MMX and the extinction theorem

[pgf]

It's well understood that extinction effects must be considered when conducting experiments to measure the velocity of light from moving sources. Fox showed that all such experiments prior to the early 60s were flawed on that basis.

I'd like to understand why early M-M type experiments aren't flawed in the same way, or, if they are, why no one mentions it.

For instance, in the often-cited FAQ on SR experiments (http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html), many of the moving source experiments are flagged with comments regarding whether extinction criticisms are warranted, but none of the M-M type experiments.

In the third cite below, Fox even seems to dismiss concerns about M-M experiments (second paragraph), but the reason for the dismissal isn't clear.

Is there something about the methodology of interferometry experiments which renders extinction irrelevant? I wouldn't have thought so, but I never see it discussed.

I'm not a physicist -- just an educated layman -- so perhaps I'm missing something subtle. Or, maybe even something obvious!

paul


* Fox, J. G. (1962), "Experimental Evidence for the Second Postulate of Special Relativity", American Journal of Physics, Volume 30, Issue 4, pp. 297-300
* Fox, J. G. (1965), "Evidence Against Emission Theories", American Journal of Physics 33 (1): 1–17
* Fox, J. G. (1967), "Constancy of the Velocity of Light," J. Opt. Soc. Am. 57, 967-968 (which seems to be available here: http://mysite.verizon.net/cephalobus_alienus/papers/Fox_1967.pdf )

 

[pgf]

I just did some more searching of the forum, and this note from pmb_phy in 2008 summarizes my thoughts on this. The original is here:
* https://www.physicsforums.com/showpost.php?p=1784283&postcount=67
but I'll quote the relevant part:

As for how the extinction theorem effects the Michelson-Morely experiment(MMX); when analyzing the results of the Michelson-Morely experiment one assumes that the Earth is moving through the ether. The MMX was at rest on the Earth.Since the source of light was at rest with respect to the MMX it also follows that the source was at rest on the Earth. The source of light therefore moved relative to the ether. Let one of the two interferometer be parallel to the direction of motion (Arm A) the other is then perpendicular to it (Arm B). According to the ether theory the speed of light relative to the MMX has one value when traveling parallel to Arm A and another value when traveling antiparallel to it. The interferometer was designed to detect these different speeds of light. However according to the extinction phenomena the light will travel at a speed which is relative to the medium, in this case air, and thus there is no expected difference in the speeds of light parallel or antiparallel to Arm A. This means that the MMX cannot detect the ether. ...

What do you believe is wrong with the above analysis of the MMX experiment?

 

[Dale]

MMX is not subject to extinction because it is not measuring the speed of light. It is measuring anisotropy in the speed of light. As such, extinction effects are irrelevant.

Extinction is only relevant for measurements of the speed of light.

 

[pgf]

MMX is not subject to extinction because it is not measuring the speed of light. It is measuring anisotropy in the speed of light. As such, extinction effects are irrelevant.

I think I need more explanation than that. If the light paths being measured for anisotropy are extinguished and reemitted (at a different velocity) by the air surrounding the device, then how can the results of the experiment not be affected?

 

[Dale]

Because they are extincted (extinctioned, extinguished?) on both paths. The MMX detects anisotropy in the speed of light, not the speed of light. If there were anisotropy in the speed of light then there would be anisotropy in 1/n times the speed of light. And vice versa.

 

[pgf]

What about the final, shared path to the detector or eyepiece? It feels like extinction there should mask any differences created in the divergent paths. But my "feeling" about this isn't worth much -- I realize that. :-)

 

[ghwellsjr]

What is extinction?

 

[pgf]

What is extinction?

here's a start on wikipedia:
http://en.wikipedia.org/wiki/Extinction_theorem_of_Ewald_and_Oseen

and this chapter intro to "Classical Optics and Its Applications":
http://books.google.com/books?id=Nrkfmf4RizIC&lpg=PP1&pg=PA168#v=onepage&q&f=false

 

[Dale]

What about the final, shared path to the detector or eyepiece? It feels like extinction there should mask any differences created in the divergent paths.

Why would you think that?

 

[pervect]

It's well understood that extinction effects must be considered when conducting experiments to measure the velocity of light from moving sources. Fox showed that all such experiments prior to the early 60s were flawed on that basis.

I'd like to understand why early M-M type experiments aren't flawed in the same way, or, if they are, why no one mentions it.

For instance, in the often-cited FAQ on SR experiments (http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html), many of the moving source experiments are flagged with comments regarding whether extinction criticisms are warranted, but none of the M-M type experiments.

In the third cite below, Fox even seems to dismiss concerns about M-M experiments (second paragraph), but the reason for the dismissal isn't clear.

Is there something about the methodology of interferometry experiments which renders extinction irrelevant? I wouldn't have thought so, but I never see it discussed.

I'm not a physicist -- just an educated layman -- so perhaps I'm missing something subtle. Or, maybe even something obvious!

paul


* Fox, J. G. (1962), "Experimental Evidence for the Second Postulate of Special Relativity", American Journal of Physics, Volume 30, Issue 4, pp. 297-300
* Fox, J. G. (1965), "Evidence Against Emission Theories", American Journal of Physics 33 (1): 1–17
* Fox, J. G. (1967), "Constancy of the Velocity of Light," J. Opt. Soc. Am. 57, 967-968 (which seems to be available here: http://mysite.verizon.net/cephalobus_alienus/papers/Fox_1967.pdf )

My understanding of the situation is that MM type experiments won't test emission theory at all, which seems to be the sort of theory that Fox is interested in testing.

For instance Wiki (which is unfortunately not necessarily reliable) says:

Emission theory, also called emitter theory or ballistic theory of light, was a competing theory for the special theory of relativity, explaining the results of the Michelson–Morley experiment.

From your URL http://mysite.verizon.net/cephalobus_alienus/papers/Fox_1967.pdf

We are not concerned here with experiments of the Michelson Morley type, which have conclusively disproved of the Ether. We are concerned rather with experiments which are meant to ... distinguish between SR and ... Ritz's emission theory.

So it appears that Fox, at least, thinks that the MM experiments do disprove some classes of theories, but not others.

There have been some replications of the MM done in vacuum (See http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html if this is of interest.

G. Joos, Ann. Phys. 7 385 (1930). An excellent repetition of the MMX, in vacuum.

 

[pgf]

What about the final, shared path to the detector or eyepiece? It feels like extinction there should mask any differences created in the divergent paths.

Why would you think that?

Why would I think that? Ignorance, no doubt. But here goes: If the only difference between the divergent light paths is expected be their velocity, then extinction in the final light path, which will presumably equalize their velocities, would seem to eradicate what is hoped to be measured.

 

[Dale]

An interferometer measures phase differences. Over the last leg they don't accumulate any more phase difference, but that doesn't alter the phase they already accumulated. Extinction doesn't change that at all.

You seem to think that extinction results in completely unknown behavior of light. It does not, it simply gives a characteristic length over which light is expected to interact with matter in the known way. It is important for measurements of the speed of light and tests of the ballistic theory of light.

 

[ChrisVer]

here's a start on wikipedia:
http://en.wikipedia.org/wiki/Extinction_theorem_of_Ewald_and_Oseen

It sounds similar to the Huygen's Principle...

 

[pgf]

An interferometer measures phase differences. Over the last leg they don't accumulate any more phase difference, but that doesn't alter the phase they already accumulated. Extinction doesn't change that at all.

You're right this seems to be the crux of my problem. I have trouble picturing light being reemitted with a new velocity without also being reemitted with a new phase. But clearly that's not what happens.

Thanks for your patience!

 

[Dale]

You are welcome. I am glad I could help.