Mass in general relativity
The concept of mass in general relativity (GR) is more complex than the concept of mass in special relativity. In fact, general relativity does not offer a single definition of the term mass, but offers several different definitions that are applicable under different circumstances. Under some circumstances, the mass of a system in general relativity may not even be defined.
In special relativity, the invariant mass or rest mass (hereafter simply "mass") of an isolated system can be defined in terms of the energy and momentum of the system by the relativistic energy–momentum equation:
Where E is the total energy of the system, p is the total momentum of the system and c is the speed of light. Concisely, in fundamental units where , the mass of a system in special relativity is the norm of its energy–momentum four vector; otherwise, it is .
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