B Michelson Morley experiment and the Doppler effect

[Luyo_66]

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Dear Sirs, I am not a physicist so my knowledge on this subject is limited and my question may be wrong so I would be grateful if someone could patiently answer me. I am an electronic engineer and I have a question regarding the Michelson Morley experiment. It seems to me that the conclusion about the non-existence of the ether based on this experiment was wrong. Namely, light is an electromagnetic wave and the speed of light is not affected by the speed of the source or the observer. Therefore, measuring the speed of the ether wind by measuring the speed of light would be meaningless. It would make sense if light had a completely corpuscular nature. The way the experiment was conceived, the only thing we got was indirect evidence that light has a wave nature and this experiment does not prove the non-existence of the ether. There are probably many other experiments that confirm this (the non-existence of the ether), but it seems to me that the conclusion about the non-existence of the ether based on the MM experiment was logically wrong. That was the same as trying to prove the non-existence of an optical cable by measuring the speed of a signal in opposite directions in a moving or accelerating environment. The only thing we can measure in that case is the Doppler effect. Am I wrong about something? Thank you in advance. Sincerely, Josip Luetich, Croatia

 

[PeroK]

Yes, you are wrong. See, for example:

https://en.wikipedia.org/wiki/Michelson–Morley_experiment

 

[Nugatory]

That was the same as trying to prove the non-existence of an optical cable by measuring the speed of a signal in opposite directions in a moving or accelerating environment.

We've never done it because the speed of light and the dimensions of optical cable make the experiment impractical... but in principle an MM-type experiment could determine whether light is being transmitted through an optical cable in motion relative to the source and detector. It's just another medium, and the behavior of waves in a a medium is well-understood.

As long as the source and detector are at rest relative to one another there is no Doppler effect and no frequency shift. MM-type experiments and interferometers in general measure phase shifts not frequency shifts.

 

[Ibix]

Also see the Croatian version: https://hr.wikipedia.org/wiki/Michelson-Morleyjev_pokus. Note that the second figure on that page (https://hr.m.wikipedia.org/wiki/Mic...media/Datoteka:MichelsonMorleyAnimationDE.gif) illustrates the flight time of light when the speed of light is fixed with respect to the ether and the experiment is stationary (left) and moving (right) with respect to the medium. The first figure on the page (https://hr.m.wikipedia.org/wiki/Mic...media/Datoteka:Michelson-Morleyev_pokus_1.png) has the mathematics.

Note that the Michelson-Morley experiment does not rule out every possible non-relativistic model of light. It does rule out a fixed ether with light having a constant speed in the ether frame.

 

[Luyo_66]

Also see the Croatian version: https://hr.wikipedia.org/wiki/Michelson-Morleyjev_pokus. Note that the second figure on that page (https://hr.m.wikipedia.org/wiki/Michelson-Morleyjev_pokus#/media/Datoteka:MichelsonMorleyAnimationDE.gif) illustrates the flight time of light when the speed of light is fixed with respect to the ether and the experiment is stationary (left) and moving (right) with respect to the medium. The first figure on the page (https://hr.m.wikipedia.org/wiki/Michelson-Morleyjev_pokus#/media/Datoteka:Michelson-Morleyev_pokus_1.png) has the mathematics.

Note that the Michelson-Morley experiment does not rule out every possible non-relativistic model of light. It does rule out a fixed ether with light having a constant speed in the ether frame.

OK, thank you very much for this answer. I'll study this article. Thanks to PeroK as well.

 

[Dale]

It seems to me that the conclusion about the non-existence of the ether based on this experiment was wrong.

It is not wrong, but it may be a bit more nuanced than at first appearance. There are at least three distinct concepts of the aether: a rigid aether, a dragged aether, and the Lorentz aether. The MMX ruled out only the rigid aether.

Namely, light is an electromagnetic wave and the speed of light is not affected by the speed of the source or the observer. Therefore, measuring the speed of the ether wind by measuring the speed of light would be meaningless. It would make sense if light had a completely corpuscular nature.

This does not follow. At the time of the MMX the wave nature of light was well understood. The experiment does not hinge on its quantum nature in any way.

The way the experiment was conceived, the only thing we got was indirect evidence that light has a wave nature and this experiment does not prove the non-existence of the ether.

What it actually provides evidence for is that the speed of light is isotropic regardless of the time of year or time of day. This is contrary to a rigid aether.

That was the same as trying to prove the non-existence of an optical cable by measuring the speed of a signal in opposite directions in a moving or accelerating environment.

It would be more similar to Fizeau’s experiment where the speed of light was measured through flowing water. That indeed was able to measure light speed anisotropy.

The only thing we can measure in that case is the Doppler effect.

I don’t see any Doppler shift. All parts of the equipment are moving at the same velocity through the aether.

 

[Luyo_66]

Dear Sir,
thank you very much for these explanations. I see I have to dig much more deeply into this subject to get a better understanding of this problem. It's good I've asked. This is a great forum. Thank you once again! :)

 

[Luyo_66]

We've never done it because the speed of light and the dimensions of optical cable make the experiment impractical... but in principle an MM-type experiment could determine whether light is being transmitted through an optical cable in motion relative to the source and detector. It's just another medium, and the behavior of waves in a a medium is well-understood.

As long as the source and detector are at rest relative to one another there is no Doppler effect and no frequency shift. MM-type experiments and interferometers in general measure phase shifts not frequency shifts.

Thank you very much. This is very valuable information. At the beginning, I saw no flaw in my reasoning and now I see that I have to read a few more books...

 

[Luyo_66]

Yes, you are wrong. See, for example:

https://en.wikipedia.org/wiki/Michelson–Morley_experiment

Pero, many thanks for the comment. I'll check the article.

 

[Hornbein]

You could look at the Fizeau experiment. Albert E. said this was a big influence on SR.

 

[Herman Trivilino]

IMO opinion it didn't disprove the existence of the aether, it rendered its existence useless and undetectable. Or in Einsein's words, "superflous".

It's not disproven, it's shown to have no experimental support for its existence. Einstein then went on to demonstrate that there was no theoretical support, either.

All of this, by the way, is of historical interest only, and stays in view because of pop sci and textbook's use of it. It happened over 120 years ago. Since then far more modern and precise experiments have supported these conclusions, and the MM experiment is of historical and educational value, but what it showed has been shown in far more precise and therefore scientifically convincing ways.

 

[PeroK]

IMO opinion it didn't disprove the existence of the aether, it rendered its existence useless and undetectable. Or in Einsein's words, "superflous".

In fact, all it did was failed to detect the ether. This is what Einstein mentions in the 1905 paper:

... the unsuccessful attempts to discover any motion of the earth relatively to the “light medium,” ...

 

[Dale]

IMO opinion it didn't disprove the existence of the aether

I would say that it did disprove the existence of the rigid aether.

The superfluous aether is the Lorentz aether.

 

[Luyo_66]

Hey, it is becoming more and more interesting... I'll learn much more than I ever thought...guys, thank you very much for your contribution!

 

[Luyo_66]

I'll study EM field theory a little bit deeper. And I'll probably come with the other strange questions in a few months. Unfortunately, I have limited time for it so I can't do it in a short time. And my English should be improved as well. If I wish to ask something more sophisticated, I can't do it with simple structures honed for engineering or trade. It is not enough. Could somebody advise some book or webinar for that? Not English basics or executive or advanced. I can get it anywhere. I need something suitable for a conversation about philosophy.

 

[Luyo_66]

You could look at the Fizeau experiment. Albert E. said this was a big influence on SR.

Great advice. I'll check that. Thanks. But in any case, I have to dig deeply in theory. There is nothing more practical than that...as I've heard that A.E. said.

 

[Luyo_66]

It is not wrong, but it may be a bit more nuanced than at first appearance. There are at least three distinct concepts of the aether: a rigid aether, a dragged aether, and the Lorentz aether. The MMX ruled out only the rigid aether.

This does not follow. At the time of the MMX the wave nature of light was well understood. The experiment does not hinge on its quantum nature in any way.

What it actually provides evidence for is that the speed of light is isotropic regardless of the time of year or time of day. This is contrary to a rigid aether.

It would be more similar to Fizeau’s experiment where the speed of light was measured through flowing water. That indeed was able to measure light speed anisotropy.

I don’t see any Doppler shift. All parts of the equipment are moving at the same velocity through the aether.

Dale, this was very informative. I've made a mistake trying to make an analogy with the measurement in optical cable. Therefore, the Doppler effect in the cable would be nonexistent. My mistake. Flowing water is much better idea. I didn't know about Fizeau's experiment. I'll check the articles regarding it. Many thanks!

 

[ahmadphy]

"It is well known that if a magnet is in motion and the conductor is at rest, an electric field is produced with a certain energy content; but if the magnet is at rest and the conductor is in motion, no electric field is produced in the space around the magnet, and yet a current is induced in the conductor. The observable phenomena here depend only on the relative motion of the magnet and the conductor; the two cases should therefore be equivalent."
This is written by Einstein in the first of 1905 paper and I think it can be considered as a disprove to the existence of aether.

 

[Ibix]

This is written by Einstein in the first of 1905 paper and I think it can be considered as a disprove to the existence of aether.

I don't immediately see how, except in the sense that relativity implicitly underlies Maxwell's equations. In fact, it was the apparent paradox that Maxwell's equations were not Galileo-covariant but mechanics (as it was then understood) was that led to the ether as a proposal for how to recover covariance in Maxwell's equations. It was something of a mis-step, of course.

 

[ahmadphy]

I don't immediately see how, except in the sense that relativity implicitly underlies Maxwell's equations. In fact, it was the apparent paradox that Maxwell's equations were not Galileo-covariant but mechanics (as it was then understood) was that led to the ether as a proposal for how to recover covariance in Maxwell's equations. It was something of a mis-step, of course.

Thanks , that helps clarify the historical context.
I was trying to connect Einstein’s point about the magnet and conductor with his motivation to discard the idea of a preferred frame. It seemed to me that this example directly challenged the concept of aether, but I wasn’t sure if that applies only to rigid aether or to all forms.

 

[Ibix]

Thanks , that helps clarify the historical context.
I was trying to connect Einstein’s point about the magnet and conductor with his motivation to discard the idea of a preferred frame. It seemed to me that this example directly challenged the concept of aether, but I wasn’t sure if that applies only to rigid aether or to all forms.

I think that was a well-known-at-the-time example of the problem ether theory was supposed to solve; Einstein was about to solve the problem in a different way.

The full issue is that it is possible to use Maxwell's equations to write a description of a magnet moving past a stationary current-carrying conductor. And it is possible to use them to write a description of a currenthcarryong conductor moving past a stationary magnet. However, with the Galilean transforms you cannot transform one solution to the other, in contrast to classical mechanics where you can (e.g.) write the equations of motion of a ball bouncing in a lab and in a train and trivially transform one to the other.

The obvious solution was that Maxwell's equations were correct in some preferred frame, and there was a more complex version of the equations that was valid in all frames. Given that you can get a wave equation out of Maxwell's equations, some kind of medium in which we are (nearly) at rest is a very sensible guess - its rest frame supplies the preferred frame and its vibrations are the wave solutions. Presumably the supposed-full form of Maxwell's equations would be Galileo-invariant.

Of course, it didn't work out that way, but that was more or less the context in which Einstein was writing.

On reflection I do see what you are getting at in the last sentence of Einstein's that you quoted, and given what Einstein was just about to say he may have intended it that way. But I think the issue he cites would also have been addressed by a Galileo-covariant extension of Maxwell's equations, if such a thing existed.

 

[Herman Trivilino]

I was trying to connect Einstein’s point about the magnet and conductor with his motivation to discard the idea of a preferred frame.

The connection is that there is no need for a preferred frame, not that the notion of a preferred frame is disproven. Einstein was keenly aware that his theory made be wrong, for example he showed an interest in the ability of astronomers to detect a dependence of the speed of light on the speed of the source.

Of course, it's been 70 years since his death and 120 since he formulated this argument. No such experimental evidence has ever been found, but that doesn't mean that it could be at some time in the future. This is how scientific theory progresses. Einstein's special theory of relativity is so well ensconced in the huge amount of theory that's come along since then, that it seems any disproof of it would just reveal its limits of validity.