442 THE QUANTUM THEORYcourse. Nevertheless, the BKS proposal contains statistical features,* as we have
seen. Could Einstein have surmised as early as 1925 that some statistical element
is inherent in the quantum mechanical description?
During the following months, Einstein vacillated in his reaction to the Heisen-
berg theory. In December 1925 he expressed misgivings [E7], but in March 1926
he wrote to the Borns, 'The Heisenberg-Born concepts leave us all breathless and
have made a deep impression on all theoretically oriented people. Instead of a dull
resignation, there is now a singular tension in us sluggish people' [E8]. The next
month he expressed again his conviction that the Heisenberg-Born approach was
off the track. That was in a letter in which he congratulated Schroedinger on his
new advance [E9]. In view of the scientific links between Einstein's and Schroe-
dinger's work, it is not surprising that Einstein would express real enthusiasm
about wave mechanics: 'Schroedinger has come out with a pair of wonderful
papers on the quantum rules', he wrote in May 1926 [E10]. It was the last time
he would write approvingly about quantum mechanics.There came a parting of ways.Nearly a year passed after Heisenberg's paper before there was a first clarifi-
cation of the conceptual basis of quantum mechanics. It began with Born's obser-
vation in June 1926 that the absolute square of a Schroedinger wave function is
to be interpreted as a probability density. Born's brief and fundamental paper goes
to the heart of the problem of determinism. Regarding atomic collisions he wrote:One does not get an answer to the question, What is the state after collision?
but only to the question, How probable is a given effect of the collision?...
From the standpoint of our quantum mechanics, there is no quantity [Grosze]
which causally fixes the effect of a collision in an individual event. Should we
hope to discover such properties later... and determine [them] in individual
events? ... I myself am inclined to renounce determinism in the atomic world,
but that is a philosophical question for which physical arguments alone do not
set standards. [Bl]One month later, Born wrote a more elaborate sequel to this paper, in which he
pointed out that the starting point of his considerations was 'a remark by Einstein
on the relation between [a] wavefield and light-quanta; he [E.] said approximately
that waves are there only to point out the path to the corpuscular light-quanta,
* Heisenberg remarked much later that 'the attempt at interpretation by Bohr, Kramers, and Slater
nevertheless contained some very important features of the later correct interpretation [of quantum
mechanics],' [H5], I do not share this view, but shall not argue the issue beyond what has been said
in Chapter 22.