268 RELATIVITY, THE GENERAL THEORYIn preparation for the subsequent short sections which deal more directly with
Einstein's work, I turn next to a general outline of the entire period from 1915 to
the present.The decade 1915-25 was a period of consolidation and of new ideas. The main
advances were the introduction in mathematics of parallel transport by Levi-Civ-
ita in 1917 [LI], a concept soon widely used in general relativity; the emergence
of a better understanding of the energy-momentum conservation laws as the result
of the work by Einstein, Hilbert, Felix Klein, Lorentz, Schroedinger, and Her-
mann Weyl; Einstein's first papers on gravitational waves; and the pioneering
explorations of general relativistic cosmologies by Einstein, Willem de Sitter, and
Aleksandr Aleksandrovich Friedmann. The number of participating theoretical
physicists is small but growing.
There were also two major experimental developments. The solar eclipse expe-
ditions of 1919 demonstrated that light is bent by an amount close to Einstein's
prediction [El] of November 18, 1915. (I shall return to this event in the next
chapter.) The first decade of general relativity ends with the announcement by
Edwin Powell Hubble in December 1924 of an experimental result which settled
a debate that had been going on for well over a century: the first incontrovertible
evidence for the existence of an extragalactic object, Messier 31, the Andromeda
nebula [H3].* Theoretical studies of cosmological models received even more
important stimulus and direction from Hubble's great discovery of 1929 that the
universe is expanding: nebulas are receding with a velocity proportional to their
distance. In Hubble's own words, there exists '... a roughly linear relation
between velocities and distances.... The outstanding feature ... is ... the pos-
sibility that numerical data may be introduced into discussions of the general cur-
vature of space' [H3a].** Still, the literature on cosmology remained modest in
size, though high in quality.f Several attempts to revert to a neo-Euclidean theory
of gravitation and cosmology were also made in this period [N4]. These have left
no trace.
The number of those actively engaged in research in general relativity continued
to remain small in the 1930s, 1940s, and early 1950s. Referring to those years,
Peter Bergmann once said to me, 'You only had to know what your six best friends
were doing and you would know what was happening in general relativity.' Stud-
ies of cosmological models and of special solutions to the Einstein equations con-
*A brief history of cosmic distances is found in [W4].
**The history of the antecedents of Hubble's law as well as of the improvements in the determination
of Hubble's constant during the next few decades is given in [N2].
•(•The most detailed bibliography on relativity up to the beginning of 1924 was compiled by Lecat
[L2]. See also [N3]. A list of the principal papers on cosmology for the years 1917 to 1932 is found
in[Rlj.