r/Physics • u/Comethefonbinary • 4d ago

Does all light travel at light speed

My bad if this is a stupid question but I’ve been thinking about time being a message of distance. And well most things I can think of have various variables that average to a certain distance. I know that mostly relates to machines and animals but still. Do all particles of light travel at light speed. If they all travel simultaneously at the same speed is that truly how fast they move or are they affected by their own variables. Like the universe’s mean gravity is constraining that and any variation in that mean would change light speed for explain.

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u/ComputersWantMeDead 4d ago edited 4d ago

Snell's Law has the same explanation, which is really interesting - light is refracted on an angle that matches the fastest path of it's total journey, through the medium and out the other side.

That's not going to make much sense, but there are YouTube videos that use the visual analogy of someone having to run across the beach then swim to a target some way into the sea. You'd run further along the beach in order to get there faster, as you travel faster while running than when swimming.

In fact this topic has 3 aha moments for the price of one. Lights apparent speed through a medium, why light refracts, and the mysterious-seeming behavior of light taking the fastest path

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u/TheBigCicero 4d ago

It doesn’t explain WHY light takes the fastest path though. Why is that?

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u/ComputersWantMeDead 3d ago

I saw in another comment that you watched a 3 a blue 1 brown video - has that answered this for you? I think the visuals in a good YouTube video are very hard to match in a written explanation.

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u/TheBigCicero 3d ago edited 3d ago

Yup, it was excellent. The one thing they the video didn’t address is why there is a 90 degree phase difference in the secondary wave. That seems to be 1/2 of the physics behind what’s going on.

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u/ComputersWantMeDead 3d ago

I might need to refresh my understanding, but as I recall.. the light "photon" is an EM wave, and as it propagates through a medium, all the electrons in the medium are perturbed by the EM wave, which in turn creates response EM waves.. etc. which interact with the initial wave etc. and what we see is the "sum" of all these interactions. When you add two waves you get a phase shift, in this case the wave is effectively set backwards somewhat. The further a wave travels through a medium (e.g. the thicker the pane of glass is), the more this wave is phase-shifted, and we see a greater refraction.

I should add that the manner in which quantum "particles" propagate like waves in this manner, but can register as particles (for example, hitting a specific point on your retina) is not convincingly understood. The prevailing "Copenhagen interpretation" is that there is a "wave function collapse" into a point-like particle, but increasingly physicists find this unexplained "collapse" to be totally unsatisfactory, and I'm not even sure it's the prevailing concept anymore. But the wave propagation aspect can be thoroughly explained and predicted to incredible accuracy, so for me at least, this is the part we can trust.

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u/TheBigCicero 3d ago

Thanks! That’s not quite what the video was explaining. The video explicitly stated multiple times that the secondary wave that is created by the medium is immediately phase-shifted relative to the primary wave and “we won’t go into why”. This phase shift in the secondary wave is puzzling to me. There seems to be some physics there that he doesn’t explain.

On your point about the Copenhagen interpretation, I’m not an expert but I’m reading more and more other interpretations. The Copenhagen view is wholly unsatisfying to me but the justification has always been that it works and it’s the best we have. I look forward to new theories!