The Negative Absorption of Radiation

IN Einstein's celebrated derivation of the Planck radiation formula, an equilibrium is considered to exist between three elementary processes: (1) a spontaneous emission from the atoms, (2) an absorption of energy by the atoms proportional to the energy density in the field, and (3) an induced emission of energy from the atoms, also proportional to the energy density. The third process can be described as a negative absorption of radiation, and is quite characteristic for Einstein's theory, as the omission of it from the equations leads to Wien's radiation formula instead of to Planck's. The negative absorption of radiation also figures prominently in the Kramers-Heisenberg theory of dispersion. The physical existence of such absorption has been up to now an article of faith rather than a proved experimental fact, and indeed some writers (Ornstein and Burger, S. N. Bose) have been tempted to question its reality.