Can poles change color?

Abstract

The definition of the total nonabelian charge (‘‘color’’) in a classical Yang–Mills theory is shown to require a careful analysis of the boundary conditions at infinity imposed on the potentials and on gauge transformations. The color current of a nonabelian plane wave is found to be different from zero in the transverse gauge, though it vanishes in the null gauge. The color charge of a single pole, described by the Liénard–Wiechert potentials, is constant by virtue of the Yang–Mills equations. An approximate computation indicates that the total color charge of a system of particles may change in time, as a result of radiation. To make this result meaningful, it is necessary to find a method of fixing the allowed gauge transformations to those having a direction-independent limit at infinity.