On van der Waals friction: I. Between two atoms

Neglecting retardation and all other relativistic effects, we find the van der Waals force between two identical atoms mimicked as harmonic oscillators, constrained to move with constant relative velocity on fixed parallel tracks, starting infinitely far apart at time t=-infinity, each in its ground state. The component parallel to the motion has an irreversible part, producing finite excitation probabilities as t ->+infinity. In this sense, the calculation demonstrates that the atoms are subject to nonzero frictional resistance, with the immediate implication that friction is experienced also by an atom moving outside and parallel to the surface of an electrically dilute and nonabsorptive half-space. The next paper in this series finds the friction experienced by an atom outside a half-space which may be absorptive and electrically dense.