Note on the Time Reversal Asymmetry of Equations of Motion

Abstract

Rohrlich's recent claim that the equation of motion for a point charge be symmetric under time reversal is shown to be the result of an unusual definition. The equation of motion for a charged sphere of finite size, which in contrast is claimed to be asymmetric because of the finite propagation time of its (retarded) self-forces, is shown to possess the same asymmetry (or the same symmetry, depending on the definition) as that for a point charge. Similar arguments apply to other effective equations of motion (such as those describing friction or decoherence).