From what I’ve learned, Special Relativity came about from the apparently simple assumption that the speed of light is the same for all observers.
What caused Einstein to assume that? Can anyone explain (without too many tensors and light-cone diagrams) why he would have come up with such an ‘axiom’? As far as I can see there wasn’t any empirical (experimental) evidence at the time to nudge him towards it.
Probably largely prompted by the Michelson-Morley experiment which was designed to detect changes in the propogation of the speed of light depending on which way you pointed… was it an interferometer?.. Given Earth moving around the Sun, it should have given different results (in the direction of Earth’s motion, or perpendicular to it).
Actually, there was empirical evidence called the Michelson Morley expirement:
http://scienceworld.wolfram.com/physics/Michelson-MorleyExperiment.html
Damn! I actually remember some of this stuff! 
Thanks for the responses - I hadn’t realized how important the MM experiment was for the genesis of Relativity - I thought the whole thing had been discredited as a misguided attempt to detect the Ether. Also hadn’t realized how much people like Lorenz contributed to SR.
There were also the Lorenz transformation equations, which “explained” Michaelson-Morley, but which no one thought more than a curiosity until Einstein gave a reason why they worked.
BTW, to complicate matters, Einstein stated at times that Michaelson-Morley didn’t inspire him, but merely confirmed what he was already working on. However, he also stated that M-M was a major influence, so who knows?
Also, there is a certain aesthetically pleasing facet to the assumption that c is constant in all frames, because then Maxwell’s Equations hold in all frames.
The M and M experiment does provide an experimental basis for relativity, but was not Einstein’s inspiration.
Einstein’s inspiration, from what I gather, was trying to make sense of some results of the Maxwell equations. I have heard, though not seen it done, that things like Lorenz transformations can be derived from the Maxwell equations.
cheers,
-Short
Anyone who is curious enough to ask this kind of a question should read the book The Elegant Universe by Brian Greene. The book is supposed to explain string theory, but starts off with explaining relativity and quantum physiscs in a way that the layman can understand.
The way I’ve heard it, Einstein did a gedankenexperiment (“Thought experiment”) where he tried to imagine what he would see if he were riding on a photon, looking at a beam of light. Exactly how he got from this to his theory I do not know, but he finally realized that the situation was itself absurd – the only way he could get consistency would be if the speed of light were the same in all reference frames. You can read his own book on it, written for laymen.
But I’ll bet the MM experiment was a major factor in the virtual instant acceptance of the theory.
As long as we’re talking book recommendations here, Einstein’s Miraculous Year: Five Papers That Changed the Face of Physics was recommended to me. Has anybody read it?
Although he was known to be the traditional absent-minded genius, I find it hard to believe that Einstein wasn’t well aware of the MM experiment!
It was to physics what Origin of Species was to biology. It put the final nail in the coffin for the ‘waves propogating thru the ether’ mindset.
Okay, I’ve been reading up on this, and here’s what I’ve ascertained. Someone more knowledgable can maybe correct any errors I have.
The Michelson-Morley experiment was first attempted in 1881. By the end of the 1880s, the theory of Lorentz transformations was well-written, even if it had no real experimental evidence. At best, all it did was account for the M-M results in a counterintuitive and inelegant way.
It was really a bit of a controversy, up to and after 1905, when Einstein first proposed SR. Einstein of course did not invent the Lorentz transformations, but he showed that the two fundamental assumptions of SR would result in them, and that they made Maxwell’s Equations invariant. This was the first solid theory of physics that accounted for M-M (in a counterintuitive and elegant way), even though most of it wasn’t really new.
Now, as my Modern Physics teacher once explained it, SR was not all that. Many physicists were on the verge of getting it anyway, and someone would have soon if Einstein did not. If you recall, he won the Nobel Prize in 1905… for the unrelated photoelectric effect. GR, on the other hand, is Einstein’s crowning accomplishment. Without Einstein, its discovery could have been delayed for decades.
That’s essentially what I heard–that he noted that we never see a photon standing still with the magnetic and electric fields just bouncin’ up and down in place. If photons could have any speed, you’d expect to have some with v=0.
Yes, and it’s an entirely worthwhile bit of publishing. Strachel is one of the editors of the ongoing definitive multi-volume edition of Einstein’s writings and this is their translation of the famous five papers from 1905, together with good introductory commentaries on them. I’d however hesitate to recommend it to anyone with less exposure to relativity and quantum physics than something at undergraduate level - these were, after all, papers written with a professional audience in mind. Dover Books have been good at keeping the four relativity and statistical physics in print in cheap editions, but until this came along there was no equivalent for the photoelectric effect one. All definitely worth reading once you understand enough to realise quite how technically dazzling they are.
Incidentally, the influence of the Michelson-Morley experiment on him is one of those things there’s no great concensus about. Even historians of science argue about it.