Defining mass in general relativity: concepts and obstacles
Generalizing this definition to general relativity, however, is problematic; in fact, it turns out to be impossible to find a general definition for a system’s total mass (or energy).
The main reason for this is that “gravitational field energy” is not a part of the energy-momentum tensor; instead, what might be identified as the contribution of the gravitational field to a total energy is part of the Einstein tensor on the other side of Einstein’s equation (and, as such, a consequence of these equations’ non-linearity).
While in certain situation it is possible to rewrite the equations so that part of the “gravitational energy” now stands alongside the other source terms in the form of the Stress-energy-momentum pseudotensor, this separation is not true for all observers, and there is no general definition for obtaining it.
— Wikipedia on Mass in general relativity
2013.08.06 Tuesday ACHK