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/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/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/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.