BKS theory

A 1924 theory by Niels Bohr, Hendrik Kramers, and John Slater proposing that energy and momentum were conserved statistically, rather than in each event, during interactions between radiation and matter.

Virtual oscillator: A mathematical oscillator representing a possible atomic transition, without asserting that the atom literally oscillates at that frequency. BKS assigned each possible transition a virtual oscillator whose radiation interfered with others.
Niels Bohr: A Danish physicist who developed an influential model of the atom and helped establish quantum theory. Bohr was a coauthor and supplied the quantum framework behind BKS.
Compton scattering: The scattering of X-rays by electrons, producing a wavelength shift explained through photon–electron collisions. Its event-by-event energy and momentum balance challenged BKS’s statistical conservation proposal.
Matrix mechanics: A formulation of quantum mechanics developed by Werner Heisenberg in 1925 using arrays of transition quantities. Its emergence displaced the old-quantum-theory setting in which BKS had been proposed.
Statistical conservation laws: Conservation rules that hold across an ensemble of events rather than necessarily in each individual event. BKS allowed energy and momentum to balance statistically across radiation–matter interactions.
Hendrik Kramers: A Dutch theoretical physicist known for work on quantum theory and the correspondence principle. Kramers’s work on dispersion and correspondence helped shape the theory’s treatment of radiation.
Bothe–Geiger experiment: A 1925 coincidence experiment showing that scattered X-rays and recoiling electrons were correlated in individual events. Its coincidences contradicted BKS’s denial of strict event-by-event conservation.
Quantum electrodynamics: The quantum field theory describing interactions between charged particles and electromagnetic fields. Later theory describes radiation–matter interactions with quantum fields rather than BKS’s statistical conservation hypothesis.
Correspondence principle: Bohr’s principle that quantum theory should reproduce classical physics in the limit of large quantum numbers. BKS used correspondence to connect transition frequencies with components of classical radiation.
John Slater: An American theoretical physicist whose work included atomic structure and quantum theory. Slater joined Bohr and Kramers in formulating the 1924 proposal.

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Arthur ComptonCompared with: Its rejection after scattering experiments sharpened the case for event-by-event photon momentum.