Relativity vs competing theories

  1. hey guys,

    I'm just wondering about the existing competing theories; it's been mentioned in a number of threads that there are a couple of theories which equally explain the results of relativity experiments, but which are based on different underlying principles. I've also read that relativity is the only one that involves relativity of simultaneity; is that true? Do the other theories involve absolute simultaneity?

    I'm also wondering what those other theories are?
  2. jcsd
  3. The only one I am aware of is Lorentz aether theory (LET). There is some disagreement on another thread about what LET is and questions like the one you raised. LET was essentially abandoned early on, so it doesn't have a real clear definitive interpretation. So the arguments tend to be difficult to resolve as far as what LET itself means.
  4. bcrowell

    bcrowell 6,397
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    There are probably quite a few theories of classical gravity that are consistent with all the empirical evidence. Whether these are particularly well motivated is a matter of taste.

    Brans-Dicke gravity is viable, but only for values of the omega parameter that make it very similar to GR.

    Østvang’s quasi-metric relativity:

    Jacobson Einstein-Aether gravity: Ted Jacobson, David Mattingly, "Gravity with a dynamical preferred frame," 2000, ; Ted Jacobson, "Einstein-aether gravity: a status report," 2008, ; (has experimental tests)

    Of these three, the only one I really know anything significant about is BD gravity, which incorporates relativity of simultaneity in the same way as GR. I can't imagine how any viable theory of gravity could fail to incorporate relativity of simultaneity, since that's a feature of SR, and SR has been verified to extremely high precision.
    Last edited: Feb 5, 2012
  5. Oh, good point. I was just talking about SR, not gravity.
  6. tom.stoer

    tom.stoer 5,489
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    I would like to add Einstein-Cartan-theory which is (as an enhancement of GR) indistinguishable from GR due to the tininess of the spin-current and torsion effects, but which is favoured by theoretical reasons.
    Last edited: Feb 6, 2012
  7. Probably you refer here to Special Relativity (SR) and physical models for it ("metaphysical" explanations). As a matter of fact, SR is based on postulates about phenomena and not directly on physical models. However, the second postulate was based on the success of the Maxwell-Lorentz model of light propagation, and that theory involved the concept of absolute simultaneity. Since SR emerged from those foundations, it is not really a "competing" theory. Einstein phrased it as follows:

    "We [...] assume that the clocks can be adjusted in such a way that
    the propagation velocity of every light ray in vacuum - measured by
    means of these clocks - becomes everywhere equal to a universal
    constant c, provided that the coordinate system is not accelerated.
    [..this] "principle of the constancy of the velocity of light," is at
    least for a coordinate system in a certain state of motion [..] made
    plausible by the confirmation through experiment of the Lorentz theory
    [1895], which is based on the assumption of an ether that is
    absolutely at rest".

    The essential point of SR is that "absolute rest" cannot be detected; and consequently also no "absolute simultaneity" can be detected either. SR only relates to things that can be detected.

    Addendum: there are competing explanations for SR, just as there are also many competing explanations for quantum mechanics. Some main explanations of SR:
    - the existence of a (3D) "ether" (also called physical space or vacuum)
    - the existence of a 4D physical "spacetime" (also called block universe or 4D ether)
    - shut up and calculate (a non-explanation, but possibly most popular) :wink:
    Last edited: Feb 6, 2012
  8. ghwellsjr

    ghwellsjr 5,122
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    Just in case you are asking about competing theories to Special Relativity (and not General Relativity), the wikipedia article on the One-way speed of light provides two, Lorentz ether theory and [STRIKE]Roger's[/STRIKE] Edwards' theory. The only difference between these three theories is the differing "second" postulate that they are based on. The "second" postulate has to do with simultaneity, that is, how they arbitrarily decide to divide the time of the round-trip measurement of the speed of light into its two parts, one for the outbound part of the trip and one for the inbound part of the trip. Special Relativity makes them equal. Lorentz ether theory makes them unequal except in one presumed state of rest. [STRIKE]Roger's[/STRIKE] Edwards' theory emphasizes that the division is truly arbitrary and let's you pick and choose any one you want in any particular situation.

    Please note that all three theories share the same "first" postulate, the principle of relativity. This adds to the confusion when you ask about "relativity" without specifying what you mean. I'm assuming that you meant "Special Relativity" which is one of the three relativity theories all based on the principle of relativity.
    Last edited: Feb 6, 2012
  9. If Einstein were still alive, he would probably call that Wikipedia article extremely misleading as it turns a fly (different ways of formulating or interpreting the same theory) into an elephant (different theories)... :tongue2:
  10. ghwellsjr

    ghwellsjr 5,122
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    You just quoted Einstein in which he referred to Lorentz theory so I think the facts do not support your speculation.
  11. ghwellsjr

    ghwellsjr 5,122
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    It is also an essential point of LET and Edwards' theory that "absolute rest" cannot be detected--that's the whole point of the "principle of relativity" that Einstein discussed in the 1907 paper that you referenced. It is true that "absolute simultaneity" cannot be detected but that doesn't mean that a "relative simultaneity" can be detected. Einstein's whole point, if you read what he says, is that no simultaneity can be detected, so for you to claim that SR relates to things that can be detected is wrong. That's why these three theories are indeed three separate theories, because they are each built on a different "second" postulate that defines the meaning of simultaneity in three different ways. Simultaneity of any type cannot be detected. All three of these theories equally comport with all experimental evidence. There is no way to detect which one of these three theories is preferred.

    Once you understand that Edwards' theory lets you decide how to define simultaneity (which was really Einstein's idea all along), then you can see that Einstein's arbitrary choice is the preferred one because it is simpler, not because it is based on something that can be detected.
    Last edited: Feb 6, 2012
  12. Here is a suvery of the experimental basis of special relativity...supporting experiments.

    In brief:
    But this article does not discuss, that I could see in a quick scan, if any other theories also meet these all these test criteria.
  13. ghwellsjr

    ghwellsjr 5,122
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    Sure it does, right here:
  14. That point about "absolute rest" is an interesting one, because the effect of Einstein's relativity (and perhaps others) appears to be the same as treating inertial reference frames as being absolute rest frames, from their own perspective.

    Einstein's theory involves relative simultaneity; do the other theories involve absolute simultaneity?
  15. ghwellsjr

    ghwellsjr 5,122
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    This is true for Einstein's Theory of Special Relativity but not for the others.
    The expression "relative simultaneity" only makes sense when the idea of "relative time" is significant. Prior to Einstein's ideas, it just didn't occur to anyone that time could be actually relative. Even though LET uses all the same equations, the terminology is that what we now call "Proper Time" (the legitimate time displayed on any clock) was called by them "local time", meaning that it was not really the time that nature operates on, but rather a "modified" or "corrupted" time. They viewed the time on their clocks as suffering from an unknown amount of time dilation caused by unavoidable motion through the ether.

    I looked up Edwards' paper and I would say he wasn't promoting a new theory but merely elaborating on Einstein's observation that the two parts of the round-trip speed of light measurement can be assigned arbitrarily in any proportion, including making the speed in one direction c/2 and ∞ in the other direction.

    He has a section on using the slow transport of clocks "for the absolute synchronization of clocks in a coordinate system" and he concludes "a clock that is moved to all positions in a coordinate system to set the other clocks does not establish an absolute synchronization" [emphasis his].

    But he concludes his paper saying, "For most problems the most convenient assumption to make is still that of isotropic space", in other words, Einstein's synchronization convention.
  16. Under LET, how is "the time that nature operates on" determined?
  17. ghwellsjr

    ghwellsjr 5,122
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    It isn't. It's just presumed that it exists. Put yourself back before Einstein's time. Would you ever imagine that time runs differently under different circumstances? Not just that your clocks might be affected by those circumstances but that time itself is relative? Or that each observer determines that the other ones time is running slower? It's one thing to believe in Galilean relativity where time and spatial distances are absolute where all you have to do is have the obvious sort of offsets to get from one frame to the other but what Einstein came up with is a giant leap and not at all obvious.
  18. I still disagree with this. I think that all aether theories, including LET, violate the principle of relativity. That is, IMO, the point of the aether.
  19. That's what I was thinking, I just wanted to make doubly sure, cheers.

    Just to try and get a better understanding of the differences, would you by any chance know how the light clock on the train experiment would be interpreted under LET?

    Would the clock on the train slow down in that reference frame?
  20. ghwellsjr

    ghwellsjr 5,122
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    Exactly the same way as under SR.
    Yes, but nobody knows by what amount.

    The difference is that LET would never consider the ground frame because it represents the stationary ether and we don't know where that is. We're all doomed to ride around on the train having our clocks running slow (by an unknown amount) and having our lengths contracted along the direction of the train (again, by an unknown amount).

    Einstein said, "You don't need the ground--just assume that you are stopped and anyone moving with respect to you is on a different train. All you care about is the relative difference in your respective trains. And everybody agreed.
  21. That's not what I meant; probably I should have written that SR only relates to things can be experimentally verified. My point was that while the predictions of a theory of physics relate to the setting of measurement instruments, no theory depends on those settings - also not SR. No theory postulates how experimenters will act. On a side note, relative simultaneity is detected continuously with GPS receivers in the sense that corrections must be made for the motion of the geoid.
    Last edited: Feb 7, 2012
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