Consider 2 (not necessarily super-massive) black holes approaching each other at relativistic (0.5 c or something) speed, such that at the closest approach, the event horizons are just overlapping by some suitably small fraction. Would such an event result in more than 1 black hole?
If the parameters are too vague for you, let me ask it this way: Are there any values for sizeA, sizeB, relative velocity, non zero overlap at closest approach, such that the 2 black holes do not merge into a single black hole?
No. The single most important quality of a black hole is that it isn’t possible to escape from it if you’re inside the event horizon. But the physical size of a black whole isn’t as easily defined as regular physical objects, so how do you define size? I believe there are more than one type of event horizon, and I don’t actually know if two strafing black holes could have overlapping even horizons or not … If so I’d guess the limit would be when the center of one horizon, the presumed position of that singularity, was inside the event horizon of the other hole.
Looking forward to the proper physicists answering this.
First we need to define “collision”. If two black holes pass by each other without overlapping their event horizons, that could still be considered a “collision” of sorts, but would still leave you with two black holes afterwards. But let’s assume that we’re just talking about cases where the horizons do overlap.
Then there is the question of other debris, ejecta, shrapnel, whatever you’d like to call it. This is certainly possible. In a black hole merger, a considerable fraction of the initial mass gets converted to gravitational waves (for instance, in the one announced a year ago, out of 50 solar masses total, 3 was carried away in waves). But gravitational waves aren’t black holes (though they could in principle form one, in the right contrived circumstances).
There’s also a set of laws governing black hole mergers, closely paralleling the Laws of Thermodynamics (in fact, they’re probably just special cases of those laws, on an immense scale). The most relevant one here is that in any black hole interaction, the total area of event horizon(s) cannot decrease. This certainly favors multiple small holes forming fewer larger holes, since a black hole’s event horizon area is proportional to its mass squared.
But I don’t think there’s any fundamental reason why you couldn’t, under just the right circumstances, have two medium-sized holes merge into one large hole plus an undetermined number of very small “shrapnel” holes. It’d probably be highly unlikely, but I can’t see any argument that it’d be completely impossible.
What does it mean that the event horizons overlap. If they overlap doesn’t it mean that some points are within the event horizons of both holes? Is that even possible?
Well, once they overlap, you don’t really have “both holes” any more, just one hole with an unusual shape (it’ll very quickly settle back down into a sphere or spheroid).