Resonance interaction between one excited and one ground state atom

The nature of the resonance interaction between two isotropic atoms in an excited configuration is reinvestigated. The currently accepted oscillatory form for the long-range retarded resonance interaction is shown to be a subtle artefact that arises due to too drastic approximations. Formulation of the resonance interaction energy problem in terms of the interacting system leads to a form that it is proportional to r(-4) in the retarded limit. We also demonstrate that the resonance interaction energy at any finite temperature goes over to purely classical long-range asymptote. This manifestation of the correspondence principle is due to thermal excitation of the electromagnetic field. We finally discuss why the textbook result for the Forster energy transfer between two atoms is incorrect for the same reasons.