l88 RELATIVITY, THE GENERAL THEORYhis first contacts with larger segments of the physics community. Such circum-
stances often lead to a slackening of creative tensions. All these events combined
might well have sufficed for others to desist from starting a truly major new pro-
gram in research. Yet, I think, all this has little if anything to do with Einstein's
silence on gravitation.
Indeed, if he was silent on that subject, he was not silent on physics as a whole.
New research continued during the years in question. There were the papers with
Laub on special relativity, the papers with Hopf on classical radiation theory, and
the difficult paper on critical opalescence. He invented his little machine. Above
all, there were the papers on quantum physics already mentioned, highly creative
in content. All this work hardly gives the impression of a man who is sidetracked
and cannot find time for serious thinking.
There is, of course, nothing unusual about the fact that Einstein did not publish
anything new about gravitation between 1908 and 1911. It could mean simply
that he thought about the problem but did not find anything novel to communicate.
More curious is the fact that he twice gave surveys of relativity theory without
mentioning gravitation or the equivalence principle and its remarkable implica-
tions: the red shift and the bending of light. The first of these surveys was his
report at the Salzburg conference, which included a survey of relativity theory, 'of
the consequences of which I would like to mention only a single one' [E3] (namely,
E = me^2 ), but quantum theory rather than relativity theory was the main issue.
The second survey was given in 1910. It is a detailed document, forty-four printed
pages long [E8]. There is no mention of relatively accelerated systems. Again this
is not too surprising. Even the special theory was still so new that it may have
seemed advisable to confine the explications to the case of uniform relative motion.
However, even such pedagogical motives fail to explain one fact which I find
truly significant. Throughout his career Einstein was accustomed to writing to one
or more colleagues or friends about scientific problems which at any given time
were important to him. With a refreshing frankness, he would share with them
not only the delights of a new insight but also the troubles of being stuck. It would
not in the least have been out of style for Einstein to write to one of his friends:
I am preoccupied with the gravitation problem, it mystifies me and I am not get-
ting anywhere. In fact, I am quite sure that he would have written in this vein if,
between 1908 and 1911, this problem had really nagged him. Yet, as far as I
know, in his scientific correspondence during this period, mention is made only
once of gravitation and the related new issues. These same letters also made clear
to me the reason for Einstein's silence on the equivalence principle and its con-
sequences: it was not gravitation that was uppermost in his mind. It was the quan-
tum theory.
Some examples may show the intensity of Einstein's concern with quantum
physics during that period. Sometime in 1908 he wrote to Laub, 'I am incessantly
busy with the question of the constitution of radiation.... This quantum problem
is so uncommonly important and difficult that it should be the concern of every-