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LIGO detects most massive black hole merger to date

(www.caltech.edu)
360 points Eduard | 1 comments | 14 Jul 25 20:06 UTC | HN request time: 0.001s | source
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perdomon ◴[14 Jul 25 20:19 UTC] No.44564794[source]▶
What happens when black holes collide? Does one black hole “consume” the other? Do they become a larger black hole? Does it get more dense or just larger?
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__MatrixMan__ ◴[14 Jul 25 20:29 UTC] No.44564894[source]▶
They become a more massive one. The volume of a black hole (assuming you're measuring at the event horizon) is determined only by its mass, so the final density is the same as you'd get for any other black hole of that mass regardless of how it came to be.

I don't know how to address the "consume" question. If you were pulling on a piece of fabric and two tears in it grew until they met each other to become one tear... would you say that the larger one consumed the smaller?

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dataflow ◴[14 Jul 25 21:09 UTC] No.44565339[source]▶
> The volume of a black hole (assuming you're measuring at the event horizon) is determined only by its mass, so the final density is the same as you'd get for any other black hole of that mass regardless of how it came to be.

Wait, really? So if you had a super massive disk that was just 1 electron away from having enough mass to become a black hole... and then an electron popped into existence due to quantum randomness... then it would become a sphere instantly? Wouldn't that violate the speed of light or something?

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1. __MatrixMan__ ◴[15 Jul 25 01:43 UTC] No.44567103{3}[source]▶
Long before your disc neared the mass where it would form an event horizon, the matter it's made of would collapse into neutron star material, which would form a sphere.

Perhaps if it were exceptionally wide the whole disc wouldn't collapse. Maybe only the parts near it's center. In that case you'd end up with a large ring around a neutron star. Add a bit more mass and maybe it's now a ring around a black hole. The gravity of the ring might distort the event horizon in some way, I'm not sure quite how, but probably its possible to get a non-spherical hole in situations where the objects distorting the shape are still in the universe.

But as for the matter lost into the hole, it's gone. If the hole were to retain some shape based on what's "inside" of it, that would be the kind of information leak that the laws of physics do not permit.