The Final Wreck
As appears from the last sentence, Bohr assumed the failure of space-time representations to extend to the radiation theory and was therefore ready
[307] Minkowski and Sponer 1924, 84. Hund's theory was soon contradicted by R. B. Brode's electronic absorption measurements on molecules (CO and N2 ), according to which the Ramsauer transparency occurred only for not-too-slow electrons. See Bohr to Heisenberg, 10 June 1925, BCW 5:[364].
[308] BKS 1924, 792; Bohr to Fowler, 21 Apr. 1925, BCW 5: [81]-[82]. At Heisenberg's suggestion, Bohr's collision paper finally appeared in ZP (Bohr 1925a) with an addendum written m July 1925.
to abandon the BKS theory and accept the alternative possibility of "coupling." The Ramsauer effect was not the only source of this radical change of outlook: Pauli the orbit-killer was in Copenhagen for the Easter holiday from 15 April to 25 April, and around that time Geiger concluded with Bothe an experiment proving the conservation of energy in individual Compton scattering events. More specifically, in the interaction between an X-ray beam and quasi-free electrons, the detection of a quantum of radiation in a given direction always coincided with the detection of an electron in the direction expected from the light-quantum explanation of the Compton effect. This implied a coupling between the Compton electrons and the atoms of an X-ray detector of a kind excluded by the BKS theory.[309]
In reply to Geiger's letter announcing these results, Bohr wrote:
Thank you very much for the great kindness of having informed me of your important results. I was quite prepared to learn that our point of view about the independence of the quantum processes in separate atoms would turn out to be wrong. The whole matter was more an expression of an endeavor to attain the greatest possible applicability of the classical concepts than a completed theory. Not only were Einstein's objections very disquieting, but recently I have also felt that an explanation of collision phenomena, especially Ramsauer's results on the penetration of slow electrons through atoms, presents difficulties to our ordinary space-time description of phenomena to such an extent that, in spite of the existence of coupling, conclusions about a possible corpuscular nature of radiation lack a sufficient basis.[310]
As we can appreciate from this letter, Bohr's reaction to the failure of the BKS theory was just as radical as Pauli's reaction to the failure of multiperiodic models of the anomalous Zeeman effect. With some delay, the two men now agreed about a general collapse of ordinary space-time descriptions. In 1922-23, before the BKS episode, Bohr had already suspected such a failure but had believed it to be restricted to the case of the interaction between atoms and radiation. He now rejected all of the space-time pictures previously used in the quantum theory: electronic orbits in stationary states, trajectories in collision processes, radiation fields, and corpuscular light-quanta.
Did the refutation of the BKS theory necessarily imply such a drastic reform of physics? Other physicists involved in the BKS program—
[309] Geiger to Bohr, 17 Apr. 1925, BCW 5: [352]-[353]. Geiger's letter reached Bohr on 21 April, as appears from the postscript of Bohr to Fowler, 21 Apr. 1925, BCW 5: [81]-[82]. That Bohr had developed strong doubts about the BKS theory even before this date is documented by Bohr to Heisenberg, 18 Apr. 1925, BCW 5: [360]. Bothe and Geiger 1925a, 1925b.
[310] Bohr to Geiger, 21 Apr. 1925, BCW 5: [353]-[354].
notoriously, Kramers, Slater, and Born—did not think so. There was a much milder remedy to the absence of the Bothe-Geiger type of correlations in the BKS theory: one just had to return to Slater's original proposal and "hang" light quanta to the virtual fields. With Jordan's help, Born even developed a fairly detailed guiding mechanism that would reproduce optical observations. Under the influence of Bohr's criticism, however, he soon abandoned this attempt.[311]
There is reason to doubt that Bohr's reiterated rejection of the Slater type of theory depended on the specific objections that he made to the Born-Jordan attempt. These objections concerned Born's particular assumptions about the guiding of light quanta, which could be adjusted. Very likely, Bohr had in mind more fundamental defects of this type of theory. As he liked to emphasize, any theory conceding too much reality to light quanta departed from the approach inspired by the correspondence principle. Further, the addition of light quanta did not cure some basic defects of the BKS theory, like the duplicate origin of the resonance radiation denounced by Pauli.
Perhaps Bohr's strongest argument against this type of theory was the general convergence of quantum-theoretical paradoxes toward a renunciation of a detailed description in space-time: on top of the early radiation paradoxes, there were the anomalous Zeeman effect, the Ramsauer effect, and, last but not least, the devastating consequence of strict energy conservation, when applied to Bohr's nonreciprocal collisions. According to the argument in the collision paper, the assumption that swiftly moving particles had a definite course in space-time implied that they lost in each collision an energy much less than the smallest transition energy of the target atoms.[312] In other words, the energy principle, the space-time mode of description, and the quantum postulate could not be simultaneously valid. If, as Geiger's experiment seemed to indicate, the energy principle retained a general validity in the quantum theory, there could be no question of a space-time description. Bohr expressed this viewpoint in an addendum to his collision paper (which he finally decided to publish):
If one wants to postulate a strict validity of the conservation laws . . . it must be emphasized that, for the collisions called nonreciprocal above, we must not only, as for reciprocal collisions, expect an interaction that is incompatible with
[311] Kramers to Urey, 16 July 1925, quoted in BCW 5:[86]; Slater 1925b; Born and Jordan [1925a]; Born to Bohr, 24 Apr. 1925, BCW 5: [308]-[310]; Born to Bohr, 1 May 1925, BCW 5: [310]-[311].
[312] In Bohr 1925a, 156, Bohr discussed a similar paradox concerning the capture of electrons by fast particles.
the properties of mechanical models, but we must in fact even be prepared to find behavior that is as alien to the ordinary space-time pictures as the coupling of individual processes in distant atoms is to a wave description of optical phenomena.[313]