l I N T R O D U C T I O N T O V O L U M E 9
critical and far-seeing, never losing sight of the overall picture” (Doc. 149); “a per-
son of a deep probing mind through whom grand connections come alive” (Doc.
160). In Einstein, this kindled a great interest in Bohr’s “kitchen of ideas” and he
humbly set out to learn, or rather relearn, quantum theory from Lorentz’s text-
books, to reread Bohr’s papers (Docs. 160, 165, 189), and to consult Sommerfeld’s
seminal book (Doc.
This effort did not, however, turn him into a supporter of Bohr’s ideas. Already
in January 1920 he was convinced that the quantum problem must be solved not by
giving up the continuum, but by finding discrete solutions to field equations
through overdetermination. Nor was he satisfied with statistical considerations: he
was reluctant to give up strictly deterministic causality (Doc. 284). As he later
explicitly wrote to Polányi, he did not think Bohr’s model was correct (Doc. 335).
Einstein and Bohr met for the first time in late April 1920.
From now on, Einstein would not consider the future of quantum theory sepa-
rately from that of relativity. When he complained to Ehrenfest that he was making
no headway in relativity, he blamed the electromagnetic field that “thwarts every
effort” (Doc. 292); and, conversely, when, in his “spare time,” he pondered quan-
tum theory, he did so “from the relativistic point of view” (Doc. 284). By March,
overdetermination had become a “pet idea” to which, however, he was unable to
give “tangible form” (Docs. 336, 371).
Despite frequent laments that in his science he had found nothing new (Doc.
189), that he was too busy to do serious work (Doc. 198), and that he felt “empty-
handed” (Doc. 265), one gleans from the letters Einstein’s preoccupation with a
theme that would mature into published papers only very much later and occupy
him for much of his professional future: the construction of a unified field theory.
Einstein’s work in this direction developed to some extent in reaction to Weyl’s
unified field theory, which geometrized the electromagnetic field by introducing a
so-called length connection, i.e., a new geometric object that modifies Riemannian
geometry. This work appeared as a competitor to his own endeavors, one that Ein-
stein perceived with an ambivalence ranging from appreciation, through suspicion,
to utter contempt. He admired Weyl’s theory “as a chain of ideas”(Doc. 59), but as
a theory of physical reality it was to him “fanciful nonsense” (Doc. 294). By in-
cluding general relativity into the third edition of his textbook on “Space-Time-
Matter,” Weyl had, according to Einstein, “messed it up” (Doc. 332).
Einstein became more seriously engaged in evaluating another proposal to unify
gravitation and electromagnetism. In April 1919, Theodor Kaluza (see Illustration
9) sent him a manuscript in which he lays out his idea of extending space-time by
an additional spacelike fifth dimension. Kaluza identified the gi5 components with
the electromagnetic vector potential and showed how the Maxwell equations can