# Consequences of light moving in a medium

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## Main Question or Discussion Point

I'm reading Special Relativity by TM Helliwell and in it he describes the second postulate and the fact that moving with respect to air changes the speed of sound, and that because light doesn't need a medium it's speed is constant. I remember my physics teacher saying that light itself(EM fields) is the medium in which it travels and I would like to know if that is true and what the consequences of that being true are in SR. I would also like to know if c truly changes in mediums or if it is just traveling a longer distance because I've heard both. Thank you for your time

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Dale
Mentor
I remember my physics teacher saying that light itself(EM fields) is the medium in which it travels and I would like to know if that is true and what the consequences of that being true are in SR.
It is not true as far as I know. At least, I am unaware of any reference which describes it that way or investigates the consequences of such a claim.

PeroK
Homework Helper
Gold Member
I'm reading Special Relativity by TM Helliwell and in it he describes the second postulate and the fact that moving with respect to air changes the speed of sound, and that because light doesn't need a medium it's speed is constant. I remember my physics teacher saying that light itself(EM fields) is the medium in which it travels and I would like to know if that is true and what the consequences of that being true are in SR. I would also like to know if c truly changes in mediums or if it is just traveling a longer distance because I've heard both. Thank you for your time
In general, I'd say the best thing is to forget everything you thought you previously knew about SR. Helliwell's book is excellent, in my opinion, and self-contained. If it contradicts in any way something you thought you knew, go with Helliwell and assume that what you previously knew must have been mistaken in some way.

That said, you may be missing his point about the difference between sound and light. Because sound needs a physical medium (such as air), it has a defined speed with respect to that medium, but the observer can be moving relative to that medium. You can analyse the motion of the sound waves relative to you as a) the constant speed of sound in air; and, b) your motion relative to the air.

Light, however, can move though the vacuum of space. And, because this is not a physical medium and you cannot detect your motion relative to space (see the discussion on the search for the imaginary ether), then the speed of light in a vacuum must be constant for all observers. This is fundamental to SR.

Finally, light can move through a transparent medium (such as water or glass). In this case, the motion of light is disrupted by the medium as light interacts with the medium itself. In this respect, the speed of light in such a medium is not $c$. This has no bearing on SR, because it's the property of light moving through a vacuum that is the key issue. The fact that light can also move through a physical medium as well as a vacuum is not relevant to the core argument.

Finally, light can move through a transparent medium (such as water or glass). In this case, the motion of light is disrupted by the medium as light interacts with the medium itself. In this respect, the speed of light in such a medium is not $c$. This has no bearing on SR, because it's the property of light moving through a vacuum that is the key issue. The fact that light can also move through a physical medium as well as a vacuum is not relevant to the core argument.
I'd also add that there's a semantic issue that arises when we talk about "light" traveling through a medium.

I'm no expert, but my understanding is that the wave propagating through the medium is actually a superposition of the incoming radiation and the electromagnetic waves emitted by the excited atoms in the material. According to the extinction theorem, this superposition results in some canceling that leaves a polarization wave whose phase velocity is lower than $c$. The semantic issue is whether to call this polarization wave "light."

(And then there are some people who reserve the word "light" for EM waves in the visible spectrum...)

I would also like to know if c truly changes in mediums or if it is just traveling a longer distance because I've heard both.
The speed of light truly changes in a medium. It needs more time for the same distance.

Thank you all for your replies, they have all been very helpful.