I N T R O D U C T I O N T O V O L U M E 1 3 l x x v i i time. Doc. 418, consisting of twenty pages with calculations, is located at the back of the bound travel diary. The calculations, both in their notation and in their con- tent, appear to be dated to just the interim between the initial manuscript version and the final published version. Docs. 417, 418, and 425, together with the context provided by the entries in the travel diary and the contemporaneous correspondence, thus provide a dense docu- mentary record that gives us insight into the workings of Einstein’s mind as it searched for a unified theory of gravitation and electromagnetism, a search that would dominate his thinking for the rest of his life. Einstein’s quiet, concentrated work on his calculations while on board the ship contrasts sharply with the hectic and public existence that had by now come to define his daily life. [1]Born 1936. [2]The satisfactory explanation of the superconductivity of metals had to wait for another thirty- four years (see Bardeen, Cooper, and Schriefer 1957). In recent times, the phenomenon has been observed in certain compound materials with a rather high transition temperature. [3]See, e.g., Vol. 4, Doc. 11, and the Introduction to it, pp. 270–273. [4]See, e.g., Vol. 12, Introduction, p. li Einstein to Hendrik A. Lorentz, 1 January 1921 and Einstein to Paul Ehrenfest, 2 September 1921(Vol. 12, Docs. 3 and 225, respectively). See also Sauer 2007. [5]See Kragh 1999, p. 86. [6]See Einstein to Paul Ehrenfest, 1 September 1921 (Vol. 12, Doc. 219). [7]See Doc. 399 and the editorial note, “Einstein’s Lecture at the University of Kyoto.” [8]Erwin Finlay Freundlich, Hauptobservator at the Astrophysical Observatory in Potsdam, and Joan Voûte were among the participants (see Doc. 46 and Freundlich 1923). About Einstein’s possible participation, see Campbell 1922 and Docs. 104, 165, 211, and Abs. 197. [9]See Docs. 227, 270, 289, 295, 302, and 310. [10]See his correspondence with Einstein in Volumes 8 and 12. [11]See Einstein’s letters to the French League for Human Rights, between September 1921 and 2 January 1922 (Vol. 12, Doc. 220) and to Paul Painlevé, 7 December 1921 (Vol. 12, Doc. 314). On Ein- stein’s 1922 trip to Paris and its impact, see Biezunski 1987, 1991, Grundmann 2004, pp. 202–220, Klemm 1998, Nordmann 1922a, 1922b, and Paty 1987. [12]Biezunski 1991, pp. 26–27. [13]However, there was also some negative fallout from the visit. The Zionist Association of Ger- many inquired whether Einstein had repeatedly denied his German origins during his visit to Paris. Einstein forcefully replied he did not (Docs. 443 and 451). [14]Borel was clearly inspired by Einstein’s visit, as witnessed by his subsequent paper and remark that Einstein’s visit had stimulated much discussion in Paris (see Norton 1999, pp. 313–314). [15]For details on Rathenau’s assassination, see Sabrow 1994a and 1999 for more on the political context of the assassination, see Rowe and Schulmann 2007, pp. 14–16. [16]See Sabrow 1994b, pp. 157–169. [17]See, e.g., Einstein to Arnold Sommerfeld, 6 September 1920 (Vol. 10, Doc. 134), and Einstein to Elsa Einstein, 14 September 1920 (Vol. 10, Doc. 149). That same month, Einstein reassured his closest political confidant, Konrad Haenisch, that “Berlin is the place in which I am most deeply rooted through personal and professional ties.” Therefore, he would not leave Berlin unless “external circumstances forced him to do so” (see Einstein to Konrad Haenisch, 8 September 1920 [Vol. 10, Doc. 137]). [18]On the International Committee on Intellectual Cooperation, see Bourgeois 1921 and Renoliet 1999.