Generalized Four-momentum for Continuously Distributed Materials

TL;DR

This paper derives a covariant expression for the generalized four-momentum of continuously distributed materials using a differential Euler-Lagrange framework, applicable to relativistic systems with fields and pressures.

Contribution

It introduces a new covariant formulation of four-momentum for continuous materials, linking Lagrangian density to four-momentum and applying it to relativistic systems with various fields.

Findings

Generalized four-momentum is an integral four-vector.

Expression matches Legendre transformation between Lagrangian and Hamiltonian.

Four-momentum depends on mass, velocity, and scalar potentials of fields.

Abstract

A four-dimensional differential Euler-Lagrange equation for continuously distributed materials is derived based on the principle of least action, and instead of Lagrangian, this equation contains the Lagrangian density. This makes it possible to determine the density of generalized four-momentum in covariant form as derivative of the Lagrangian density with respect to four-velocity of typical particles of a system taken with opposite sign, and then calculate the generalized four-momentum itself. It is shown that the generalized four-momentum of all typical particles of a system is an integral four-vector and therefore should be considered as a special type of four-vectors. The presented expression for generalized four-momentum exactly corresponds to the Legendre transformation connecting the Lagrangian and Hamiltonian. The obtained formulas are used to calculate generalized four-momentum of stationary and moving relativistic uniform systems for the Lagrangian with particles and vector fields, including electromagnetic and gravitational fields, acceleration field and pressure field. It turns out that the generalized four-momentum of a moving system depends on the total mass of particles, on the Lorentz factor and on the velocity of the systems center of momentum. Besides, an additional contribution is made by the scalar potentials of the acceleration field and the pressure field at the center of system. The direction of the generalized four-momentum coincides with the direction of four-velocity of the system under consideration, while the generalized four-momentum is part of the relativistic four-momentum of the system.