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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?

Qmechanic
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wander95
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1 Answers1

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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.

gandalf61
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