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I was recently reading about how physicists were heavily relying on the existence of Luminiferous Aether for their physics to work. I read about how the Michelson-Morley experiment attempted to prove the existence of the Luminiferous Aether by causing different light beams to travel upwind and downwind in a way that would create a shift in the interference pattern, that is if the ether existed.

Bare with me for a second and pretend that the ether actually existed and that the experiment was successful and showed the desired results. I still don't understand how this would prove the existence of the ether.

I mean, the results definitely seem to agree with the theory of the Luminiferous Aether but it doesn't necessarily prove it. Because if the ether wind didn't exist, shouldn't the shift in the interference pattern be observed either way because the earth, the mirrors, and the detectors all move relative to the light beams and (considering the speed of light respects the Principle of Galilean Relativity) depending on the direction of movement of Earth, the light beam would pick-up or lose speed c +/- v.

What I mean is, in a sense, after the light beams have been 'launched' from their sources, and due to the movement of the Earth, one of the mirrors would 'move' closer to the light beam, and hence decreasing the distance the light needs to travel, while the other mirror moves further and hence increasing the distance the light needs to travel.

So basically light, if it followed the Principle of Galilean Relativity as was postulated back then, would experience some form of 'earth movement shift', regardless of whether the ether existed or not.

What am I missing? Did they consider that the 'launched light' would have some original velocity added to it due to the movement of Earth, treating light as some sort of mechanical particle, and therefore the 'Earth shift' wouldn't matter and the results would point directly to the Ether Wind?

Or alternatively, if light was proven to follow Galilean Relativity, was there any consequential relationship between the principle of Galilean Relativity and the Luminiferous Aether that would prove the existence of the ether once light was noticed to follow Galilean Relativity?

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closed as off-topic by Aaron Stevens, John Rennie, Jon Custer, tpg2114 Aug 16 at 23:15

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    $\begingroup$ "I read about how the Michelson-Morley experiment attempted to prove the existence of the Luminiferous Aether by causing different light beams to travel upwind and downwind in a way that would create a shift in the interference pattern, that is if the ether existed." Forgive me if I'm misinterpreting, but it seems you think the experiment had one light beam go one direction (and then bounced back), and the other was going in the opposite direction (and then bounced back). In other words, one beam pointed west and the other beam pointed east. However, this is not the experiment... $\endgroup$ – SpiralRain Aug 15 at 1:53
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    $\begingroup$ The experiment was about two perpendicular beams. In other words, one pointed east and the other pointed north. This is a crucial difference, because if light moved at a constant speed with respect to an absolute inertial frame or a stationary medium (the story is similar for either one), there will be a time difference according to kinematics. Now as you rotate the two perpendicular beams, the roles of them switch (one is parallel to aether wind while the other is perpendicular), and the fringe shift should be observed then. If I misinterpreted what you said, I absolutely apologize. $\endgroup$ – SpiralRain Aug 15 at 1:57
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    $\begingroup$ @SpiralRain Please post answers as answers, not comments. $\endgroup$ – Aaron Stevens Aug 15 at 3:19
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    $\begingroup$ @SpiralRain Thanks for the clarification, but I already knew that they were perpendicular. My question is: Wouldn't there be a shift due to kinematics even if they were perpendicular, that is given the light respected Galilean Relativity? $\endgroup$ – Fahd_19 Aug 15 at 16:07
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    $\begingroup$ @Fahd_19 You're right. One important thing here is that the Michelson-Morelay experiment only rules out a specific class of possible theories. For others, you need other experiments. If light respected Galilean relativity, then its speed would be dependent on the speed of the emitter and therefore it would not be constant. In this case, your proposition would affect how distant binary star systems would look. See this link and this post for an explanation. $\endgroup$ – SpiralRain Aug 15 at 16:29
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Michelson and Morley did not set out to "prove the existence of the aether". That's at best a spin that is put on by later popularizers, writing with the benefit of hindsight, and in particular knowledge of special relativity.

At their time it was known that

  • Newton's laws of mechanics are invariant under what we now call Galilean relativity.
  • Maxwell's equations for electrodynamics are not.

Both of these were already wildly successful theories. It was generally assumed that the way they fit together needed to be that there is one particular inertial frame where the equations of electrodynamics hold exactly. The "luminiferous aether" is one way to speak of that frame, which among other things acknowledges that it might move in different ways at different times and places (as long as the variation was minute enough not to upset how light moves at astronomical scales).

The goal of the experiment was to find out by measuring it what this privileged frame was. Presumably the idea was that after we knew how the privileged frame was, we'd be able to start figuring out why that particular frame and no other was privileged with nice equations for electromagnetism. For example, was it stationary with respect to the sun or not? And if not, would there be any known astronomical phenomenon it might be aligned with instead?

The failure to identify any aether frame even though the instrument appeared to be more than sensitive enough, was a complete surprise -- not just the "not expected" among the anticipated outcomes of the experiment, but not even something that was taken seriously enough to need ruling out in the first place.

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    $\begingroup$ Thanks for your clarification about the origin of what the experiment was intended to do. I now understand a little better what the Aether meant to them back then. Your answer is great but there are other aspects of the question I need answered so I won't mark as 'Answered' yet. Thanks anyways! $\endgroup$ – Fahd_19 Aug 15 at 16:10

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