At least at the level of isolated gravitational systems (where one can define the so-called ADM mass of a spacetime), mass cannot be negative under pretty gener

At least at the level of isolated gravitational systems (where one can define the so-called ADM mass of a spacetime), mass cannot be negative under pretty general assumptions: https://en.m.wikipedia.org/wiki/Positive_energy_theorem https://en.m.wikipedia.org/wiki/Positive_energy_theorem