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Why the apparatus is rotated by 90 degree? If it is not rotated, there is still path difference between two lights. So why it is rotated?

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    $\begingroup$ Please can you provide a reference which states that the apparatus must be rotated? $\endgroup$ Commented Apr 3, 2018 at 17:47

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Superfrankie, You were right about the first part. It doesn’t matter what direction the arms of the experiment go as long as they are the same length. For many experiments like gravitational waves it’s better to cover the full 90°. That way when you rotate experiment you will have a full range or a full contrast to compare the two readings.

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I believe there was no such rotation by 90 degrees of the experimental setup, relative to the floor at least. The aim of the experiment was to measure the influence of moving through the hypothetical Aether, a substance which was hypothesized to be the medium in which light-waves travel.

The original measurement took place at two different times in the year, in which the Earth would be moving in different directions with respect to this elusive Aether. This movement of the medium should then be measurable in the speed of light, in the framework of Galilean relativity. In the end, they weren't able to find any such difference, concluding that light is not traveling in such an Aether material!

Check out the Wikipedia, especially the diagram! https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment

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  • $\begingroup$ The first picture on that page shows the experiment sitting on a mercury trough. The purpose of that trough was to allow the experiment to be easily rotated. $\endgroup$
    – BowlOfRed
    Commented Apr 3, 2018 at 15:55
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Simply to account for the case where they might be measuring perpendicular to the flow of the postulated ether, which was assumed to be the medium that the light "waves" were "oscillating in.

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