Angular momentum of radiation from ultrarelativistic charge

TL;DR

This paper analyzes the angular momentum of electromagnetic radiation emitted by an ultrarelativistic charge, providing explicit formulas and exploring its distribution relative to different points, including the charge's instantaneous curvature center.

Contribution

It introduces explicit expressions for the angular momentum flux components of radiation from a relativistic charge and clarifies their dependence on the radiation field and observer's position.

Findings

Angular momentum relative to the charge depends only on the radiation field properties.

Angular momentum relative to a distant point involves the displacement and radiation pressure.

In the ultrarelativistic limit, canonical angular momentum matches that from the symmetrized energy-momentum tensor.

Abstract

The angular momentum of radiation from an arbitrarily moving relativistic charge is studied. The angular momentum is presented as the sum of the angular momentum relative to the point where the charge is located at a retarded moment of time and the angular momentum relative to an arbitrary stationary center. In particular, the instantaneous center of curvature of the trajectory is considered as such a center. Explicit expressions for the angular distribution of these components of angular momentum fluxes are obtained and studied. It is shown that the angular momentum of the field relative to the position of the charge is determined only by the properties of the electromagnetic radiation field, and the angular momentum relative to an arbitrarily distant point is the vector product of the displacement of this point and the force corresponding to radiation pressure. It is shown that in the ultrarelativistic limit, the canonical angular momentum of the radiation coincides with the angular momentum following from the symmetrized energy-momentum tensor of the electromagnetic field.