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


 * $$m_H(\Sigma) := \sqrt{\frac{\text{Area}\,\Sigma}{16\pi}}\left( 1 - \frac{1}{16\pi}\int_\Sigma H^2 da \right), $$

where $$H$$ is the mean curvature of $$\Sigma$$.

Properties
In the Schwarzschild metric, the Hawking mass of any sphere $$S_r$$ about the central mass is equal to the value $$m$$ of the central mass.

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


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

where $$H$$ is the mean curvature of $$\Sigma_t$$ and $$\nu$$ is the unit vector opposite of the mean curvature direction, then


 * $$\frac{d}{dt}m_H(\Sigma_t) \geq 0.$$

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

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