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Regarding constancy of speed of light

  1. Sep 15, 2011 #1
    i am just a beginner in studying SPTOR..dunno whether thr question is trivial...

    as per the second postulate of SPTOR the speed of light is constant for all observers

    but why did einstein while stating this postulate refer to the speed of light in vaccum to be constant and not anything else...why is speed of light in vaccum taken as the ultimate limit and not anything elses..what is the so peculiar about the speed of light.
  2. jcsd
  3. Sep 15, 2011 #2


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    He restricted the speed of light to be in a vacuum because if there is any matter in the way (gas molecules in particular) the apparent speed of light is slowed down. The apparent speed of light also is slower through transparent solids and liquids.

    But you should know that Einstein was not concerned with the actual value of the measured speed of light in his second postulate because that was already covered in his first postulate. All measurements of the speed of light require a round-trip involving a mirror and a single clock located at the source and final destination of the reflected light. Instead what he was concerned about in his second postulate was something that could not be measured and that is the one-way speed of light. By stating that it was equal to the measured round-trip speed of light, he was defining a Frame of Reference which is what the Theory of Special Relativity is all about.

    You can read his 1905 paper introducing Special Relativity to learn more about this.
  4. Sep 16, 2011 #3

    Now we know there are some massless or almost massless particles moving with speed c, e.g. gluon, graviton, electron neutrino, muon neutrino, tau neutrino, W and Z in early universe, however at the begging of the last century physicists know only photon or electromagnetic wave which undertake phenomena of theoretical maximum speed c. That is the historical reason why the theoretical maximum speed is called speed of light.

    Last edited: Sep 16, 2011
  5. Sep 16, 2011 #4
    As far as I know the gluon and the graviton are still hypothetical... You should not mix facts with beliefs... What are you saying about the early universe?
  6. Sep 16, 2011 #5
    You should note that the speed of light is measured in "length units per time unit". The fact that the number of meters travelled per second, to a local observer, always appears to be c does not in fact mean that the velocity of light is constant... That is to strong a conclusion.

    Actually, according to general relativity, light travels slower deeper in a gravitational field but time also slows down with the same factor so locally the speed of light will always be c, even though the speed of light is not really constant.
  7. Sep 16, 2011 #6


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    Some points: As Einstein was formulating SR, two facts were already available.

    Firstly the negative result of the Michelson-Morley experiment which sought to measure the Earth's motion through the aether (via differential speeds of light causing interference). The Lorentz transformations in which the (apparent) speed of light in vacuum is a invariant were formulated to explain this negative result in terms of shrinking distances and slowing of clocks due to motion through the aether. (These transformations form a group of rotations and frame transformations on velocities, the Lorentz group).

    The second fact was Maxwell's equations which describe the propagation of light and other electromagnetic radiation and the fact that these are invariant under the Lorentz transformations.

    Einstein didn't invent the Lorentz transformations but rather re-interpreted them as relativity transformations rather than physical effects. He thereby showed other implications such as energy-mass equivalence. He eliminated the need to assume an aether as a medium for electromagnetic propagation.

    Now a third mathematical fact also comes into play. If you consider all the possible groups of velocity transformations which include rotations, there are three possibilities.
    1. Firstly there is a larger rotation group for which increasing velocities would be periodic. Go fast enough and you're back to zero velocity. This doesn't fit basic facts nor does it fit certain consistency requirements (you'd be able to turn your time axis backward).
    2. The other simple group is a Lorentz group with some absolute maximum velocity which is the same for all observers.
    3. The third is the classic Galilean velocity transformations where you simply add velocities. It is a singular boundary case between the first two applying at various scales, (the flat line between classes of ellipses and hyperbolas.)
    This third is what we thought until Einstein's explanation of the the Michelson-Morley experiment and the subsequent experiments verifying both the second group type and that this maximum velocity is the speed of light in vacuum.

    Finally note that now with Einstein's unification of space and time, we should use common units for both distances and durations. Imagine we measured height in fathoms and lateral distances in furlongs. We'd need to explain oblique rotations by introducing a unit conversion constant with units of fathoms per furlong. This is the case with space-time and we need a unit conversion factor in units of meters per second. That unit conversion factor is c and in common units the "speed of light in vacuum" is 1. (as in 1 light-second per second).

    Now we assign a value to c instead of treating it as a measured physical constant. This is a matter of using the standard second to define the standard meter by virtue of light traveling c standard meters in a second. It becomes immaterial whether the speed of light varies or not as it is now by definition fixed. We instead treat any variation in terms of variation of space-time geometry which is part and parcel with Einstein's GR and its explanation of gravity.
  8. Sep 16, 2011 #7


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    I disagree, I think that GR does not make that claim. What you are describing is the coordinate speed of light. And you can make a coordinate system to give you any value you like for that at any given point. The only coordinate independent thing that GR will say about the speed of a particle is whether it is timelike, lightlike, or spacelike, and the speed of light is always lightlike.
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