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Light speed

  1. Mar 8, 2005 #1
    Why light itself cannot travel faster than c = 299792458 m/s?
  2. jcsd
  3. Mar 9, 2005 #2
    Because that's the speed of light. Why is the charge of an electron what it is? You can derive the answer from Maxwell's equations and combined with the wave equation you can find what c is.
    But your question is more philosophical than anything
  4. Mar 9, 2005 #3
    if we consider light as a particle then why doesnt special theory of relativity act on it?
  5. Mar 9, 2005 #4


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    SR is not a theory with "action", it's a theory of how things act. SR tells you how light acts; it has the same speed in every inertial frame.
  6. Mar 9, 2005 #5
    Nobody knows why. One of the postulates of special relativity states that the speed of light is the same in all frames of reference. It can be derived in other similar ways. For example; one can find the expression of a light wave and it will have a phase velocity of c = sqrt(1/eu) where e is the permativity of free space and u is the permeability of free space. It we now demand that Maxwell's equations are the same in all frames then we have in this sense proven that the speed of light is a constant. It depends on what postulates you start with.

  7. Mar 15, 2005 #6
    What is speed of light measured from a accelarated frame?
  8. Mar 15, 2005 #7
    Speed of light remains same i.e. c under all circumstances....One of Consequence of Einstein's theory of Relativity
  9. Mar 15, 2005 #8


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    The constancy of the speed of light is an apparent fact of nature. It is just that way. This is not because of Einstein's Relativity, rather Einstein's Relativity is because of the constancy of c.
  10. Mar 15, 2005 #9
    try ansrin that

    but who told that to him and how ?
  11. Mar 15, 2005 #10


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    The "Michaelson-Morley" experiment told him!

    That's the way it is with Physics- everything eventually goes back to "experimental evidence".
  12. Mar 15, 2005 #11


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    Special relativity only deals with inertial frames, ie frames which are not accelerating. I dunno if there's a single established way to define an accelerating coordinate system, but light will not necessarily travel at constant velocity in a non-inertial coordinate system--an observer with constant acceleration will even perceive a sort of "event horizon" beyond which light can never reach him, known as the "Rindler horizon" (although of course if he stops accelerating the light will catch up).
  13. Mar 15, 2005 #12
    This is very much different from the way that I think of physics. If Einstein had confined himself just to considering the results of experiments, he would never have had the confidence to overthrow classical notions of time and space. (this was of course what happened to lorentz, who submitted his transformations as empirical formulas that would explain the michelson morely experiment.)

    The thought experiment that led Einstein to special relativity was this, based on a study of maxwel''s equations at age 16:

    "I see an EM wave pass by at speed c. Then, I catch up with it so that I am in the rest frame of the light wave. What I see are static E and B fields, at rest. There are no sources around, so how can these fields exist?"

    This theoretical paradox suggested it was impossible to travel at the speed of light.

    Also, consider that there is a maximum speed in the universe. Then something must move at this speed (or something slower would have the title of fastest). Furthermore, the speed of this thing must look the same from all frames of reference (or else moving in the opposite direction as the thing mean its speed is greater than the maximum). From these apriori considerations, special relativity follows.
  14. Mar 15, 2005 #13


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    ...and Maxwell's equations.
    Why not? In physics (as Halls said), experiments are everything. What else is there? Preconceptions and biases?
    But Maxwell's equations are themselves based on experimental evidence.
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