  A positive mass theorem for General Relativity Theory is proved. The proof is
4-dimensional in nature, and relies completely on arguments pertaining to
causal structure, the basic idea being that positive energy-density focuses
null geodesics, and correspondingly retards them, whereas a negative total mass
would advance them. Because it is not concerned with what lies behind horizons,
this new theorem applies in some situations not covered by previous positivity
theorems. Also, because geodesic focusing is a global condition, the proof
might allow a generalisation to semi-classical gravity, even though quantum
violations of local energy conditions can occur there.

General Relativity and Quantum Cosmology;High Energy Physics - Theory