UNIFIED FIELD THEORY 347
Einstein's colleagues were not impressed. Eddington [E63] and Weyl [W6]
were critical (for other views, see [L2] and [W7]). Pauli demanded to know what
had become of the perihelion of Mercury, the bending of light, and the conser-
vation laws of energy-momentum [P7]. Einstein had no good answer to these
questions [ E64], but that did not seem to overly concern him, since one week later
he wrote to Walther Mayer, 'Nearly all the colleagues react sourly to the theory
because it puts again in doubt the earlier general relativity' [E65]. Pauli on the
other hand, was scathing in a review of this subject written in 1932: '[Einstein's]
never-failing inventiveness as well as his tenacious energy in the pursuit of [uni-
fication] guarantees us in recent years, on the average, one theory per annum....
It is psychologically interesting that for some time the current theory is usually
considered by its author to be the "definitive solution" ' [P8].
Einstein held out awhile longer. In 1930 he worked on special solutions of his
equations [E66] and began a search for identities which should play a role (with-
out the benefit of a variational principle) similar to the role of the Bianchi ident-
ities in the usual theory [E67]. One more paper on identities followed in 1931
[E68]. Then he gave up. In a note to Science, he remarked that this was the wrong
direction [E26] (for his later views on distant parallelism, see [S5]). Shortly there-
after, he wrote to Pauli, 'Sie haben also recht gehabt, Sie Spitzbube,' You were
right after all, you rascal [E69]. Half a year after his last paper on distant par-
allelism he was back at the five dimensions.
1931-2\. Work on the Einstein-Mayer theory of local 5-vector spaces.- The Spencer lecture, referred to in Chapter 16, in which Einstein
expressed his conviction that pure mathematical construction enables us to dis-
cover the physical concepts and the laws connecting them [E70]. I cannot believe
that this was the same Einstein who had warned Felix Klein in 1917 against
overrating the value of formal points of view 'which fail almost always as heuristic
aids' [E2]. - Work with Rosen and Podolsky on the foundations of the quantum the-
ory.
1935-8. Work on conventional general relativity—alone on gravitational
lenses, with Rosen on gravitational waves and on two-sheeted spaces, and with
Infeld and Hoffmann on the problem of motion.
1938-41\, Last explorations of the Kaluza-Klein theory, with Bergmann and
Bargmann.
The early 1940s. In this period, Einstein became interested in the question of
whether the most fundamental equations of physics might have a structure other
than the familiar partial differential equations. His work with Bargmann on
bivector fields [E71, E72]* must be considered an exploration of this kind. It was
not meant to necessarily have anything to do with physics. Other such investiga-
tions in collaboration with Ernst Straus [S6] remained unpublished.**
*See Chapter 29.
**I am grateful to Professors Bargmann and Straus for discussions about this period.