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publication of the Princeton lectures (Einstein 1922c [Doc. 71]), the closest he
came to writing a textbook on relativity.
For much of the rest of his career, Einstein’s work was dominated by his search
for a unified field theory of gravity and electromagnetism, an interest stirred by
Weyl’s unified field theory (Weyl 1918a), the first instance of what later became
known as gauge theory. In a supplement, Einstein 1918g (Doc. 8), to Weyl 1918a,
he praises the ingenuity of Weyl’s proposal, but emphasizes that he finds the theory
physically untenable. At this point, Einstein was still very hesitant to use formal
criteria such as Weyl’s gauge principle instead of physical principles (as he had laid
out in Einstein 1918e [Doc. 4] for general relativity). In his subsequent work on
unified field theory, however, formal criteria gradually assumed greater impor-
tance. Weyl’s influence on Einstein’s thinking can also be seen in Einstein 1921e
(Doc. 54). In this paper, he generalizes Weyl’s gauge theory to what is now called
a conformal theory, in which the metric is only determined up to a scalar factor.
While Einstein remarked that its usefulness for physics was unclear, conformal
transformations have become an important tool in contemporary general relativity,
e.g. for treating asymptotic behavior.
In his inaugural lecture in Leyden on “Ether and Relativity” (Einstein 1920j
[Doc. 38]), Einstein clearly outlines his unification program: Through the unifica-
tion of electrodynamics and general relativity, “the whole of physics would become
a complete system of thought, similarly as geometry, kinematics, and the theory of
gravitation had become in general relativity.” He observes, nonetheless, that quan-
tum theory might present an obstacle to the unification of gravity and electromag-
netism on the basis of general relativity (Doc. 38, p. 15), a reservation which may
have arisen out of his work on gravitational waves. In Einstein 1918a (Doc. 1),
immediately after writing down the quadrupole formula, he repeats a sentiment
expressed in his 1916 paper (Einstein 1916g [Vol. 6, Doc. 32]) that the result
“would require a loss of energy of bodies due to thermal agitation—[which] must
raise doubts as to the general validity of the theory. It seems that a more complete
quantum theory would also lead to a modification of the theory of gravitation.” In
1916, he had written: “Nevertheless, due to the inneratomic motion of electrons,
atoms would have to radiate not only electromagnetic but also gravitational energy,
if only in tiny amounts. As this is hardly true in nature, it appears that quantum
theory would have to modify not only Maxwellian electrodynamics, but also the
new theory of gravitation” (Einstein 1916g [Vol. 6, Doc. 32, p. 696]). Einstein
1919a (Doc. 17) is another instance of his interest in unification theories, in which
he takes a single approach to the problems of the stability of fundamental particles
and to cosmology. He employs the notion of a universal pressure, present every-
where in space, inside and outside of material bodies, to stabilize both the electron