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Why no "Slow Light" (or slow something)

  1. Aug 16, 2014 #1
    I was watching a Science Video explaining why Ligtht travels slower in a Transparent Medium than in a Vacuum.

    I understand that nothing can exceed 186 272 Miles Per Second. I even have a reasonable handle on how the Relativity Effects create Apparent foreshortening and slowing of time in the frame of reference of an observer outside the system—and since there are no preferred frames of reference, "Apparent" is as Objective as one can ever be.

    I've never understood why 186 272 mps is the "Magical Limit".

    Someone once told me that IF C were different...

    As C approached ∞, motion would approach the Newtonian Ideal and as C approached 0 the more we'd notice Relativistic Weirdness in our Daily life.

    Obviously a well-ordered Universe and life itself would become impossible with C no more than a few Miles Per Hour.

    Be all that as it may—Why is a kind of "Tired Light" loping along at—O, say 100 000 Miles Per Second or even 50 000 Miles Per Second not possible?

    {Yeah, by definition "Light" may have to travel at "Light Speed" in a vacuum.}

    But what aspect of our Universe prevents Light-Like Propagating Energetic Phenomena noticeably slower than C from existing?

    Sometimes questions like this make no sense at all in terms of the "Real World"*—but sometimes they do...

    Can we ascertain any hypothetical properties of "Slow Light"?

    Sound waves propagate at about 761.2 Miles Per Hour—but Sound obviously cannot exist in a Vacuum. Nonetheless I came across some articles about how Scientists hoped that studying "Dumb-Holes" might give some insight ito the mechanics of Black Holes...


    Saxon Violence

    * I realize that many of you don't believe that there is an actual "Real World" and I'm neither Ignorant or Insensitive—but I Do like to Think and Theorize in terms of a "Real World"—as do most readers of a "Science Forum"...

  2. jcsd
  3. Aug 16, 2014 #2


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    That the speed of light in vacuum is always c is simply an experimental observation. One can go back and forth with theories, but in the end, this is a piece of experimental data, nothing more.

    By the way, the speed of sound is material dependent. It is about 760 mph in air at our atmospheric pressure and densities.
  4. Aug 16, 2014 #3


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    It's simply an observed fact that there exists an absolute speed limit in the universe. We don't know why, nor do we know why this speed happens to be the value that it is. The reason that light travels at this speed is because it is massless, so there is no other speed that it could travel.
  5. Aug 16, 2014 #4


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    (You mean in a vacuum, I presume? Light can propagate at speeds noticeably slower than ##c## in a medium, as you've said.)

    Nothing prevents such a thing from existing, but no such thing has ever been observed.

    It would be [STRIKE]totally amazing[/STRIKE] very surprising if we were to find such a thing, as it would allow us to define a preferred reference frame and invalidate a bunch of other stuff that we believe to be true and has a tremendous amount of experimental support. But in the end, it's all about experimental observations - if such a thing were to be observed we'd have to update our theories to accommodate it, just as an observation of winged flying pigs would force us to update our theories of porcine biology. I'd bet long odds against either of these happening.
  6. Aug 16, 2014 #5


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    I'm not sure what this is supposed to mean. There are plenty of phenomena in our universe that move slower than light. We don't call those things "light" (at least not in vacuum), but that's just a matter of terminology, not physics. Physically, there are things that move at the speed of light (in vacuum), and things that move slower. What's the problem?

    If your question is, why can't something that shares all of light's properties (other than speed) move slower than light, then the answer to that is that light (in vacuum) has zero invariant mass, and anything that has zero invariant mass *must* move at the speed of light (in vacuum). Put another way, if "light" moves slower than light (in vacuum), that means it has somehow acquired nonzero invariant mass, which means it no longer shares all of light's other (vacuum) properties.
  7. Aug 16, 2014 #6


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    There are also cases where particles travel faster than the light would within materials (in which case they emit Cerenkov Radiation). In the vacuum however that's impossible as PeterDonis post makes clear (massless particles will travel at c, massive will travel at v<c).
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