# Hawking energy

The Hawking energy or Hawking mass is one of the possible definitions of mass in general relativity. It is a measure of the bending of ingoing and outgoing rays of light that are orthogonal to a 2-sphere surrounding the region of space whose mass is to be defined.

## Definition

Let ${\displaystyle ({\mathcal {M}}^{3},g_{ab})}$ be a 3-dimensional sub-manifold of a relativistic spacetime, and let ${\displaystyle \Sigma \subset {\mathcal {M}}^{3}}$ be a closed 2-surface. Then the Hawking mass ${\displaystyle m_{H}(\Sigma )}$ of ${\displaystyle \Sigma }$ is defined[1] to be

${\displaystyle m_{H}(\Sigma ):={\sqrt {\frac {{\text{Area}}\,\Sigma }{16\pi }}}\left(1-{\frac {1}{16\pi }}\int _{\Sigma }H^{2}da\right),}$

where ${\displaystyle H}$ is the mean curvature of ${\displaystyle \Sigma }$.

## Properties

In the Schwarzschild metric, the Hawking mass of any sphere ${\displaystyle S_{r}}$ about the central mass is equal to the value ${\displaystyle m}$ of the central mass.

A result of Geroch[2] implies that Hawking mass satisfies an important monotonicity condition. Namely, if ${\displaystyle {\mathcal {M}}^{3}}$ has nonnegative scalar curvature, then the Hawking mass of ${\displaystyle \Sigma }$ is non-decreasing as the surface ${\displaystyle \Sigma }$ flows outward at a speed equal to the inverse of the mean curvature. In particular, if ${\displaystyle \Sigma _{t}}$ is a family of connected surfaces evolving according to

${\displaystyle {\frac {dx}{dt}}={\frac {1}{H}}\nu (x),}$

where ${\displaystyle H}$ is the mean curvature of ${\displaystyle \Sigma _{t}}$ and ${\displaystyle \nu }$ is the unit vector opposite of the mean curvature direction, then

${\displaystyle {\frac {d}{dt}}m_{H}(\Sigma _{t})\geq 0.}$

Said otherwise, Hawking mass is increasing for the inverse mean curvature flow.[3]

Hawking mass is not necessarily positive. However, it is asymptotic to the ADM[4] or the Bondi mass, depending on whether the surface is asymptotic to spatial infinity or null infinity.[5]

## References

1. ^ Page 21 of Schoen, Richard, 2005, "Mean Curvature in Riemannian Geometry and General Relativity," in Global Theory of Minimal Surfaces: Proceedings of the Clay Mathematics Institute 2001 Summer School, David Hoffman (Ed.), p.113-136.
2. ^ Geroch, Robert. 1973. "Energy Extraction." doi:10.1111/j.1749-6632.1973.tb41445.x.
3. ^ Lemma 9.6 of Schoen (2005).
4. ^ Section 4 of Yuguang Shi, Guofang Wang and Jie Wu (2008), "On the behavior of quasi-local mass at the infinity along nearly round surfaces".
5. ^ Section 2 of Shing Tung Yau (2002), "Some progress in classical general relativity," Geometry and Nonlinear Partial Differential Equations, Volume 29.
• Section 6.1 in Szabados, László B. (2004), "Quasi-Local Energy-Momentum and Angular Momentum in GR", Living Rev. Relativ., 7, retrieved 2007-08-23