https://www.youtube.com/watch?v=KTzGBJPuJwM
https://www.youtube.com/watch?v=Cz4Q4QOuoo8
The first video is really a tour de force. Too bad it saw such limited traction here: https://news.ycombinator.com/item?id=38482549
This video uses the explanation that light interacts with electrons in a medium which causes the electrons to produce an electromagnetic wave whose interference pattern changes the phase of the light wave, kicking it back.
The problem is that an interference pattern can never change the speed of a wave, it can change the phase of the wave and adjust it, but that won't explain how light passing through a medium takes longer to traverse that medium than light travelling in a vacuum.
I don't know of a visualization or explanation that captures why light slows down in a medium, and the video linked is still a good way to get a sense of what's happening, but it's worth knowing that there is still a great deal being left out of that explanation.
It is nevertheless better than explanations involving the absorption an reemission of photons or that light bounces around within the internal structure of a medium. Both of those explanations manage to explain how light takes longer to pass through a medium but fails to explain how light keeps a consistent direction as opposed to scattering randomly.
The explanation in the video manages to explain how light maintains a consistent direction but fails to explain how light will actually take longer to pass through a medium compared to a vacuum.
Doesn't mechanical oscillation already occupy all three spatial dimensions plus a time dimension?
A photon is EM only.
So you’re correct that it occupies all four dimensions, but we’re discussing an excitation in one field (photon) changing to an oscillation in multiple (phonon) and then back to only one field (photon).
I really can't quite grasp how the photon/phonon switches back and forth like this.
2. The photon enters a material, where the disturbance in the EM field couples to the electron waves — creating a phonon.
3. The phonon travels through the material, as an oscillation in both electrons and EM.
4. The phonon reaches the edge of the material, where there are no more electrons and reverts to a wave of just EM — a photon.
5. The photon continues in free space.
The reason this happens is the photon changes the electron behavior, which in turn changes the photon behavior. This is because EM interacts with charged particles like electrons.
For the time it is in a material, a EM wave can’t be separated from the behavior of the electrons present: both and their mutual interaction are required to explain what happens.
That disturbance of both is “coupled” — and called a phonon.
Something that might be comforting about light going slower than the speed of light is that the speed of light is not a special property of light, but it is a property of space time. It just so happens that light is the only way we have experience on this magic speed. Light isn't physically bound to go "the speed of light".
[Edit]In the second video above, he notes that phase kicks don't change the speed of the wave, even though the wavelengths can stretch out, it's still lower than the speed of light in vacuum.
If you want an actual non-layman explanation for why light slows down ib a medium you need to express the light and medium interaction in terms of polaritons. Of course this is incredibly difficult to do so for even simple cases, so alas a whole suite of simplified explanations exist that seek to explain some aspect of the situation while failing to explain others.
"Phase kicks" explain the phase velocity. The speed of a wave is the group velocity. Phase velocity is omega(k)/k, group velocity is omega'(k). So if "phase kicks" explain phase velocity sufficiently so it's valid for a range of frequencies, then it also explains group velocity by taking the derivative of the dispersion relation.
The group velocity of these pulses through a medium will still be lower than the speed of light in a vacuum and phase kicks won't influence the group velocity.
Explaining how the group velocity of light can be slower in a medium than in a vacuum requires analyzing the coupling of photons with electrons to form polariton quasiparticles. You can then calculate the mass of these polaritons which in turn gives you the speed and get the full picture. Doing this, however, is incredibly complex and so it's much easier to consider simplified scenarios like either the steady-state case where you can simply reason about the scenario in terms of interference patterns between the light wave and the electromagnetic waves produced by oscillating electrons, or you can consider some non-steady state scenarios involving photons themselves being absorbed and reemitted by electrons but neither of these explanations fully capture the phenomenon.
I agree that the phase-kick classical model is a very simplified material model, which probably breaks down in certain ways. But it does yield a dispersion relation, therefore both phase and group velocities.
I can imagine things get really weird with pulses, but that's out of scope for the video.
So, there no slowdown by phase shift at all? But.. are you sure, because it looks like the phase shift is the real effect and logically speaking result of the phase shift will be the effect that look like slowdown for outside observer.
Also, how can you simulate similar effect in water and sound if there no sound polarization?
But this model won't be useful for understanding pulses of light through a medium. For example if I emit a very short pulse of light through a medium and measure how long it takes the head of the pulse to traverse the medium, using this model won't work. A phase shift won't move the head of a pulse of light backwards, it will just adjust the amplitude of the head as it propagates through the medium.
You still need a way to explain how it is that the head of the pulse actually takes longer to travel through the medium and that's not something a phase shift can explain.
If there was a phase shift but no slowdown, wouldn't we expect individual photons to propagate through a 10km loop of fibre optic cable without any slowdown, which would be measurable?
The best answer I could find on physics stack exchange was that the single photon's wavefunction is delocalized, so that the photon's wavefunction, in fact, interacts with the entire medium, instead of at a single point.
Is this the correct way to state this phenomenon or is there a better understanding of how the light emitted from a single photon interacting with a medium would bend?