In this context, “negative mass” is a mathematical convenience rather than an actual particle having negative mass.
Think of it more like “energy required to pull apart a matter-antimatter pair”. In the vacuum of space, the energy that created the pair gets returned when they annihilate. But when near a blackhole, it had to “burn” some of its energy to interrupt that process. Energy is mass, so the blackhole gets less massive.
Mind you this is a very basic explanation of it. It’s just another quantum whackyness of our universe.
How can stuff have negative mass in the true sense of that?
In this context, “negative mass” is a mathematical convenience rather than an actual particle having negative mass.
Think of it more like “energy required to pull apart a matter-antimatter pair”. In the vacuum of space, the energy that created the pair gets returned when they annihilate. But when near a blackhole, it had to “burn” some of its energy to interrupt that process. Energy is mass, so the blackhole gets less massive.
Mind you this is a very basic explanation of it. It’s just another quantum whackyness of our universe.