Poincare was primarily known as a mathematician. So did he do any other physics, besides relativity? Yes, he was also a leader in celestial mechanics and quantum mechanics. See and The algebraic cast of Poincaré's Méthodes nouvelles de la mécanique céleste and Poincaré's proof of the quantum discontinuity of nature. (And also articles here and here on quantum mechanics, and Lorentz 1914.)
Poincare introduced his 1911 quantum paper by saying:
Here is the profoundest revolution that natural philosophy has undergone since Newton.This was from the guy who revolutionized space, time, electromagnetism, and gravity several years earlier. That paper, more than any other single paper, convinced Europe of Planck's quantum hypothesis. Poincare was the first to compare special relativity to Copernicus, and the first to say that quantum mechanics was revolutionary. Now these are considered the two great physics revolutions of the 20th century.
Mark Yasuda wrote:
Although it's rather well known that Henri Poincare anticipated a number of results in special relativity prior to Einstein's 1905 publication, it seems that fewer people are aware that Poincare also played a role in another revolution in physics at the beginning of the 20th century -- namely, quantum mechanics (I guess there are some people who might also argue that he participated in another revolution for his pioneering work in dynamical systems). Recently, I had the good fortune to come across Russell McCormmach's excellent article "Henri Poincare and the Quantum Theory" (Isis; Volume 58 (191); 1967; pages 37-55). Besides discussing Poincare's work, it offers some fascinating glimpses into the first Solvay Conference that took place in October and November of 1911. Below are a few (six) excerpts that I've selected from McCormmach's article, which I highly recommend for people interested in the historical development of physics during this time period:The 1927 Solvay Conference was famous for getting together the founders of quantum mechanics and discussing what it meant. But that first 1911 Solvey Conference was for getting quantum mechanics started. It must have seemed strange at the time for some rich business to invite a few scholars in an obscure field to a conference.
1. At the time, Maurice de Broglie remarked to F. A. Lindemann that of all those present Poincare and Einstein were in a class by themselves (p 40).
2. Lorentz recalled that in the discussions Poincare had shown "all the vivacity and penetration of his spirit, and that one had admired the facility with which he entered vigorously into even those questions of physics which were new to him" (p. 40).
3. ... it was Planck, however, who stimulated Poincare's most penetrating, questioning spirit ....
4. In a descriptive essay he spelled out the essence of Planck's theory as it appeared to him: "A physical system is capable of only a finite number of distinct states; it jumps from one of those states to another without going through a continuous series of intermediate states." The image of a physical system jumping from one discrete state to another put him in a speculative frame of mind. He considered the possibility that a particle might trace only certain allowed paths in phase space, shifting discontinuously between them. And he supposed that the universe as well as an electron ought to experience quantum jumps. Since there would be no distinguishable instants within the motionless states between universal jumps, there should exist an "atom of time." Such were the kinds of ideas going through Poincare's mind shortly before he died; there was nothing timid or grudging about his late acquaintance with the quantum theory (p 50).
5. ... above all it was the unquestioned authority of Poincare in mathematical matters which secured him an attentive audience. Jeans undoubtedly voiced a majority sentiment when he said that "we shall probably feel inclined to trust to the accuracy of Poincare's mathematics." (pp 51-52).
6. Whereas Jeans had strongly opposed the quantum theory in Brussels ..., he came out vigorously in support of quanta at the Birmingham meeting of the British Association in September 1913, fourteen months after Poincare's death. There is no doubt about what caused him to change his mind. Jeans had read Poincare's paper and been converted by it. ... The French scientist's arguments had been so completely persuasive that from this time on every theory would have to "logically involve either the belief that Poincare is wrong, or the belief that he is right, together with all that this involves. ... And Jeans himself felt compelled to accept the quantum hypothesis in its entirety." (p. 53).