillations in   this field are the photon - our first gauge  boson and carrier of electromagnetic ...
  • 04:37: ... gauge field still has bosons that look a bit like  the photon and the three weak force bosons, but   the latter are still ...
  • 15:06: ... boson manages to escape  unscathed and massless, becoming the photon   that we know and love. It flew free of its  heavier cousins, the ...

2022-04-20: Does the Universe Create Itself?

  • 05:27: ... the double-slit experiment. Here’s one example of this. Imagine a single photon hitting a beam splitter - a semi-reflective mirror that has a 50-50 ...
  • 05:56: ... we add a pair of detectors we see that each photon randomly arrives in detector 1 or detector 2, revealing whether it ...
  • 06:13: ... this case, quantum mechanics states that the photon is in a state of having been both transmitted and reflected until we ...
  • 06:24: ... to scramble the beams so that we don’t know which path brings the photon to detectors 1 versus 2. Now detector 1 always registers a photon and ...
  • 07:08: ... principle, the second beamsplitter could be put in place only after the photons passed through the first. After they’d made their decision of a path to ...

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

  • 02:43: And spin is conserved - flip an electron’s spin and the difference has to be transferred by a photon.

2022-02-16: Is The Wave Function The Building Block of Reality?

  • 11:50: ... measurement, the scientists in Trieste were able to measure single photons emitted from the germanium ...
  • 12:20: After watching the crystal for two months, they had detected a grand total of 576 photons.
  • 17:42: ... a growing black hole means that the event horizon envelopes that final photon, not the object. By some definitions of the event horizon that actually ...

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

  • 00:03: ... time and uh as an example uh we have cosmological redshift okay so a photon of light traveling from one galaxy to the other has to travel through ...

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

  • 06:16: ... force is communicated between charged particles by transferring virtual photons - ephemeral excitations in the electromagnetic ...
  • 07:08: They interact by exchanging a virtual photon.
  • 07:10: Or more precisely, they exchange the sum of all possible virtual photons.
  • 07:16: But those photons don’t follow a well defined path between the interacting particles.

2021-12-10: 2021 End of Year AMA!

  • 00:02: ... for my pronunciation um and the question is when an electron emits a photon and another particle let's say a proton absorbs it what causes the ...

2021-09-21: How Electron Spin Makes Matter Possible

  • 01:13: ... particles are bosons, and they are the force carrying particles like photons with spin 1 or the Higgs particle with spin ...
  • 01:44: ... you like. For example in a laser beam, there’s no limit to the number of photons you can add - all of them in the same quantum state. But not fermions - ...
  • 05:59: ... through space. If you have two such wavefunctions overlapping - like two photons in a laser beam, a shift in one of them by half a wave cycle puts the ...

2021-09-15: Neutron Stars: The Most Extreme Objects in the Universe

  • 02:00: ... pairs are created out of the extreme  energy photons in the magnetic field.   That field then becomes a particle ...

2021-08-18: How Vacuum Decay Would Destroy The Universe

  • 02:52: ... is zero.   For example, for an electromagnetic wave  - a photon - the electric and magnetic   fields rise and fall between ...

2021-07-21: How Magnetism Shapes The Universe

  • 07:35: If the electric and magnetic fields of a collection of photons all tend to point in the same direction, we say the light is linearly polarized.
  • 15:28: ... of a spin-1/2 state we get 2 worlds, but for every measurement of photon number we get countably infinite splits, and for particle position it’s ...

2021-07-13: Where Are The Worlds In Many Worlds?

  • 06:37: ... through the detector, along wires, through computer circuitry, as photons from the screen, as action potentials down our optical nerves, and ...

2021-07-07: Electrons DO NOT Spin

  • 02:25: ... electrons. That came from looking  at the specific wavelengths of photons emitted when electrons jump between energy levels  in atoms. Peiter ...
  • 12:13: ... are called bosons, and include the force-carrying particles like the photon, gluons, etc. These are not described by spinors but instead by vectors, ...

2021-06-23: How Quantum Entanglement Creates Entropy

  • 17:55: ... of   the particle is roughly equal to the momentum of the photon used to measure that position.   F points out that surely we ...

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

  • 01:16: ... Rather came in quanta - chunks of energy that we now call photons.   Planck’s discovery hinges on a single  number that appears in his ...
  • 03:47: ... the minimum intensity is when you have only a   single photon. And because you can’t know exactly how the momentum transfer will ...
  • 04:30: ... photon’s momentum is the Planck  constant divided by its ...
  • 06:02: ... momentum. We keep decreasing the wavelength of our measuring photon - ultraviolet - X-ray -   gamma-ray - which also increases the ...
  • 06:39: ... Let’s replace the mass with the effective  mass of our photon - its energy over c^2,   and the energy of a photon is ...
  • 07:10: ... I hope, it gets interesting. As you pump up the energy of your photon,   reducing its wavelength also reduces the regular Heisenberg ...
  • 07:48: ... the distance across a one-Planck-length   object. You need a photon with a  wavelength smaller than one-Planck-length.   But ...

2021-05-25: What If (Tiny) Black Holes Are Everywhere?

  • 03:08: ... such black holes the Hawking radiation is just photons - electromagnetic waves with kilometers-long wavelengths, so really, ...
  • 05:23: It jiggles in its little crystal lattice cage with very specific vibrational modes, producing photons of specific energies.
  • 05:39: The black hole loses energy one photon at a time, but the process seems smooth and continuous.
  • 06:20: In other words, when you get to the point where a single photon would take away the rest of the black hole’s mass.
  • 13:44: JG 46 wants to know how to build a divide to split photons to make them entangled.
  • 13:57: ... might know that regular laser light is produced when an incoming photon causes an electron in a crystal to drop in energy to produce an ...
  • 14:09: In certain materials known as non-linear crystals, the incoming photon is absorbed and the energy is instantly emitted as two photons.
  • 14:16: Those photons are entangled with each other because various properties are correlated - in particular phase, polarization, and momentum.

2021-05-19: Breaking The Heisenberg Uncertainty Principle

  • 02:31: He hit on it while considering what would happen if he wanted to measure the position of a particle with a photon.
  • 02:37: ... reasoned that the photon would give the particle a momentum kick, which would account for a ...
  • 02:47: ... to measure the position more precisely, you would need a higher energy photon, which would kick the object even harder, causing an even greater ...
  • 07:03: By amplitude I meant the number of photons making up the beam.
  • 08:35: The laser is sent through a special material called a non-linear crystal, which converts incoming photons into pairs of photons.
  • 08:43: Those outgoing photons have entangled phases - the relative positions of their peaks and troughs are correlated.
  • 09:26: ... for the improved phase precision with more uncertainty in the number of photons traveling in your laser ...
  • 09:34: And that introduces its own type of noise - radiation pressure noise as these photons transfer energy to the mirrors in the interferometer.

2021-04-07: Why the Muon g-2 Results Are So Exciting!

  • 04:33: In this theory, electromagnetic interactions result from charge particles communicating by exchanging virtual photons.
  • 04:47: For example, a pair of electrons could repel each other by exchanging one virtual photon, or two virtual photons, or three et cetera.
  • 04:56: All those virtual photons could do something weird like momentarily becoming an electron-positron pair.
  • 05:31: We have an electron being deflected by a single photon from that field.
  • 05:48: ... next simplest is for the electron to emit a virtual photon just prior to absorbing the magnetic field photon, and then reabsorbing ...

2021-03-23: Zeno's Paradox & The Quantum Zeno Effect

  • 06:25: ... the electron is in state 1 when the laser pulse hits, it absorbs a laser photon and jumps to 3, and then immediately drop back down to 1, releasing the ...
  • 06:38: On the other hand, if the atom is in state 2 during the pulse, it would not absorb a photon and the atom would stay dark.
  • 09:11: ... showed that each laser photon perturbed the system in such a way that the electron had an increased ...
  • 09:21: ... quantum Zeno-like freezing you’d need to hit the atom with many, many photons - and that was certainly not a “subtle” ...

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

  • 01:47: But the photon doesn’t experience the flow of time - it doesn’t even have any mass.
  • 03:56: ... process that generates a photon can be thought of as a clock - be it an electric charge pulsing up and ...
  • 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.
  • 04:37: ... time dilation is so strong that clocks stop and the frequency of photons trying to escape is brought to ...
  • 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.
  • 10:41: Of course for someone actually inside the gravitational field, the photon is still traveling at the speed of light as it whizzes past them.

2021-02-24: Does Time Cause Gravity?

  • 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: ... rotated out of the time direction into space - although technically photons and other massless particles don’t have a 4-velocity, which is defined ...
  • 07:39: If photons are already fully rotated into the spatial direction, how is it that they’re also affected by gravitational fields?

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

  • 03:48: Einstein’s thought laboratory - his gedankenlab - was filled with many incredible imaginary devices, but one of his favorites was the photon clock.
  • 03:57: This is a simple pair of perfectly reflective, massless mirrors between which bounces a single photon of light.
  • 04:03: A counter ticks over every time the photon does a full cycle.
  • 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.
  • 04:59: ... the stationary perspective, the photon seems to travel further but it has to keep the same speed - so it ...
  • 05:09: Add an identical but stationary photon clock.
  • 06:29: We have a photon clock in the lab and an identical one with the physicist.
  • 08:31: The photon in the accelerating clock has to chase the upper mirror some, increasing the distance it needs to travel.
  • 11:03: ... the photon - or whatever light-speed quantum components make up matter - actually ...
  • 11:14: So that photon clocks and matter do evolve more slowly in gravitational fields.

2021-01-26: Is Dark Matter Made of Particles?

  • 02:05: Any electrically charged particle experiences the electromagnetic force and can communicate with other charged particles by exchanging photons.
  • 02:21: Neutrinos are unaffected by that force, and so they are quite literally invisible to photons.
  • 08:42: ... particle’ where the electrically neutral superpartners of the Z boson, photon, and Higgs particle, all mix ...

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

  • 13:48: The energy levels are represented by a very small number of photons in a cavity - 0 to 5 - so quite quantum.

2021-01-12: What Happens During a Quantum Jump?

  • 01:22: ... that light is made up of irreducible packets of energy that we now call photons. ...
  • 01:49: Electrons would then jump between energy levels by emitting or absorbing a photon that corresponded to the difference in energy.
  • 05:01: ... rejected the idea of the “photon” as an irreducible energy packet, and even dismissed the notion that ...
  • 05:54: ... consequences…” That was in 1952 - and in 1952 we had never seen a single photon produced by a single quantum jump in a single ...
  • 06:41: If the electron is in level 1, it should jump to level 2 by absorbing a photon from the laser light.
  • 06:46: If the electron then falls back to level 1 it should emit an identical photon in a random direction.
  • 07:06: The individual photons emitted in this process couldn’t be seen - instead the single atom just glowed, or fluoresced.

2020-11-04: Electroweak Theory and the Origin of the Fundamental Forces

  • 02:36: ... particle that transmits the force - the mitichlorian - I mean the photon. ...
  • 05:36: When we quantize that field - when we let it oscillate with discrete packets of energy - we get the photon.
  • 07:22: It had 1 degree of freedom - corresponding to a gauge field with a single mode - the electromagnetic field and its photon.
  • 08:16: The bosons of the version of SU(2) that I just described are simple light-speed oscillations in their fields, just like photons.
  • 08:50: ... example, adding mass to a photon means adding an extra term to the electromagnetic field stuff in the ...
  • 11:48: ... broken - leaving an independent, massless s(1) field for the photon and a massive, broken SU(2) field that gives the massive weak force ...
  • 12:13: Back then, electromagnetism and photons and the weak force didn’t exist.

2020-10-05: Venus May Have Life!

  • 04:07: One possible biosignature in this range is phosphine, which absorbs photons of around 1.1mm wavelength.

2020-09-08: The Truth About Beauty in Physics

  • 09:42: ... the simplest thought experiments - he imagined falling off a roof, or a photon bouncing between mirrors - but the resulting theory predicts black ...
  • 14:58: In that case you'll see all of those photons produced when absorbed light is reemitted - emission lines.

2020-09-01: How Do We Know What Stars Are Made Of?

  • 03:06: ... amount of light we receive at different colors - or in other words, from photons of different energies of ...
  • 03:44: Those are where photons of very specific energies have been plucked out of this thermal light.
  • 03:51: A photon trying to escape from inside the Sun encounters a lot of obstacles.
  • 04:03: Electrons deflect the path of a photon very easily.
  • 04:07: So any given photon has to bounce its way between many electrons before finding its way to the surface.
  • 04:12: ... photon coming from the core of the Sun will be or scattered so many times that ...
  • 04:28: By the time a photon reaches the photosphere it has a 50-50 chance of traveling the final 100km to space without bumping into anything.
  • 04:39: But some photons encounter a new obstacle.
  • 04:49: And if free electrons are good at stopping photons in their tracks, these atoms are even better.
  • 04:54: An atom can absorb a photon if doing so would cause one of its electrons to jump up to a higher energy level.
  • 05:02: The energy of the photon and the energy of the electron jump have to be exactly the same.
  • 05:07: So any photons trying to escape the Sun that happen to have one of these particular energies are going to get sucked up on its way out.

2020-08-10: Theory of Everything Controversies: Livestream

  • 00:00: ... they don't exist in the absence of one so you have in the case of a photon you have the c in which the photon is a wave if you will and then you ...

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

  • 00:00: ... you know um and the electro you can do this with light as well with photons and then what you see is that the electrons deposit themselves as ...

2020-07-08: Does Antimatter Explain Why There's Something Rather Than Nothing?

  • 00:22: ... canceling each other out completely, and leaving only two photons to carry away the energy. And it works in reverse too. Particle and ...
  • 01:25: ... almost everything would have annihilated, leaving a universe full of photons and only very few particles that couldn’t find an annihilation partner. ...

2020-06-30: Dissolving an Event Horizon

  • 05:33: That radiation cian be any type of elementary particle - but in the case of the most massive black holes, it’s mostly just photons.
  • 05:45: In very massive black holes the Hawking radiation has trouble mustering the energy for anything but weak photons.
  • 14:40: ... the case of a universe full of photons - I THINK the idea is that when you rescale both space and time by the ...
  • 15:14: ... mentioned that in conformal cyclic cosmology, photons and gravitational waves can pass the boundary from universe end to new ...

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

  • 02:54: Let’s say these universes contain no matter - only photons - light.
  • 03:39: For those photons, the beginning of their journey is the same as their end, and these universes are equivalent.
  • 06:17: Both space and time lose meaning for a photon.
  • 07:16: ... but it may be the case that we’re left with only a universe of photons, electrons and positrons, and neutrinos, as well as gravitons - the ...
  • 07:27: The photons and gravitons are massless - you can’t build clocks with them.
  • 14:02: It turns out that, as well as photons, gravitational waves should be able to pass between aeons.

2020-05-11: How Luminiferous Aether Led to Relativity

  • 16:14: ... same size as the local bubble of galaxies. Those poor cosmic background photons should have reached us billions of years ago - it's not that they had ...

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

  • 00:55: ... are no rocks from 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 ...

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

  • 00:00: ... Lopez also asks if we can explain how what happens to the energy of photons that are redshifted okay and the restrict of gravitational waves what ...

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

  • 08:22: ... do you see? Light can reach you from the universe behind - those are photons that overtake you heading towards the central singularity. Light can ...
  • 12:02: ... for the spin state. If the electron then interacts with, say, a photon so that the photon and electron spin states are entangled, then that ...

2020-03-24: How Black Holes Spin Space Time

  • 10:27: ... black hole with mirrors. Then you just shine a flashlight at it and its photons pass through the ergosphere again and again, becoming exponentially ...

2020-03-16: How Do Quantum States Manifest In The Classical World?

  • 04:28: ... is the so-called Einstein-Podolsky-Rosen, or EPR paradox. A high energy photon decays into an electron and a positron. These particles both have spin ...

2020-03-03: Does Quantum Immortality Save Schrödinger's Cat?

  • 13:33: ... yes - in a typical quantum eraser experiment you use entangled photon or other particle pairs - one of the pairs goes through a double-slit ...
  • 14:38: Well these days double-slit experiments are usually done with single photons or other particles.
  • 14:54: ... actually, most visible-light photons will travel the length of a typical double-slit experiment without ...

2020-02-24: How Decoherence Splits The Quantum Multiverse

  • 04:04: This time we'll use particles of light - photons as our quantum particle.
  • 04:09: So imagine a single photon traveling to a single spot in the center of the screen.
  • 04:28: The probability of the photon having reached this particular spot is determined by the sum of all possible trajectories to that spot.
  • 05:44: In general we can see an interference pattern if there is coherence between different parts of the photon wavefunction.
  • 05:52: The key in this experiment is that all photons exit the slits with the same phase relationship.
  • 06:06: But it still works if they don’t match exactly, as long as we get the same relative phase offset between the two slits for every subsequent photon.
  • 06:46: We have to say that the photon passed equally through both slits, in what we call a superposition of states.
  • 07:30: ... photon remains in a superposition of states - it passed through both slits AND ...
  • 07:58: The part of the wavefunction - corresponding to a possible path of the photon - is now disturbed by those particles.
  • 08:05: ... can think of that wavefunction slice as the “possible photon” being absorbed and reemitted by those particles, and so the wavefunction ...
  • 08:18: ... random phase offset would just shift the pattern left or right for that photon. ...
  • 08:27: ... that shift would then change for each subsequent photon - new photons land in unpredictable places - so in the end we would just ...
  • 09:04: By the way, this is why any attempt to observe which slit the photon passes through destroys the interference pattern.
  • 09:35: Let’s now leave the slits alone and let the coherent photon wavefunction reach the screen again.
  • 09:43: ... the photon energizes electrons in a pixel on the screen, which results in an ...
  • 09:54: We can think about the photon wavefunction becoming mixed with the wavefunctions of the quantum particles along this chain.
  • 10:34: Imagine just two potential locations for the original double-slit photon.
  • 10:38: Two branches of the wavefunction will represent histories where the photon landed in different locations.
  • 11:00: Perhaps instead we could use that electrical current to generate a new pair of photons, which could then interfere.

2020-02-18: Does Consciousness Influence Quantum Mechanics?

  • 00:36: The rules governing the tiny quantum world of atoms and photons seem alien.
  • 04:17: ... that information travels via photons to light-sensitive molecules in our retinas, which initiate electrical ...
  • 06:35: You know the experiment has been completed with a single photon reaching the detector, and your friend is aware of the result, but you are not.

2020-02-11: Are Axions Dark Matter?

  • 07:53: ... they can still interact with the electromagnetic field and produce photons via the strong ...
  • 08:05: ... do this by generating pairs of virtual quarks which then decay into photons - the so-called Primakoff effect. This would look like an axion turning ...
  • 08:23: ... a strong magnetic field and then blocked by a metal wall. But some photons get converted to axions in the field, and so pass directly through the ...
  • 08:58: ... fields of its own to try to turn those axions back into detectable photons. No luck yet, but the range of possible properties of axions is being ...
  • 09:45: ... that some gamma rays get converted back and forth between axions and photons by the magnetic fields of entire galaxies. That makes them invisible for ...
  • 12:47: ... are indistinguishable from each other - swapping two electrons or two photons doesn't change anything, so that number might be an over estimate. But ...

2020-02-03: Are there Infinite Versions of You?

  • 15:00: Imagine an interaction where an electron emits a virtual photon which then deflects another particles - say, a proton.
  • 15:08: ... exactly the same interaction as if the proton emitted the photon to deflect the electron - in other words, the direction of the flow of ...

2020-01-06: How To Detect a Neutrino

  • 05:05: ... the weak force bosons is that they are massive, ♪ ♪ unlike the massless photon or gluon, which carry the electromagnetic and strong nuclear ...
  • 07:38: ... have perfectly annihilated each other ♪ ♪ leaving a universe of only photons. ...

2019-10-15: Loop Quantum Gravity Explained

  • 12:41: ... that the speed of light should depend very slightly on the energy of the photon, with, for example, high-energy gamma rays travelling a wee bit slower ...

2019-10-07: Black Hole Harmonics

  • 09:53: ... doesn’t matter what fell in to make the black hole – atoms, photons, dark matter, monkeys – all that information should be lost, leaving only ...

2019-08-19: What Happened Before the Big Bang?

  • 12:02: That energy would then end up in the cosmic background radiation photons, but not for a while.

2019-07-15: The Quantum Internet

  • 03:19: We can already send photons of light very long distances using lasers or fiber optics - and those photons are pretty quantum.
  • 03:27: The problem is that to transmit quantum information we have to pay attention to individual photons - quanta of light.
  • 03:34: ... classical information using light, each bit is encoded with many photons, and many can be lost or altered en route without compromising the ...
  • 03:44: If too many photons are lost you can just run the channel through a repeater, which reads the signal and boosts it with extra photons.
  • 03:52: It’s much harder to transmit single photons in a way that perfectly maintains their quantum state.
  • 03:59: And it’s fundamentally impossible to boost that signal by duplicating those photons.
  • 06:50: Qubits A and B could be the polarization states of two photons.
  • 07:03: Now Bill takes photon A and entangles it with photon C using a Bell measurement so that now A and C have opposite polarization.
  • 07:12: Photon B, which was opposite to A, must now be the same polarization as the original photon C.
  • 07:18: ... this point the original quantum state of photon C, which contains the message, has been almost completely teleported to ...
  • 08:01: ... minor technical caveat is that you we are only using photonic qubits then it's not so easy to perform a Bell measurement that will ...
  • 09:07: ... typically means transferring a quantum state between a photon and a matter particle – say, an electron whose up or down spin direction ...
  • 09:27: ... up with a number of ingenious solutions, ranging from storing entangled photon quantum states in a cloud of caesium atoms, a kind of quantum atomic ...
  • 10:07: There are also proposals for removing the need for physical storage all together, with repeaters that are entirely photonic.
  • 10:14: These are great because they’re much, much faster than repeaters that have to transfer quantum states between photons and matter particles.
  • 10:22: So the current state of the art is that entangled quantum states have been transmitted with photons using fibre optics and lasers.
  • 10:29: Some researchers have even succeeded in bouncing entangled photons off a satellite.
  • 10:34: ... photons can then transfer their entangled states into a variety of matter ...

2019-06-06: The Alchemy of Neutron Star Collisions

  • 12:15: ... when electrons were free of their atoms and so could block the paths of photons then it was transparent during the dark ages because electrons bound in ...
  • 13:02: ... was 100^3 times lower than at recombination and so the mean free path of photons was a million times larger in a related question, LobbySeatWarmer asks, ...

2019-05-16: The Cosmic Dark Ages

  • 05:13: ... transparent, but it did block some very particular types of light. Any photon whose energy happened to exactly match an electron energy transition in ...
  • 05:45: ... hydrogen gas flips its spin direction it either absorbs or emits a radio photon with a wavelength of 21cm. When the first stars ignited they heated the ...
  • 06:39: ... different photon tells us about the progress of the epoch of reionization. But before we ...
  • 08:11: ... to the second photon of interest. It’s the Lyman-alpha photon – one with a wavelength of ...
  • 08:30: Neutral hydrogen gas is hungry for Lyman-alpha, gobbling up any such photon that it encounters.
  • 08:36: ... – a quasar shines out from the epoch of reionization. Lyman-alpha photons from that quasar can travel a short distance because the quasar has ...
  • 09:21: ... towards us, but the universe keeps expanding. Wavelength by wavelength, photons get absorbed as they are shifted into the danger zone of Lyman-alpha ...
  • 10:47: ... end of the trough is where reionization ended, so that photons to the left of it could potentially reach us. The jagged region is the ...
  • 12:02: ... extremely sensitive radio telescope to catch more of those elusive 21cm photons. ...

2019-05-09: Why Quantum Computing Requires Quantum Cryptography

  • 04:52: Another example is the polarization of a photon, a quantum of electromagnetic wave.
  • 05:19: An unmeasured photon exists in a state of maybe-vertical maybe-horizontal – a superposition of the two, with maybe a preference for one or the other.
  • 05:32: You can measure the state by, for example, sending the photon through a polarizing filter.
  • 05:39: That act forced the photon to make a choice – first which basis – rectilinear or diagonal – then which actual direction.
  • 05:48: ... for example, pass a randomly-polarized photon through a horizontal polarization filter like a polarized sunglasses ...
  • 06:06: ... that now-horizontally-polarized photon through a vertical filter and it’ll be blocked because its vertical ...
  • 06:46: ... any photon coming out of the first filter has its rectilinear polarization ...
  • 07:43: ... a random string of bits, 0’s and 1’s and encodes these bits using photons polarized in a particular basis, and uses a randomly chosen basis, ...
  • 07:58: These bits are then sent over an open channel to Niels who then randomly picks a basis of his own for each photon and projects onto that.
  • 08:22: Over that same public channel they randomly pick a subset of those bits and Albert reveals which basis was used for those photons, and what she sent.
  • 09:06: That’s because Werner, like Niels, can only pick a random basis each time on which to project the photons.
  • 09:14: ... he picks the right one, the photon state is unchanged, otherwise it will project the photon onto a random ...
  • 10:06: But the chance is 1 in 2 to the power of the number of photons, which quickly gets close enough to impossible given that Werner only gets one shot.
  • 11:04: ... between the two – for example, electrons with opposite spin axes or photons with 90-degree ...
  • 16:09: What was imaged was light escaping from the photon sphere that was produced a the magnetically driven jet.

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

  • 06:17: What we’re seeing here is the black hole shadow inside the bright ring of the photon sphere.
  • 06:24: The photon sphere is where gravity is so strong that light itself can orbit the black hole.
  • 06:29: That orbiting light will eventually leave the photon sphere– either falling into the black hole or escaping outwards.
  • 06:36: We only see light that escapes directly towards us, so the photon sphere looks like a photon ring.
  • 06:47: If the black hole is rotating then you can get photon orbits over a range of distances.
  • 06:53: Measuring the radius of the photon sphere potentially gives you both black hole mass and spin.
  • 06:58: There are two main sources of light feeding the photon sphere.
  • 08:03: So that synchrotron light shines from the jet vortex, it gets trapped briefly in the photon sphere, and then makes its way to us.
  • 08:14: The ring is the photon sphere, blurred due to the fact that this observation is incredibly difficult and the EHT is “only” the size of the Earth.
  • 08:38: ... towards us – or at least forwards before the light gets deflected in the photon ...

2019-03-20: Is Dark Energy Getting Stronger?

  • 07:58: ... works like this: when photons of ultraviolet light radiate from the accretion disk, they bump into ...

2019-02-20: Secrets of the Cosmic Microwave Background

  • 11:14: ... of years the Universe was in the radiation-dominated epoch Basically photons produced more gravity than matter Fluctuations that were small enough to ...

2019-02-07: Sound Waves from the Beginning of Time

  • 02:01: There's also light, in fact, around a billion photons for every electron.
  • 02:06: but no photon is safe from a free electron.
  • 02:17: A photon could barely travel any distance before colliding with an electron.
  • 02:26: We say that in this state, light was coupled with matter, And baryons and photons formed a single strange fluid: A Baryon-Photon plasma.
  • 03:08: ... interaction between the charged particles of the plasma via the trapped photons meant that ripples in the plasma travelled at over half the speed of ...
  • 03:16: Mixed in this soup of baryons and photons was dark matter.
  • 03:38: Okay, so, the universe is filled with this hot ocean of baryons, photons, and dark matter.
  • 04:24: But also at that density peak, the imprisoned photons exerted an enormous outward pressure.
  • 04:52: ... matter became more diffused, and the photons themselves were stretched, redshifted to ower energies, themselves were ...
  • 05:49: As the wave of plasma and photons decoupled, light began to stream freely through the universe as the cosmic background radiation.
  • 11:30: Our understanding of the behaviour of the original baryon photon plasma is excellent.

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

  • 00:02: ... weak interaction into nickel by emitting an electron and some gamma ray photons and neutrinos the cobalt-60 nucleus also happens to have an unusually ...

2018-11-21: 'Oumuamua Is Not Aliens

  • 15:43: It just explained the force carrying bosons like the graviton and the photon.
  • 16:21: Hiccup Haddock asks, why ice, as in why does IceCube use photon detectors in ice instead of any other material.
  • 16:41: That radiation travels a short distance through ice to the nearest photon detector.

2018-11-14: Supersymmetric Particle Found?

  • 04:50: When ultra high energy cosmic rays travel through space, they bump into the photons of the cosmic microwave background.
  • 05:45: For example, the IceCube Observatory is a one kilometer cube of the Antarctic glacier laced with photon detectors.

2018-11-07: Why String Theory is Right

  • 11:57: ... you can only get the right particles, including the graviton and the photon, out of string theory for a very specific number of spatial dimensions, ...
  • 14:26: Uri Nation asks about the photons that mediate the magnetic field or the contact force between two bodies.

2018-10-31: Are Virtual Particles A New Layer of Reality?

  • 01:15: That math hack turned out to represent the very real quantum nature of the photon.
  • 03:25: ... with the EM field, transferring between them energy momentum and one photon worth of quantum properties in a single packet that we call a virtual ...
  • 06:00: One electron throws a virtual photon at the other one causing them to be deflected from each other like a game of quantum dodgeball.
  • 06:12: How can throwing photons between particles cause them to be drawn together?
  • 06:17: Let's look at the fine and diagram of a single virtual photon passing from electron to positron.
  • 06:23: ... of this, you add together the possible effect of every possible virtual photon being emitted by the electron and absorbed by the ...
  • 06:32: Bizarrely, that includes photons that are pointing in the wrong direction to even make the journey.
  • 06:45: And you also count photons emitted by the positron but pointing away from the electron.
  • 06:50: These are the virtual photons that ultimately provide the attractive force.
  • 07:53: Our virtual photon doesn't have a location, so it doesn't travel a real path.
  • 08:03: ... a bit like the photon starts out moving in the wrong direction and then quantum tunnels ...
  • 08:16: No individual virtual photon can be credited with producing the attractive force.
  • 08:22: In fact, you only see that force in the sum of all possible virtual photons over all possible Feynman diagrams.
  • 12:18: ... in the case of Max Planck discovery of the quantum nature of photons, it turned out that a mathematical artifact represented new real ...

2018-10-10: Computing a Universe Simulation

  • 05:55: ... you want to include photons, neutrinos, dark matter, et cetera, and not just atoms, you need to scale ...

2018-09-20: Quantum Gravity and the Hardest Problem in Physics

  • 05:28: You would typically do that by bouncing a photon or other particle off the object.

2018-09-12: How Much Information is in the Universe?

  • 06:17: Neutrinos and photons formed in the big bang are probably a billion times more abundant than protons.
  • 06:24: That's verified experimentally in the case of photons.
  • 06:27: The cosmic microwave background has around 10 to the power of 89 photons across the observable universe.
  • 06:34: So almost all of the information, and for that matter, the entropy in particles is in neutrinos and in the cosmic microwave background photons.

2018-08-23: How Will the Universe End?

  • 06:18: But there are several beyond the standard-model mechanisms that would allow them to decay into positrons, neutrinos, and gamma ray photons.
  • 06:58: The universe will contain only photons, electrons, and black holes.

2018-08-15: Quantum Theory's Most Incredible Prediction

  • 01:07: QED talks about the electromagnetic field whose excitations give us the photon.
  • 07:22: So one way to think about this quantum buzz is with virtual photons.
  • 07:36: In the case of electromagnetism, those interactions are mediated by virtual photons, which are just a mathematical way to describe quantum buzz.
  • 07:45: Every interaction with virtual photons that can happen, does, at least in a sense.
  • 08:17: They represent the possible interactions of the quantum field by way of virtual photons.
  • 08:36: An electron encounters a real photon that could represent an external magnetic field.
  • 08:47: The electron first emits a virtual photon, then gets deflected, then re-absorbs the virtual photon.

2018-07-11: Quantum Invariance & The Origin of The Standard Model

  • 09:23: Those oscillations in our new electromagnetic field turn out to be the photon.
  • 10:12: These are, the gauge bosons, the photon for electromagnetism, the W and Z bosons for the weak interaction, and the gluon for the strong interaction.

2018-06-20: The Black Hole Information Paradox

  • 03:44: The black hole radiates particles, mostly photons, that contain no information.
  • 13:41: ... electric charge given that the electromagnetic field is communicated by photons and photons can't escape the black ...
  • 14:01: But quantum-field theory imagines the electromagnetic force as being transmitted by virtual photons.
  • 14:07: Now it's important to note the distinction between virtual photons and real photons.

2018-06-13: What Survives Inside A Black Hole?

  • 02:28: ... could have formed from a collapsed star or entirely out of antimatter or photons or monkeys, but the only thing we can know about the material that went ...
  • 12:47: It's encoded in the energy, phase, polarization, et cetera of the two gamma-ray photons that are created.

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

  • 12:05: The energy density of photons is much, much lower than the energy density of dark energy.
  • 12:17: Radiation, including photons and neutrinos, dominated the energy density until around 50,000 years after the Big Bang.

2018-05-16: Noether's Theorem and The Symmetries of Reality

  • 01:24: And so the energy of each photon drops.
  • 01:27: Where does the energy from red-shifted photons go?

2018-05-02: The Star at the End of Time

  • 03:09: The layer above the Sun's core is what we call "radiative." All of the energy travels in the form of photons bouncing their way upwards.
  • 05:03: First, the hotter something is, the more thermal photons it produces.
  • 05:08: So increasing the surface temperature allows a red dwarf to shed all of those excess photons produced by its rising fusion rate.
  • 05:17: And rule two, the hotter something is, the more energetic its individual thermal photons.
  • 05:23: The black-body spectrum of a hot object emits relatively more photons at short energetic wavelengths than a cooler object.

2018-04-11: The Physics of Life (ft. It's Okay to be Smart & PBS Eons!)

  • 08:01: The most random possible form for energy is thermal radiation, and the lower the energy of its component photons, the higher the entropy.
  • 11:38: That emission looks like a straightforward quantum process, analogous to photon emission by an accelerating electric charge.
  • 12:44: But the accelerating expansion of the universe will prevent any photons emitted today from galaxies at that distance or beyond from ever reaching us.

2018-04-04: The Unruh Effect

  • 01:57: Einstein taught us that an object without mass, like a photon, can only travel at the speed of light and no slower.
  • 02:38: ... because photons fired from anywhere in the past light cone can reach our observer either ...
  • 02:59: If you wait long enough, photons from anywhere in the universe can catch up to you.
  • 04:03: ... I fire a photon at the point of closest approach, say to send a message, that photon can ...
  • 04:15: The photon will always be inching closer.
  • 04:40: But until that happens, they stay just ahead of my photon.
  • 04:43: They also stay ahead of any photon emitted from this diagonal line or any point on the other side of it.

2018-03-21: Scientists Have Detected the First Stars

  • 01:31: ... do that by looking for a very particular type of photon-- the one that is released or absorbed, when the ground state electron and ...
  • 01:42: That photon has a wavelength of 21 centimeters, which is radio light.
  • 01:47: ... spin flip was in equilibrium with the CMB, meaning that for every CMB photon that was absorbed by the spin flip, another one was ...
  • 02:31: That change in equilibrium meant the gas was suddenly absorbing more 21 centimeter photons, than it was emitting.
  • 02:38: ... gas and eventually, became too hot to emit, or absorb, 21 centimeter of photons at ...
  • 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.
  • 04:44: Colder gas is better at absorbing 21 centimeter photons.

2018-03-15: Hawking Radiation

  • 09:39: By the way, Hawking radiation is mostly going to be photons and other massless particles.

2018-01-24: The End of the Habitable Zone

  • 11:06: Are they photons or what?
  • 11:41: Of course, you would always see photons because photons are massless.
  • 11:45: Those photons would have a perfect black body spectrum.

2018-01-17: Horizon Radiation

  • 03:08: Imagine I fire a pair of photons, which annihilate to produce an electron, positron pair.
  • 03:14: ... a black hole, should agree on the basic result of that interaction-- two photons in, one electron, one positron ...
  • 08:28: ... old ones, for example, to describe a particle interaction like those two photons annihilating into an electron, positron ...

2018-01-10: What Do Stars Sound Like?

  • 12:48: Those particles can then fire photons in our direction in a couple of different possible ways.
  • 12:56: The charged particles spiral around the axial magnetic fields and emit photons as they do.
  • 13:05: ... in the jet bump into existing photons, perhaps synchrotron photons, and scatter them to higher energies, and ...
  • 13:14: In both cases, photons are emitted in different directions.

2017-12-13: The Origin of 'Oumuamua, Our First Interstellar Visitor

  • 10:30: How can a photon's frequency be generalized as momentum?
  • 10:47: But photons have constant speed and no mass.
  • 10:56: That last fact explains the increasing spread in the direction of photons after they pass through a narrowing slit.
  • 11:03: ... increasing our certainty in the location of the photon passing through the slit, we increase the uncertainty in its momentum ...

2017-12-06: Understanding the Uncertainty Principle with Quantum Fourier Series

  • 06:49: In the early days of quantum mechanics, it was realized that photons are electromagnetic wave packets whose momentum is given by their frequency.

2017-11-29: Citizen Science + Zero-Point Challenge Answer

  • 06:57: ... the vacuum energy density comes from assuming that there are no virtual photons above a certain cut of ...
  • 07:06: ... you choose that to be the frequency corresponding to a photon with the Planck energy, you get a vacuum energy density of a ...
  • 07:49: The frequency of a photon with the Planck energy is the Planck energy divided by the Planck constant, or an insane 3 by 10 to the power of 42 hertz.
  • 08:22: That corresponds to a photon wavelength of a tenth of a millimeter, which is in the far infrared part of the spectrum.
  • 08:32: ... us that the simplistic approach of choosing the right maximum virtual photon frequency definitely can't give us the vacuum energy that we see as dark ...
  • 08:42: ... Photons with wavelengths shorter than 0.1 millimeters definitely exist, and we ...
  • 09:06: That proves the existence of virtual photons with wavelengths smaller than the plate separation.

2017-11-08: Zero-Point Energy Demystified

  • 06:41: ... cavity, then they're giving it up again to real particles, probably photons, on the ...
  • 06:50: This would certainly produce thrust, but no more than the rather anemic photonic thruster.
  • 09:09: And is this a possible maximum virtual photon frequency, given the results of Casimir experiments?

2017-11-02: The Vacuum Catastrophe

  • 02:55: What if there's a maximum possible frequency for virtual photons?
  • 03:07: And anyway, there's an almost sensible cutoff for photon frequency.
  • 03:12: It's where photon energy is equal to the Planck energy, or 10 to the power of 19 giga electron volts.
  • 03:24: Until we develop a theory of quantum gravity, we can't say whether the photons above this energy are possible.
  • 03:31: ... if we add up the vacuum energy, including virtual photons, all the way up to the Planck energy, we get a finite number-- a very, ...
  • 06:32: ... basic supersymmetry only allows us to cancel out photons down to the so-called electroweak energy, which brings the predicted ...

2017-10-25: The Missing Mass Mystery

  • 04:14: See, before the photons of the cosmic background radiation were released, they were trapped in the searing hot plasma of baryonic matter.
  • 04:23: The interplay between baryonic and photons resulted in density oscillations.
  • 08:32: As photons from the CMB pass through a giant filament, the hot plasma in the filament grants it a little energy boost.
  • 08:41: In fact, the electrons in that plasma scatter CMB photons to higher energies.
  • 12:16: On particle annihilation, it's given back without producing a real photon.

2017-10-19: The Nature of Nothing

  • 02:15: ... energies, and those oscillations are the electrons, quarks, neutrinos, photons, gluons, et cetera, that comprise the stuff of our ...
  • 04:33: For example, QFT describes the electromagnetic force as the exchange of virtual photons between charged particles.
  • 05:50: For example, the massless photon can have the tiniest of possible energies.
  • 05:55: And so virtual photons can exist for any amount of time, long enough to carry the electromagnetic force to any distance.
  • 08:43: He imagined two conducting plates, brought so close together that only certain virtual photons could exist between the plates.
  • 08:51: ... resonates with waves of certain frequencies, any non-resonant virtual photon would be excluded, reducing the vacuum energy between the ...
  • 09:05: However, on the outer surface of the plates, all frequencies of virtual photon are allowed.
  • 13:32: ... so electrons and quarks, while the force-carrying particles like photons, gluons, et cetera, are spin-1 ...

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

  • 04:41: When electrons move between levels, they emit or absorb photons with energies equal to that lost or gained by the electron.
  • 06:48: When a quasar's light passes through giant clouds of gas on its way to us, elements in those clouds absorb photons to produce spectral lines.
  • 08:26: ... challenge here is that the measurement is really, really difficult. Photons from these extremely distant quasars and gas clouds are massively ...

2017-09-20: The Future of Space Telescopes

  • 08:46: ... light into colorful arcs across the sky, scientists have proposed we use photon pressure to suspend a cloud of tiny reflective particles in Earth's ...

2017-08-16: Extraterrestrial Superstorms

  • 13:17: There should be diagrams with photons connecting across the vertices of the two-vertex photon deflection diagram, like this.

2017-08-10: The One-Electron Universe

  • 02:50: The direction of an electron's worldline can shift as the electron is scattered by photons.
  • 06:34: ... example, this one diagram for electron and photon scattering represents both the double deflection of an electron or of a ...
  • 09:44: ... an electron influencing each other's momentum by exchanging a virtual photon-- similar to electron-electron ...
  • 09:53: ... and positron actually annihilate each other, producing a virtual photon, which then creates a new electron-positron ...

2017-08-02: Dark Flow

  • 03:55: As the photons of the CMB pass through that plasma, they steal a little bit of its energy.
  • 04:24: ... Hubble flow, then the SZ effect adds an extra Doppler shift to the CMB photons that pass through that ...
  • 10:04: ... vertex, representing interactions between an electron, positron, and photon, is not by itself a valid ...
  • 10:35: For example, in order to conserve momentum, an annihilating electron and positron must produce two photons, not one.

2017-07-26: The Secrets of Feynman Diagrams

  • 02:12: That means interactions between electrons; their anti-matter counterparts, the positron; and photons.
  • 02:34: The photon is shown as a wavy line.
  • 02:36: Time direction is irrelevant for the photon.
  • 03:08: ... QED-- one with an arrow pointing in, an arrow pointing out, and a single photon ...
  • 03:31: ... upwards, this vertex represents an initial electron that emits a photon, after which, both particles move off in opposite ...
  • 03:42: But if we rotate this vertex so that photon is coming in from below, we have a picture in which an electron absorbs that incoming photon.
  • 03:52: The photon vanishes as its momentum is completely transferred to the electron.
  • 03:56: ... again, and the picture is of a photon coming in and giving up its energy to produce an electron-positron pair, ...
  • 04:07: ... again, and now we have a positron absorbing a photon, and a positron emitting photon, and finally, an electron and a positron ...
  • 05:01: So a zero charge photon must leave.
  • 05:04: Similarly, if a photon creates a negatively charged electron, it must also create a positively charged positron.
  • 07:13: Simple examples are the exchange of a single photon to transfer momentum between electrons, or the exchange of two or more photons.
  • 07:21: ... as many of these vertices as we like, including the electrons exchanging photons with themselves at different stages in the process, or photons ...
  • 08:16: ... example, for two electrons exchanging a single photon, it doesn't matter if we draw the photon going from the first to the ...
  • 08:28: We can think of the differences just being the photon traveling forward in time in one case and backwards in the other.
  • 08:45: An incoming electron and an incoming photon bounce off each other.
  • 08:49: One way that can happen is for the electron to emit a new photon and later absorb the old incoming photon.
  • 08:56: ... take, as long as they lead to producing the same final electron and photon. ...
  • 09:25: Instead of an electron emitting and absorbing a photon, we have on one side that incoming photon creating an electron-positron pair.
  • 09:37: But the positron annihilates with the incoming electron to produce the outgoing photon.
  • 10:41: The most important Feynman diagrams for Bhabha scattering are the two cases involving a single virtual photon.
  • 11:12: For the latter, don't bother with what we call the self-energy diagrams, in which electrons or positrons emit and then reabsorb a photon.

2017-07-19: The Real Star Wars

  • 05:05: The excited electron releases its energy is a photon that exactly matches the phase and direction of the seed photon.

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

  • 02:13: Vibrations in the EM field are called photons, what we experience as light.
  • 02:35: One electron excites a photon, and that photon delivers a bit of the first electron's momentum to the second electron.
  • 02:43: It's arguable exactly how real that exchanged photon is.
  • 02:49: In fact, we call it a virtual photon, and it only exists long enough to communicate this force.
  • 03:31: They exchange a virtual photon-- this squiggly line here-- and the two electrons move apart at the end.
  • 03:58: ... squiggle represents the quantized fueled excitation of the photon, and the connecting points, the vertices, represent the absorption and ...
  • 04:09: ... the ways that two electrons can deflect involving only a single virtual photon. ...
  • 05:26: For example, the electrons might exchange just a single virtual photon, but they might also exchange two, or three, or more.
  • 05:34: The electrons might also emit and reabsorb a virtual photon.
  • 05:39: Or any of those photons might do something crazy, like momentarily split into a virtual anti-particle-particle pair.
  • 06:32: In the case of electron scattering, the most likely interaction is the exchange of a single photon.
  • 07:08: So the most probable interaction for electron scattering is the simple case of one photon exchange with its two vertices.
  • 07:43: ... include exchanging two virtual photons, or one electron emitting and reabsorbing a virtual photon, or the ...
  • 08:21: ... is especially true for so-called loop interactions, like when a photon momentarily becomes a virtual particle-anti-particle pair and then ...
  • 08:41: Electrons are constantly interacting with virtual photons.
  • 09:07: ... due to one of these self-energy loops, you need to add up all possible photon energies, but those energies can be arbitrarily large, sending the ...
  • 09:21: In reality, something must limit the maximum energy of these photons.

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

  • 01:10: ... didn't watch for the double-slit experiment is this-- a particle, say a photon or an electron, travels through a barrier containing two slits to a ...
  • 09:07: ... example, a photon traveling between two points could spontaneously become a virtual ...
  • 09:17: And a traveling electron could emit and reabsorb a photon, which itself could make its own particle-antiparticle pair ad infinitum.
  • 09:54: So a photon is an excitation of vibration in the electromagnetic field.
  • 10:10: ... includes motion of the photon, but it also includes the probability amplitude of a photon's energy ...
  • 14:16: Up to around a millionth of a second after the Big Bang, the universe was hot enough for photons to be continuously forming matter-antimatter pairs.

2017-06-28: The First Quantum Field Theory

  • 04:43: The smallest possible oscillation above zero is an indivisible little packet of energy that we call a photon.
  • 06:08: If you take a pair of electrons or photons in two quantum states and make them swap places, then nothing changes.
  • 07:06: Paul Dirac's solution was to not try to track the changing states of individual photons.
  • 07:31: ... a number of minimum amplitude quantum oscillations, which is to say, photons. ...
  • 08:00: ... the math doesn't even try to keep track of the movement of individual photons-- only the shifting number in each quantum ...
  • 09:07: An electron can absorb or emit a photon.
  • 09:10: An electron and a positron can annihilate each other and create two photons.
  • 10:26: ... per quantum state, rather than infinite particles in the case of the photon. ...
  • 10:48: Remember, this approach began with thinking of photons as oscillations in the electromagnetic field.
  • 11:28: ... pair, for every type of force-carrying particle-- so-called bosons, like photons and gluons-- and of course for the famous Higgs boson, which is just an ...

2017-06-07: Supervoids vs Colliding Universes!

  • 04:05: A photon entering a matter-rich galaxy cluster gets an energy boost as it falls into the cluster's gravitational well.
  • 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.
  • 04:27: The photon exits with a net energy gain, which would register as a higher temperature on our CMB map.
  • 04:35: But the opposite happens when the photon enters a void.
  • 05:25: So if there are giant voids in the direction of the cold spot, then these could have sapped energy from the CMB photons as they passed through.

2017-05-31: The Fate of the First Stars

  • 06:29: Those electrons then lose that energy by emitting light at specific wavelengths-- signature photons that are different for every element or molecule.
  • 06:39: Those photons quickly escape the cloud, taking energy with them, and helping to cool things down.

2017-04-19: The Oh My God Particle

  • 06:58: Empty space isn't really empty, it's full of low-energy microwave photons leftover from the heat glow of the very earliest of times.
  • 07:11: ... volts, about 8 joules, can't travel far before smacking into these photons and giving up some of their ...

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

  • 07:45: We also begin to encounter a new set of photons from the past.
  • 09:57: Every photon that reaches us was emitted at some larger radius than wherever we encounter.

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

  • 07:06: Well, you just trace the photon paths, assuming for a moment that an FTL ship doesn't produce infinitely red or blue shifted photons.
  • 07:14: The Paradox outraces its own photons as it catches up to the Annihilator, and then it continues to emit light backwards behind it after it passes.
  • 07:24: So the Annihilator sees a series of photons coming from both directions that arrive simultaneously.
  • 08:30: To do that, we first need to outrace photons that were admitted at the space time point that we want to perceive.

2017-03-01: The Treasures of Trappist-1

  • 02:59: ... of the habitable zone for a given star based on the intensity of its photon flux and the effect of atmospheric greenhouse ...

2017-01-25: Why Quasars are so Awesome

  • 03:05: For one thing, its spectrum was redshifted, the wavelength of its light stretched out as those photons traveled through the expanding universe.

2017-01-19: The Phantom Singularity

  • 09:42: ... its perspective, a photon exists in a single instant, and so it can hang out at the event horizon, ...

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

  • 03:01: ... those photons actually escape the cavity, then any momentum exchange between the ...
  • 03:12: And if photons do escape, then you've just built a photon thruster.
  • 04:50: That's vastly smaller than the thrust observed by Shawyer's experiments, but still much, much larger than for a photon thruster.
  • 07:36: Photons would need to give up their energy, producing particle anti-particle pairs.
  • 07:49: Or if they escaped, they'd be a propellant, and momentum would be exchanged with no more efficiency than a photon thruster.

2016-12-21: Have They Seen Us?

  • 08:06: To spot these radio photons, we need a truly gigantic interferometer, both for extreme sensitivity and to eliminate our own radio buzz.
  • 14:32: So a distant immortal observer, with a ridiculously good telescope, will detect photons from the falling monkey at all future times.
  • 14:42: Eventually, those photons will come billions, even trillions, of years apart from each other and be hugely redshifted.
  • 14:49: For an eternal non-leaking black hole, there is no last photon.

2016-10-26: The Many Worlds of the Quantum Multiverse

  • 01:09: But to summarize, a stream of photons or electrons, or even molecules, travels from some point to a detector screen via pair of slits.
  • 06:50: For example, many histories lead to photons landing on the bright bands of the interference pattern, and very few to the dark bands.

2016-09-21: Quantum Entanglement and the Great Bohr-Einstein Debate

  • 04:45: When spontaneously created from a photon, these particles will always be spinning in opposite directions to each other.
  • 07:57: Instead of looking at the entangled spins of an electron positron pair, he used photon pairs with entangled polarizations.
  • 08:05: Polarization is just the alignment of a photon's electric and magnetic fields.
  • 08:11: ... correlation between the choice of polarization measurement axis for one photon and the final polarization direction of its entanglement ...
  • 08:25: The experiment was even set up so that the influence had to travel between the photons at faster than the speed of light.

2016-09-07: Is There a Fifth Fundamental Force? + Quantum Eraser Answer

  • 00:45: And when they settle down again, they give off that energy as photons, but also sometimes as a particle or a particle-antiparticle pair.
  • 01:19: The same sort of excess in the photons emitted after proton collisions in the Large Hadron Collider led to the discovery of the Higgs boson.
  • 04:14: ... the landing location of each individual photon passing through to the interference screen of this experiment does seem ...
  • 04:28: ... decision was whether we would know the path of the original photon, thus eliminating any interference pattern, or to erase our knowledge of ...
  • 04:55: ... reason is that there's absolutely no way to tell if any given photon at the interference screen has a known path until you compare the ...
  • 05:06: In fact, the distribution of photons at the screen always looks like a single blurred distribution.
  • 05:15: It's only when you flag which photons had twins arriving at detectors A, B, C, or D that you see patterns arise.
  • 05:23: In fact, even if you remove all of the A and B photons, you still don't see an interference pattern until you distinguish C versus D.
  • 05:33: And this is because those photons have interference bands that are exactly out of phase.
  • 05:49: The photon positions are decided and presumably those patterns are embedded in the distribution.
  • 05:55: Those embedded patterns are set by the eventual destination of the entangled partners of those photons.
  • 06:09: Yet, we can't extract the patterns of the photons that landed at the screen until we get the information of which detectors their entangled twins hit.
  • 07:05: The delayed choice in this experiment is whether or not to know the path of the original photon or whether to erase that knowledge.
  • 07:41: And so it's way less out there than photons somehow knowing that in the future some conscious mind will know its path.

2016-08-24: Should We Build a Dyson Sphere?

  • 11:57: ... point is that in order to resolve the photon distributions at the screen, you need to know which detector was ...
  • 12:11: ... the screen and a hit at one of the detectors, that means that those two photons were an entangled ...
  • 12:21: ... the experiment is done, we can pick out off the screen all of the photons that had twins hitting, say, detector A. Those photons turn out to show ...
  • 12:33: But the photons associated with C or D do have an interference pattern.
  • 12:38: ... there is no way to figure out which photons correspond to which detectors until the arrival times at the screen are ...
  • 13:24: ... at detectors A, B, C, and D. Well, the screen just looks like a blur of photons. ...
  • 13:37: You see, it's not just that the blur of photons connected to detectors A and B are overlaid with an interference pattern from C and D, no.
  • 14:06: It adds up to a flat distribution, and it's only when you look at the photons connected to C and D separately that you see the bands.

2016-08-17: Quantum Eraser Lottery Challenge

  • 00:00: [MUSIC PLAYING] The quantum eraser experiment tantalizes us with the apparent instantaneous flow of information between entangled photon pairs.
  • 00:45: Photons are fired one at a time through the two slits.
  • 00:49: On the opposite side, each photon is split into an entangled pair of photons.
  • 00:53: ... is recorded, while the other is used to identify which slit the original photon passed ...
  • 01:16: One-- a photon finds its way to detector A. In that case, we know the original photon must have passed through slit A.
  • 01:31: And the result is the photons whose entangled twins land at A produce no interference pattern.
  • 02:05: But you see the clear patterns when you separate those C and D photons.
  • 02:11: So the patent that forms at the screen depends on whether we gain knowledge of the path of the original photon.
  • 02:17: But crazily, all of this detecting or erasing of path information happens after each photon lands at the screen.
  • 02:25: ... those photons land according to an interference distribution, or a single pile ...
  • 03:11: ... it was executed, photons can travel to the "which way" section-- so detectors A and B-- or to the ...
  • 03:34: With the mirrors in place, photons are reflected to the which way detectors, and no interference pattern is formed.
  • 03:44: Photons travel through to the eraser section, resulting in an interference pattern at the screen.
  • 03:50: We make the decision of whether to know the path of the original photon or whether to erase that knowledge.
  • 04:17: Before the photons get to the which way end, we freeze them for a day.
  • 04:25: Actually, you can't really freeze photons.
  • 04:35: Photons start hitting the screen, building up some pattern.

2016-08-10: How the Quantum Eraser Rewrites the Past

  • 04:32: ... made use of a very special type of crystal that absorbs an incoming photon, and creates two new photons, each with half the energy of the ...
  • 04:43: These new photons are twins of each other.
  • 04:54: Place this crystal in front of the double slit to make coherent entangled pairs of any photons passing through.
  • 05:06: And use the other to figure out which slit the original photon passed through.
  • 05:14: Detector A lights up if the original photon passed through slit A. And detector B lights up for slit B.
  • 05:21: If we run this for a bunch of photons, we see that whenever detectors A or B light up, we get a simple pile of photons here at the screen.
  • 05:33: As though any knowledge of which way the original photon traveled stops it from acting like a wave during its passage through the slits.
  • 05:41: And crazier, this experiment was set up so that photons reach A or B after their twins reach the screen.
  • 05:49: So a photon lands on the screen according to the pattern defined by its wave function.
  • 06:03: It's like the second photon is saying, whoa, whoa, whoa.
  • 06:32: Its job is to destroy any information about the path of the photons.
  • 06:42: They work by allowing 50% of the photons through, while reflecting the other 50%.
  • 06:50: Instead of being reflected to detectors A or B, half of the photons end up in detectors C or D.
  • 06:57: But this clever arrangement ensures that if C or D light up, we have no idea which slit that photon came from.
  • 07:06: If we only look at the photons whose twins end up at detector C or D, we do see an interference pattern.

2016-07-27: The Quantum Experiment that Broke Reality

  • 02:13: ... comes in indivisible little bundles of electromagnetic energy called "photons." Einstein demonstrated this through the photoelectric effect but his clue ...
  • 02:33: So each photon is a little bundle of waves, waves of electromagnetic field, and each bundle can't be broken into smaller parts.
  • 02:43: That means that each photon should have to decide whether it's going to go through one slit or the other.
  • 02:53: That shouldn't be a problem as long as you have at least two photons.
  • 02:58: One photon passes through each slit and then the two photons interact with each other on the other side and produce our interference pattern.
  • 03:11: The interference pattern is seen even if you fire those photons one at a time.
  • 03:19: The first photon is detected as having arrived at a very particular location on the screen.
  • 03:25: ... second, third, and fourth photons, also-- they deliver their energy at a single spot and so they appear to ...
  • 03:37: If you keep firing those single photons, you start to see our interference pattern emerge once again.
  • 03:56: This pattern has nothing to do with how each photon's energy gets spread out, as was the case with the water wave.
  • 04:03: Each photon dumps all of its energy at a single point.
  • 04:07: No, the pattern emerges in the distribution of final positions of many completely unrelated photons.
  • 04:16: ... photon has no idea where previous photons landed or where future photons will ...
  • 04:40: It turns out that the photon isn't the only thing that does this.
  • 05:14: We have to conclude that each individual photon, electron, or buckyball travels through both slits as some sort of wave.

2016-06-22: Planck's Constant and The Origin of Quantum Mechanics

  • 02:12: ... and importantly, the relationship between the energy and frequency of a photon. ...
  • 03:30: And so the average frequency of the resulting particles of light, of photons, increases with temperature.
  • 03:42: The sun is yellow because its 6000 Kelvin surface produces more photons in the green yellow part of the electromagnetic spectrum than anywhere else.
  • 04:04: Your temperature is around 310 Kelvin, so your heat glow is mostly in low frequency infrared photons.
  • 05:28: This simple idea allowed our good Englishmen to figure out the frequencies of the photons produced by all of this thermal motion.
  • 09:52: ... was the clue Einstein needed to hypothesize the existence of the photon-- part wave, part particle, carrying a quantum of energy equal to the now ...

2016-06-15: The Strange Universe of Gravitational Lensing

  • 00:39: We just catch photons with our eyes and trace their paths backwards.
  • 08:23: The photon sphere hovers at about half, again, the height of the event horizon.
  • 08:29: This is a region where light paths are so strongly curved that photons can actually orbit the black hole, forming a shell of light.
  • 08:41: So photons will inevitably spiral inwards or outwards.
  • 08:46: ... outspiraling light escapes the photon sphere, it joins with severely lensed light from any surrounding ...

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

  • 05:46: The photon wave packets interact with each other and produce an interference pattern that is incredibly sensitive to differences in path lengths.
  • 06:00: We want to send individual photons instead of lasers.
  • 06:11: In the absence of quantum tunneling, that barrier should reflect its photon 100% of the time.
  • 06:18: ... just like with the alpha particle, as the photon approaches the barrier the wave packet defining its possible location ...
  • 06:29: About 99% of the time the photon is reflected.
  • 06:39: ... those rare tunneling photons really do travel instantaneously through the width of the barrier, then ...
  • 07:39: At that point, you can get an incredibly precise measurement of any differences in photon travel time.
  • 07:54: The tunneling photon does arrive a tiny bit earlier than its partner.
  • 08:07: So scale up from photons to people and we have transporter beams, right?
  • 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 ...

2016-05-25: Is an Ice Age Coming?

  • 12:46: ... have wondered whether the energy lost in the cosmological redshift of photons could account for the energy gained by dark ...
  • 13:09: But photons also get spread out and they get red shifted, so they do lose energy inversely proportional to the increasing scale factor.
  • 13:28: ... Photons make up only a tiny energetic contribution to the modern universe-- far ...
  • 13:42: These days, photons just don't have enough energy left to contribute.

2016-04-27: What Does Dark Energy Really Do?

  • 01:23: That history is coded in every photon of light that reaches our telescopes from the distant universe.
  • 01:43: ... we also know how long a given photon was traveling through that expanding universe, then its redshift tells ...
  • 01:59: So we need to figure out how far it traveled, the actual physical amount of space the photon had to traverse to get to us.
  • 02:26: Redshift is the amount the universe expanded during a photon's journey, and distance is the amount of physical space it travelled through.
  • 05:13: ... small side, that would mean that the universe expanded less while that photon was ...

2016-03-30: Pulsar Starquakes Make Fast Radio Bursts? + Challenge Winners!

  • 02:28: How far did the cosmic microwave background photons that we see now have to travel in order to reach us?
  • 03:08: ... asked you what average distance a photon could travel through the universe before the moment of recombination, ...
  • 03:27: You see, free electrons are really, really good at getting in the way of photons.
  • 03:33: ... that even though the electrons themselves are infinitesimally small, photons don't have to get too close before they interact via the electromagnetic ...
  • 03:54: Photons passing inside the circle interact and are scattered.
  • 04:03: But the approximation does allow us to estimate how far a photon can travel before encountering an electron.
  • 06:27: So how far would a photon have to travel before bumping into one of these electrons?
  • 06:37: Now, imagine the photon is able to look ahead and see all its possible paths along a column.
  • 06:46: And the further ahead the photon looks, the more of its possible paths are blocked by these targets.
  • 06:52: There's a distance forward, at which the photon's view ahead is completely blocked.
  • 06:58: Any photon traveling that distance is probably going to have hit an electron.
  • 07:24: ... area and we have the number of these 1-meter segments before all the photon's possible paths forward are ...
  • 07:44: Some photons travel further, some not so far.

2016-03-09: Cosmic Microwave Background Challenge

  • 00:23: ... photons of the cosmic background radiation were released when the universe was ...
  • 00:43: In the process, the distance that the average photon could travel went from not very far to greater than the length of the entire observable universe.
  • 01:12: Today, I have two questions for you about how far those CMB photons actually traveled.
  • 02:53: That plasma was effectively opaque because photons couldn't travel far without bouncing off all those free electrons.
  • 03:11: The question is what average distance could a photon travel before being scattered by an electron just before recombination?

2016-03-02: What’s Wrong With the Big Bang Theory?

  • 01:52: ... particles that carry the weak nuclear force, they become just like the photon, which itself carries the electromagnetic ...
  • 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.

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, would seem to ...
  • 04:21: So a photon clock that stationary on the spacetime diagram moves straight up.
  • 04:48: To understand that, you need to draw lights like photon paths between the moving clock and the stationary observer.
  • 05:14: ... way of saying that the approaching clock sort of chases after its own photons, condensing the distance between the light signals that carry those ...
  • 05:25: ... the receding clock is backing away from the photons traveling in your direction, stretching out the distance, and hence the ...

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

  • 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.
  • 03:50: Stationary, the world line of the photon clock looks like this.
  • 03:59: However, the internal photon still has to travel those 45 degree light like paths, because photons can only travel at the speed of light.
  • 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.
  • 07:15: Just as with the photon clock, it's only the ensemble that can travel slower than light, or be still.
  • 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: ... episode, we showed you how the ticking of a clock-- and in particular a photon clock-- slows down if that clock is moving with respect to ...
  • 00:21: The photon in the clock has further to travel from your stationary perspective.
  • 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

  • 02:46: ... compared the nucleons of atoms-- protons and neutrons-- to the imaginary photon box, a massless mirrored box filled with light, which despite being ...
  • 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.
  • 03:19: The tick rate depends on the speed of light and on the distance that the photon has to travel between the mirrors.
  • 03:46: All observers, regardless of their own speed, will report seeing the same speed for any particle of light-- any photon.
  • 03:57: So from my point of view, the photon takes longer to make the up-down journey.
  • 04:05: ... the ticks take longer in the moving clock compared to an identical photon clock standing still right next to me, which ticks at the normal ...
  • 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.
  • 04:49: 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 ...
  • 05:07: The overall distance that the photon has to travel is larger in an accelerating frame.
  • 05:11: Because that top mirror is running away from the rising photon faster than the bottom mirror catches up to the falling photon.
  • 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.
  • 05:50: ... a fast moving photon box, we perceive that its internal particles have further to travel to ...
  • 06:00: Note that this is not the same thing as accelerating the photon box, which gives it the feeling of mass.
  • 06:11: And from the last two episodes, we know that atoms and their nucleons are all kind of like photon boxes.
  • 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.
  • 08:02: Paramdeep Singh, and a few others, questioned the plausibility of massless walls in our photon box thought experiment.
  • 08:18: In that case, the box's mass increases by the amount equal to the energy of the contained photons, divided by the speed of light squared.

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

  • 01:33: A good place to start is with a thought experiment that we'll call a photon box.
  • 01:45: Now fill it with photons, also massless, that bounce around inside the box in all directions.
  • 02:01: Now the back wall of the box moves into the incoming photons.
  • 02:08: In the meantime, the front of the box, moving away from the incoming photons, feels less pressure.
  • 02:20: The photons exert a force on the box, the box also exerts a force on the photons-- Newton's Third Law, which gives us the conservation of momentum.
  • 02:29: Momentum lost by the box is transferred to the photons.
  • 02:33: Now, if the box stops accelerating, then everything jiggles around and momentum gets shared out evenly between the box and the photons again.
  • 02:54: The photon box is massive, even though none of its components-- not the photons, not the walls-- have any mass.
  • 03:10: It's the energy of the photons divided by the square of the speed of those photons.
  • 03:14: And you can derive the famous E equals Mc squared just by looking at how momentum transfers between the photons in the box under acceleration.
  • 03:23: But E equals Mc squared describes the universal relationship between mass and confined energy, not just confined photons.
  • 04:16: ... seemingly very different physical effects-- the box of photons and the compressed spring-- both give the same translation between mass ...
  • 04:36: Photons in the photon box, but even in the spring, the density wave is ultimately communicated by electromagnetic interactions between the atoms.
  • 05:08: ... the proton is a lot like a combination of our photon box and our compressed spring-- quarks, bouncing off the walls in the ...
  • 06:17: But how does the inertial mass of our photon box end up translating to gravitational mass?
  • 06:23: ... as described by general relativity, it's not so surprising that the photon box feels the pull of ...
  • 06:44: Holding up our photon box against Earth's surface gravity has to be just as hard as trying to accelerate it at 1 g in empty space.
  • 06:53: The photon box feels heavy.
  • 07:31: Individual photons affect space-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.
  • 08:05: The individual photons don't have it when they travel from one side of the box to the other.
  • 08:13: Does the ensemble of photons somehow feel time that individual photons do not?
  • 10:25: Then the weak force carriers gained mass and became differentiated from the electromagnetic carrier-- the photon.

2015-12-16: The Higgs Mechanism Explained

  • 03:12: Now, take the photon.
  • 03:26: A photon only changes if it bumps into something else.
  • 03:30: But the photon and the electron are both just excitations in their own fields, so why does the electron have mass and the photon not?
  • 03:40: ... ways to interpret it, but perhaps the simplest is to say that while the photon can cross the entire observable universe without bumping into a single ...

2015-11-05: Why Haven't We Found Alien Life?

  • 11:14: ... the bubble, you'd see nothing unless it stopped, in which case all the photons and particles that are captured on its journey would blast you into ...

2015-10-22: Have Gravitational Waves Been Discovered?!?

  • 06:08: Even quantum fluctuations in the photon rate causes noise.

2015-10-07: The Speed of Light is NOT About Light

  • 00:14: ... does the universe seem to conspire to, one, keep photons from traveling at any speed but 300,000 kilometers per second in a ...
  • 08:57: So lights or photons, also gravitational waves and gluons all have no mass.

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

  • 08:30: ... anyway, the photons from the future universe will never catch up to the monkey because that ...
  • 10:13: And with that radiation comes all of the remaining photons that the monkey emitted before crossing the horizon.

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

  • 07:28: ... even though it's true that everything inside a black hole, including a photon, will always move radially inward, it's not being "pulled." Instead, the ...
  • 07:54: Remember, from our point of view, there are no photons inside.

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

  • 10:09: ... as being mediated by some kind of particle like electromagnetism by the photons, strong nuclear forces by the gluon, and so ...

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

  • 05:37: Fire a laser pulse from the ground floor of a building up to a photon detector on the roof.
  • 05:44: On a flat spacetime diagram the world lines of those photons should be parallel and congruent.
  • 05:49: ... light, that would be true even if it turned out that gravity slowed photons down and bent their world lines, since both photons would be affected ...
  • 06:04: Thus, the vertical lines at the ends of the photon world lines should also be parallel and congruent.
  • 06:09: But if you actually do this experiment you find the photons arrive on the roof slightly more than five seconds apart.

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

  • 02:51: And at that same moment, I shoot a photon from a laser pointer to the right.
  • 02:58: Anyway, say I plot the values of ct on my clock as the photon passes different marks on the x-axis.
  • 03:10: But that one is more vertical since he moves slower than the photon.
  • 03:18: Those lines that we just drew link all the events at which the photon and the red guy respectively are present.
  • 03:57: See, according to him, the photon also moves rightward at speed c.
  • 04:18: ... at two events represented by points on such a line an observer or a photon would have to be moving faster than light, which normal objects and ...
  • 06:12: Second, the world lines of the red guy, the monkey, the photon, and me are all straight.

2015-05-27: Habitable Exoplanets Debunked!

  • 09:04: Natalia B, Pablo Herrero, and Gorro Rojo all asked whether photons actually have mass if they have energy.
  • 09:24: Gareth Dean asked, if all the photons in the universe have been red shifting as the universe expands, that means they're losing energy.
  • 10:17: ... total amount of effective mass you'd have from putting some number of photons in a mirrored box, more or ...

2015-04-01: Is the Moon in Majora’s Mask a Black Hole?

  • 08:49: With each passing moment of time, any observer sitting anywhere will see photons that were emitted from progressively more distant locations.
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