INTERLUDE: THE BKS PROPOSAL 417
like an extremely sensitive child who moves around in this world in a sort of
trance' [E4]. The next month, Bohr wrote to Einstein, 'To meet you and to talk
with you was one of the greatest experiences I ever had' [B2]. Some years later,
Einstein began a letter to Bohr, 'Lieber oder viehmehr geliebter Bohr,' Dear or
rather beloved Bohr [E5]. Once when I talked with Helen Dukas about the strong
tie between these two men, she made the comment that is at the head of this
chapter: 'They loved each other warmly and dearly.'
Those also were the years of scientific harmony between the two men. In 1922
Einstein wrote to Ehrenfest, 'At present, I am reading a major lecture by Bohr*
which makes his world of thought wonderfully clear. He is truly a man of genius.
It is fortunate to have someone like that. I have full confidence in his way of
thinking' [E6]. Einstein was particularly impressed at that time with Bohr's enun-
ciation and handling of the correspondence principle [E6], a concept on which he
and Bohr were able to see eye to eye, then and later.
All who have known Bohr will be struck by the perceptive characterization
Einstein gave of him much later. 'He utters his opinions like one perpetually grop-
ing and never like one who believes to be in the possession of definite truth' [E7].
Bohr's style of writing makes clear for all to see how he groped and struggled.
'Never express yourself more clearly than you think,' he used to admonish himself
and others. Bohr's articles are sometimes dense. Having myself assisted him on a
number of occasions when he was attempting to put his thoughts on paper, I know
to what enormous lengths he went to find the most appropriate turn of phrase. I
have no such first-hand information about the way Einstein wrote. But, again for
all to see, there are the papers, translucent. The early Einstein papers are brief,
their content is simple, their language sparse. They exude finality even when they
deal with a subject in flux. For example, no statement made in the 1905 paper on
light-quanta needs to be revised in the light of later developments.
The first meeting of Einstein and Bohr took place in 1920, some years before
they found themselves at scientific odds on profound questions of principle in
physics. They did not meet very often in later times. They did correspond but not
voluminously. I was together a few times with both of them some thirty years after
their first encounter, when their respective views on the foundations of quantum
mechanics had long since become irreconcilable. Neither the years nor later events
had ever diminished the mutual esteem and affection in which they held one
another.
Let us now turn to the BKS proposal.
As already stressed in Section 19f, it was the position of most theoretical phys-
icists during the first decades of the quantum era that the conventional continuous
description of the free radiation field should be protected at all cost and that the
quantum puzzles concerning radiation should eventually be resolved by a revision
'This was presumably the text of Bohr's contribution to the third Solvay conference (April 1921).
Because of ill health, Bohr did not deliver that lecture in person [B3].