London: Taylor & Francis, 1924. First edition of Bohr’s attempt to explain the Compton effect without invoking light quanta. The authors took the bold step of abandoning “the classical form of causality in favor of a purely statistical description. Even the distribution of energy and momentum between the radiation field and the ‘virtual oscillators’ constituting the atomic systems was assumed to be statistical, the conservation laws being fulfilled only on the average” (DSB).
First edition of this celebrated paper. Its idea of modeling atomic behaviour under incident electromagnetic radiation using ‘virtual oscillators’ led Born and Heisenberg to explore mathematics that strongly inspired the subsequent development of matrix mechanics. The most striking feature of this remarkable paper, “The Quantum Theory of Radiation,” was the renunciation of the classical form of causality in favor of a purely statistical description. Even the distribution of energy and momentum between the radiation field and the “virtual oscillators” constituting the atomic systems was assumed to be statistical, the conservation laws being fulfilled only on the average” (DSB). Rare in original printed wrappers.
Following the discovery of the Compton effect late in 1922, many physicists had tried to account for the frequency shift of the scattered X-rays observed by Compton. Some found it difficult to accept Compton’s explanation in terms of light quanta (photons) with definite energy and momenta. Bohr, in particular, could not accept the existence of photons, writing in his Nobel lecture in December 1922, “In spite of its heuristic value,… the hypothesis of light-quanta, which is quite irreconcilable with so-called interference phenomena, is not able to throw light on the nature of radiation.” When the young American physicist John Slater arrived in Copenhagen just before Christmas 1923, and explained his new ideas on radiation theory, Bohr saw a way to explain the Compton effect without invoking light quanta.
“BKS begin by recalling that ‘the exchange of energy and momentum between matter and radiation claims essentially discontinuous features. These have even […] led to the introduction of light-quanta…’ They abandon light-quanta in their own paper, replacing this concept by a new one ‘due to Slater … the atom, even before the process of transition between stationary states takes place is capable of communicating with distant atoms through a virtual radiation field’, a field distinct from the conventional, real radiation field. This virtual field, carried by the atom in a given stationary state, was supposed to know and carry all the possible transition frequencies to lower states, one might say, to release one of these frequencies. Emission of light in an atomic transition is, BKS posited, not spontaneous but rather induced by the virtual fields ‘by probability laws analogous to those which in Einstein’s theory hold for induced transitions’. Accordingly, ‘the atom is under no necessity of knowing what transition it is going to make ahead of time’.
“Does communication with a distant atom, the receipt of a light signal emitted by another atom ‘even before transition takes place’ not violate causality? It does. ‘We abandon any attempt at a causal connection between the transition in distant atoms, and especially a direct application of the principles of conservation of energy and momentum, so characteristic of classical theories … Not only conservation of energy … but also conservation of momentum [reduce to] a statistical law.’ Regarding Compton’s results, BKS noted, correctly, that so far his experiments had only confirmed energy-momentum conservation averaged over many individual processes, which was not in conflict with their statistical viewpoint” (Pais, Niels Bohr’s Times, pp. 236-7).
Einstein and Pauli both reacted negatively to the BKS proposal, and “the paper was hardly in print before A. H. Compton and A. W. Simon had established by direct experiment the strict conservation of energy and momentum in an individual process of interaction between atom and radiation. Nevertheless, this short-lived attempt exerted a profound influence on the course of events; what remained after its failure was the conviction that the classical mode of description of the atomic processes had to be entirely relinquished” (DSB II, p. 247).
The BKS paper was printed simultaneously in German as ‘Über die Quantentheorie der Strahlung’ in Zeitschrift für Physik, Bd. 24 (1924), pp. 69-87.
Pp. 785-802 in Philosophical Magazine, Sixth Series, Vol. 47, No. 281, May 1924. 8vo (221 x 145 mm), pp. [ii], 785-1056, with two plates (numbered VI and VII). Original printed wrappers, some chipping and wear to capitals and corners.