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Geroch energy
The Geroch energy or Geroch mass is one of the possible definitions of mass in general relativity. It can be derived from the Hawking energy, itself 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, by leaving out certain (positive) terms related to the sphere's external and internal curvature.
See also
Mass in general relativity
Robert Geroch
References
Section 6.2 in Szabados, László B. (2009), "Quasi-Local Energy-Momentum and Angular Momentum in GR", Living Rev. Relativity
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