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MMX explanation question

  1. Apr 7, 2013 #1
    I am wondering whether MMX was based on faulty assumptions that light waves would behave like sound waves.
    1) What if MMX detected a phase shift? It would just mean that speed of light is c and it goes through different distance, with or without Ether justifying the shift.
    2) Alternatively they would not detect any shift even when Ether was present since speed of light is always the same as they do not behave as sound waves.
    It does not prove existence or non existence of Ether or constant speed of light.

    What am I missing here?
    Last edited by a moderator: Apr 7, 2013
  2. jcsd
  3. Apr 7, 2013 #2


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    Yes, it was, in the sense that M&M as well as most all scientists of the time believed that light did propagate in a medium (but differently than sound waves).
    If they did detect a phase shift, then we'd be living in a universe with different physics so anybody's guess is as good (or bad) as anybody else's.
    You're not missing anything.

    In fact, Lorentz, et al, came up with a perfectly logical and consistent theory that was based on an absolute rest state for the presumed ether only in which light traveled at c. Since they believed that the surface of the earth must always be traveling through this ether, length contraction (and later time dilation) of the experimental apparatus would explain the null result.
  4. Apr 7, 2013 #3


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    For #1: The experiment does not observe a phase shift, so it doesn't much matter what it might be telling us if it had observed a phase shift.

    For #2: Yes, the M-M experiments are consistent with a constant speed of light, ether or no. These experiments disprove only those theories in which light has a constant speed with respect to a hypothetical ether that would define a universal rest frame and therefore not with respect to other observers.

    Experiments never prove any theory. All they do is disprove competing theories.
  5. Apr 8, 2013 #4
    What do you mean differently than sound waves? Are you just referring to the fact that they thought light was a transverse oscillation in the aether, as opposed to sound which is a longitudinal oscillation in air?
    I wish you wouldn't put so much emphasis on the "absolute rest" part, because it can be a bit misleading. To be clear, they believed that the principle of relativity was true, which to them meant that the "true" laws of physics were invariant under Galilean transformations. So when they saw that Maxwell's equations weren't Galilean invariant, their conclusion was that these weren't the true laws of physics; instead, they thought these laws were akin to the wave equation for sound, i.e. laws that were only true in one frame, not the real laws of physics that hold for all inertial frames.

    Then the Michelson Morley experiment showed that Maxwell's equations were true in all frames despite not being Galilean invariant. This posed a threat to the principle of relativity, and Lorentz resolved this by saying that Maxwell's equations appeared to be true in all frames because of length contraction and time dilation, but the real laws of physics are actually something else, equations that really ARE Galilean invariant and really do hold in all frames. (These modified equations were found by Hertz, and can be found by expressing Maxwell's equations in terms of "uncontracted length" and "undilated time".)
    Last edited: Apr 8, 2013
  6. Apr 8, 2013 #5


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  7. Apr 8, 2013 #6
    ghwellsjr, I think you may have replied in the middle of me editing my post. Did you see the rest of it?
  8. May 5, 2013 #7
    Thanks for replies. As I get it so far:

    1) MMX always assumed that speed of light was c in Ether, just like speed of sound in constant in a media. It also assumed that Galileo's relativity was correct.

    2) So if earth moves through it, it will take longer or shorter for it to travel along Ether than perpendicular direction, assuming Galileo's relativity was correct.

    3) That was not observed. So the theory suggested that there is no Ether. (I am not clear of this conclusion yet since even if you replace Ether with Vacuum, Galileo's relativity can hold.)

    4) But then how do you equate the equations for time in horizontal and perpendicular direction? So the hteory said that if we "assume" time dilation/length contraction they equate. There was no explanation given by MMX. Lorentz tried to explain by saving that electron in direction of motion can contract the size of atom but had no proof.

    5) Later Einstein "assumed" that speed of light is not only c but also independent of relative motion of observers and worked back words to come at same result but had a better explanation for length contraction/time dilation. He used invariant of length to come to same result.

    Is this correct so far?
  9. May 6, 2013 #8
    I read it and learned something from it. Thanks! :smile:
  10. May 6, 2013 #9
    splitting hairs, but Einstein didn't assume. The assumption was a medium of sorts. Einstein went on what the evidence (experiments) was showing (proving). Minkowski put a pretty bow on it; no Ether, just spacetime.
  11. May 8, 2013 #10


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    Didn't Lorentz, et al, believe in a "luminiferous ether"? Or was Einstein ill-informed?
  12. May 8, 2013 #11
    Yes, he definitely did. What in my post contradicts that? Lorentz believed that light was a propagation of the aether, which did not contradict the principle of relativity any more than sound being a propagation of air.
  13. May 9, 2013 #12


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    After I said:

    ...you said:

    ...and since you now say you agree with my first comment, I don't understand what is misleading about putting an emphasis on the "absolute rest" part which is the significant aspect of the "luminiferous ether", isn't it?
  14. May 9, 2013 #13
    When you use the term "absolute rest", it suggests that Lorentz and his contemporaries didn't believe in the principle of relativity, which is emphatically not true. (That kind of absolute rest was what Aristotle believed, not the aether theorists.). They thought that Maxwell's equations were only true in the aether frame, just as the sound wave equation is only true in the air frame, but they thought that neither of these facts contradicted the principle of relativity. This is because they thought that neither of these sets of equations were "true" laws of physics. In their mind, the true laws of physics were still Galilean-invariant and were true in all frames.
  15. May 9, 2013 #14


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    To me, the term "absolute rest" implies "absolute time" and "absolute space", referring to the concept that a second here is the same as a second there and everywhere, and a foot here is the same as a foot there and everywhere. It doesn't mean that they believed they could ever locate the state of "absolute rest", just that they believed nature operated according to it. In other words, nature conspired to hide the state of "absolute rest" by modifying the rate at which moving clocks ticked and the lengths of moving objects along the axis of motion so that the it gave the illusion of relativity. It never occurred to them to think in terms of time and space being relative in the sense that Einstein did. If you have ever followed my comments, I have always said that the difference between SR and LET is that they both accept the same first postulate of the Principle of Relativity but they accept a different second postulate regarding the propagation of light.

    By the way, I agree with the rest of your comments and so I'm wondering if there is a way to express these ideas succinctly without going into all the details every time the subject comes up.
  16. May 9, 2013 #15
    If all you're saying is that they had absolute notions of space and time, I agree with that wholeheartedly. They thought that the length of an object, and the duration between two events, had definite, objective values, that did not depend on how observers were moving around. But "absolute rest" has a different meaning: it means that there is such a thing as being "truly" at rest, independent of the motion of different observers. That is something that the physicists of the late nineteenth century did not believe. They thought the universe treated all frames equally, in the sense of thinking myself as at rest and you as in motion is equally valid as the other way around.
    If you replaced "the state of absolute rest" with "the rest frame of the aether", I would have no quibbles with it. But the way you say it makes it seem like they thought the universe has a preferred frame, and it's because of length contraction and time dilation that it seems that it doesn't have a preferred frame. But that's the exact opposite of what they believed. They thought the universe *appeared* to have a preferred frame, thus seeming to violate the principle of relativity. But they thought that if you were able to accurately measure lengths and times, then you would find out that the universe really doesn't have a preferred frame. Specifically, you would find that the real laws of electromagnetism are Galilean-invariant and hold in all frames.
    Yes, I agree with that; due to their absolute notions of space and time, they thought the principle of relativity meant Galilean invariance, so they thought that length contraction and time dilation must just be things that lead to inaccurate measurements, as opposed to things that lead to a new, genuinely valid coordinate system.
    I have absolutely no problem with that, other than the minor proviso that Lorentz's historical theory had physical explanations for length contraction, time dilation, and mass increase.
    We do seem to have an awful lot of agreement, but I think there is still something disagree about, because there was something we were arguing about in this thread.

    As far as expressing these ideas succinctly, what ideas in particular do you want to express?
  17. May 10, 2013 #16


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    I'm saying both. Belief in absolute notions of space and time is the same as belief in an "absolute rest", it's just that they came to realize that they could never identify that state. Thus, as I have repeatedly said, they believed that nature operated on the state of absolute ether rest but conspired to hide that state from us so that it was impossible to perform any experiment that would violate the Principle of Relativity.

    Yes, that is what I'm saying.

    Now that I don't understand and don't agree with. I thought they had a split understanding of the Principle of Relativity, one set of transforms, Lorentzian, that applied to electromagnetism and Maxwell's equations and one set of transforms, Galilean, that applied to mechanics. But in all cases, there were no experiments that violated the Principle of Relativity and so I don't know what you mean by the universe "appeared" to have a preferred frame. Maybe that thought would be the case before MMX, but it never panned out.

    That's because he was trying to resolve the differences in the two sets of transforms that he believed applied to different laws.

    Yes, that is a very interesting thread and I read it all. I really liked my arguments.

    I think the best way to deal with this is to look at Einstein's 1920 book on relativity. If you look at chapter 5, The Principle of Relativity (In the Restricted Sense), you will see that he deals directly with this topic. By the way, when he says, "in the restricted sense", he's talking about the Galilean transformation. And he points out that since there has never been any example of an experimental violation of the PoR, "This is a very powerful argument in favour of the principle of relativity".

    Then in chapter 7, The Apparent Incompatibility of the Law of Propagation of Light with the Principle of Relativity, he once again addresses this same issue that he brought up in his first paper on SR. At the end of the second to last paragraph, he says: "Prominent theoretical physicists were therefore more inclined to reject the principle of relativity, in spite of the fact that no empirical data had been found which were contradictory to this principle." So once again, he is pointing out that there has not been any experimental evidence against the Principal of Relativity. But he is stating the exact opposite of what I have been saying, namely that LET affirms Einstein's first postulate, the Principle of Relativity but rejects his second postulate. I believe he is saying this because Lorentz's transformation process when applied to Maxwell's equations are compatible with Einstein's second postulate. However, Einstein's important point is that having two sets of transformations is rejecting the Principle of Relativity on theoretical grounds, even if there is no evidence against it. And this is where it seems we differ, you believe that having two different sets of transforms is compatible with the Principle of Relativity.

    Finally, look at chapter 14 where Einstein points out that the resolution to the apparent conflict between his two postulates is to apply the Lorentzian transformation to all laws, not have two sets of transforms for different laws. In other words, change all the laws that previously conformed to the Galilean transformation so that they would now conform to the Lorentzian transformation and that's what they did.

    Now getting back to the difference in what Einstein says and what I have been saying, when I'm talking about the moving target called LET, I'm referring to where it ended up, as wikipedia says:

    You correctly identified this as my emphasis in your reference thread. And the reason I'm doing this is because I want to emphasize that Einstein's theory of Special Relativity affirms all Inertial Reference Frames (IRF's) and any one of them is sufficient to explain any scenario, just like the one illusive LET state of absolute ether rest. Some people think that each observer needs their own IRF to understand what they observe or that it provides additional insight over some other IRF in which they are not at rest. Some people think that SR proves that LET is wrong or that it proves that an absolute ether rest state cannot exist. Some people think that Special Relativity means that you have to have multiple IRF's in any given scenario. In fact, the only difference between (modern) LET and SR is the second postulate: SR says there is no preferred IRF and light propagates at c in all of them whereas LET says that there is a preferred IRF, the only one in which light propagates at c. And there is no experimental evidence to help us decide between these two theories so if we want to choose between them, we choose on philosophical grounds.

    So I like to point out that even if someone believed in an "absolute rest" and its inherent "absolute time" and "absolute space", they would still be better off forgetting about LET and wholeheartedly adopting Einstein's theory of Special Relativity as being a simpler theory.

    You wanted me to use "the rest frame of the aether" instead of "the state of absolute rest" but I'm wondering if you would have any problem with "the absolute rest frame of the aether"?
  18. May 11, 2013 #17


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    I think the historical point that lugita15 is making is that Lorentz and his contemporaries made a subtle distinction between properties of space & time (what we would nowadays call spacetime) and properties of the supposed aether that filled spacetime. That distinction is often lost in modern-day presentations of LET. If my memory is correct, Lorentz interpreted the Lorentz transform as[tex]\begin{align}
    \begin{pmatrix}\gamma && - \gamma v \\ -\gamma v && \gamma \end{pmatrix}
    \begin{pmatrix}1 && - v \\0 && 1 \end{pmatrix}
    \begin{pmatrix}\gamma^{-1} && 0 \\ 0 && \gamma \end{pmatrix}
    \begin{pmatrix}1 && 0 \\-v && 1 \end{pmatrix}
    \textbf{L} &=\textbf{T} \cdot \textbf{E} \cdot \textbf{G}
    [/tex]I'm using a matrix notation that Lorentz wouldn't have used, with [itex]c=1[/itex] and [itex]\begin{pmatrix}t\\x\end{pmatrix}[/itex] coordinates.
    • G is the Galilean transform, a relative property of Galilean spacetime
    • E represents time dilation and length contraction, an absolute property of the supposed aether. Note that EG alone is enough to explain the Michelson-Morley experiment.
    • T expresses Lorentz's notion of "local time" (what we would now call a clock synchronisation convention), and I'm not sure if Lorentz had any explanation for it other than something needed to make Maxwell's equations invariant.
  19. May 11, 2013 #18
    No, that's not true. Newton and Galileo believed that there was such a thing as the true length of a rod or the true duration between two events. But they did not believe that there was such a thing as being "truly" at rest or "truly" in motion. (That's what Aristotle believed, though.) They thought that all inertial frames are equally valid. The physicists of the late nineteenth century still shared all these beliefs.
    They did indeed believe that nature used length contraction and time dilation to hide the rest frame of the aether from us. But they did not see the aether as being "truly" at rest. They thought that the universe treated all frames as equal. The role of the aether frame for light seemed no more unusual to them then the role of the air frame for sound.
    No, they thought the Principle of Relativity exclusively meant Galilean invariance. The fact that the laws of electromagnetism seem Lorentz invariant just indicated to them that their measuring equipment was faulty due to length contraction and time dilation. They were still confident that accurate measuring rods and clocks would show Galilean invariant laws of electromagnetism.
    As I said, to them the Principle of Relativity and Galilenan invariance were the same. So before Michelson-Morley, they put Maxwell's equations in the same category that they put the wave equation for sound: equations that weren't Galilean invariant, and thus were not the true laws of physics. But then Michelson-Morley showed that Maxwell's equations really do seem to be actual laws of physics. But if the actual laws of electromagnetism were not Galilean invariant, in their mind this would mean that the Principle of Relativity was false. That's what I meant when I said the universe appeared to have a preferred frame; Michelson-Morley seemed to indicate that the universe doesn't respect the Principle of Relativity.

    Lorentz's solution to this was to say "Don't worry, the laws of physics are Gailiean invariant, but length contraction and time dilation make it seem like they're not. If Michelson-Morley conducted their experiment with accurate equipment, they would find that electromagnetism obeys Galilean invariance. So the Principle of Relativity is still true."

    Einstein's solution was to say "Michelson-Morley was accurate, so Galilean invariance really is wrong, but the Principle of Relativity is still true. You just need to throw out your absolute notions of space and time."
    No, he did not believe that the Lorentz transformations genuinely applied to the laws of electromagnetism. He believed that they appeared to apply, but Galilean transformations are what really applied.
    I'm sure you do. :smile:
    I think he is referring to the fact that after the Michelson-Morley experiment, many physicists (other than Lorentz) started to doubt the Principle of Relativity. That's because Michelson-Morley seemed to show that the speed of light is c in all reference frames, so Maxwell's equations seemed to accurately describe electromagnetic phenomena in all reference frames, which would mean that the real laws of electromagnetism are not Galilean invariant.
    Yes, he is pointing out that even though some physicists thought the Michelson-Morley experiment rejected the Principle of Relativity, it didn't actually identify a preferred frame or show that different laws of physics hold in different frames, which is what you'd expect for a refutation of the Principle of Relativity.
    I think Einstein is not referring to Lorentz ether theory. He's referring to Lorentz's analysis of Michelson-Morley, where Lorentz concluded that Maxwell's equations appeared to hold in all frames, so the speed of light appeared to hold in all frames. Of course Lorentz's reaction to this conclusion was to say that this appearance was deceptive, based on length contraction and time dilation.
    When did I say anything like that?
    Lorentz believed that the Lorentz transformations appeared to apply to all laws of physics, but they actually applied to none of them. Einstein's resolution to say that the Lorentz transformations genuinely applied to all laws of physics, which meant that the notions of space and time that gave rise to the Galilean transformations were in need of rethinking.
    Once again I'd quibble with the word "absolute" if you mean it in the sense of one frame being truly at rest or one frame being preferred by the universe over all others. But other than that, I agree with you. In Lorentz's theory, if you were in any frame other than the aether frame, then your rods and clocks were inaccurate, but in Einstein's theory you could use the readings from your own rods and clocks without fearing that you were making a mistake.
    We're in complete agreement in all that.

    EDIT: Except possibly for the part about "preferred". If by "preferred" you mean that the aether frame was the only one in which Maxwell's equations held and light propagates at c, then I'm fine with that. But if by preferred you mean you mean that it's its privileged by the universe over other frames, then that's not quite right. It is true that people in other frames make inaccurate frames according to LET, but that's not due to a problem with the frame, that's due to physical effects from the aether that makes their measurements off. If they were able to use the "true" coordinates of their frame, as opposed to the "apparent" coordinates of their frame, then the universe would treat that frame the same way it treated the aether frame. So in that sense Lorentz saw himself as firmly in the Principle of Relativity camp.
    Again, I disagree that absolute rest is the same as absolute space and time.
    I'd still object to the word absolute. Would you similarly say "the absolute rest frame of air"?
    Last edited: May 11, 2013
  20. May 12, 2013 #19
    Lorentz didn't have much of an explanation, but Poincare came up with something: he said that due to length contraction and time dilation, observers in frames other than the aether frame would synchronize their clocks incorrectly if they used light signals (basically Einstein synchronization).
  21. May 14, 2013 #20
    Maybe we should look what Lorentz himself wrote. In 1895 he argued in the introduction of his influential "Versuch..." paper:
    http://en.wikisource.org/wiki/Attempt_of_a_Theory_of_Electrical_and_Optical_Phenomena_in_Moving_Bodies/Introduction [Broken]

    However, in 1910 he argued that it is a matter of taste, whether one adopts Lorentz's own view of "true" times and lengths in the "preferred" aether system, or Einstein's and Minkowski's relativistic ones:
    So even though Lorentz remained a believer in the aether his entire life, he also admitted that his aether-and-true-time ideas were the cause for the failure of his pre-1905 papers to reach exact Lorentz covariance of electrodynamics. He wrote in 1914:

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