On the definition of energy for a continuum, its conservation laws, and the energy-momentum tensor

TL;DR

This paper reviews the concept of energy in continua and fields, analyzing local conservation laws in Newtonian and relativistic contexts, and clarifies the definitions and properties of energy-momentum tensors.

Contribution

It provides a rigorous analysis of energy conservation, the energy-momentum tensor, and their frame dependence, including proofs of uniqueness and tensoriality in a general spacetime setting.

Findings

Derived local conservation equations for energy in Newtonian gravity.

Clarified the definitions and properties of canonical and Hilbert energy-momentum tensors.

Proved the uniqueness and tensoriality of the Hilbert tensor and energy density.

Abstract

We review the energy concept in the case of a continuum or a system of fields. First, we analyze the emergence of a true local conservation equation for the energy of a continuous medium, taking the example of an isentropic continuum in Newtonian gravity. Next, we consider a continuum or a system of fields in special relativity: we recall that the conservation of the energy-momentum tensor contains two local conservation equations of the same kind as before. We show that both of these equations depend on the reference frame, and that, however, they can be given a rigorous meaning. Then we review the definitions of the canonical and Hilbert energy-momentum tensors from a Lagrangian through the principle of stationary action in a general spacetime. Using relatively elementary mathematics, we prove precise results regarding the definition of the Hilbert tensor field, its uniqueness, and its tensoriality. We recall the meaning of its covariant conservation equation. We end with a proof of uniqueness of the energy density and flux, when both depend polynomially of the fields. Keywords: energy conservation; conservation equation; special relativity; general relativity; Hilbert tensor; variational principle