448 THE QUANTUM THEORYwhere t is the time taken to readjust the pointer and g is the gravitational accel-
eration. Thus, tgAmAq > h. Next, Bohr used the red shift formula*: the uncer-
tainty Aqr of the position of the clock in the gravitational field implies an uncer-
tainty At = c~^2 gtAq in the determination of t. Hence, c^2 Am&t = AEAt > h.
Thus the accuracy with which the energy of the photon is measured restricts the
precision with which its moment of escape can be determined, in accordance with
the uncertainty relations for energy and time.
Note that every one of the many details in the figure serves an experimental
purpose: the heavy bolts fix the position of the scale along which the pointer
moves, the spring guarantees the mobility of the box in the gravitational field, the
weight attached to the box serves to readjust its position, and so on. There was
nothing fanciful in Bohr's insistence on such details. Rather he had them drawn
in order to illustrate that, since the results of all physical measurements are
expressed in classical language, it is necessary to specify in detail the tools of mea-
surement in that language as well.
After this refutation by Bohr, Einstein ceased his search for inconsistencies. By
1931 his position on quantum mechanics had undergone a marked change.
First of all, his next paper on quantum mechanics [El 5], submitted in February
1931, shows that he had accepted Bohr's criticism.** It deals with a new variant
of the clock-in-the-box experiment. Experimental information about one particle
is used to make predictions about a second particle. This paper, a forerunner of
the Einstein-Podolsky-Rosen article to be discussed below, need not be remem-
bered for its conclusions.!
A far more important expression of Einstein's opinions is found in a letter he
wrote the following September. In this letter, addressed to the Nobel committee
in Stockholm, Einstein nominated Heisenberg and Schroedinger for the Nobel
prize. In his movitation, he said about quantum mechanics, 'Diese Lehre enthalt
nach meiner Uberzeugung ohne Zweifel ein Stuck endgiiltiger Wahrheit.'^: Ein-
stein himself was never greatly stirred by honors and distinctions. Even so, his
nominations reveal a freedom of spirit and a generosity of mind. In addition, they
show most clearly his thoughts: he came to accept that quantum mechanics was
not an aberration but rather a truly professional contribution to physics.
'Recall that the only ingredients for the derivation of this formula are the special relativistic time
dilation and the equivalence principle.
**The Gedankenexperiment in this paper involved a time measurement. The authors take care to
arrange things so that 'the rate of the clock ... is not disturbed by the gravitational effects involved
in weighing the box.'
•f-The authors are 'forced to conclude that there can be no method for measuring the momentum of
a particle without changing its value,' a statement which, of course, is unacceptable.
^'1 am convinced that this theory undoubtedly contains a part of the ultimate truth.' Einstein had
already proposed Heisenberg and Schroedinger in 1928, and proposed Schroedinger again in 1932
(see Chapter 31).