In the almost one hundred writings and more than one thousand letters included in this volume, Einstein is revealed yet again as the consummate puzzler of myriad scientific problems as well as the invested participant in social and political engagements. He continues to explore the light quantum, whose reality is confirmed by new experiments, and to attempt to formulate a unified theory of gravitation and electromagnetism. He travels to South America, where he lectures widely on relativity, rejoins the International Committee on Intellectual Cooperation, and supports the idea of a European union. Einstein has a fourteen-month romantic relationship with his secretary, Betty Neumann, which ends in October 1924.

A translation of selected non-English texts included in Volume 15 is available in paperback. Since this supplementary paperback includes only select portions of Volume 15, it is not recommended for purchase without the main volume. Every document in The Collected Papers of Albert Einstein appears in the language in which it was written, and this supplementary paperback volume presents the English translations of select portions of non-English materials in Volume 15. This translation does not include notes or annotation of the documentary volume and is not intended for use without the original language documentary edition which provides the extensive editorial commentary necessary for a full historical and scientific understanding of the documents.

This volume covers one of the most thrilling two-year periods in twentieth-century physics, as matrix mechanics-developed chiefly by W. Heisenberg, M. Born, and P. Jordan-and wave mechanics-developed by E. Schroedinger-supplanted the earlier quantum theory. The almost one hundred writings by Einstein, of which a third have never been published, and the more than thirteen hundred letters show Einstein's immense productivity and hectic pace of life. Einstein quickly grasps the conceptual peculiarities involved in the new quantum mechanics, such as the difference between Schroedinger's wave function and a field defined in spacetime, or the emerging statistical interpretation of both matrix and wave mechanics. Inspired by correspondence with G. Y. Rainich, he investigates with Jakob Grommer the problem of motion in general relativity, hoping for a hint at a new avenue to unified field theory. Einstein falls victim to scientific fraud when, in a collaboration with E. Rupp, he becomes convinced that the latter's experiments, aimed at deciding whether excited atoms emit light instantaneously (in quanta) or in a finite time (in waves), confirm a wave-theoretic explanation. While it was known that the teenage Einstein had been romantically involved with Marie Winteler in 1895, newly discovered documents reveal that his love for Marie was rekindled in 1909-10 while he was still married to Mileva Maric. The 1925 Locarno Treaties renew Einstein's optimism in European reconciliation. He backs the "International manifesto against compulsory military service" and continues his participation in the League of Nations' International Committee on Intellectual Cooperation. He remains intensely committed to the shaping of the Hebrew University in Jerusalem, although his enthusiasm for this cause is sorely tested.