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My question is motivated from studying the null result of the Michelson Morley experiment.

  1. Does the experiment prove that the aether does not exist? Wikipedia says that it was more broadly interpreted that the aether was not needed, rather than it definitely not existing. If so, how could it be possible that the aether exists, when one of its properties is that it must be stationary, but we know that according to special relativity, there is no such thing as an absolute frame of reference.

  2. Why was the aether not observed in the MM experiment?

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  • $\begingroup$ the answers here might help researchgate.net/post/… $\endgroup$ – anna v Jan 17 '18 at 8:39
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    $\begingroup$ Aether exists in the same sense that the universe is full of invisible ponies: I can propose detailed theories which contain aether and/or invisible ponies (the tfb aetheric pony theory being merely the most well-known of these), but such theories, to be compatible with experiment, require that detection of the aether / ponies be impossible, even in principle. Physicists have little time for theoretical constructs which are not detectible, even in principle. $\endgroup$ – tfb Jan 17 '18 at 9:40
  • $\begingroup$ A relevant and very like question is this one here; I address some of your questions in my answer there. It doesn't fully answer the question of the relationship between a putative aether and the MM experiment, but you may find it helpful. $\endgroup$ – Selene Routley Jan 17 '18 at 12:03
  • $\begingroup$ @WetSavannaAnimalakaRodVance thank you for the link. The part about Lorentz aether theory is very interesting and sort of what I was looking for when I asked for why, assuming that aether exists, might not be detected. $\endgroup$ – Curiouslearner Jan 17 '18 at 12:06
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I'll try to be brief: the null result of the MM experiments demonstrated that the conceptual picture held by most physicists of the day about how light is propagated had something wrong with it on a very basic level, and the ideas about how exactly to repair that picture to square it with the experimental results (ether drag, fitzgerald contraction and so forth) were ad-hoc and lacked satisfactory theoretical justification.

The easiest explanation of why the aether was not revealed by MM is that it wasn't there in the first place.

I hope this helps, and I invite others here to weigh in with a more complete answer than this, should mine be deemed insufficient.

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  • $\begingroup$ Hello Niels, thank you for your response. Yeah everything makes sense to me if I simply accept that aether doesn't exist, but then wiki says that the general interpretation isn't that aether doesn't exist, but rather that its existence is not needed. I am not sure why they don't automatically conclude that there is no room for aether to exist since it should contradict with special relativity $\endgroup$ – Curiouslearner Jan 17 '18 at 7:49
  • $\begingroup$ Because if you can not interact with it in any way, then there is no way of knowing it is "there", hence all the speculation about it becomes philosophy, rather than science. If something is not detectable, then the probability of it's existance is hard to define against a sea of other possibilities. There are only a few things we can know for certain (100%). But even if a thing is not 100 % proven, does not justify to be working on it equaly than something that is likely to exist by 50 %. $\endgroup$ – MaDrung Jan 17 '18 at 7:57
  • $\begingroup$ @Curiouslearner this is also known as Occam's razor. en.wikipedia.org/wiki/Occam's_razor $\endgroup$ – Klodd Jan 17 '18 at 9:55
  • $\begingroup$ @MaDrung Thank you for your comment. I am still a bit confused, because shouldn't the probability of the aether's existence be zero, since if it were to exist, then it would have be stationary its by definition, which contradicts special relativity which implies that no frame of reference is absolute? Or are you saying that the probability of the aether's existence isn't necessarily zero since there's a possibility that special relativity might be wrong? I assume that this is what you mean? $\endgroup$ – Curiouslearner Jan 17 '18 at 11:13
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    $\begingroup$ @Curiouslearner: for physics 'does not exist' and 'can not be detected, even in principle' are the same thing: physics deals with the results of experiments, and if no experiment can detect something it can make no difference to physics. Philosophers, perhaps, might still want to talk about the thing, but whatever they are doing is not experimental science. $\endgroup$ – tfb Jan 17 '18 at 11:59
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The aether is linked with the history of gravity.

When Newton conceived of his theory he admitted that it used action at a distance and to him this was problematic. Action ought to be local, but he couldn't see his way around the conceptual problems.

Other physicists later hypothesised the aether to get a local theory. They conceptualised it as mechanical but the theories were like Ptolemys, cumbersome.

These problems were swept away by Einstein by using the field concept; he showed that the aether was the gravitational field and this in turn was simply spacetime.

In short, the aether was needed but it was wrongly conceptualised.

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  • $\begingroup$ I don't pretend to really know local theory or spacetime, but did Einstein redefine aether by any chance in the theory you outlined? From what I understand aether was originally assumed to be stationary (absolute frame of reference) - I just don't see how this doesn't contradict ideas put forward in special relativity. $\endgroup$ – Curiouslearner Jan 17 '18 at 11:16
  • $\begingroup$ @CuriousLearner: Sure. The usual description of the aether doesn't emphasise it's role in being a carrier of the gravitational force in order for it to be a local theory so that's not surprising. The aether does contradict SR, so you're not wrong there. If it's mentioned at all it's usually in relation to Michelson-Morley experiment and SR. Einstein didn't directly handle the aether theory, he used the field concept developed by Faraday and Maxwell. The gravitational force is a field. It's only now, looking back, after all the dust has settled that we can see what went on. $\endgroup$ – Mozibur Ullah Jan 17 '18 at 11:26
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    $\begingroup$ Uh, no. The aether the question asks about is linked with the history of electrodynamics, not gravity. This question asks about the MM (Michelson Morley) experiment, so it is asking about the luminiferous aether, the medium supposedly needed to transport electromagnetic phenomena. $\endgroup$ – David Hammen Jan 17 '18 at 20:53
  • $\begingroup$ @David Hammen: Yes, I know that history. I'm pointing out that the aether was also used to describe the mechanism of gravity as a local phenomenon. $\endgroup$ – Mozibur Ullah Jan 17 '18 at 21:04
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  1. Does the experiment prove that the (luminiferous) aether does not exist?

No, it just proved that the luminiferous aether is not needed. The wikipedia article is correct. Physicists were working on the problem of the apparent conflict between Maxwell's electrodynamics and Newtonian mechanics prior to Einstein's development of special relativity. There is an alternative to special relativity, Lorentz ether theory. The latter says that the ether does exist, but that it is not observable. There is no difference between the two theories in terms of predicted experimental outcomes. There is a huge difference between the two in terms of the assumptions those theories make.

Einstein simply took Maxwell's equations at face value: That the speed of light is the same to all inertial observers, with all the repercussions that has with regard to the limitations of Newtonian mechanics. The modern view is of course that the luminiferous aether is not needed, bolstered by the fact that photons do just fine in vacuum

  1. Why was the aether not observed in the MM experiment?

Because it doesn't exist.

Physicists at that time looked at electromagnetic radiation as being a wave phenomenon, somehow analogous to other wave phenomena known at that time. All wave phenomena known at that time needed some kind of medium to transport the waves. Physicists at that time did not know about quantum mechanics. Photons do just fine in vacuum.

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Matter is always in motion. Aether is a non-baryonic form of matter and thus is always in motion as is baryonic matter. The concept of a stationary aether is as absurd as the concept of a stationary planet.The Michelson-Morley (MM) experiment did not detect the presence of stationary aether, just as no experiment is ever going to detect any stationary planet. To conclude from the MM experiment that aether does not exist, is unjustified.

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