Mine is a very basic question yet I haven't got a satisfactory answer so I turn to you. Special Relativity is based on two assumptions, one of which is that the speed of light is constant for all inertial frames. While I know the disprove of universal reference frame, Aether, I want to know how we came to the conclusion that the speed of light REALLY is constant for all inertial frames (light traveling in vacuum).
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I want to know how we came to the conclusion that the speed of light REALLY is constant for all inertial frames (light traveling in vacuum)
When physicists want to know how things REALLY are, they do experiments. The list of experiments supporting the second postulate is quite long. So I will just give a small sample. For a more complete list please see The Experimental Basis of Special Relativity
A.A. Michelson and E.W. Morley, "On the Relative Motion of the Earth and the Luminiferous Ether", Am. J. Sci. (3rd series) 34 333–345 (1887). The famous MMX experiment. Shows that the speed of light is independent of the orientation of the apparatus.
R.J. Kennedy and E.M. Thorndike, "Experimental Establishment of the Relativity of Time", Phys. Rev. 42 400–418 (1932). Shows that the speed of light is independent of the orientation and velocity of the apparatus. Rules out rigid aether theories.
Michelson and Gale, Nature 115 (1925), pg 566; Astrophys. J. 61 (1925), pg 137. A large ring interferometer which detected the rotation of the earth. Rules out dragged aether theories.
K. Brecher, "Is the Speed of Light Independent of the Velocity of the Source?", Phys. Rev. Lett. 39 1051–1054, 1236(E) (1977). Observations of binary pulsars. Shows that the speed of light is independent of the speed of the source. Rules out ballistic theories of light.
Alvaeger F.J.M. Farley, J. Kjellman and I Wallin, Physics Letters 12, 260 (1964). Measured speed of light from relativistic pions in the lab.
Beckmann and Mandics, "Test of the Constancy of the Velocity of Electromagnetic Radiation in High Vacuum", Radio Science, 69D, no. 4, pg 623 (1965). Measured the speed of light reflected off a moving mirror. Beckmann was a known critic of SR.
Brown et al., Phys. Rev. Lett. 30 no. 16 (1973), pg 763. Compared gamma rays to light and found the speed of light was the same.
Petley, "New Definition of the Metre", Nature 303 (1983), pg 373. A review article describing the reasons why the meter was redefined in terms of the speed of light. At this point the experimental techniques used to measure the speed of light were so precise and reliable that the main source of uncertainty in the speed of light was the uncertainty in the length of the platinum bar used to define the meter.
This is a very small selection of the available experimental evidence. But it should give you a “flavor” for why scientists consider this issue settled.
Dale
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You said you "know the disprove of universal reference frame, Aether". I am guessing you mean the famous Michelson-Morley experiment from 1887. In this experiment, the experimenters measured the speed of light in two supposedly different frames, and the result was the same speed. I guess you could say that this is not a proof that if you measured the speed of light in 100 other frames, maybe one of those would differ. But this is not how physics works - the fact that the two frames got the same speed of light (contrary to the theory of the Aether) was a reason to come up with a different model - and once this model was completed (Einstein's theory of special relativity) its predictions could be tested - and so far all of them have held up.
By the way, there is an interesting video on Youtube, Why No One Has Measured The Speed Of Light, which explains that although everyone thinks that we measured that the speed of light is the same in every inertial frame, we actually never measured it. Where "it" is the speed of light in one direction. The video makes the point that all experiments measure the speed of light going back and forth (and special relativety makes it impossible to define the meaning of its one-way speed), so you can't actually prove in experiments that the speed of light is the same in different frames.
Nadav Har'El
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