van der Waals dispersion forces in electromagnetic fields.

By relating the van der Waals dispersion interaction between two neutral polarizable particles to a field correlation function, we show that the interaction is modified by applied electromagnetic fields. The modified interaction, which has a simple physical interpretation, can vary as {ital r}{sup {minus}3} rather than {ital r}{sup {minus}6} if the applied field is nearly unidirectional. Approximate numerical estimates indicate that the effect is very small for static or low-frequency fields, but that it might be significant, particularly in cold traps, when the frequency of a quasimonochromatic applied field is near a strong absorption line. {copyright} {ital 1996 The American Physical Society.}