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Why doesn't light travel infinitely fast ?

  1. Feb 22, 2013 #1
    Light doesn't have mass so what's stopping it from going infinitely fast ?
  2. jcsd
  3. Feb 22, 2013 #2

    Simon Bridge

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    Welcome to PF;
    Science doesn't really do "why" questions of this form but I'll try to answer you - you see:
    the speed of light in a vacuum is a Property of the Universe.

    It's not so much that anything is stopping it from going faster so much as that there is no way to make it, or anything, go faster.

    Science investigates and attempts to improve our knowledge of the Laws of Nature - we can say what happens and how it happens, and most "why" questions are actually about them ... for instance: "why is the sky blue" is really about how the sky gets it's color. We can then say, "yes, but why!" It's basically because that is how our Universe works.
    Last edited: Feb 22, 2013
  4. Feb 22, 2013 #3
    I believe it is geometry that "limits" length divided by time.
  5. Feb 22, 2013 #4
    I don't really get what you're saying
  6. Feb 22, 2013 #5
    either do I in any really deep sense!

    but I do know how length & time are defined, and it involves c.

    Read about the light-like interval, and how it is distinctly different from space / time like intervals.

    There is no proper time / proper length at the speed c, in turn no metric can be described for something traveling at c, at least not according to how length / time is defined by c.

    So if I travel at 0.9999..... c I can still measure a proper time / length & in turn calculate a speed for what ever I wish to measure. But at c there is no proper time / length. No way to calculate a speed if you can't take those measurements.
  7. Feb 22, 2013 #6

    Simon Bridge

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    He's saying that c is part of the way the Universe works - but he is going into a bit of detail about the "how". That's where the interesting stuff is.

    The upshot is he's telling you to learn about relativity.
  8. Feb 22, 2013 #7

    Simon Bridge

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    He's saying that c is part of the way the Universe works - but he is going into a bit of detail about the "how". That's where the interesting stuff is.

    The upshot is he's telling you to learn about relativity.
    If you want to understand the answers, that's pretty much what you need to do.

    Meantime - this question gets asked a lot around here.
    Have a look at how other people's questions got answered.
    Last edited: Feb 22, 2013
  9. Feb 22, 2013 #8


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    The speed of light is determined by the permittivity and permeability of free space.
  10. Feb 22, 2013 #9
    Also didn't Poincare show that if space is homogenous, then there must exist maximum posible speed in nature, because otherwise the causality would be reversed?
  11. Feb 22, 2013 #10
    What do you mean by "space is homogenous"?
  12. Feb 22, 2013 #11
    A quick wiki search will provide that answer, Homogeneous essentailly means the same throughout. In cosmology it essentailly means that at a certain size portion usually at 100 Mpc (megaparsecs). On the average one region of space is the same as another. Isotropic means that there is no preferred direction. regardless of which way you look its all the same.

    Obviously we see large scale structures etc in different directions, however their energy densities will average out per region.

    A good example of both is spacetime expansion. no matter which region of space or direction. expansion is the same rate provided that region is not gravitationally bound such as around a large scale structure.
  13. Feb 22, 2013 #12
    Guess I'm trying to understand what is the same throughout. Beside the constant c I am unsure what it means?

    Just don't get what "same" is representing.
  14. Feb 22, 2013 #13
    Yeah its a little tricky of a term in regards of cosmology this is a cut and paste expaination from "Introduction to Cosmology " by Barbera Ryden.

    On large scales, the universe is isotropic and homogeneous
    What does it mean to state that the universe is isotropic and homogeneous?
    Saying that the universe is isotropic means that there are no preferred directions
    in the universe; it looks the same no matter which way you point your
    telescope. Saying that the universe is homogeneous means that there are no
    preferred locations in the universe; it looks the same no matter where you set
    up your telescope. Note the very important qualifier: the universe is isotropic
    and homogeneous on large scales. In this context, \large scales" means that
    the universe is only isotropic and homogeneous on scales of roughly 100Mpc
    or more.
    The isotropy of the universe is not immediately obvious. In fact, on small
    scales, the universe is blatantly anisotropic.
  15. Feb 22, 2013 #14
    Ah I see,

    I misunderstood homogeneous, always thought of it in the sense of mixtures; and was wondering what are the components that are thoroughly mixed :smile: sorry for such bad terminology.

    In that text you generously posted, it reads as though it is strictly via comparison that space is called homogeneous.
  16. Feb 22, 2013 #15
    An interesting side note is that in Newtonian physics if you went an infinite speed you would get there at the same time you left. In special relativity if you could go the speed of light you would get there the same time you left. Only, under relativity, the same time for you could be years later for somebody else.

    That can be characterized as a reason why exceeding the speed of light is tantamount to time travel, because it would mean you could get to your destination before you left.
  17. Feb 22, 2013 #16


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    I don't feel that any of the answers here are going to satisfy you, as they do not answer "why" in a fashion you would accept. The only thing I think you can do is to try to accept the following fact.

    There is a maximum speed that anything can travel in our universe. That speed is represented by the letter c and is approximately 300,000 kilometers per second. (About 186,000 miles per second)

    Why is it like this? We have no idea. But we do know that a great many things would be much, much different if this value were different. Especially if it were infinite.
  18. Feb 22, 2013 #17

    Thats one valid way to look at it. Its also commonly used in models that represent the overall universe, This is largely a means of simplification. If for example we want to find the average vacuum energy of the universe, sample regions of space and apply the mathematics of that region in a manner that represents the average as homogeneous and isotropic. Many of your Freidman equations apply the same methodology.
  19. Feb 22, 2013 #18
    light can travel at arbitrarily large values in appropriate units. You can make the numbers as big as you want by changing units.
  20. Feb 22, 2013 #19


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    How does this have any relevance to the thread?
  21. Feb 22, 2013 #20

    Simon Bridge

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    It strikes me that a lot of the answers are just changing the wording of the question.
    Technically any one of those is determined by the other two isn't it? Anyway - doesn't that observation just change the question into why the permittivity and permiability are that way?
    ... and this changes the question to "why doesn't causality reverse?" It's the same question in different words.

    I do vaguely recall something like that from Poincare - havn't been able to find a reference.
    Wikipedia has a mention of something similar.

    But I think Drakkith is right here:
    ... I don't see how empirical science can answer this sort of "why" question in the spirit it appears to have been asked. It seems to be a philosophy question more than anything. The exact same question can be asked of any physical constant - or, indeed, all of them together. We see the values we do because we live here. If we lived in a different Universe then perhaps we'd see different values and wonder about them? But there may be limits on the sets of values that make sense - that produce Universes with physicists for example. That's a triple-whiskey discussion right-there!

    The whats and hows are usually more interesting - see how engaged OP is with Mordreds posts about the large-scale nature of the Universe.

    So there may be a way forward ... the first post suggests that OP is thinking like this:
    Since F=ma, the smaller the mass, the higher the acceleration for a given force.
    If the mass is zero, then any force produces infinite acceleration, and so an infinite final speed.
    Light has zero mass...

    If this is, in fact, the reasoning involved then we can answer this in two ways:
    1. pointing out that F=ma is incorrect ... only works for small relative speeds.
    2. asking how OP imagines accelerating light - and explore the reasoning process more.
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