x x x v i I N T R O D U C T I O N T O V O L U M E 1 4
The letters and writings presented here reveal novel and previously unnoticed
connections between seemingly disparate aspects of Einstein’s scientific activities
during these two years. In addition to his well-known, fundamental papers on the
quantum theory of the ideal gas, we find him deeply involved with scrutinizing the
Compton effect and the Bohr-Kramers-Slater theory. The volume also documents
his preoccupation with a large number of new experiments designed to test these
subjects, as well as an entire series of speculations, theories, and proposed obser-
vations meant to interrogate what he called the “electromagnetic bookkeeping” of
the Earth (Doc. 87).
An idea to test an empirical consequence of his approach to unified field theory
based on Eddington’s affine theory engaged his attention in summer and fall of
1923; he collaborated with Hermann Mark on how to design the corresponding ex-
periment (Doc. 152). In Berlin, as well as in Kiel and Lautrach, he also worked on
the interpretation and experimental confirmation of the Compton effect. After giv-
ing up on this approach toward unified field theory, he kept thinking about the re-
lationship between quantum theory and classical field theory, respectively. This
process resulted by the end of the year in a paper relating quantum aspects to an
overdetermination of classical field equations (Doc. 170).
While in Berlin from January through April 1924, Einstein continued work on
the implications of the Compton effect (Doc. 236). In May 1924, he traveled to Kiel
for a month, and upon his return presented his objections to the Bohr-Kramers-
Slater theory (Doc. 256) at the Berlin physics colloquium. He embarked on an au-
tomobile trip to Göttingen to visit Max and Hedwig Born, and, during the summer,
between a meeting of the International Committee on Intellectual Cooperation in
Switzerland and his holiday stay once more in Lautrach, he devoted himself with
extraordinary alacrity to the translation of S. N. Bose’s paper (Doc. 278) and the
composing of a new quantum theory of the ideal gas (Docs. 283, 315, 316). The
paper engendered disputes and exchanges, leading Einstein to offer clarifications
regarding the revolutionary new quantum statistics in further publications
(Docs. 385, 427) before departing for South America, where he lectured on relativ-
ity and wrote several articles (Docs. 463, 467, 479, 481, 485).
The current volume testifies yet again to the ever-increasing amount of corre-
spondence that would be the hallmark of the coming decades in Einstein’s life. For
the twenty-six months covered here, the volume presents a total of 1,103 incoming
and outgoing letters that Einstein exchanged with almost 500 different correspon-
dents (see the Alphabetical List of Correspondence, pp. 783–804). Of the total cor-
respondence, 411 letters are presented as full texts, and 692 in abstract. Among the
total 411 letters written by Einstein during this period, 261 are presented as full
texts, and 149 in abstract.
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