The present volume opens in April 1923 upon Einstein’s return to Berlin from his
trip to Japan, Palestine, and Spain, and closes with his return at the end of May
1925 from yet another extended voyage, this time to South America. During the
intervening two years, Einstein maintained an intense schedule of visits and lec-
tures beyond Berlin and Germany’s borders both within Europe and further afield.
He had become a “traveler in relativity” (Doc. 482).
Einstein visited the University of Leyden, where he held an appointment as spe-
cial professor, in the first half of May and at the end of September 1923. Between
these two visits, on his way to Gothenburg in Sweden to deliver his postponed
Nobel Prize lecture in early July (Doc. 75), he stopped over in Copenhagen and
spent “a couple of terrific hours” with Niels Bohr (Doc. 113). He then attended the
meeting of the Deutsche Physikalische Gesellschaft in Bonn in September. During
the summers of 1923 and 1924, Einstein spent several weeks in Lautrach, near
Munich, and in Kiel. These sojourns, as will become clear below, were particularly
conducive to the formulation of various new scientific projects.
Unexpectedly, Einstein traveled to Leyden for a third time in November 1923,
when, in the wake of a rumored assassination attempt, he fled Berlin for the safety
of the Netherlands.
In late September 1924 Einstein attended a Zionist fundraising event in Vienna
and the 88th conference of the Gesellschaft Deutscher Naturforscher und Ärzte in
Innsbruck, then continued on to Zurich where he visited his sons and lectured “On
the Ether” in Lucerne at the annual conference of the Schweizerische Natur-
forschende Gesellschaft (Doc. 332). He then proceeded to Antwerp, where he
visited his uncle Caesar Koch, and on to Leyden (Docs. 325, 326, 327). A fifth trip
to Leyden occurred in February 1925, when Einstein attended the 350th anniversa-
ry of the city’s university (Doc. 436).
Einstein was remarkably productive in the breadth and depth of his scientific
contributions, even though he was away from home for almost half the time
covered by the present volume, and despite much turmoil in his personal life.
Among the almost one hundred writings included here are important papers con-
ceived during a crucial period in the development of modern physics. Much of this
work was formulated in an intensely collaborative exchange with numerous inter-
INTRODUCTION TO VOLUME 14