ADM and Bondi Masses in Asymptotically Flat Space-times
If a system containing gravitational sources is surrounded by an infinite vacuum region, the geometry of the space-time will tend to approach the flat Minkowski geometry of special relativity at infinity.
Such space-times are known as "asymptotically flat" space-times.
For systems in which space-time is asymptotically flat, the ADM and Bondi energy, momentum, and mass can be defined. In terms of Noether's theorem, the ADM energy, momentum, and mass are defined by the asymptotic symmetries at spatial infinity, and the Bondi energy, momentum, and mass are defined by the asymptotic symmetries at null infinity. Note that mass is computed as the length of the energy-momentum four vector, which can be thought of as the energy and momentum of the system "at infinity".
Read more about this topic: Mass In General Relativity, Types of Mass in General Relativity
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