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Another question about speed of light

  1. Jul 9, 2008 #1
    (I've searched and couldn't find anything about this in the forums, but please excuse me if this is a duplicate of another post).

    I clearly understand that "c" is the maximum speed at which "things" can move (with certain restrictions, caveats, etc). But it seems to be also true that other entities, such as massless particles, can travel at the speed of light.

    Which leads me to the question: Is it actually the speed of light that's inviolate or is light just one of the many entities that must adhere to this upper boundary?

    Although I understand "c", it's a complete mystery to me as to why this limit is "the" limit. Can someone help nudge my understanding along? I'm not suggesting that there shouldn't be a limit, such as "c", nor arguing against "c" being the limit, but rather, what physical mechanism causes this to be a limit.

  2. jcsd
  3. Jul 9, 2008 #2
    The second you have written. Every massless particle which have non zero energy must move at c.
    The speed of light c is related to the electric properties of the void. The issue is still not completelly understood; someone speculates that those properties could in theory be changed someway, giving as result a different value for c in that modified medium.
    Last edited: Jul 9, 2008
  4. Jul 9, 2008 #3


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    It's the other way round: there is a speed limit, and every single force or interaction must obey it. That's why the electric properties of the void happen to assure that electromagnetic waves travel at c.
    The speed limit itself is thought to be the result of how space and time are connected, the spacetime concept. Mathematically, arguably the simplest spacetime must have a conversion factor between both units (length and time), c, which acts as a natural unit of speed and as its limit, too.
  5. Jul 9, 2008 #4
    Many have various ideas about how to explain or understand this limit - but describing a cause in the form of a real physical mechanism, I don’t think anyone has done that.
  6. Jul 9, 2008 #5
    So, what do you think of the fact that void's electric properties could depend, let's say, on the virtual particles density and that in the Casimir effect it's claimed that density is modified?
  7. Jul 10, 2008 #6


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    "c" is not "the speed of light". When the electric properties of a medium (be it "the void" or anything else) change, the speed of light will be different, but not the limiting velocity "c".
    If you have a different energy density like in the Casimir effect, "c" will be different if suitably compared wth the outside world. It's not very helpful to hold a fine-tuned simultaneous change in the electric, weak, strong, and gravitational properties of the void responsible for that. That's the opposite of relativity.
  8. Jul 10, 2008 #7
    The limiting value c is light's speed in the void; in a different "void" light's speed would be different = c', as you say too. The question is another: if this different kind of "void" could really exist, Maxwell's equations and Lorenz's transforms, there, should be written with c or c'?
    Last edited: Jul 10, 2008
  9. Jul 10, 2008 #8
    It makes more sense when you look at c as the conversion factor between space and time units. From that perspective, it is what it is because of our choice of units.

    There has to be _some_ conversion factor between the two if you believe Einstein and Minknowski. The fact that c is the maximum spatial velocity flows logically from that.
  10. Jul 10, 2008 #9
    Hello peter0302


    ---It makes more sense when you look at c as the conversion factor between space and time units. From that perspective, it is what it is because of our choice of units.---

    Surely light has the same speed whatever units you use to express this speed. The speed of light is by definition the speed of light and c is this speed in vacuo, usually expressed in meters per second.

  11. Jul 11, 2008 #10
    Since massless particles with energy move at this same upper limit, how do the electrical properties of the void figure into this?
  12. Jul 11, 2008 #11
  13. Jul 11, 2008 #12
    Thanks, all, for thoughtful replies. I now understand that there does not appear to be any general consesus as to what causes the physical manifestation. Does anyone know of a group or individual that I might find on the Internet that is working on this?

  14. Jul 11, 2008 #13


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    The speed 'c' follows from Maxwell's equations. If you write Maxwell's equations in the case of no sources, you get a wave equation with
  15. Jul 12, 2008 #14
    It's a good question, infact I think this is the connection between em interaction and GR (it's just a speculation, of course).
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