According to conventional wisdom, Poincar? failed to derive a relativity theory mainly as a result of his stubborn adherence to the ether. In (1905) Einstein constructed a relativity theory that was based on the assertion that the ether was superfluous. In 1908 Minkowski formulated the theory of the ?absolute world?. The nineteenth century ether no longer existed. A new kind of ether (space-time) came into being. One could keep on maintaining the ether, and at the same time strip it of the notion of absolute rest. Einstein seemed to agree, and after 1916 he returned to the ether. In 1920 he combined Minkowski?s absolute world concept and Mach?s ideas on rotational movements: in order to cancel action-at-a-distance, the inertial interactions between matter and fixed stars should be mediated by a medium. Einstein called Mach?s medium ?Mach?s ether?. In this paper I demonstrate that Einstein?s 1920 reasoning hardly differed from the one Poincar? had presented prior to 1905. Thus, whil Einstein was a hero because he did away with the ether, this situation lasted a few years only. This is not to underestimate the magnitude of Einstein?s achievement, but to emphasize the limits of simplistic comparisons between Einstein and Poincar?.
In my paper ?Einstein's ether part A? I mainly re-examined the bucket experiment and earth's daily rotation (the problems that had been occupying Mach and Poincar?) from Einstein's General Relativistic point of view. In this paper I further discuss Einstein's General Relativistic solution to the problems that had been occupying Mach and Poincar?.
Prof. Ludwig Kostro is the most influential historian and philosopher of science who has written about Einstein's post 1916 return to the ether concept (1988, 1992). He has endeavored to show that (Kostro, 1988, p. 239): ?the notion of the ether was not destroyed by Einstein, as the general public believe.? In addition, Kostro showed (1988, p.238): ?Lorentz wrote a letter to Einstein in which he maintained that the general theory of relativity admits of a stationary ether hypothesis. In reply, Einstein introduced his new nonstationary ether hypothesis.?
In parts A, B and C I suggest a new view of the problem tackled by Kostro. I ask the following question: Did Einstein respond to Poincar? too when returning to the ether concept? In parts A and B I first introduce the problem by re-examining the problems that had been occupying Poincar? from Einstein's point of view.
In his 1900 lecture ?On the Principles of Mechanics,? Poincar? imagined the following fable (1900b, p. 480; 1902, p.131): imagine beings living on an imaginary cloudy planet. They can never see the stars and therefore may think that their planet is the only object in the universe. How can they find out whether their planet rotates or stands still? Poincar? answers that for these beings the two conventions; ?the earth turns round? (Copernican) and ?the earth does not turn round? (Ptolemaic) are equivalent. Therefore no absolute motion can exist. Poincar? seemed to have been inspired by Mach's ideas towards offering his conventionalist above point of view (see Mawhin, 1995). Mach protested against Newton's interpretation to his famous bucket experiment in terms of absolute motions and space. He philosophically demonstrated relative motions by stating the logical equivalence of the Copernican and Ptolemaic systems. However, unlike Mach, Poincar? examined the experimental equivalence of the two conventions for beings living in the cloudy planet. According to Poincar?, beings living in the cloudy planet and performing experiments in order to discover whether their planet turns round or stands still, would always find out that the two conventions are completely experimentally equivalent. As a result of this suggestion, Poincar? had to philosophically and physically respond to a realist understanding by his audience over the years of the above conventionalist position: imagining a being standing outside Poincar?'s cloudy world and knowing that thick clouds forever cover this planet, he could readily choose between the two conventions, and he might conclude that the earth rotates or else stands still with respect to absolute space. Therefore, Poincar?'s audience could not have accepted his reasoning.
In this paper I demonstrate that in light of Poincar?'s special efforts to save his conventionalist view, he himself was also eventually not persuaded by his own arguments. He probably understood very well that his conventionalism was open to a kind of criticism regarding the possibility of the existence of an external being (standing outside his cloudy planet) for whom conventionalism did not hold any more. This criticism was embodied in the na?ve and realist response of Poincar?'s audience to his explanations. Poincar?'s last resource was therefore the ether: the cloudy planet does not rotate with respect to absolute space but with respect to the ether. In my previous paper, ?What characterizes Poincar?'s ether?? I characterized Poincar?'s obscure notion of the ether, which is based on Lorentz's stationary ether. I start this paper by analyzing Newton's famous bucket experiment and Mach's solution in terms of relative motions. I then trace Poincar?'s response to Mach's ideas and, his inner struggles and special efforts to save his conventionalist view.
In this paper I discuss Poincar?'s solution to the following problem: the principle of relativity is not valid for rotations and we thus can claim for absolute rotation. The principle of relativity was experimentally not valid for uniform rotations, and therefore it lost of its complete validity. Logical conventionalism (the philosophical principle of relativity) also lost of its complete validity. Therefore, since the principle of relativity was not a priori completely valid, we could disclose absolute motions with respect to absolute space, or speak of a reality independent of the observer. Poincar? could not accept this. He therefore postulated the ether as a material body in absolute rest. By doing so he felt that the principle of relativity regained its complete validity, because by no experimental means could we disclose absolute space (1908a, p. 567): ?it is impossible to escape this impression that the principle of relativity is a general law of nature, that one can never by any imaginable means get evidence of any but relative velocities, and by this I mean not merely the velocities of bodies in relation to the ether, but the velocities of bodies in relation to other bodies?.
This paper is divided into three parts, in which I suggest five answers to the question: Why did Poincar? retain the ether? These answers are based on Poincar?'s own reasoning: the ether was required for the explanation of stellar aberration, to remove action-at-a-distance, to remove absolute rotation and absolute space from physics, to save broken theories and to save Poincar?'s conventionalism. Poincar?'s first reason can be seen as related to rectilinear and uniform motions. In 1905 Einstein managed to explain aberration without resource to ether. Special relativity crowned the final oblivion of the ether. Poincar?'s four other reasons are centered on the solution to the following old problem: the principle of relativity is not valid for rotations and we thus can claim for absolute rotation. Poincar? struggled with this problem and could not solve it without resource to the ether. In General Relativity, Einstein could not solve it without returning to some kind of ether, either. I first discuss Poincar?'s reasoning and in a future paper ?Why did Einstein come back to the ether?? I discuss Einstein's solution to the problem of absolute rotation and his return to a revised form of Poincar?'s ether.