EINSTEIN S RESPONSE TO THE NEW DYNAMICS 441and new. Eugene Wigner, who was in Berlin in 1925, told me that Einstein had
at that time the idea of wavefields serving as 'Fiihrungsfelder,' guiding fields, for
light-quanta or other particles, one field for each particle. 'Einstein, though in a
way he was fond of [this idea], never published it' [Wl] since his idea of one field
per particle was incompatible with strict energy-momentum conservation—a dif-
ficulty which was overcome when Schroedinger introduced one guiding field, the
Schroedinger wave function, for joint particle configurations.
As was mentioned earlier, Einstein considered his work on the quantum gas
only a temporary digression. During the very early days of quantum mechanics,*
we find him 'working strenuously on the further development of a theory on the
connection between gravitation and electricity' [E3]. Yet the great importance of
the new developments in quantum theory was not lost on him. Bose, who visited
Berlin in November 1925, recalled that 'Einstein was very excited about the ne
quantum mechanics. He wanted me to try to see what the statistics of light-quanta
and the transition probabilities of radiation would look like in the new theory'
[Ml]. It was not Bose but Dirac who answered that question by giving the
dynamic derivation of expressions for Einstein's A and B coefficients in a paper
in which he laid the foundations of quantum electrodynamics [Dl]. Initially, Ein
stein's reaction to Dirac's contributions was decidedly negative. In 1926 he wrote
to Ehrenfest, 'I have trouble with Dirac. This balancing on the dizzying path
between genius and madness is awful' [E4], and again, a few days later, 'I don't
understand Dirac at all (Compton effect)' [E5]. Some years later, however, he
wrote admiringly of 'Dirac, to whom, in my opinion, we owe the most logically
perfect presentation of [quantum mechanics]' [E6].
Let us return to the fall of 1925. Einstein's deep interest in quantum mechanics
must have led him to write to Heisenberg soon after the publication of the latter's
paper [H2].** All the letters from Einstein to Heisenberg have been lost. How-
ever, a number of letters from Heisenberg to Einstein are extant. One of these. (dated November 30, 1925) is clearly in response to an earlier letter from Einstein
to Heisenberg in which Einstein appears to have commented on the new quantum
mechanics. One remark by Heisenberg is of particular interest. 'You are probably
right that our formulation of quantum mechanics is more adapted to the Bohr-
Kramers-Slater attitude, but this [BKS theory] constitutes, in fact, one aspect of
the radiation phenomena. The other is your light-quantum theory, and we have
the hope that the validity of the energy and momentum laws in our quantum
mechanics will one day make possible the connection with your theory' [H4]. I
find it remarkable that Einstein apparently sensed that there was some connection
between the BKS theory and quantum mechanics. No such connection exists, of
'Recall that Heisenberg's first paper on this subject was completed in July 1925, Schroedinger's in
January 1926.
**The two men met for the first time in the spring of 1926. See [H3] for an attempt at reconstruction
of their early discussions.