100 STATISTICAL PHYSICSBrownian motion. This difference seemed to him to be 'disquieting, since one must
say that the theoretical foundation of Planck's formula is fictitious.'The foregoing account of Einstein's work on Brownian motion emphasizes its
role in securing general acceptance of the reality of molecules. That, however, was
not the only thing nor, in Einstein's own opinion, the most important thing that
his theory of Brownian motion did for the development of physics. In 1915, he
wrote about this work:
[It] is of great importance since it permits an exact computation of TV. ... The
great significance as a matter of principle is, however,... that one sees directly
under the microscope part of the heat energy in the form of mechanical energy.
[E6]
and in 1917:
Because of the understanding of the essence of Brownian motion, suddenly all
doubts vanished about the correctness of Boltzmann's interpretation of the ther-
modynamic laws. [El6]5e. Einstein and Smoluchowski; Critical Opalescence
If Marian Ritter von Smolan-Smoluchowski had been only an outstanding theo-
retical physicist and not a fine experimentalist as well, he would probably have
been the first to publish a quantitative theory of Brownian motion.
Smoluchowski, born to a Polish family, spent his early years in Vienna, where
he also received his university education. After finishing his studies in 1894, he
worked in several laboratories abroad, then returned to Vienna, where he became
Privatdozent. In 1900 he became professor of theoretical physics in Lemberg (now
Lvov), where he stayed until 1913. In that period he did his major work. In 1913
he took over the directorship of the Institute for Experimental Physics at the
Jagiellonian University in Cracow. There he died in 1917, the victim of a dys-
entery epidemic.*
It is quite remarkable how often Smoluchowski and Einstein simultaneously
and independently pursued similar if not identical problems. In 1904 Einstein
worked on energy fluctuations [El7], Smoluchowski on particle number fluctua-
tions [S5] of an ideal gas. Einstein completed his first paper on Brownian motion
in May 1905; Smoluchowski his in July 1906 [S3]. Later on, we shall encounter
a further such example. Let us first stay with Brownian motion, however.
Unlike Einstein, Smoluchowski was fully conversant with the nineteenth cen-
*For a detailed account of the life and work of Smoluchowski, the reader is referred to the biography
by Teske [T4], in which the Einstein-Smoluchowski correspondence referred to hereafter is repro-
duced. My understanding of Smoluchowski's contributions was much helped by my reading of an
unpublished manuscript by Mark Kac.