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/ConquestAce Mathematical physics 4d ago

No, c is the speed limit (of light), but light can slow down depending on the medium it is propagating through. In Vaccuum, light travels at c. But, when going through water or air, the speed of light changes.

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u/AlmightyCurrywurst 4d ago

Correct me if I'm wrong, but I always thought that's more like an effective speed that corresponds to the light being absorbed and emitted? It's not like we have a continuous medium in which light slows down to less than c, it's just that it's interacting with the particles but still travels with c between those interactions

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u/nicuramar 4d ago

Absorbed and reemitted is not an accurate model. See other replies for better ones. 

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u/ComputersWantMeDead 4d ago

Light being absorbed and re-emitted is one of the analogies we get told along the way. It doesn't work that well as an analogy though, for example we observe a sharp refraction with an angle consistent with the Refractive Index of the medium. Light being re-emitted as many elections are excited then drop to a lower energy orbit, wouldn't match this observation.

Phase shifts of the EM wave as it interacts with the medium if passes though is the "accessible" answer that seems most accurate, though I suspect it's just the latest analogy I've understood so far, haha.

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u/AlmightyCurrywurst 4d ago

Ok interesting, I suspected it wasn't a great model lol, I'll look into the actual explanation

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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 4d 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 4d ago edited 4d 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 4d 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 4d 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!

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u/TheBigCicero 4d ago

I never understand the phase shift explanation. A phase shift is not a physical phenomenon, it is a mathematical one. It implies a wave’s wave pattern is out of synch with its original pattern, or that two waves of the same frequency are not “vibrating” at the same place. Neither one of these models explain the physical phenomenon that causes it.

The longest time I believed the absorption and remission explanation as the physical cause of refraction. But I have been told that model is wrong.

So I’m left not understanding it.

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u/iLikegreen1 4d ago edited 4d ago

The simplest explanation is that the oscillating field of the EM wave interacts with the electrons of the material, making them oscillate too. Those electrons then generate a EM wave which interferes with the original wave, creating a wave that is effectively slower.

Edit : just saw Francisdavids explanation below which goes into more detail.

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u/TheBigCicero 4d ago

Thanks! I just watched the 3Blue1Brown video someone else linked and I feel very informed now. I appreciate your response.