328 THE LATER JOURNEY
his search for unification. In fact, by then he already believed that the need to
unify forces and the need to resolve the quantum paradoxes were connected
desiderata. In later years, he was one among few to search for unification and one
among few to be critical of quantum mechanics. He was unique in holding the
view that there was a link between these problem areas. In this chapter, nothing
further will be said on Einstein and quantum physics. However, in Chapter 26
I shall return to his hopes for a new dynamics, based on a generalization of general
relativity, in which quantum mechanics would be explained rather than
postulated.17b. Another Decade of Gestation
Einstein completed his first paper on unified field theory in January 1922.
Much had happened to him since the strenuous days of November 1915, when
he completed his general relativity theory. He had done his share of applying this
theory to the energy-momentum conservation problem, to gravitational waves,
and to cosmology. He had introduced the A and B coefficients in quantum theory.
He had been ill. He had remarried. After November 1919 he had become a world
figure. He had been in the midst of turmoil in Germany. And he had made his
first trip to the United States. The problem of unification had been on his mind
in the intervening years, even though he had not published on this subject. In 1918
he wrote to Weyl, 'Ultimately it must turn out that action densities must not be
glued together additively. I too, concocted various things, but time and again I
sank my head in resignation' [E3]. His statement to Ehrenfest in 1920, 'I have
made no progress in general relativity theory. The electromagnetic field still stands
there in unconnected fashion' [E4], expressed both his disbelief in Weyl's theory
(to be described in Section 17d) and his conviction that unification is a worthy
cause. When he wrote to Weyl in 1922 about unified theories, 'I believe that in
order to make real progress one must again ferret out some general principle from
nature' [E5], he was still taking his cues from physics.
Nor were his interests in physics in those years confined to general relativity,
whether of the orthodox or of the unified variety. His letters of that period to
Ehrenfest, always filled with physics ideas that intrigued him, deal largely with
the quantum theory. In 1921 he was excited about his new proposal for an exper-
iment to test quantum aspects in Doppler phenomena [E6]. In 1922 he was
intrigued by the Stern-Gerlach experiment [E7]. His January 1922 paper on
unified field theory, written with Grommer [E8], is never mentioned in these let-
ters, but a few weeks after its completion he wrote of his work with Grommer on
quantum problems [E9]. In 1923 he and Ehrenfest worked on the quantum the-
ory of radiative equilibrium [E10], and, together with another friend,* he pub-
lished his last paper on experimental physics, a determination of the width of
*See the entry about Miihsam in Chapter 29.