x x v i i i I N T R O D U C T I O N T O V O L U M E 7
and the universe itself. The same approach had been taken by Poincaré in the case
of the stability of the electron (Poincaré 1906), and by Schrödinger in the case of
cosmology (Schrödinger 1918b). In fact, in Einstein 1918d (Doc. 3) Einstein
argues against Schrödinger’s cosmology. He views his own approach of 1919,
which unifies the very small with the very large, as more fundamental than his ear-
lier introduction of the cosmological constant term (Einstein 1917b [Vol. 6, Doc.
43]), though the two approaches are physically equivalent. He uses the language of
his “cosmic pressure” term in lectures and books throughout this period (see Docs.
19 and 63 and Einstein 1922c [Doc. 71]). During the following decades, while the
rest of physics parted company with Einstein in pursuit of the new quantum theory,
cosmology would remain the only field of physics where some knowledge of gen-
eral relativity was required.
Einstein’s work on general relativity and cosmology led him to take an interest
in astronomy and astrophysics. In Einstein 1921c (Doc. 52), Einstein discusses the
possibility of measuring the cosmological constant through astronomical observa-
tions of the gravitational constant. Einstein 1921f (Doc. 56) probably grew out of
such an attempt, using recent estimates for the size of globular star clusters. The
empirical data, however, were insufficient to demonstrate a deviation from the
Newtonian value. Consequently, Einstein presents his calculation using Newtonian
theory without even mentioning general relativity or the cosmological constant.
Some of Einstein’s notes and calculations for this paper are reproduced in Appen-
dix A. An earlier astronomical paper, Einstein 1919b (Doc. 18), also had a connec-
tion to general relativity: Kurt Bottlinger had given an explanation of certain
fluctuations of the orbital velocity of the moon based on Hugo von Seeliger’s the-
ory of gravitational absorption, which was incompatible with general relativity
(Bottlinger 1912b). Einstein, who had become aware of this work probably in con-
nection with his cosmological considerations, attempted to give an alternative
explanation based on fluctuations in the rotation of the earth. However, this expla-
nation was based on a misunderstanding of time measurement in astronomy, and
Einstein had to retract it in Einstein 1919c (Doc. 22).
While this volume contains only one paper on quantum theory (Einstein 1922a
[Doc. 68]), this article shows that Einstein by no means had stopped thinking about
quantum theory since his papers of 1916 (Einstein 1916j [Vol. 6, Doc. 34] and Ein-
stein 1916n [Vol. 6, Do. 38]). Convinced more than ever before that light quanta
are real, Einstein thinks that he has found an experiment that shows the breakdown
of wave optics directly through a nonstatistical phenomenon, as opposed to the sta-
tistical evidence for the quantization of light produced so far. Einstein assumes that,
according to wave optics, light emitted by fast-moving ions should be deflected in
a dispersive medium. He takes the result that no such deflection was found as a cor-
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