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atoms’ light emission in a thermal radiation bath. But there is no extant explicit
account of Einstein’s project, nor of its execution.
Another investigation into experimental evidence for implications of the emerg-
ing quantum theory is mentioned in a letter to Ehrenfest of 1 September (Doc. 219),
in which Einstein mentions a joint investigation with Grommer into existing exper-
imental data that would decide whether helium shows any evidence of a zero-point
energy.
During 1921, accumulating evidence suggested that Einstein’s and Wander J. de
Haas’s initial work on the gyromagnetic effect was beset by a factor-of-two prob-
lem. The initial agreement between theory and experiment could not be repro-
duced. The factor-of-two discrepancy would eventually be satisfactorily accounted
for only a few years later, after the discovery of the electron’s spin. In June 1920,
Einstein had accepted an invitation to present a review of the problem at the 1921
Solvay conference (Vol. 10, Doc. 56), and in December 1920, he corresponded
with De Haas on the available experimental literature (Vol. 10, Doc. 215). Al-
though Einstein withdrew from the Solvay conference due to his trip to America,
and asked De Haas to speak on his behalf, he nevertheless kept up an interest in the
issue. When the Hamburg experimentalist Johannes Classen wrote Einstein about
his own measurements of the gyromagnetic effect (see Calendar entry of 3 Febru-
ary), Einstein remained skeptical as to their conclusiveness, and recommended
“great caution” in their interpretation (Doc. 50; see also Doc. 57). Similarly, he cor-
responded with Sommerfeld about experiments and an interpretation of the factor-
of-two problem that the Munich-based physicist Karl Herzfeld had proposed
(Docs. 89, 98, 168, and 175). But, again, Einstein rejected the alternative interpre-
tation: “The devilry is rooted deeper down” (Doc. 175).
While the aforementioned experiments all deal in one way or another with prob-
lems associated with quantum theory, Einstein also engaged a former correspon-
dent in discussions concerning experiments relating to the theory of relativity.
Ludwik Silberstein, whom Einstein met in person in Princeton in May 1921, joined
the faculty at Chicago for a term later that summer (Doc. 178). Silberstein had
inspired Michelson—the “master,” according to Einstein (Doc. 136)—to again take
up an experiment on ether drag: according to relativity, and the hypothesis of a
stationary ether, the Earth’s rotation implies that a phase difference was to occur
between light traveling in opposite directions along a closed triangular trajectory.
Although Michelson had proposed such an experiment already in 1904, Einstein
was excited about this new test about whose progress Silberstein kept him informed
throughout the year (Docs. 187, 197, 229, 320). Silberstein further wrote Einstein