I N T R O D U C T I O N T O V O L U M E 9 x x x i x
general relativity, proposed to Einstein that Hermann Weyl’s unified field theory of
gravity and electromagnetism might be such an alternative. He was not alone. Ed-
dington and Arnold Sommerfeld wrote to Weyl himself in a similar vein (Doc. 75,
note 18).
Even though he maintained the position that a negative result would be fatal for
his theory, Einstein retained confidence that, in the long run, it would be vindicated.
He drew reassurance from other observational results. In 1919, his collaborator
Freundlich examined the spectra of fixed stars in an attempt to gather statistical ev-
idence for the existence of a gravitational redshift. If most nearby stars show a no-
ticeable redshift, it would indicate that their gravitational fields themselves are re-
sponsible for the effect, rather than the unlikely scenario in which these stars are all
being repelled by the Earth, as would be required by the Doppler shift hypothesis.
Earlier attempts by Freundlich in this direction had drawn criticism, but Einstein
welcomed Freundlich’s investigations of the spectra of bright stars of spectral type
B and O, especially those in the Orion
Nebula.[31]
Einstein’s knowledge of astro-
physics had apparently become quite sophisticated. His reading of a review of Ed-
dington’s new stellar theory in early 1919 convinced him that giant stars of types B
and O should have a very pronounced redshift due to their rather high densities
(Doc. 8). Freundlich’s work in this area, described in Doc. 14, seems to have greatly
encouraged Einstein (see his reply, Doc. 15).
Of much greater significance, however, for both Einstein and the astrophysics
community, was the work of Leonhard Grebe (see Illustration 3) and Albert
Bachem. In response to the public announcement of funding opportunities through
the Kaiser Wilhelm Institute of Physics (see the Appendix), Grebe applied for fi-
nancial help for their work (Doc. 25). It is hardly surprising that Einstein chose to
support their research and that he made the additional offer that they use Freundli-
ch’s photometer (see entry for 26 April 1919 in Calendar). By June 1919, Grebe
informed Einstein (Doc. 57) that asymmetrical broadenings of lines in the cyano-
gen band had in some cases been misinterpreted as a shift toward the blue. This had
resulted in an underreporting of the redshift in some lines. By the end of 1919,
Grebe and Bachem also pointed out that in some instances the absence of redshift
is simply the result of a misidentification of the lines (Doc. 232). Einstein’s excite-
ment at this fresh evidence is palpable in a series of letters written to colleagues
shortly thereafter (Docs. 235, 254, 256, 293).
Although the balance was about to tip in favor of the Einstein effect, change
came only gradually, unlike in the case of the eclipse expedition. The reception ac-
corded Grebe and Bachem’s work was not uncritical. Eddington himself responded
in reserved fashion, both to Einstein personally (Doc. 352), and in
print.[32]
Robert
Lawson forwarded Einstein’s brief comment endorsing Grebe and Bachem’s work