[chris (OP)]

If a black hole that formed from the collapse of a normal matter star were to feed on antimatter, what would be the consequence?

[Kryptid]

It would be exactly the same as a black hole being fed "normal" matter. Black holes "have no hair", meaning that their only properties are spin, electric charge and mass. You cannot distinguish a black hole created by the collapse of antimatter from one that formed from the collapse of matter.

[chris (OP)]

So it would just shred the antimatter and gain mass?

[Kryptid]

Pretty much, yeah.

[jeffreyH]

The black hole event horizon is not a surface but a mathematical construct. We cannot know what happens to the anti matter. Bill_s made a good point recently. Due to energy conservation it would not matter if annihilation took place since the energy content of the black hole would still increase by the same amount. It depends upon how forces behave beyond the horizon.

[Bill S]

The black hole event horizon is not a surface but a mathematical construct.

This is one of the things that puzzles me about Hawking radiation.  If a particle/antiparticle duo appears and one of them enters a BH, while the other heads off into space; does this not suggest that the event horizon is so well defined that it can separate the two particles?

[jeffreyH]

The event horizon is well defined. The radius could be used to trace out a sphere around the singularity (easier for a non-rotating black hole) but this would only be a mathematical surface. Objects can theoretically pass through it unhindered so it cannot be solid.

[Bill S]

The event horizon is well defined. The radius could be used to trace out a sphere around the singularity (easier for a non-rotating black hole) but this would only be a mathematical surface.

So, do we distinguish between a mathematical surface and a physical surface? 

Is it well defined, only, mathematically, or do scientists have reason to believe that there is a surface, in space, that is so well defined that it can separate a pair of virtual particles?

[jeffreyH]

It is the potential that will separate the particles. The strange thing about the horizon is that it is within the photon sphere. This is another 'surface' where only photons can orbit. This means that nothing else can move perpendicular to the direction of the field. So only particles pointing outward, away from perpendicular, can escape. This almost guarantees Hawking radiation.