On the Dynamics of the Electron
By H. Poincaré (Paris).
Meeting of July 23, 1905.
Introduction.
It seems at first sight that the aberration of light and the related optical and electrical phenomena will provide us a means of determining the absolute motion of the Earth, or rather its motion, not in relation to the other stars, but in relation to the ether. Fresnel had already tried it, but he recognized soon that the motion of the earth does not alter the laws of refraction and reflection. Similar experiments, like that of a telescope filled with water and all those which take into consideration only terms of first order in respect to aberration, give no other but negative results; soon an explanation was discovered; but Michelson, having imagined an experiment where the terms depending on the square of the aberration became sensitive, failed as well.
It seems that this impossibility of demonstrating an experimental evidence for absolute motion of the Earth is a general law of nature; we are naturally led to admit this law, which we will call the Postulate of Relativity and admit it without restriction. This postulate, which is up to now in accord with experiments, may be either confirmed or disproved later by more precise experiments, it is in any case interesting to see which consequences follow from it.
An explanation was proposed by Lorentz and FitzGerald, who introduced the hypothesis of a contraction undergone by all bodies into the direction of the motion of earth and proportional to the square of aberration; this contraction, which we will call Lorentz contraction, would give an account of the experiment of Michelson and all those which were carried out up to now. The hypothesis would become insufficient, however, if one were to assume the postulate of relativity in all its generality.
Lorentz sought to supplement and modify it in order to put it in perfect agreement with this postulate. He succeeded in doing so in his article entitled Electromagnetic phenomena in a system moving with any velocity smaller than that of light (Proceedings de l’Académie d’Amsterdam, May 27, 1904).
The importance of the question determined me to take it up again; the results which I
