Einstein's invention of Brownian motion

1 The impact of Einstein's work Einstein's 1905 paper on Brownian motion was an essential contribution to the foundation of modern atomism (20). Atomism as understood in science today presupposes, like its predecessor rooted in the theories of nature from Greek antiquity and from early modern times, that matter is constituted by small entities. But it no longer assumes that the properties and the behavior of these entities can simply be inferred from the familiar physical laws governing our macroscopic environment, nor that a description of matter in terms of its atomistic constituents can be exhaustive. Einstein succeeded in interpreting the irregular movements of small particles suspended in a liquid as visible evidence for the molecular motions constituting the heat of a ponderable body according to the kinetic theory of heat. But he did so by radically changing the understanding of these irregular motions which he no longer conceived as being characterized by a velocity in the classical sense but as a stochastic process that can only be described with the help of statistical methods. It is therefore not surprising that Einstein's work on Brownian motion also became one of the pillars of modern statistical thermodynamics and, more generally, of the physics of stochastic processes. In the sequel to his groundbreaking work, Einstein published several other related articles, extending the subject to Brownian motion in condensers and the fluctuations of heat radiation. His work aroused widespread interest among physicists and chemists, as indicated by Einstein's correspondence with other scientists interested in the subject, in particular Conrad Rontgen, Richard Lorenz, Marian von Smolu- chowski, and The Svedberg.2 In 1906 the Polish physicist von Smoluchowski submitted a paper on the kinetic theory of Brownian motion to the Annalen that was stimulated by Einstein's papers but represented results which he had derived independently. While Smoluchowski's argument was different from Ein- stein's, his results were - apart from a numerical factor - essentially equivalent. Einstein's interpretation of Brownian motion soon also received striking experimental confirmation by Jean Perrin and others. This success furthered the general acceptance of atomism and helped to convert the then still numerous skeptics. Indeed, while in the nineteenth century atomism was widely employed as a working hypothesis in numerous fields of physics and chemistry, it was accepted as a physical reality only after the impressive accumulation