In 1947, Theodore Holstein attacked the problem of imprisonment of resonance radiation in a novel way. Foregoing the classical method presented in the prior section, Holstein asserted that there could not exist a mean free path for the photons. His treatment begins with the introduction of a probability function G ( r , r ′ ) d r , which describes the probability that a photon emitted at r is absorbed within the volume element d r about the point r . Additionally, one can enforce atom number conservation to write