Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

SR postulates

  1. Oct 5, 2007 #1
    Hi all
    I was recently told that certain postulates of SR are equivalent or one can be derived from the other. Namely 1) The speed of light in vacuum is constant and is independent of the speed of emitting body; 2) The speed of light is impassable limit;

    I did some search and found quite different statements though. I found that postulate #1 was first postulated by Einstein in his "ON THE ELECTRODYNAMICS OF MOVING BODIES". One article said even that Einstein did not say light speed is impassable limit. Postulate #2 was first proposed by Poincare.

    My question is: Can #1 be derived from #2 or vice versa?

    Thank you all.
  2. jcsd
  3. Oct 5, 2007 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The two postulate of SR are (paraphrased):

    1. The speed of light in a vacuum is a constant for all inertial observers.
    2. The laws of Physics are the same for all inertial observers.

    The speed of light limit for massive objects is a conclusion arrived at from these postulates, not a postulate itself.

    The "in a vacuum" part is important, as the apparent speed of light changes through different mediums. In fact, it is given a special symbol, "c". And it is c that the limit refers to.

    Thus it is possible to travel faster than the speed of light in a medium in which the speed of light is less than c, for example, water. All relativistic effects however would be calculated in reference to c, not the speed of light through the medium.

    It can also be said that velocities greater than c are not strictly forbidden by Relativity. There are hypothetical particles called tachyons, which travel at speeds greater than c. Tachyons, however, can only travel at velocities greater than c. The speed of light barrier is still in force. Objects that travel at less than c can not be accelerated to c or greater than c, and tachyons can not be slowed to c or less than c.

    Also, no one has ever detected a tachyon, and we don't know if they really exist.
    Last edited: Oct 5, 2007
  4. Oct 5, 2007 #3
    Thank you. That was a very clear explanation.
  5. Oct 5, 2007 #4
    First off Einstein did call the constancy of light a postulate in that paper, i.e. after stating the principle of relativity Einstein went on to say
    Einstein knew quite well that if Maxewell's equations hold then the second postulate follows. However this assumes the correctness of Maxwell's equation. If, for example, the proper mass of the photon was non-zero then Maxwell's equations are no longer valid and must be replaced by the equations derivable from the Proca Lagrangian which has a photon proper mass term in it. A decent relativity textbook will explain this. This is stated quite nicely in Special Relativity; A Modern Introduction, by Hans C. Ohanian, Physics Curriculum & Instruction, (2001). In the footnote (to the second postulate) on page 41 Ohanian writes
    There was a time when I agreed with you Janus. About 5 years ago I think. So I wrote this up in a letter to the Editor of the Am. J. Phys.. The letter was rejected for the exact reasons given above by Ohanian and myself.
    The reason Einstein

    The "in a vacuum" part is important, as the apparent speed of light changes through different mediums. In fact, it is given a special symbol, "c". And it is c that the limit refers to.
    As you mention below, what is strictly forbidden is that a particle which is originally at rest in the inertial frame S could be accelerated past the speed of light. However it may be possible that a hypothetical a particle, known as a tachyon, could be created and for which is already moving faster than light. This would not violate special relativity and it could be done with a finite amount of energy. It was for that reason that the article

    Possibility of Faster-Than-Light Particles, G. Feinberg, Phys. Rev. 159(5), 25 July 1967, was written. As the author says in the abstract
  6. Oct 5, 2007 #5
    Please explain me in more detail your statement in boild. I know many papers where the problems are studied in the rest frame of the medium where the light signals propagate with c/n.
  7. Oct 6, 2007 #6
    What do you mean "the speed of light for a massive object"? Do you mean a object that has non-zero proper mass??? If so then that violates conclusions reached with the postulates of relativity.

  8. Oct 6, 2007 #7
    Janus said, "The speed of light limit for a massive object...".
  9. Oct 6, 2007 #8
    Thank you very much for the correction neutrino. I am much obliged. I was sure Janus wouldn't make sure a mistake. Hence my confusion!

    Darn dyslexia! :rofl:

    Best wishes

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook