Can black holes lose enough mass to stop being black holes?

12 August 2012

Question

I have a question regarding Black Holes. Since Black holes can lose mass by Hawking Radiation, is it possible for them to lose sufficient mass to no longer be a Black Hole, and become, say, a Neutron Star?

Mark Anderson
Windsor, Ontario, Canada

Answer

Dominic - There is a process by which black holes can lose mass and what happens is that over time, in the vacuum of space, there's a quantum process by which particles and anti-particles are popping out of the vacuum of space. If one of those two particles, the anti-particle, falls into the black hole and the other particle, the real mass particle, does not fall into the black hole then you've got a particle out of the black hole. The anti-particle has fallen in and will annihilate with matter inside the black hole, and that causes the black hole to lose mass.

This is a very slow process for the kinds of astrophysical black holes you see in the centres of galaxies and that form out of supernova explosions. But if you were, for example, to take a small black hole - I did a calculation a while back for one the size of a double decker bus - then this is actually quite a fast process and a black hole the size of a double decker bus would last about 1000th of a second before it would just evaporate into the vacuum of space. But obviously, real black holes are around about the mass of the Sun or many million times that mass and they will lose mass very slowly compared to the amount of mass they have.

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