Two Black Holes' collision thought experiment

[George K]

Once, someone had asked two interesting (though absolutely hypothetical) questions:

1) What should happen during the collision of two BHs, one consisting of matter (BH+) and the other consisting of antimatter (BH-)? Should they form a larger BH or should they be annihilated?

My personal opinion is that -probably- the EH of the new BH is formed before the singularities (of the original BHs) will meet each other. So, as the mass of a BH is concentrated on its center (i.e. on its singularity), the annihilation of their masses will take place after the formation of the new EH (and the vast amount of photons that will be produced from the annihilation will never manage to escape). As a 2nd thought, we cannot really know what will happen when the two singularities will meet each other, as we don't know the "form" of the mass on a singularity or even the laws of Physics that govern it. What's your opinion?

2) Suppose that there are two neutron stars (NS), one consisting of matter (NS+) and the other consisting of antimatter (NS-), and each of them having a mass somewhat lower than the critical mass for the formation of a BH. What should happen during their collision? Should they form a BH or should they be annihilated?

My personal opinion is that -probably- there will be an annihilation right on the contact point and this will lead to the gradual annihilation of the whole mass of the two objects before an EH has the chance to be formed. What's your opinion?

 

[mathman]

Your guesses seem reasonable, but as you point out they are only guesses. The neutron star description is well within theory, but the black situation is fairly speculative, since the inside of a black hole is unknown physics (quantum theory meets general relativity).

 

[PAllen]

1) Per the black hole has no hair theorem, any result to a BH collision is only based on their mass, charge, and angular momentum (assuming GR). Whether one formed from antimatter is wholly irrelevant. So your final answer is correct, for a more fundamental reason. As to what happens inside, it obviously depends on how much you trust GR in the interior. If you do, then for 'old' BH, there is only vacuum on the inside with a singularity, so no interaction to occur. For new BHs, where not all matter has collapsed, your suggestion of annihilation with everything continuing to collapse anyway, is what would be expected.

2) It all depends. For example, if the neutron and anti-neutron star were approaching each other at exceeding close to the speed of light, their combined COM energy could be large enough that they are inside their SC radius before they are touching (note: total energy/c² is the M to use for computing the combined SC radius). Then, if the hoop conjecture is true, they would inevitably form a BH. A minimum energy collision would be difficult to compute. I don't think they could possibly meed the criteria of the hoop conjecture, so detailed dynamics would have to be modeled. I wouldn't even venture a guess as to the outcome.