446 THE QUANTUM THEORYment. This delimitation differs incomparably from the restrictions on information
inherent in the coarse-grained description of events in classical statistical mechan-
ics. There the restrictions are wisely self-imposed in order to obtain a useful
approximation to a description in terms of an ideally knowable complete specifi-
cation of momenta and positions of individual particles. In quantum mechanics,
the delimitations mentioned earlier are not self-imposed but are renunciations of
first principle (on the fine-grained level, one might say). It is true that one would
need action at a distance if one were to insist on a fully causal description involving
the localization of the electron at every stage of the experiment on hand. Quantum
mechanics denies that such a description is called for and asserts that, in this
experiment, the final position of an individual electron cannot be predicted with
certainty. Quantum mechanics nevertheless makes a prediction in this case con-
cerning the probability of an electron arriving at a given spot on the second screen.
The verification of this prediction demands, of course, that the 'one-electron exper-
iment' be repeated as often as necessary to obtain this probability distribution with
the desired accuracy.
Nor is there a conflict with Geiger-Bothe, since now one refers to another
experimental arrangement in which localization in space-time is achieved, but
this time at the price of renouncing information on sharp energy-momentum
properties of the particles observed in coincidence. From the point of view of quan-
tum mechanics, these renunciations are expressions of laws of nature. They are
also applications of the saying, 'II faut reculer pour mieux sauter,' It is necessary
to take a step back in order to jump better. As we shall see, what was and is an
accepted renunciation to others was an intolerable abdication in Einstein's eyes.
On this score, he was never prepared to give up anything.
I have dwelt at some length on this simple problem since it contains the germ
of Einstein's position, which he stated more explicitly in later years. Meanwhile,
the debate in the corridors between Bohr and Einstein continued during the sixth
Solvay Conference (on magnetism) in 1930. This time Einstein thought he had
found a counterexample to the uncertainty principle. The argument was inge-
nious. Consider a box having in one of its walls a hole that can be opened or closed
by a shutter controlled by a clock inside the box. The box is filled with radiation.
Weigh the box. Set the shutter to open for a brief interval during which a single
photon escapes. Weigh the box again, some time later. Then (in principle) one
has found to arbitrary accuracy both the photon energy and its time of passage,
in conflict with the energy-time uncertainty principle.
'It was quite a shock for Bohr ... he did not see the solution at once. During
the whole evening he was extremely unhappy, going from one to the other and
trying to persuade them that it couldn't be true, that it would be the end of physics
if Einstein were right; but he couldn't produce any refutation. I shall never forget
the vision of the two antagonists leaving the club [ of the Fondation Universitaire]:
Einstein a tall majestic figure, walking quietly, with a somewhat ironical smile,