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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Harish Chandra Rajpoot
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The speed of light was c long before Einstein special relativity https://web.pa.msu.edu/courses/2000fall/phy232/lectures/emwaves/maxwell.html . It was that maxwell equations fitted all data on electromagnetism that established it as an observable fact. (otherwise stated, Lorentz transformations are inherent in the Maxwell equation mathematics) – anna v Jul 11 '23 at 14:55
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1@annav Maxwell himself did not believe in an invariant speed of light. He believed in the ether and expected a positive outcome of the Michelson-Morley experiment. So the Lorentz transformation (in the sense of Special Relativity) is not inherent in Maxwell's equations. There are further postulates required. – Thomas Jul 11 '23 at 18:03
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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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All of these are experiments done after the 1905 paper that supported the statement except for MMX, but how did Einstein reach the conclusion in 1905?? He must have reached it theoretically?? – Rudransh Joshi Jul 11 '23 at 17:46
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@RudranshJoshi this question asks how we reached that conclusion. The question about how Einstein knew it is a different question. I would recommend asking it on the History of Science and Math StackExchange. I don’t know enough history to answer it – Dale Jul 11 '23 at 18:02
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These are all very good experiments and relevant, but it is actually impossible to rule out the class of frame dragging æther theories from just doing experiments. You can bring up infinitely many experimental verifications of SR predictions and get nowhere. One has to entertain theoretical and philosophical issues to really understand why we have to pay attention to SR. – naturallyInconsistent Jul 12 '23 at 05:07
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@naturallyInconsistent also thank you for your valuable insight – Rudransh Joshi Jul 12 '23 at 06:24
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@naturallyInconsistent I disagree. Ring interferometers do rule out dragged aether theories. A ring interferometer in a dragged aether would not detect the earth’s full astronomical angular velocity because the aether is dragged with the earth to some degree. It should be 0 angular velocity for a fully dragged aether, but in all cases something less than the astronomical angular velocity. The only experimentally viable aether theory is Lorentz aether. – Dale Jul 12 '23 at 10:51
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@Dale, I stand corrected, but that still goes to show that some horribly contrived æther theory could survive experimental testing. The ability of such a theory to predict observations would be reduced, but it would still survive. Only from considerations of theoretical aspects can we make the point that we should not be considering such theories. – naturallyInconsistent Jul 12 '23 at 11:19
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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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1Thanks for the video kind sir. I deeply appreciate it since I'm knife new and just beginning to drive my feet into relativity. Thank you very much for your deeply appreciated help. – Rudransh Joshi Jul 11 '23 at 15:43
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1You correctly point out that SR makes it impossible to define the meaning of its one-way speed, so that one can never actually prove in experiments yada yada, but that is precisely why I have serious reservations with recommending Veritasium's video due to the word choice. It is incredibly likely that students would come away with the idea that physicists were lazy and did not do the experiment, as opposed to understanding that it is impossible to do. Constructing via Minkowski diagrams make the concept much more obvious. – naturallyInconsistent Jul 12 '23 at 04:58
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I think you might also know that neither Michelson nor Morley gave up their conceptions of the æther, because what they were doing their experiment for, was to distinguish between two different types of æther theories, one with frame dragging and one without. The idea that there is no need for æther was just not even on the table for them to consider. – naturallyInconsistent Jul 12 '23 at 05:00