The retarded interaction between neutral systems with no permanent moments, i.e., relativistic dispersion forces, is analyzed by quantum electrodynamics and the twohyphen;photon exchange potential is computed for three cases: (i) When both systems are in their ground states, where the potential is the Casimirmdash;Polder potential which falls off for separationsRgreater than lgr;slash;, the characteristic reduced wavelength in dipole transitions, asRminus;7; (ii) when one system is in an optically allowed excited state where the potential at short distances is the wellhyphen;known resonance potential proportional toRminus;3; and (iii) when both molecules are in excited states. The interaction energies are computed at all separations larger than a few molecular radii, so all overlap and Pauli effects are not considered. The asymptotic forms are given both forRlgr;slash; andRlgr;slash; (but still larger than molecular size). The latter give the familiar London and resonance potentials. The former are new results and show a very much slower falloff like a modulated inverse distance or distance squared. This effect is due to the true radiation field of a dipole containing in the radiation zone fields which fall off asRminus;1. The London potentials, calculated normally by Coulomb interaction only, arise in the fully retarded theory from the nearhyphen;zone fields which fall off asRminus;3.
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