Compendium of Quantum Physics pp 62-64
The Bohr—Kramers—Slater theory (or BKS theory) was proposed in 1924 as an attempt to explain problems in physical optics and to provide a unified picture of the continuous electromagnetic field and the discontinuous quantum transitions in atoms. Although the theory was short-lived it proved most important in the subsequent development of quantum theory, not least because it replaced causal spatio-temporal description of the transitions between stationary states with statistical considerations. Moreover, it followed that energy and momentum was only conserved statistically, not for individual atomic processes.
In early 1924 atomic physics was in a state of crisis (► quantum theory, crisis period), one of the critical problems being the interaction between matter and radiation. In a paper published in Nature in February 1924, John Clark Slater (1900– 1976) suggested the radical idea that when an atom was in a stationary state, it would “communicate with other atoms… by means of a virtual field of radiation originating from oscillators having the frequencies of possible quantum transitions and the function of which is to provide for statistical conservation of energy and momentum by determining the probabilities for quantum transitions.” Note that the field was thought to be emitted by atoms in their stationary states and not, as in Bohr's original theory, during the ► quantum jumps from one state to another.
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