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.
[1] The equation for the mass of a 2-sphere is based on the equation for the ingoing and outgoing ray of light corresponding to an electron on its surface, expressed as: γ = 2 γ.
The inverse of this equation represents the value of the Hawking energy, and the result is the mass of an entangled pair of photons: the two photons can not be entangled if only they are located at the same location on the surface.
But in this case, both photons must be propagating simultaneously.