I also argue that Einstein had no role in either discovering or popularizing this crucial idea. He did not even understand it until after many other physicists did.
Briefly, here is the history of special relativity. Maxwell discovered the first relativistic theory, following the work of Gauss, Faraday, and others, and coined the word "relativity". Michelson did the crucial experiment on the relativity of motion, following a suggestion of Maxwell. Lorentz built on Maxwell's theory, and discovered the transformations that reconciled the theory with Michelson's experiments. Poincare perfected Lorentz's work, and discovered the 4-dimensional spacetime geometry and electromagnetic covariance in 1905. Minkowski extended and elaborated Poincare's ideas, emphasizing the geometry, and published the 1908 paper that got everybody excited about relativity.
Einstein played no part in any of this. Historians say that he paid no attention to the relativity experiments that inspired Lorentz, Poincare, and Minkowski, that the main point of his paper was that he postulated what Lorentz had proved, and that the physics community was not impressed by his 1905 paper at the time.
More importantly, Einstein's 1905 paper and subsequent papers lack the crucial concepts of spacetime geometry and electromagnetic covariance. I explain this in my book, and refute scholars who say otherwise.
I neglected to address a comment from a 1905 Einstein private letter where he seems to say that he had a spacetime theory. Noted science writer James Gleick writes about Einstein's 1905 papers:
The name echoes through the language: It doesn't take an Einstein. A poor man's Einstein. He's no Einstein. In this busy century, dominated like no other by science—and exalting, among the human virtues, braininess, IQ, the ideal of pure intelligence -— he stands alone as our emblem of intellectual power. We talk as though humanity could be divided into two groups: Albert Einstein and everybody else.The quote is from a letter to Einstein's friend, Conrad Habicht. Here is a little more context:
... Einstein said in a letter to a friend, it "modifies the theory of space and time." Ah, yes. Relativity.
Such movement of suspended bodies has actually Been observed by biologists who call it Brownian molecular movement. The fourth work is based on the concepts of electrodynamics of moving bodies and modifies the theory of space and time; ... [quoted in Einstein: the life and times, by Ronald W. Clark, p.87]He says that his paper is based on electrodynamics. Compare that to what Minkowski said in 1908:
The views of space and time which I wish to lay before you have sprung from the soil of experimental physics, and therein lies their strength. They are radical. Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.But Einstein misses all of these points. He fails to say that he has a spacetime theory, that it is a consequence of experiments, that it makes space and time inseparable, and that it is a radical new idea.
Einstein's 1905 paper does give formulas for modifying space and time, but they are the same formulas previously given for the FitzGerald contraction and the Larmor dilation of Lorentz local time. Einstein later acknowledged that he got Lorentz transformations out of the Lorentz 1895 paper, and may have also read subsequent papers on the subject by Lorentz and Poincare.
Some say that Einstein followed experiments, but Clark's biography documents on p.128-130 that Einstein said contradictory things about the Michelson-Morley experiment. Sometimes he said that it was important, and other times he said that it was unimportant or that he never even heard of it. Most Einstein historians now say that he ignored experiments like Michelson-Morley, and that they praise him for using postulates instead. I think that Einstein ignored experiments because he was just reciting Lorentz's theory, and he knew that the experimental evidence would be the same as that for Lorentz's theory. Einstein did not even claim (until years later) that he was doing anything different from Lorentz. Lorentz's approach was very different because he was creating a new theory to explain the experiments.
It sounds as if Einstein was getting close when he says his paper "modifies the theory of space and time", but he is basing it on electrodynamics just as Lorentz did 10 years earlier. Lorentz also modifies space and time with his Lorentz transformations. The core of Lorentz's theory was that the experiments could be explained by coupling motion to changes in space and time. It is silly to assume that Einstein meant something different from Lorentz unless Einstein actually said that he meant something different from Lorentz. He did not, and he was happy to see other physicists call it the "Lorentz-Einstein theory".
If Einstein had said, "I show that a new non-Euclidean geometry of space and time can be used to explain the electrodynamics of moving bodies", then I would have to agree that Einstein understood the essence of special relativity, and that he could be called a co-discoverer of the theory. But he did not. Poincare and Minkowski said it, and everyone else got it from them, and not Einstein. Einstein did not even understand or accept it until after the mainstream European physicists did after 1908.