On some points, such as the principle of relativity or the physical interpretation of the Lorentz transformations, Poincaré’s contributions preceded by at least 5 years those of Einstein’s published in 1905. On the other hand, many of their contributions were practically simultaneous. In 1905 Poincaré published an abridged version of his “Sur la dynamique de l’électron” [3] (which preceded the work of Einstein); the expanded version of the article appeared in 1906 [4].No, it was not simultaneous. Poincare was five years ahead of Einstein. Poincare was years ahead of Einstein with the relativity principle, rejection of the aether, local time, synchronizing clocks, interpreting Michelson-Morley, Lorentz group, mass-energy equivalence, four-dimensional spacetime, and relativistic theories of gravity. Einstein's only claim to originality is to certain epistemological differences of no physical significance.What are the conceptual differences? According to Darrigol,
Einstein completely eliminated the ether, required that the expression of the laws of physics should be the same in any inertial frame, and introduced a “new kinematics” in which the space and time measured in different inertial systems were all on exactly the same footing. In contrast, Poincaré maintained the ether as a privileged frame of reference in which “true” space and time were defined, while he regarded the space and time measured in other frames as only “apparent.” …Einstein derived the expression of the Lorentz transformation from his two postulates (the relativity principle and the constancy of the velocity of light in a given inertial system), whereas Poincaré obtained these transformations as those that leave the Maxwell–Lorentz equations invariant [1].These are conceptual differences that have no actual experimental consequences as far as electromagnetism and optics are concerned. As Lorentz commented, the difference is purely epistemological: it concerns the number of conventional and arbitrary elements that one wishes to introduce in the definitions of the basic physical concepts.Are we then dealing with a case of simultaneous discovery?
Credit Einstein with those obscure conceptual differences if you want, but there is no mathematical or physical value to any of them. Mostly they consist of mathematical misunderstandings by Einstein and other physicists. For example, there is nothing erroneous about choosing a privileged frame on a symmetric space. It does not break the symmetry. Those who criticize Poincare for occasionally choosing a privileged frame are just mathematically ignorant.
The essay concludes:
As Darrigol suggests, it seems wiser to concede that Lorentz, Poincaré and Einstein all contributed to the emergence of the theory of relativity, that Poincaré and Einstein offered two different versions of the theory, and that Einstein gave form to what today is considered the best one.No, Einstein's is not considered best today. Nearly everyone prefers the spacetime formulation that Poincare advanced in 1905 and Minkowski perfected in 1907. Einstein was still rejecting it in 1911, and did not even speak positively about it until after that, and never really accepted the geometrical significance.
A lot of historians concede that Lorentz and Poincare had all the mathematics of special relativity, and all the physical consequences, but insist that Einstein had a more modern viewpoint or superior understanding, leading to how we understand the theory today. But that is false. The Poincare-Minkowski geometrical interpretation has been preferred by nearly everyone but Einstein, since 1908.
