I N T R O D U C T I O N T O V O L U M E 1 4 x x x i x Whatever the idea and execution of the experiment, its outcome was negative (as the title of Doc. 152 indicates), and no joint paper by Einstein and Mark was ever published. Indeed, when asked by Emil Warburg in November 1923 about the status of “his theory of terrestrial magnetism” (Doc. 154), Einstein replied that it had led to a negative result “a long time ago,” and that “the theory was untenable” (Doc. 158). By then, he had dismissed the approach for good, as evident in his re- sponse to Théophile de Donder in early December 1923, where he states that he is “now tending toward not attaching any physical significance at all to this whole theoretical development, which is based on a generalization of the Riemann metric through generalization of ” (Doc. 166). Many months later, remarking on the Weyl-Eddington approach during a lecture in Lucerne in the summer of 1924, Einstein concluded that, despite its mathemati- cal elegance, this route would not lead to any progress in physical understanding. In his diary of the South America trip, while reading and criticizing Emile Meyer- son’s account of recent developments in physics, he called Weyl’s and Eddington’s work “escapades” that should not be counted as belonging to the core of relativity theory (Doc. 455, entry of 12 March 1925). Einstein addressed other issues in general relativity. At the end of May 1923, he published a “Correction to Friedmann” (Einstein 1923g [Doc. 51]), in which he ad- mits that his earlier criticism (in Einstein 1922p [Vol. 13, Doc. 340]) of Alexander Friedmann’s solutions to the gravitational field equations as constituting a class of dynamical solutions to the Einstein field equations was incorrect, due to a calcula- tional error. The draft of this article ([1 026]) contains an interesting qualification: after writing that the Friedmann solutions do exist after all, Einstein adds the phrase: “to which, most probably, no physical significance may be attributed.” This sentence was deleted for publication. Viktor Y. Frenkel, commenting on this dele- tion, pointed out that, similarly to the first admission of having made a mathemat- ical mistake, the deletion of the physical qualification may have been a result of Einstein’s conversations with Friedmann’s colleague Y. A. Krutkov, who is men- tioned in Einstein’s published paper.[2] According to Frenkel, Krutkov’s diary and letters indicate that Einstein had extensive discussions with Krutkov while in Ley- den, and then again in Berlin. Krutkov also made notes of a lecture in Leyden in which Einstein and Ehrenfest described their most recent research. The episode is important for two reasons. First, the Friedmann solution became the foundation for the theoretical side of relativistic big bang cosmology. It contains as special cases many solutions found earlier and later, including Willem de Sitter’s spherical and Einstein’s cylindrical solutions. Second, Einstein’s initial reaction to Friedmann’s solution mirrors quite strongly his initial reaction to De Γμν σ