UNIFIED FIELD THEORY 329
capillaries in membranes [Ell]. Late in 1924 and early in 1925, his three papers
on the Bose-Einstein gas were completed (see Chapter 23).
Meanwhile he was not altogether idle in regard to unified field theory. There
is the Einstein-Grommer paper of 1922 in response to the Kaluza theory (see
Section 17c). There are several papers in 1923 (to be discussed in Section 17e)
elaborating an attempt at unification due to Eddington. But it is not until 1925
that we witness his first truly deep immersion in this subject, as he came forth
with an invention all his own of a new version of unification.
From that time on, the character of Einstein's scientific output changes. In 1926
he wrote three papers of that playful but not at all flippant variety which he had
so often produced in earlier years, one on the meandering of rivers [E12], two on
the light emission by canal rays [El3, El4]. They were his last in this genre. The
later period begins. He is nearly fifty years old. Occasionally there are papers on
conventional general relativity, such as those on the problem of motion. But uni-
fied field theory now becomes the main thrust of his efforts, along with the search
for an alternative that deprives quantum mechanics of its status as a fundamental
theory. I have already alluded to the fact that these two themes were—in his
view—intimately related, a subject to which I shall return at more length in Sec-
tion 26e.
Heisenberg's first paper on matrix mechanics [HI] and Einstein's first privately
created unified field theory [E15] were both completed in July 1925; Schroedin-
ger's first paper on wave mechanics in January 1926 [SI]. Einstein's gestation
period before he made the real plunge into unified field theory had lasted about a
decade, just as it had been for the special and the general theories of relativity.
This time, however, it was not he but others who in the end ushered in the new
physics. So it was to remain in the next decade, and the next and the next, until
he laid down his pen and died. His work on unification was probably all in vain,
but he had to pursue what seemed centrally important to him, and he was never
afraid to do so. That was his destiny.
Let us see next what he did, first with five-dimensional theories.
17 c. The Fifth Dimension
- Kaluza and Oskar Klein. The two pioneers of unified field theory were both
mathematicians. The first unification, based on a generalization of Riemannian
geometry in the usual four space-time dimensions, was proposed by Hermann
Weyl in 1918 (see Section 17d). With the same aim in mind, and inspired by
Weyl's paper, the mathematician and consummate linguist Theodor Kaluza
became the first to suggest that unification might be achieved by extending space-
time to a five-dimensional manifold.* His one paper on this subject was published
*In 1914 Nordstrom had already proposed to use a five-dimensional space for the unification of
electromagnetism with a scalar gravitational field [Nl].
