When Robert Shankland asked Einstein how he had learned of the Michelson-Morley experiment, Einstein told him that he had become aware of it through the writings of Lorentz, but only after 1905 had it come to his attention. "Otherwise", he said, "I would have mentioned it in my paper". He continued to say that the experimental results which had influenced him most were stellar aberration and Fizeau's water tube experiment. "They were enough". Indeed the famous Michelson-Morley experiment is not mentioned in the 1905 relativity paper; but curiously Einstein did not mention Fizeau's experimental result either, and this is puzzling in light of the importance of the experiment in Einstein's pathway to his theory.No, the stellar aberration and Fizeau experiments were not enough, as Einstein explained in his 1909 review paper:
This contradiction was chiefly eliminated by the pioneering work of H. A. Lorentz in 1895. Lorentz showed that if the aether were taken to be at rest and did not participate at all in the motions of matter, no other hypotheses were necessary to arrive at a theory that did justice to almost all of the phenomena. In particular, Fizeau's experiments were explained, as well as the negative results of the above-mentioned attempts to detect the Earth's motion relative to the ether. Only one experiment seemed incompatible with Lorentz's theory, namely, the interference experiment of Michelson and Morley.It was because of the Michelson–Morley experiment that Lorentz and Poincare improved the 1895 theory to a full relativity theory during 1899-1904, and they said so in their papers before Einstein wrote anything.
So Einstein certain knew in 1905 that Michelson-Morley was the crucial experiment for special relativity, but he may not have understood exactly why.
The one relativity formula that Einstein had in 1905 that had not been published by Lorentz in 1904 or earlier was the velocity-addition formula. (Poincare had it before Einstein, and included it in a letter to Lorentz, so Lorentz knew it before Einstein.) Weinstein says:
In 1907 Max Laue showed that the Fresnel dragging coefficient would follow from a straightforward application of the relativistic addition theorem of velocities. This derivation is mathematically equivalent to Lorentz's derivation of 1895. From 1907 onwards Einstein adopted Laue's derivation.When the Einstein fans praise his 1905 work, they often argue that the previous work of Lorentz and Poincare was just mathematical, while only Einstein truly appreciated the physical significance of relativity. Others say the opposite, and argue that Einstein paid no attention to the physical experiments, and that his contribution was to turn relativity into a mathematical theory with postulates. I think that both views are desperate and misinformed attempts to over-credit Einstein.
Weinstein wonders why Einstein did not use the velocity addition formula to explain Fizeau:
John Norton writes, "It might seem surprising that Einstein could devise and publish the relativistic rule of velocity composition in his 1905 paper (§5) without recognizing that the result of the Fizeau experiment is a vivid implementation of the rule." ... Stachel explains that Einstein was under the spell that everything could be solved using Maxwell's equations that he failed to notice the kinematic nature of Fresnel’s formula, resulting from direct application of the relativistic law of combination of relative velocities; it was left for Laue to make this observation in 1907.These are all big Einstein idolizers who have looked really hard for originality in Einstein's work. They say that Einstein had a superior understanding of relativistic kinematics, so they are baffled he was unable to apply that understanding to Fizeau, as Laue did. The simple explanation is that Einstein did not have a superior understanding.
Weinstein also posts Did Mileva Maric assist Einstein in writing his 1905 path breaking papers?
Writers read Einstein's letter to Maric from 1901 in which he wrote: "bringing our work on relative motion to a successful conclusion!" What came afterwards was boosted by a claim that Joffe had seen the original relativity paper manuscript, and that it was signed "Einstein-Marity" (i.e., "Maric"). This drew the attention of some writers to develop a theory according to which Mileva Maric assisted Albert Einstein in solving his physics problems, but her name was left out of the published article and only Einstein's name appears in the journal as author. Historical and primary sources do not support this scenario.Mileva was a physics grad student, was married to Einstein, and did read and discuss his papers with him. So she surely had some influence. The question is how much.
The main argument against Mileva's influence is that she never again did anything so original. But some of Einstein's strongest admirers also say that he never again did anything so original as that 1905 relativity paper. My view is that the 1905 was not very original at all, and that we cannot trust Einstein about his sources because he did not believe in crediting his sources. So the amount of her influence is a mystery, but not a very interesting one. Even if it were somehow shown that she came up with the main ideas, it would not change my opinion of him very much because it would just mean that she stole them from Lorentz and Poincare instead of him stealing them.