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Consistency of the speed of light

  1. Sep 12, 2005 #1
    Hi:

    Einstein's second postulate states that the speed of light is constant as viewed from any frame of reference. Most of the books on relativity that I have been reading usually ask the reader to accept that fact because proving it is behind the scope of the book. Can anyone help me understand the actual reason behind the second postulate?

    thanks
     
  2. jcsd
  3. Sep 12, 2005 #2

    Aether

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    Moneer81, a "postulate" is something that you assume to be true at first for the sake of argument. In this case, the whole special theory of relativity follows from the postulates that the laws of nature are the same in all inertial frames, and that the speed of light is the same in all inertial frames.

    So, the acutal reason behind the second postulate is this: special relativity follows from it + the other potulate. That does not mean that you could ever prove that it is true by doing experiments, and as it turns out you can't. On the other hand: nobody has ever proved that it is not true either, but they could.
     
    Last edited: Sep 12, 2005
  4. Sep 12, 2005 #3
    hmm....that's interesting. I thought it was experimentally proven that the speed of light is constant from all frames of reference. I could swear that Brian Greene said that too in this book "The Elegant Universe".
     
  5. Sep 12, 2005 #4

    Aether

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    I can't speak as to exactly what it was that Brian Greene said in his book, but nope, that could never be proven. However, as a postulate, it is simply true by definition within the framework of special relativity. However, don't let me give you the wrong idea: accepting that postulate for the sake of argument is usually a safe bet so long as you keep in mind that it's a postulate and not something that you could ever prove by making measurements.
     
    Last edited: Sep 12, 2005
  6. Sep 12, 2005 #5

    LURCH

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    It has been proven that the speed of light has been the same from all refference frames in which the speed of light has ever been measured. That doesn't meab that it couldn't be different in some instance that we simply haven't measured, or it may even be different in every instance when we are not looking. Maybe we're like a cop sitting at the side of the road with a radar-gun, who concludes at the end of the day that "all drivers everywhere in the universe travell at or below the posted speed limit", not realising that all those drivers are just waiting untill he;s not looking so they can floor it! Maybe light travells at all different speeds depending on what mood it's in, and only regulates itself to c when we're watching.

    But it seems unlikely to me.
     
  7. Sep 12, 2005 #6

    russ_watters

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    It has been. Aether (as his name would imply) is incorrect. Every attempt to measure the speed of light has always come up with the same answer, to within the margin for error of the experiment. Based on that, it is reasonable to theorize (and for the sake of further theories, postulate) that the speed of light is, in fact, always constant in inertial reference frames.

    While Aether is correct that postulates are assumptions for the purpose of the theory, they are based on experimental evidence - and in this case, also the math of Maxwell's equations. It is important to note that theories require postulates. While it is theory that the speed of light is constant, for the sake of logical consistency, it is necessary to assume it to be universallly true for the sake of building other theories on it.

    Postulates aren't just some random assumption you pull out of your a- er, um air.... They have to be grounded in reality to actually be useful.
     
  8. Sep 12, 2005 #7

    Aether

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    Special relativity is empirically equivalent to "...an ether theory taking into account time dilation and length contraction but maintaining absolute simultaneity..." R. Mansouri & R.U. Sexl, A Test Theory of Special Relativity: I. Simultaneity and Clock Synchronization, General Relativity and Gravitation, Vol. 8, No. 7 (1977), pp. 497-513. In such an empirically equivalent ether theory, the speed of light varies with direction.

    This is not just some cherry-picked statement from an article that I pulled out of thin a- er, um air, it is the very crux of a famous paper referenced by most if not all of the experiments published over the past 30 years which measure local Lorentz invariance.
     
    Last edited: Sep 12, 2005
  9. Sep 12, 2005 #8

    ZapperZ

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    This is not correct. A postulate does not mean one cannot verify it with experiment. One can and this is continually done on SR's postulates. A postulate on the other hand cannot be derived from First Principles! If it can, one doesn't need a postulate in the first place!

    Zz.
     
  10. Sep 12, 2005 #9

    Aether

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    This postulate has so far proven to be consistent with experiment, but so too has "...an ether theory taking into account time dilation and length contraction but maintaining absolute simultaneity...". No experiment has ever been able to distinguish between these two points of view, and if one ever does it could only favor the ether view.
     
    Last edited: Sep 12, 2005
  11. Sep 12, 2005 #10

    Tom Mattson

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    Of course the speed of light postulate can be verified. Just measure the speed of light from any moving source.

    In fact this has been done: Alvaeger F.J.M. Farley, J. Kjellman and I Wallin, Physics Letters 12, 260 (1964).
     
  12. Sep 12, 2005 #11

    JesseM

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    Perhaps we could at least agree that it is possible to experimentally test the statement that the speed of light will be measured to be the same by all observers, assuming these observers all use the same type of procedure to make their own measurements (ie if one observer uses rulers and clocks which are at rest relative to himself to make his measurements, then all observers must do so).
     
  13. Sep 12, 2005 #12
    The second postulate is an immediate consequence of Newton's first law of motion and the homogeneity of time.

    http://www.everythingimportant.org/relativity/
    http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=AJPIAS000043000005000434000001 [Broken]
    http://arxiv.org/PS_cache/physics/pdf/0302/0302045.pdf [Broken]
     
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  14. Sep 12, 2005 #13

    JesseM

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    No it isn't--it may be that you can use those assumptions to show that coordinate transformations must have the same general form as the Lorentz transformation equations, except with the constant c having the option of being infinite (and if you plug infinite c into the Lorentz transform you get the Galilei transform), as is suggested on that http://arxiv.org/PS_cache/physics/pdf/0302/0302045.pdf [Broken] that you linked to. But the c in this equation has no necessary connection to the speed of electromagnetic waves--it is perfectly compatible with Newton's first law and the homogeneity of time that there would be no time dilation so you could synchronize clocks just by setting them to the same time in one location and moving them apart (in which case, assuming there's no Lorentz contraction you'll be using the Galilei transform to convert between different observer's measurements), and yet light would only have a constant speed in one preferred frame, while in other frames it would be measured to go faster in one direction than the other.
     
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  15. Sep 12, 2005 #14

    Aether

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    It is the convention used for the synchronization of clocks which determines the outcome of a one-way speed of light measurement. If you postulate that the speed of light is isotropic, then clocks are synchronized by Einstein's convention, and...voila...all subsequent experiments measure a constant speed of light. That does not constitute experimental proof that the speed of light is constant; it is not possible to ever prove that by experiment. It is possible to disprove it upon the identification of a locally preferred frame, but that hasn't been done yet, and maybe it never will be.

    To prove that the speed of light is constant, you would have to prove a negative: e.g., "there is no locally preferred frame".
     
    Last edited: Sep 12, 2005
  16. Sep 12, 2005 #15

    ZapperZ

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    You have missed my objection. You said:

    "So, the acutal reason behind the second postulate is this: special relativity follows from it + the other potulate. That does not mean that you could ever prove that it is true by doing experiments, and as it turns out you can't."

    A postulate does not mean you can't verify it with experiments. Read this again. This has nothing to do with SR.

    And I'd like to see you come up with "time dilation" and "length contraction" based on your ether postulate, make just JUST fit together that they happen to cancel any variation to make it agree with the tons of experimental observations, and then publish it. Till then, you'll understand if I don't buy this.

    Zz.
     
  17. Sep 12, 2005 #16
    Merriam-Webster's dictionary defines postulate as:

    to assume or claim as true, existent, or necessary : depend upon or start from the postulate of b : to assume as a postulate or axiom (as in logic or mathematics)

    I guess the whole jargon is getting me a little confused, but what I know is that if we're gonna have believe a theory like SR then it better be based on something that makes sense, and for those of us that don't believe in faith, it better be something we can convince ourselves that it is valid.
     
  18. Sep 12, 2005 #17

    Aether

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    I agree that "a postulate does not mean that you can't verify it with experiments", but I don't see where I implied otherwise.

    I haven't suggested that you should buy an ether theory, yet. What I recommend that you should buy today is that the possibility has not been, and never will be, ruled out by experiment.
     
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  19. Sep 12, 2005 #18
    LOOOOL

    Sorry Aether, but that was a real good joke...original :rofl:
     
  20. Sep 12, 2005 #19

    HallsofIvy

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    There is a difference between a "postulate" in mathematics and a "postulate" in physics. In fact those blasted physicists should know better than to use the word "postulate" at all. I suspect that those who use it in a physics book are really saying "I don't want to bother to explain how this has be shown".

    The fact that the speed of light is constant in all frames goes back to the Michaelson-Morley experiment- it and versions of it are probably one of the most "repeated" experiments in the history of science.
     
  21. Sep 12, 2005 #20

    Aether

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    The round-trip speed of light, such as in MM experiments, is a constant in all frames because the issue of clock synchronization doesn't arise. However, that does not allow one to distinguish between special relativity and "...an ether theory taking into account time dilation and length contraction but maintaining absolute simultaneity...".
     
  22. Sep 12, 2005 #21

    JesseM

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    That's sort of true, but it's also true that as long as all observers use the same procedure to synchronize their clocks and construct their coordinate system (with no references to external markers--assume each observer is setting up their coordinate system in a windowless box), then if this procedure involves a definition of synchronization such that at least one observer measures light moving at the same speed in all directions, then all observers will measure light to move at the same speed in all directions in their own coordinate systems. Perhaps you could come up with some weird procedure that would result in all observers measuring light to have different speed in different directions, but it would probably be a pretty weird one--can you think of any examples of such a procedure? I suspect the only way to do it would be to use rulers that don't measure lengths the same even when placed side-by-side, or clocks that don't tick at the same rate even when placed side-by-side (and even then, I can't think of a universal procedure that all observers could use to decide how the different rulers and clocks should be oriented--if you have one ruler that's shorter than the other when they're put together, should the shorter ruler go on your left or your right? There's no experiment you can do in a windowless box that'll pick out a preferred axis in space, and you can't just pick randomly or you'll violate the assumption that each observer uses the same procedure to set up his own coordinate system).
     
  23. Sep 12, 2005 #22

    ZapperZ

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    And you should also never rule out the possibility that you could be entirely wrong (see? I can play that game too).

    And you should also never rule out that our knowledge of the semiconductors in our modern electronics are wrong despite of the huge amount of experimental verifications, so at any given time, your life could be in extreme danger, such as when you are in an airplane. You should never rule out that your house could just crash on you because, franky, newton's laws are based on principles that are never derived, but rather built on a bunch of postulates that were accepted to be valid only based on experiments.

    Apply that to EVERYTHING that you know, and you'll see the absurdity of the situation. You ACCEPT a bunch of things that you take for granted to be valid. How come you never raise any question on them, but simply use this mantra on the postulates of SR?

    Rather than showing a consistent and comprehensive theory of mechanics on the SAME grounds that SR has done, all you can do here is say "well, it could be wrong, even though I have no experimental evidence for it". Where exactly in the history of physics have things been proven to be wrong simply based on a matter of tastes and preference? Physics isn't a fad nor a fashion show!

    Zz.
     
  24. Sep 12, 2005 #23

    Hans de Vries

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    This is not true.


    Some very common effects depend entirely on the non simultaneity of Special
    Relativity. Take for example the EM wave front of electromagnetic radiation
    (say light) which is at right angles with de direction of motion. No matter
    from which reference frame you look a it.

    see: http://physicsquest.homestead.com/plane-wave.gif

    Now if you look from a reference frame which moves at 90 degrees with
    the light then you see that the direction of the light changes more the
    faster you go.

    But the wavefront also rotates. It always stays at right angles to the
    direction of motion of the wave.

    Now how can you ROTATE the pattern with only Lorentz contraction ??
    You can't. This is easy to see because the moving frame's speed is at
    90 degrees with the direction of the light, in parallel with the wavefront.

    It is the non-simultaneity of SR that performs the trick. In the moving
    frame one side of the wavefront has propagated further while the other
    side has propagates less, therefor the direction of the wavefront rotates.


    Regards, Hans
     
    Last edited: Sep 12, 2005
  25. Sep 12, 2005 #24

    Aether

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    I am not playing a game. I don't rule out the possibility that I could be entirely wrong, but I did quote a strong reference in my initial post and you have ignored that.

    God, I hope so...really. Not altogether wrong, but missing something important.

    Hmm, I would not rule that out either. For example, see the attached picture of the Mt. Palomar observatory complex...I took this picture a couple of weeks ago from the outside of a glider at 11,000ft!

    Please see the article that I quoted. It is referenced by most if not all of the published experiments testing local Lorentz invariance over the past 30 years.
     

    Attached Files:

    Last edited: Sep 12, 2005
  26. Sep 12, 2005 #25

    Aether

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    If you will take the time to look up the Mansouri-Sexl reference that I cited earlier, you will find a more general transformation than the Lorentz transformation which accounts for every so-called relativistic effect while maintaining absolute simultaneity.

    This article is referenced by most, if not all, of the published experiments testing local Lorentz invariance over the past 30 years!
     
    Last edited: Sep 12, 2005
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