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The varying speed of light (Calc+ needed)

  1. Jul 6, 2005 #1
    I read in Wired a while ago about how light is affected by gravity or some other such fundamental force, and thus the speed of light varys throughout the universe. Does this hold any water anymore, or is it just baloney? I did a quick search on VSL through google and found an article, but the math was way above my head; perhaps someone could read this article and maybe explain it in laymans terms for my benefit.


    Thanks :smile:
  2. jcsd
  3. Jul 6, 2005 #2


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    The link you quoted is not quite baloney - I'd call it more of a dancing bear. It's definitely not standard theory, it's highly speculative. I've nicknamed it a dancing bear because in my opinion it was motivated by the author's desire to be "clever" ("Look how good I am - I can even make a bear dance") rather than to actually propose a serious theory which is likely to be true.

    The author in my opinion does make a few good philosophical points, but it would be unwise to treat his theory as being a good representation of reality.

    As far as the speed of light goes, according to current theory (General Relativity) wherever you are, the speed of light is 'c', and wherever you go, the speed of light will also be c. You may think that the speed of light is not equal to 'c' someplace else far away from you, but if you actually go there and measure it, you'll find that it's equal to 'c'.

    This may sound odd, but that's the way it works according to current theory. Details of calculating the "apparent speed of light" far away from you are highly technical, and depend on the exact coordinate system used. Philosophically, one can view the speed of light as being always constant, and the coordinate systems themselves as being warped - this is more or less the standard view.

    Mathematically, given a pre-defined coordiante system, the path of light must obey certain equations, called geodesic equations, the path that light takes is one particular form of geodesic called a "null geodesic".
  4. Jul 6, 2005 #3


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    To be clear, it is uncontroversial and widely shown by experimental evidence that light is affected by gravity. It is the part about having varying speeds that is doubtful.
  5. Jul 6, 2005 #4


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    There is a caveat here, if we believe that quantum theory is correct. Quantum physics predicts that the vacuum of "empty space" is anything but empty and is in fact a seething field of virtual particle pairs. If light has to cross this medium and the medium's density is NOT absolutely uniform, we must have to consider that the speed of light in a "vacuum" is not absolute and is not constant. Light slows and is refracted as it enters denser transmissive media, in accord with classical optics.

    If "empty" space is not empty, the "speed of light in a vacuum" is a theoretical ideal that can never be met, and the universe is a lot messier than envisioned in GR.
  6. Jul 6, 2005 #5


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    But I could also play the game in a different way. You are making an explicit assumption of a normal, dispersive medium. There's nothing here to indicate that this "exotic medium of virtual particles" cannot be an anomolous, non-linear medium. We already have such stuff today with regular materials, exhibiting negative permitivity and permeability. Such medium doesn't follow the normal rules that light (at least the phase velocity) has to slow down in a medium.

    Furthermore, your scenario immediately would need to reconcile with the fact that we DO measure a constant value for c. If there are such variations due to what you describe, one would expect the value of c to deviate by a very large amount the further that light has travelled. Has this been observed? Over a large frequency spectrum?

  7. Jul 11, 2005 #6
    But, if c varies with time, the relativistic Lagrangians are no longer invariant under time translatations, hence no Noether's theorem and no conservation of energy. Shouldn't have this been observed in the non-relativistic range even if the variation of c is very small?
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