x l i v I N T R O D U C T I O N T O V O L U M E 1 5 Einstein had already revisited these equations in Einstein 1922r (Vol. 13, Doc. 387), where he had argued that the vacuum equations of 1919 are equivalent to the vacuum equations with cosmological constant introduced in Einstein 1917b (Vol. 6, Doc. 43).[7] Now, with Rainich’s results in hand, Einstein saw the opportu- nity to find an equally “mathematically natural interpretation” to the left-hand side of the 1919 equations (and thus also to that of the 1917 equations), as Gustav Herglotz had done for the original Einstein tensor of 1915 (Herglotz 1916). In Einstein 1927a (Doc. 158), Einstein described how Herglotz had related the Einstein tensor to the median curvature of three-dimensional slices through space- time now Einstein himself argued that the trace-free tensor of 1919 vanishes if and only if Rainich’s antisymmetric part of the Riemann curvature tensor vanishes. In a letter to Michele Besso (Doc. 138) he praised the 1919 field equations as the “best of what we have today,” and gave a similar endorsement in a letter to Edding- ton. But he also pointed out that the question remained whether the equations, and with them general relativity, “fail in the face of quantum phenomena” (Doc. 179). The first part of Einstein’s correspondence with Rainich may be surprising in that Einstein was much less enthusiastic than one might expect. After all, if true, Rainich’s results would mean that the unified field theory that Einstein had been seeking for years was already at hand. But Einstein’s reaction was consistent with his previous conduct of this search. Again and again, Einstein had abandoned ap- proaches to a unified field theory not because he felt that they were lacking as unifiers of the gravitational and the electromagnetic field, but because they did not also solve “the quantum problem” by allowing for solutions that could be interpret- ed as electrons. Indeed, in Einstein 1924d (Vol. 14, Doc. 170), Einstein had spelled out a research program he had already implicitly followed for years: every satisfac- tory unified field theory of gravity and electromagnetism needed to have “at least the static spherically symmetric solution that describes the positive, or the nega- tive, electron.” Einstein had then recently developed another candidate unified field theory. In Einstein 1925t (Doc. 17), which he described to Ehrenfest as a captivating paper (see Doc. 71), he both continued and departed from what he had called “the Weyl- Eddington approach” to unified field theories. The approach starts from Hermann Weyl’s and Tullio Levi-Civita’s realization that the affine connection can be de- fined independently of the metric, and from Eddington’s idea of basing a unified field theory only on the affine connection that recovers the metric as a derivative concept. Einstein had followed this doctrine in a series of papers published be- tween 1921 and 1925, and investigated different candidate field equations for the affine connection (see Vol. 14, Introduction, sec. I). In Einstein 1925t (Doc. 17), he departed from the tenet of introducing only the connection as the fundamental
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