EINSTEIN'S RESPONSE TO THE NEW DYNAMICS 445
Einstein came down to breakfast and expressed his misgivings about the new
quantum theory, every time [he] had invented some beautiful experiment from
which one saw that [the theory] did not work.... Pauli and Heisenberg, who
were there, did not pay much attention, 'ach was, das stimmt schon, das stimmt
schon' [ah, well, it will be all right, it will be all right]. Bohr, on the other hand,
reflected on it with care and in the evening, at dinner, we were all together and
he cleared up the matter in detail.Thus began the great debate between Bohr and Einstein. Both men refined and
sharpened their positions in the course of time. No agreement between them was
ever reached. Between 1925 and 1931, the only objection by Einstein that
appeared in print in the scientific literature is the one at the 1927 Solvay confer-
ence [E13]. However, there exists a masterful account of the Bohr-Einstein dia-
logue during these years, published by Bohr in 1949 [B6]. I have written else-
where about the profound role that the" discussions with Einstein played in Bohr's
life [PI].
The record of the Solvay meeting contains only minor reactions to Einstein's
comments. Bohr's later article analyzed them in detail. Let us consider next the
substance of Einstein's remarks.Einstein's opening phrase tells more about him than does many a book: 'Je dois
m'excuser de n'avoir pas approfondi la mecanique des quanta,' I must apologize
for not having penetrated quantum mechanics deeply enough [El3]*.He then went on to discuss an experiment in which a beam of electrons hits a
fixed screen with an aperture in it. The transmitted electrons form a diffraction
pattern, which is observed on a second screen. Question: does quantum mechanics
give a complete description of the individual electron events in this experiment?
His answer: this cannot be. For let A and B be two distinct spots on the second
screen. If I know that an individual electron arrives at A, then I know instanta-
neously that it did not arrive at B. But this implies a peculiar instantaneous action
at a distance between A and B contrary to the relativity postulate. Yet (Einstein
notes) in the Geiger-Bothe experiment on the Gompton effect [B7], there is no
limitation of principle to the accuracy with which one can observe coincidences in
individual processes, and that without appeal to action at a distance. This circum-
stance adds to the sense of incompleteness of the description for diffraction.
Quantum mechanics provides the following answer to Einstein's query. It does
apply to individual processes, but the uncertainty principle defines and delimits
the optimal amount of information obtainable in a given experimental arrange-
*The original German text reads, 'Ich [bin] mir des Umstandes bewusst dass ich in das Wesen der
Quantenmechanik nicht tief genug eingedrungen bin' [El4],