INTERLUDE: THE BKS PROPOSAL 421(which had already plagued him for seven years by then) was the one to which he
took exception most strongly. He confided to Born that the thought was unbear-
able to him that an electron could choose freely the moment and direction in which
to move [El5]. This causality question would continue to nag him long after
experiment revealed that the BKS answers to both paradoxes were incorrect.
The Experimental Verdict on Causality. The BKS ideas stimulated Walther
Bothe and Hans Geiger to develop counter coincidence techniques for the purpose
of measuring whether, as causality demands, the secondary photon and the knock-
on electron are produced simultaneously in the Gompton effect [B6]. Their result:
these two particles are both created in a time interval < 10~^3 s [B7, B8]. Within
the limits of accuracy, causality had been established and the randomness of the
relative creation times demanded by BKS disproved. Since then, this time interval
has been narrowed down experimentally to < 10~" s [B9].
The Experimental Verdict on Energy-Momentum Conservation. The valid-
ity of these conservation laws in individual elementary processes was established
for the Compton effect by Compton and A. W. Simon. From cloud chamber obser-
vations on photoclectrons and knock-on electrons, they could verify the validity of
the relation
in individual events, where (f>, 6 are the scattering angles of the electron and pho-
ton, respectively, and v is the incident frequency [Cl].
And so the last resistance to the photon came to an end. Einstein's views had
been fully vindicated. The experimental news was generally received with great
relief (see, e.g., [P2]*). Bohr took the outcome in good grace and proposed 'to give
our revolutionary efforts as honorable a funeral as possible' [BIO]. He was now
prepared for an even more drastic resolution of the quantum paradoxes. In July
1925 he wrote, 'One must be prepared for the fact that the required generalization
of the classical electrodynamic theory demands a profound revolution in the con-
cepts on which the description of nature has until now been founded' [B4].
These remarks by Bohr end with references to de Broglie's thesis and also to
Einstein's work on the quantum gas (the subject of the next chapter): the profound
revolution had begun.
References
Bl. N. Bohr, H. A. Kramers, and J. C. Slater, Phil. Mag. 47, 785 (1924).
B2. , letter to A. Einstein, June 24, 1920.
B3. Niels Bohr, Collected Works (L. Rosenfeld, Ed.), Vol. 3, pp. 28, 357. North Hol-
land, New York, 1976.*Pauli's own description of BKS, written early in 1925, can be found in his collected works [P3].