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MMX and the extinction theorem

  1. Aug 28, 2014 #1

    pgf

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    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 )
     
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  3. Aug 28, 2014 #2

    pgf

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    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:
     
  4. Aug 28, 2014 #3

    Dale

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    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.
     
    Last edited: Aug 28, 2014
  5. Aug 28, 2014 #4

    pgf

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    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?
     
  6. Aug 28, 2014 #5

    Dale

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    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.
     
  7. Aug 28, 2014 #6

    pgf

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    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. :-)
     
  8. Aug 28, 2014 #7

    ghwellsjr

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    What is extinction?
     
  9. Aug 28, 2014 #8

    pgf

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  10. Aug 28, 2014 #9

    Dale

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    Why would you think that?
     
  11. Aug 28, 2014 #10

    pervect

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

     
  12. Aug 28, 2014 #11

    pgf

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    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.
     
  13. Aug 29, 2014 #12

    Dale

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    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.
     
    Last edited: Aug 29, 2014
  14. Aug 29, 2014 #13

    ChrisVer

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  15. Aug 29, 2014 #14

    pgf

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    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!
     
  16. Aug 29, 2014 #15

    Dale

    Staff: Mentor

    You are welcome. I am glad I could help.
     
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