2O INTRODUCTORYalso be remembered that the main applications of his first work (1904) on energy
fluctuations (4c) are in the quantum domain. His analysis of these fluctuations in
blackbody radiation led him to become the first to state, in 1909, long before the
discovery of quantum mechanics, that the theory of the future ought to be based
on a dual description in terms of particles and waves (21 a). Another link between
statistical mechanics and the quantum theory was forged by his study of the
Brownian motion of molecules in a bath of electromagnetic radiation. This inves-
tigation led him to the momentum properties of light-quanta (21c). His new der-
ivation, in 1916, of Planck's blackbody radiation law also has a statistical basis
(21b). In the course of this last work, he observed a lack of Newtonian causality
in the process called spontaneous emission. His discomfort about causality origi-
nated from that discovery (21d).
Einstein's active involvement with statistical physics began in 1902 and lasted
until 1925, when he made his last major contribution to physics: his treatment of
the quantum statistics of molecules (23). Again and for the last time, he applied
fluctuation phenomena with such mastery that they led him to the very threshold
of wave mechanics (24b). The links between the contributions of Einstein, de
Broglie, and Schroedinger, discussed in (24), make clear that wave mechanics has
its roots in statistical mechanics—unlike matrix mechanics, where the connections
between the work of Bohr, Heisenberg, and Dirac followed in the first instance
from studies of the dynamics of atoms (18c).
Long periods of gestation are a marked characteristic in Einstein's scientific
development. His preoccupation with quantum problems, which began shortly
after Planck's discovery of the blackbody radiation law late in 1900, bore its first
fruit in March 1905. Questions that lie at the root of the special theory of relativity
dawned on him as early as 1895 (6d); the theory saw the light in June 1905. He
began to think of general relativity in 1907 (9); that theory reached its first level
of completion in November 1915 (14c). His interest in unified field theory dates
back at least to 1918 (17a). He made the first of his own proposals for a theory
of this kind in 1925 (17d). As far as the relativity theories are concerned, these
gestation periods had a climactic ending. There was no more than about five weeks
between his understanding of the correct interpretation of the measurement of
time and the completion of his first special relativity paper (7a). Similarly, after
years of trial and error, he did all the work on his ultimate formulation of general
relativity in approximately two months (14c).
I focus next on special relativity. One version of its history could be very brief:
in June, 1905, Einstein published a paper on the electrodynamics of moving bod-
ies. It consists of ten sections. After the first five sections, the theory lies before us
in finished form. The rest, to this day, consists of the application of the principles
stated in those first five sections.
My actual account of that history is somewhat more elaborate. It begins with
brief remarks on the nineteenth century concept of the aether (6a), that quaint,
hypothetical medium which was introduced for the purpose of explaining the