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Ether hypothesis and M/M Experiment not needed

  1. Oct 11, 2008 #1
    ...For Relativity...
    In fact, Relativity should have come about even if there was no ether hypothesis to justify the Law of the Constancy of the Velocity of Light. In fact, I'm kind of surprised James Clerk Maxwell didn't come up with Special Relativity himself in 1857...
     
  2. jcsd
  3. Oct 11, 2008 #2

    JesseM

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    Relativity comes about if you assume Maxwell's laws work in every inertial frame, but there's no reason to assume this should be true in the absence of experimental evidence. The idea of the ether hypothesis is that they would only work precisely in the rest frame of the ether, in other frames you'd have to modify them with a Galilei transform.
     
  4. Oct 11, 2008 #3
    Hello Bible Thumper

    The existence of the ether as a propagating medium for EM radiation led to the prediction that the speed of light, while constant relative to the ether, would be observer dependent. That is dependent on the observers motion relative to it (the ether), which is what the MM experiment was expected to find. I don't see how the ether hypothesis, and i take this to mean the existence of the ether, was required to justify the constancy of c.

    However one could use the term constancy of the speed of light to mean that it is constant in any particular frame, but different between frames. In the light of SR this interpretation would be wrong.

    Matheinste.
     
  5. Oct 11, 2008 #4
    You'd think that the ether would be the first and most intuitive explanation for the constancy of c. But! Suppose Maxwell did consider time and space, and that he came up with the Principle of Relativity, which is an idea completely independent of any experiment (Michelson/Morley) or hypothesis (ether).
    All he'd have to do is intuitively connect his nascent Law to the idea of the Principle of Relativity, and voila! The Special Theory of Relativity! :)
     
  6. Oct 11, 2008 #5
    No ether or experimentation needed.
     
  7. Oct 11, 2008 #6
    The Principle of Relativity, which A. Einstein said "sounded so simple and natural" that it had to be true, automatically generates independent frames of reference. No ether needed.
     
  8. Oct 11, 2008 #7

    Hurkyl

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    If you're going to make bold assertions, then you need to back them up.
     
  9. Oct 11, 2008 #8

    JesseM

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    To get the principle of relativity he'd also have to postulate that all the other laws of physics are invariant under the Lorentz transformation too, not just Maxwell's laws. This is a pretty big leap, considering that the non-electromagnetic Newtonian laws known in Maxwell's time didn't have this property (though they turned out to just be approximations to Lorentz-invariant laws).
    Did Einstein really say it "had to be true"? Where?
     
  10. Oct 11, 2008 #9

    atyy

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    Schutz, Gravity from the Ground Up, p6, http://assets.cambridge.org/97805214/55060/sample/9780521455060ws.pdf

    Apparently the Principle of Relativity was known before Maxwell, having been enunciated by Galilei. Do you think Maxwell was unaware of it?

    Galilei also formulated the Principle of Equivalence. Newton was aware of this, stated it in his Principia, and used it to solve a problem.
     
  11. Oct 11, 2008 #10
    This book:
    Written by The Man himself.
    If you can get it on-line thru Google books, read the part about the Principle of Relativity. In it, he tells us he came by the idea before he read about the results of the Michaelson/Morley experiment. He described the idea (as best my memory can serve) as the Principle whereby all natural laws have to be maintained, regardless of what reference frame the observation of the experiments on the natural law is taking place.
    And since c=300K regardless, it stands to reason that time and space have to go thru contractions relative to observers.
     
  12. Oct 11, 2008 #11
    I thought the Principle of equivalence was strictly 1906 thought-experiment work?
     
  13. Oct 11, 2008 #12

    JesseM

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    The book is online here, can you find the part where he said it "had to be true"? I note that on this page he seems to say there is no way to rule out the possibility the principle of relativity might be wrong a priori, although he then gives some reasons why we might consider "a priori not very probable" that it'd be false:
     
  14. Oct 11, 2008 #13

    atyy

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  15. Oct 11, 2008 #14

    atyy

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    I don't know about Special Relativity, but there's a famous quote to this effect for General Relativity: "Then I would feel sorry for the good Lord. The theory is correct anyway. (1919, reply to his assistant, Ilse Rosenthal-Schneider, who asked what he would have done had Eddington's eclipse measurements not supported general relativity)" http://physicsworld.com/cws/article/print/23008

    I think JL Martin's GR text says that Maxwell knew gravity was a "kinematic force" like the centrifugal force, and was working on a modification of Newton's gravity that had waves like electromagnetism or something like that. He even wonders what would have happened if Maxwell had not died at age 48.

    I personally would not accept the Principle of Relativity without experimental evidence, though I think I'd be quite disturbed if it had been found to be false. But I'm fond of absolute space too - relativity lets us have our cake and eat it, since every inertial frame is as good as absolute space.
     
  16. Oct 12, 2008 #15
  17. Oct 12, 2008 #16

    Dale

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    The invariance of c was not known at the time of Maxwell. Why would he have tried to justify a strange experimental result that had not yet been discovered and was not predicted by any theory of the time?
     
  18. Oct 12, 2008 #17
    Here's what I paraphrased:
    Here, Einstein said the Principle should be retained by sheer virtue of its simplicity and naturalness.
    Immediately after, he says this:
    Which stemmed from this:
    If you have followed me up to this point, you will note that A. Einstein favored the Principle of Relativity, but it was the then-recent research from Lorentz that caused him to create the Special Theory; to unify Lorentz's work and the Principle of Relativity.

    Could he have done this without Lorentz's work? As my quoting shows, he could have; the only thing required is a formal, rigorous definition of time in physics. Here's what Einstein says with regards to the idea:
    And that quote came after this one:

     
  19. Oct 12, 2008 #18
    Maxwell created the "invariance of c". He called it, The Law of the Constancy of the Velocity of Light. This law states that c is invariant. It stemmed from Maxwell dividing the equations for electricity and the equations fro magnetism.
    A constant, c, was the result of that division.
     
  20. Oct 12, 2008 #19

    Dale

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    Could you give a reference for Maxwell's "Law of the Constancy of the Velocity of Light" that you mentioned above?

    As far as I know Maxwell supported the idea of the luminiferous aether. I.e. he believed EM waves require a medium and that his equations predicted the speed of light only wrt that medium. So in a reference frame moving wrt the aether the speed of light would be something other than c.
     
  21. Oct 12, 2008 #20
    I got the reference from a lecturer at Cal Tech. He said the Law was initially Maxwell's idea. Maxwell allegedly got the idea after dividing his two equations and unifying magnetism/electric fields.
    I tried googling it; couldn't find anything on paper, either.
     
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