Modern readers turning to Einstein’s famous 1905 paper on special relativity may not find what they expect. Its title, “On the electrodynamics of moving bodies,” gives no inkling that it will develop an account of space and time that will topple Newton’s system. Even its first paragraph just calls to mind an elementary experimental result due to Faraday concerning the interaction of a magnet and conductor. ...That protocol is known as Poincaré–Einstein synchronization, as Poincare published it in 1898, 1900, and 1904.
It has also received considerable attention outside physics. It is the first port of call for philosophers and other thinkers, seeking to understand what Einstein did and why it changed everything. It is often also their last port. ...
It contains Einstein’s analysis of simultaneity, probably the most celebrated conceptual analysis of the century.
Many have tried to emulate Einstein and do in their fields just what Einstein did for simultaneity, space and time. For these reasons, many have sought to understand how Einstein worked his magic and came to special relativity. These efforts were long misled by an exaggeration of the importance of one experiment, the Michelson-Morley experiment, even though Einstein later had trouble recalling if he even knew of the experiment prior to his 1905 paper. ...
Why did special relativity emerge when it did? The answer is already given in Einstein’s 1905 paper. It is the fruit of 19th century electrodynamics. It is as much the theory that perfects 19th century electrodynamics as it is the first theory of modern physics. 4 Until this electrodynamics emerged, special relativity could not arise; once it had emerged, special relativity could not be stopped. Its basic equations and notions were already emerging in the writings of H. A. Lorentz and Henri Poincaré on electrodynamics. ...
All efforts to recount Einstein’s path face one profound obstacle, the near complete lack of primary source materials. ...
To demonstrate this result, Einstein imagined two places A and B, each equipped with identically constructed clocks, and a simple protocol to synchronize them using light signals
We actually have all Einstein's source materials. Besides the Poincare synchronization, Einstein is known to have gotten the electrodymanics, space-time transformations, postulates, and everything else from Lorentz and Poincare.
Einstein himself agreed that Michelson-Morley was crucial to the development of relativity, as he explains in a 1909 paper:
The Michelson and Morley interference experiment showed that, in a special case, second-order terms also cannot be detected, although they were expected from the standpoint of the ether-at-rest theory. To include this experiment in the theory, Lorentz and FitzGerald introduced the postulate that all objects, including the parts of Michelson and Morley's experimental set-up, changed their form in a certain way, if they moved relative to the ether. ...Einstein was unsure about Michelson-Morley in 1905 because he did not realize the experiment's importance at the time. Lorentz and Poincare emphasized that the experiment was crucial to their reasoning, but Einstein was just giving an expository account of their conclusions, not their reasoning. So as Einstein explained in 1909 how Michelson-Morley was crucial to the development of relativity, it was crucial to Lorentz and Poincare, but not to Einstein's 1905 paper.
Michelson's experiment suggests the axiom that all phenomena obey the same laws relative to the Earth's reference frame or, more generally, relative to any reference frame in unaccelerated motion. For brevity, let us call this postulate the relativity principle.
I just discovered the Marauders of the Lost Sciences blog, that regularly posts excerpts of old math and physics papers that are particularly interesting, such as anticipating later results. A recent one is Poincaré discussing relativity a year before Einstein. Poincare traveled to the 1904 St. Louis Missouri World's Fair, and gave a lecture about relativity and other topics. He speaks of a "wholly new mechanics which would be characterized above all by this fact, that there could be no velocity greater than that of light, any more than a temperature below that of absolute zero."
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