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INTRODUCTION 219
or the structure of any other field (for example, that of the
electromagnetic field). Therefore, several scientists (e.g., Wey!,
Kaluza, Eddington) tried early to extend or to generalize the
theory so as to achieve a unified theory of all fields-or at least
the gravitational and the electromagnetic fields. For various
reasons the early attempts were not satisfactory. Einstein himself
has steadily worked on this problem since 1923, repeatedly
modifying the form of the theory. The latest version was initiated
in 1945 and received its definitive form in 1953 (published
as Appendix II to the fourth edition of The Meaning of
Relativity).
II. Quantum Theory.
Soon after the inaugnration of the quantum theory by Max
Planck in 1900, Einstein became the foremost pioneer in the
new field. His first contribution appeared in the same year
(1905)-and even in the same volume of the Annalen der
Physik-as his first paper on relativity. It introduced the concept
of light quanta or photons and provided the basis for much
of the further work in quantum theory, in particular for Bohr',
theory of the atom. In 1917 there appeared one of Einstein's
most significant later papers on this subject, in which, in addition
to a penetrating analysis of the properties of photons, he
gave a new derivation of Planck's law of radiation based on the
concept of transition probabilities. This concept has remained
basic ever since.
Among Einstein's other contributions we mention the first
application of the quantum theory to the theory of specific
heats (1907), and the particularly important papers on the
quantum theory of gases (1924-25). These introduced in full
generality the new type of statistics which is now known as
Bose-Einstein statistics, and also contained far-reaching ideas
on electron waves, which Schroedinger credited with guiding
him in his work on wave mechanics.
III. Kinetic Theory of Matter.
In the years 1902-04 Einstein wrote a series of papers in