Gravitational energy in stationary spacetimes

TL;DR

This paper explores the concept of gravitational energy in stationary spacetimes, defining it via mechanical energy flux measurements by static observers and deriving a non-local gravitational energy density using Einstein's equations and conformal transformations.

Contribution

It introduces a novel method to define and compute gravitational energy density in stationary spacetimes, extending previous spherical models to more general observer frameworks.

Findings

Total gravitational energy can be negative for certain systems.

A non-local gravitational energy density is derived using Einstein's equations.

The approach applies to observers both static and moving orthogonally to spatial slices.

Abstract

Static observers remain on Killing-vector world lines and measure the rest-mass+kinetic energies of particles moving past them, and the flux of that mechanical energy through space and time. The total mechanical energy is the total flux through a spacelike cut at one time. The difference between the total mass-energy and the total mechanical energy is the total gravitational energy, which we prove to be negative for certain classes of systems. For spherical systems, Misner, Thorne and Wheeler define the total gravitational energy this way. To obtain the gravitational energy density analogous to that of electromagnetism we first use Einstein's equations with integrations by parts to remove second order derivatives. Next we apply a conformal transformation to reexpress the scalar 3-curvature of the 3-space. The resulting density is non-local. We repeat the argument for mechanical energies as measured by stationary observers moving orthogonally to constant time slices like the "zero angular momentum" observers of Bardeen who exist even within ergospheres.