THE FIELD EQUATIONS OF GRAVITATION 259credited Amalie Emmy Noether (who was in Goettingen in 1915) with the proof
of the theorem about the four identities; Noether's theorem had meanwhile been
published, in 1918 [N4]. By 1924 Lorentz [L4], Felix Klein [K7], Einstein [E55],
and Weyl [Wll] had also written about the variational methods and the identities
to which they give rise (see further Section 15c).
I must return to Einstein and Hilbert, however. The remarkable near simul-
taneity of their common discovery raises the obvious question of what exchanges
took place between them in 1915. This takes me back to the summer of that year.
As was mentioned earlier, in late June-early July, Einstein had spent about a
week in Goettingen, where he 'got to know and love Hilbert. I gave six two-hour
lectures there' [E9].* The subject was general relativity. 'To my great joy, I suc-
ceeded in convincing Hilbert and Klein completely' [E56]. 'I am enthusiastic about
Hilbert. An important figure.. .,' [E39], he wrote upon his return to Berlin. From
the period in which Einstein lectured, it is clear that his subject was the imperfect
theory described in his paper of October 1914. I have already mentioned that
Einstein made his major advance in October-November 1915. I know much less
about the time it took Hilbert to work out the details of the paper he presented
on November 20. However, we have Felix Klein's word that, as with Einstein,
Hilbert's decisive thoughts came to him also in the fall of 1915—not in Goettingen
but on the island of Rugen in the Baltic [K8].
The most revealing source about the crucial month of November is the corre-
spondence during that period between Einstein and Hilbert. Between November
7 and 25, Einstein, otherwise a prolific letter writer, did not correspond with any-
one—except Hilbert (if the Einstein archive in Princeton is complete in regard to
that period). Let us see what they had to say to each other.
November 7: E. to H. Encloses the proofs of the November 4 paper 'in which
I have derived the gravitational equations after I recognized four weeks ago that
my earlier methods of proof were deceptive.' Alludes to a letter by Sommerfeld
according to which Hilbert had also found objections to his October 1914 paper
[E40]. The whole November correspondence may well have been triggered, it
seems to me, by Einstein's knowledge that he was not the only one to have found
flaws in this earlier work of his.
November 12: E. to H. Communicates the postulate \fg = 1 (the November
II paper). Sends along two copies of the October 1914 paper [E47].
November 14: H. to E. Is excited about his own 'axiomatic solution of your
grand problem. ... As a consequence of a general mathematical theorem, the
(generalized Maxwellian) electrodynamic equations appear as a mathematical
consequence of the gravitational equations so that gravitation and electrodynamics
are not distinct at all.' Invites E. to attend a lecture on the subject, which he plans
to give on November 16 [H8].
*Einstein and Hilbert began corresponding at least as early as October 1912, when Einstein was
still in Zurich.