Geometric field theory and weak Euler-Lagrange equation for classical relativistic particle-field systems

TL;DR

This paper develops a covariant geometric field theory for relativistic particle-field systems, establishing space-time symmetry and conservation laws without coordinate splitting, and introduces a weak geometric Euler-Lagrange equation for particles.

Contribution

It introduces a geometric formulation of the Euler-Lagrange equations for relativistic particles and fields, including a weak form for particles, and derives a geometric energy-momentum tensor.

Findings

Established space-time symmetry-conservation law connection geometrically.

Derived a geometric energy-momentum tensor for particles.

Generalized Euler-Lagrange equations to a covariant geometric form.

Abstract

A manifestly covariant, or geometric, field theory for relativistic classical particle-field system is developed. The connection between space-time symmetry and energy-momentum conservation laws for the system is established geometrically without splitting the space and time coordinates, i.e., space-time is treated as one identity without choosing a coordinate system. To achieve this goal, we need to overcome two difficulties. The first difficulty arises from the fact that particles and field reside on different manifold. As a result, the geometric Lagrangian density of the system is a function of the 4-potential of electromagnetic fields and also a functional of particles' world-lines. The other difficulty associated with the geometric setting is due to the mass-shell condition. The standard Euler-Lagrange (EL) equation for a particle is generalized into the geometric EL equation when the mass-shell condition is imposed. For the particle-field system, the geometric EL equation is further generalized into a weak geometric EL equation for particles. With the EL equation for field and the geometric weak EL equation for particles, symmetries and conservation laws can be established geometrically. A geometric expression for the energy-momentum tensor for particles is derived for the first time, which recovers the non-geometric form in the existing literature for a chosen coordinate system.