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While watching YouTube, I came across the following popular video:

Why no one has measured the speed of light

It appears to say that speed of light could depend on direction it is traveling and we would never know.

I was under the impression that the Michelson-Morley experiment was intended to verify that very assumption. If velocity of light were different in different directions, we would see a pattern. If memory serves me right, the experiment was repeated several times in various locations. None of the observations were inconsistent with constancy of the speed of light in all directions. Where is my logic wrong?

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  • $\begingroup$ There is a huge literature on tests of relativity, including measurements of both the two-way and one-way speeds of light. It's true that a Michelson-Morley-type experiment is a two-way test, but it is by no means the only way of measuring the propagation speed of light. $\endgroup$
    – Buzz ♦
    Commented Oct 31, 2020 at 17:21
  • $\begingroup$ Note that with the current International System of Units you cannot measure the speed of light, which is defined exactly. Every experiment aiming at measuring the speed of light would actually be an experiment measuring a length instead. $\endgroup$
    – Massimo Ortolano
    Commented Oct 31, 2020 at 18:45

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The objection raised in the video is that fundamental measurements of the speed of light are based on a round trip and assume that the speed of light is the same in both directions. This would be a reasonable objection if the speed of light had only been measured on one occasion in one direction at one location.

But, as you say, Michelson and Morley’s experimental set up compared the round trip times in different directions over a period of three months, during which time the Earth had moved millions of miles in its orbit. And the experiment has been repeated many times since then. Any directional dependence in the speed of light would have shown up - especially since this was exactly the sort of effect that Michelson and Morley were trying (unsuccessfully) to detect to prove the existence of the aether.

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    $\begingroup$ Have you actually watched the full video? It goes on to discuss how measurements that try to detect the one-way speed of light would be obscured by time dilation, so that even if it wasn’t c, it would appear to be c. $\endgroup$
    – nick012000
    Commented Nov 1, 2020 at 3:41
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    $\begingroup$ @nick012000 Yes I watched the full video. It is nonsense. There are many ways to measure the speed of light, including astronomical methods, which do not depend on synchronised clocks. They all give consistent results. For example, if the speed of light depended on direction then pulsars in different directions would show different doppler shifts. $\endgroup$
    – gandalf61
    Commented Nov 1, 2020 at 9:30
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    $\begingroup$ "For example, if the speed of light depended on direction then pulsars in different directions would show different doppler shifts." Really? The speed of light varying would result in different amounts of time dilation, right? Wouldn't that cancel out the differences in the Doppler effect? $\endgroup$
    – nick012000
    Commented Nov 1, 2020 at 9:35
  • $\begingroup$ @nick012000 No, time dilation does change the Doppler factor but it does not make it independent of $c$. See en.wikipedia.org/wiki/…. $\endgroup$
    – gandalf61
    Commented Nov 1, 2020 at 9:40
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    $\begingroup$ Anyway, the point is that it's not nonsense. Here's an answer talking about the original papers the video was based on: physics.stackexchange.com/questions/257665/… $\endgroup$
    – nick012000
    Commented Nov 1, 2020 at 9:47

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