l x x x v i i i A C K N O W L E D G M E N T S University of Jerusalem Claire Guttinger, Collège de France, Paris Hans Haubold, UN Office for Outer Space Affairs, Vienna Klaus Hentschel, University of Stutt- gart Danian Hu, the City College of the City University of New York Michio In- oue, Caltech Herbert Karbach, Politisches Archiv des Auswärtigen Amtes, Berlin Catherine Kounelis, Library and Archives of the ESPCI, Paris Claudia Lienhard, Federal Institute of Technology Library, Zurich Yiwen Ma, École Polytechnique, Paris Christine Nelson, The Morgan Library & Museum, New York Masako Ohnuki, Pasadena Franziska Rogger, University of Bern Kerstin Schimmeck, Bundesarchiv, Berlin Adàn Sus, Autonomous University of Barcelona Victor Wei, Chinese Embassy, Brussels Joan Bider, University of California, Berkeley Alexandre Cunha, Caltech Charlotte Erwin, Judy Goodstein, and Loma Karklins, the Caltech Institute Archives. Intensive work has been carried out at the Einstein Papers Project by talented and dedicated students. We thank Mike Ferrara, Dawn Xiao Jin, Iris Liu, Sevana Moses, Lernik Ohanian, Thea Wade, Carol Wang, and Tatjana Wiese. Pavel Khazanov and Dilyara Valeeva carried out research in Russian archives. Our activities and well-being at Caltech have been furthered by the assistance and generosity of Susan Davis, Jonathan Katz, Gail Nash, Ed Stolper, and many other colleagues. We thank our longtime copy editor, Alice Calaprice, who has freelanced for us since retiring from Princeton University Press. Many thanks to everyone at Princeton University Press, especially to Linny Schenck, Terri O’Prey, and Leslie Flis, our production team Brigitta van Rhein- berg Neil Litt Adam Fortgang Martha Camp and Peter Dougherty, its director. We dedicate this volume to the memory of Herbert S. Bailey, Jr., Martin J. Klein, and Harold McGraw, Jr., without whose decades-long foresight and devotion the Collected Papers series would not have been possible. Herb Bailey directed Princeton University Press from 1954 to 1986 and was instrumental in setting up the Einstein Papers Project, among other notable publication projects. He collaborated closely with Harold McGraw, Jr., chairman of McGraw Hill publishers, who sat on the board of trustees of Princeton University Press for twenty-five years, most of them as chairman, and generously endowed the funds necessary for continuing the editorial work of the Collected Papers of Albert Einstein. Martin Klein was one of the foremost historians of physics in the world. He was associated for more than three decades with the Einstein Papers Project and served as general editor from 1988 to 1998. His erudition and generosity of spirit were unrivaled. He is sorely missed by his colleagues and disciples.