Fierz bilinear formulation of the Maxwell-Dirac equations and symmetry reductions

TL;DR

This paper reformulates the Maxwell-Dirac equations using gauge-invariant spinor bilinears, derives symmetry-reduced equations for specific cases, and finds exact solutions for certain Poincaré subgroups, advancing the understanding of gauge-invariant formulations.

Contribution

It introduces a Fierz bilinear gauge-invariant formulation of the Maxwell-Dirac equations and explicitly derives symmetry-reduced equations for spherical, cylindrical, and other Poincaré subgroups.

Findings

Symmetry reduction leads to coupled third-order nonlinear PDEs.

Magnetic monopoles are necessary under spherical symmetry but cancel in the system.

Exact solutions are found for non-splitting Poincaré subgroup classes.

Abstract

We study the Maxwell-Dirac equations in a manifestly gauge invariant presentation using only the spinor bilinear scalar and pseudoscalar densities, and the vector and pseudovector currents, together with their quadratic Fierz relations. The internally produced vector potential is expressed via algebraic manipulation of the Dirac equation, as a rational function of the Fierz bilinears and first derivatives (valid on the support of the scalar density), which allows a gauge invariant vector potential to be defined. This leads to a Fierz bilinear formulation of the Maxwell tensor and of the Maxwell-Dirac equations, without any reference to gauge dependent quantities. We show how demanding invariance of tensor fields under the action of a fixed (but arbitrary) Lie subgroup of the Poincar\'e group leads to symmetry reduced equations. The procedure is illustrated, and the reduced equations worked out explicitly for standard spherical and cylindrical cases, which are coupled third order nonlinear PDEs. Spherical symmetry necessitates the existence of magnetic monopoles, which do not affect the coupled Maxwell-Dirac system due to magnetic terms canceling. In this paper we do not take up numerical computations. As a demonstration of the power of our approach, we also work out the symmetry reduced equations for two distinct classes of dimension 4 one-parameter families of Poincar\'e subgroups, one splitting and one non-splitting. The splitting class yields no solutions, whereas for the non-splitting class we find a family of formal exact solutions in closed form.