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Speed of light not constant

  1. Aug 10, 2010 #1
    hey guys i just saw a weirdo on TV claiming that the speed of light is not constant......since that is a direct attack on Einstein's postulate of relativity is there any truth in that statement?
  2. jcsd
  3. Aug 10, 2010 #2


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    Welcome to PF.

    It's difficult to comment without knowing exactly what was said. If the postulate you are referring to is "all observers measure locally the same speed for light", then contradicting that would be clashing with relativity. But this concept is not easy to apply on large scales and the 'weirdo' may have meant something different.
  4. Aug 10, 2010 #3
    The "speed of light" IS (c) as Mentz posts...for a vacuum... In some sense, "the speed of light" is different in materials than in a vacuum, but insofar as is known individual photons always move at c even in materials. There are numerous discussions in these forums.

    Another variation is 'phase velocity' which can moves faster than an individual photon...

    The speed of light also typically refers to the tranmission of information as a maximum speed...so while phase velocity can exceed c, it cannot be used to transmit information.
    Last edited: Aug 10, 2010
  5. Aug 10, 2010 #4
  6. Aug 11, 2010 #5
    yeah am familiar with the phase velocity bit but this guy claimed that ,the speed of light not being constant ,then one had to cast doubts on Einstein's theory of relativity.
    does different speeds of light in different times of our history have any effect on the theory of relativity?does the theory of relativity have any experimental findings against it?
    yes Mentz i was talking about that postulate
  7. Aug 11, 2010 #6
    If the speed of light were different for different observers, that would affect the theory of relativity as it is based on the fact "c" is constant, time and distance are variables. This has lots of experimental verification...at least indirect but generally overwhelming evidence.

    The speed of light is a measured characteristic, like the mass of a proton, for example; nobody has the theoretical knowledge to derive either from basic principles yet.

    But we do know (http://en.wikipedia.org/wiki/Speed_of_light#Propagation_of_light)

    Relativity has well known theoretical faults: It doesn't work at the big bang and black hole singularities...neither does quantum mechanics...that's how we can tell neither is entirely correct. Relativity has no experimental contradictions...nor does quantum mechanics...the former is good at macroscopic scales, the latter at microscopic scales....
    Last edited: Aug 11, 2010
  8. Aug 12, 2010 #7
    On rereading that question, I now think it meant has light speed varied over the history of the universe and would that affect relativity....."c" is a constant in the Einstein Field equations.....so depending on what you were studying, yes it would change solutions.

    I am unsure if the speed of light did change over history how we would detect it, but I suspect there are ways...the cosmic background radiation for example comes from some 13.3 billions years or so ago (about 380,000 years after the bang) and seems 'consistent' with "current" light, say as from our sun....

    so far its believed the expansion speed of the universe that has been variable rather than the speed of light.
  9. Aug 12, 2010 #8


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    Perhaps you encountered something from Joao Magueijo, a for real Physicist, who has done some speculations by treating c as if it were a function of time. He has written an interesting and readable book about his experiences with this speculation:

    Faster then the Speed of Light

    If you have time give it a read.
  10. Aug 12, 2010 #9
    Very curious...Hello by the way...

    Umm...traveling faster than the speed of light would perhaps have a very interesting result.
    Once one approaches light speed, his mass becomes infinite...thus necessitating an infinite force to propel the infinite mass. Only one force that I know of is infinite...but how can one harness force?

    From what I've read, light does only travel at a constant, regardless of the velocity of the observer. However, the postulate of relativity necessitates that we all agree on the constant speed of light. If, however, relativity is not applied, then it may be possible that light travels at different velocities under different variable factors.
  11. Aug 16, 2010 #10
    does the book have any thing to do with tachyons?
  12. Aug 16, 2010 #11
    If the speed of light were significantly different during some interval well after the Big Bang then shouldn't chemistry, ultimately an electromagnetic phenomena, be different as well? I'd be surprised if the world can be what it is now with different chemical behavior...
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