Energy of test objects on black hole spacetimes: A brief review

TL;DR

This review discusses how energy for test particles in black hole spacetimes can be defined, emphasizing the role of conserved quantities, horizon energy, and the implications for energy conservation in various black hole backgrounds.

Contribution

It clarifies the definitions of energy in black hole spacetimes and compares different approaches, including the isolated horizons formalism and recent proposals.

Findings

Conserved energy can be well-defined using time-like isometries.

Horizon energy provides a consistent energy measure during particle absorption.

Global energy conservation is maintained with appropriate definitions.

Abstract

In this paper, we review the issue of defining energy for test particles on a background stationary spacetime. We revisit the different notions of energy as defined by different observers. As is well known, the existence of a time-like isometry allows for the notion of total conserved energy to be well defined. We use this well known quantity to show that a gravitational potential energy can be consistently defined. As examples, we study the case of the exterior regions of an asymptotically flat black hole and of the $\Lambda>0$ Schwarzschild de Sitter case, where an asymptotic region is not available. We then consider the situation in which the test particle is absorbed by the black hole, and analyze the energetics in detail. In particular, we show that the notion of horizon energy as defined by the isolated horizons formalism provides a satisfactory notion of energy compatible with the particle's total conserved energy. With these choices, there is a global conservation of energy. Finally, we comment on a recent proposal to define energy of the black hole as seen by a nearby observer at rest, for which this feature is lost.