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Why is the speed of light constant regardless of perspective?

  1. May 10, 2004 #1
    I understand why it needs to be for relativity, but I dont understand why it is always c regardless if we're speeding toward it or away from it as some insane velocity, can someone tell me without a reference to relativity?
  2. jcsd
  3. May 11, 2004 #2
    I think the short answer is a resounding "No."

  4. May 11, 2004 #3


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    That is the whole point of relativity- it IS relativity so it can't "be explained without relativity". Essentially (yes, I'm leaving out a whole lot- don't jump all over me!), Maxwell's equations implied that it should be possible to determine an "absolute" speed (as opposed to only the "relative" speed implied by Gallilean physics) by measuring the speed of light relative to oneself in different directions. The "Michaelson-Morley" experiment was an attempt to do so and showed no change- that is, that the speed of light is constant from all frames of reference. Lorenze came up with a very cute theory that the change in electric fields of moving atoms themselves contracted physical objects in the direction of motion just enough to defeat the measurement. The "Kennedy" experiment, a variation of Michaelson-Morley, showed that the was NOT sufficient, leading to Einstein's theory that space itself contracts. That has met all of the experimental challenges so far.
  5. May 11, 2004 #4


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    I've never heard of this!
    Can you recommend a book detailing the immediate prehistory of SR?
  6. May 11, 2004 #5


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    I would echo arildno's puzzlement here. While the MM experiment was published in 1886[1], the Kennedy-Thorndyke experiment (if that's what you meant by the "Kennedy" experiment) wasn't published till 1932.[2] This is waaaay after SR was published in 1905. So I'm not sure how the KT experiment could lead to Einstein's theory that space itself contracts.

    Furthermore, the KT experiment itself doesn't have the ability to measure "space contraction". In fact, the length contraction in SR is merely a consequence of the postulates. It is several layers down and not very apparent if one simply look at the formulation and the postulates. One has to do some work in actually getting it.


    [1] A. A. Michelson and E. H. Morley, Am. J. Sci. v.34, p.333 (1887).
    [2] R. J. Kennedy and E. M. Thorndike, Phys. Rev. v.42, p.400 (1932) .
  7. May 11, 2004 #6
    I'm missing something here.

    To my understanding, regarding the classic light clock example. When it is stationary the photons go vertically up and down. However, from our perspective when the light clocks are moving at some immense velocity, it appears the photons are moving diagonally. Since the speed of light is constant no matter who is observing it, the derivation of time dilation makes sense. But this is if we assume c to be constant from our perspective...hmmm

    I thought SR proved c is constant, it seems rather it is a requisite for relativity to work.
  8. May 12, 2004 #7


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    SR did not "prove" c is a constant in all reference frame. SR made several POSTULATES, and this is one of them. We have no way of deriving these. They are only verified (there is a difference) either via direct experimental observations of the postulates themselves or via experimental observations of the various consequences of the postulates, which time-dilation is only but one of many.

  9. May 12, 2004 #8
    Can any of you reccomend a book that explains all the stuff that you said about light in the previous posts. It's all gone far over my head.
  10. May 12, 2004 #9
    So it is all based experimental data? So I guess my pursuits of reasoning why light is constant in all frames is constant was a fruitless endeavor this afternoon. I am relieved, I thought I was missing something.

    "The Elegant Universe" by Briane Green has a few good chapters on relativity, you should check that out.
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