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Constancy of Velocity of Light for all Observers

  1. Mar 9, 2015 #1
    Having decided to try to understand the mathematics underlying Special Relativity, I have come up against an initial problem. I read that one of its postulates is that measurements of the speed of light, in any inertial frame of reference, return the same value, the value c.

    What I have so far failed to find is the basis for this postulate. To me, the term postulate suggests that the notion of the constancy of the velocity of light was not derived by experiment or theory. Is that, in fact, the case? If so, was it plucked out of thin air, so to speak (and by whom?), and If not, then what is its basis?
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  3. Mar 9, 2015 #2


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  4. Mar 9, 2015 #3
    Thanks. A.T., for pointing me to that source. My physics background is pretty thin, and it would take me some time to get to grips with some of the terminology in it, but my initial impression was that it mainly describes experiments confirming the constancy of the velocity of light for all observers rather than explaining the thinking behind the idea itself, which arose rather earlier - although I concede that initial impressions can often be misleading.
  5. Mar 9, 2015 #4
    Maxwell came up with a theoretical c that was constant, and experimentalists such Michelson and Morley could only come up with c in their measurments, so Einstein said : heck, might as well stick with c, and go on from there.
  6. Mar 9, 2015 #5


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    There were two lines of thought that led Einstein to adopt the light-speed postulate and run with it.

    One is that experiments seeking to measure variation in the speed of light when the source or receiver was in motion came up dry. These experiments can be explained in more than one way, but a constant speed of light is one of the simpler explanations. (There is some question as to whether Einstein was aware of the most convincing of these failures when he first came up with relativity but he certainly knew that no one had succeeded in finding a light-speed variation).

    Second (and this was the great unsolved problem of 19th century physics from 1861 on) is that it is natural to believe that the laws of physics don't change with the speed of the physicist. The earth's rotation carries you in one direction at noon and in exactly the opposite direction at midnight, and the earth's orbit carries you in one direction in June and in the exactly the opposite direction in December, yet the laws of physics stay the same and our experiments yield the same result. However, in 1861 James Maxwell calculated the speed of light from the laws of electricity and magnetism, and Maxwell's equations don't include a term for the speed of the laboratory in which the experiment is done. Thus, we are forced to choose between different laws of electricity and magnetism for people moving at different speeds, or a constant speed of light so that we can all have the same laws of E&M.

    Einstein's great achievement with special relativity was to produce a consistent theory that respected both Maxwell's laws of electricity an magnetism and the intuitive notion that the laws of physics shouldn't change with the speed of the physicist. It is worth noting that the title of Einstein's 1905 paper was "On the electrodynamics of moving bodies" - it's pretty clear that he viewed this as a problem in reconciling Maxwell's electrodynamics with our notions of motions.
  7. Mar 10, 2015 #6
    Nugatory already explained it rather well, so here just a precision:

    Many textbooks oversimplify the postulates (only saving one paragraph of space!), with your question as a result. The postulates of SR are based on two assumptions that followed from many experiments of that time (you can read it in Einstein's 1905 paper*):

    1. the laws of nature are the same in every inertial frame, and the laws of electrodynamics are laws of nature (late 19th century experiments)
    2. the speed of light is a constant that is independent of the motion of the source (Maxwell and later experiments)

    You will likely agree that each of these postulates sounds, if taken by itself, quite plausible. Moreover they were both supported by the experimental evidence. However they are incompatible in classical physics, and SR was the solution.

    * introduction of http://fourmilab.ch/etexts/einstein/specrel/www/
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