444 THE QUANTUM THEORY
as 1909 Einstein had been the first to stress the need for incorporating a particle-
wave duality in the foundations of physical theory (see Section 2la).
March 1927. Heisenberg states the uncertainty principle [H6]. (In this paper,
Heisenberg, too, referred to 'Einstein's discussions of the relation between waves
and light-quanta.') In June 1927 Heisenberg writes a letter to Einstein which
begins, 'Many cordial thanks for your kind letter; although I really do not know
anything new, I would nevertheless like to write once more why I believe that
indeterminism, that is, the nonvalidity of rigorous causality, is necessary [his ital-
ics] and not just consistently possible' [H7]. This letter is apparently in response
to another lost letter by Einstein, triggered, most probably, by Heisenberg's work
in March. I shall return to Heisenberg's important letter in Chapter 26.1 mention
its existence at this point only in order to emphasize once again that Einstein did
not react to these new developments as a passive bystander. In fact, at just about
that time, he was doing his own research on quantum mechanics (his first, I
believe). 'Does Schroedinger's wave mechanics determine the motion of a system
completely or only in the statistical sense?'* he asked. Heisenberg had heard
indirectly that Einstein 'had written a paper in which you ... advocate the view
that it should be possible after all to know the orbits of particles more precisely
than I would wish.' He asked for more information 'especially because I myself
have thought so much about these questions and only came to believe in the uncer-
tainty relations after many pangs of conscience, though now I am entirely con-
vinced' [H8]. Einstein eventually withdrew his paper.
September 16, 1927. At the Volta meeting in Como (Einstein had been invited
but did not attend), Bohr enunciates for the first time the principle of comple-
mentarity: 'The very nature of the quantum theory... forces us to regard the
space-time coordination and the claim of causality, the union of which character-
izes the classical theories, as complementary but exclusive features of the descrip-
tion, symbolizing the idealization of observation and definition, respectively' [B5].
October 1927. The fifth Solvay Conference convenes. All the founders of the
quantum theory were there, from Planck, Einstein, and Bohr to de Broglie, Hei-
senberg, Schroedinger, and Dirac. During the sessions, 'Einstein said hardly any-
thing beyond presenting a very simple objection to the probability
interpretation.... Then he fell back into silence' [B5a]. As was mentioned in
Chapter 23, Einstein had declined an invitation to give a paper on quantum sta-
tistics at that conference.
However, the formal meetings were not the only place for discussion. All par-
ticipants were housed in the same hotel, and there, in the dining room, Einstein
was much livelier. Otto Stern has given this first-hand account**:
"This is the title of a paper Einstein submitted for the May 5, 1927, meeting of the Prussian Acad-
emy in Berlin. The records show that the paper was in print when Einstein requested by telephone
that it be withdrawn. The unpublished manuscript is in the Einstein archive. See also [Kl],
**In a discussion with Res Jost, taped on December 2, 1961. I am very grateful to Jost for making
available to me a transcript of part of this discussion.