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LIGO detects most massive black hole merger to date

(www.caltech.edu)
360 points Eduard | 3 comments | 14 Jul 25 20:06 UTC | HN request time: 0.019s | 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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gus_massa ◴[14 Jul 25 21:35 UTC] No.44565623{3}[source]▶
It's the https://en.wikipedia.org/wiki/No-hair_theorem , but it only applies after a while, not instantly.

Your disk will emit a lot of gravitational on electromagnetic radiation, and after a while it will be a nice sphere. (Unless it's rotating and it will be a nice somewhat-elipsoidal ball.)

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> and then an electron popped into existence due to quantum randomness

I feel there is a huge can of worms of technical problems in this sentence that nobody know how to solve for now. Just in case replace the quantum randomness with a moron with a broken CRT used as an electron cannon.

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ars ◴[14 Jul 25 22:12 UTC] No.44565910{4}[source]▶
> and after a while it will be a nice sphere

Time doesn't exist for black holes, so "after a while" is not something you can say about them.

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gus_massa ◴[15 Jul 25 00:51 UTC] No.44566858{5}[source]▶
Simulations linked in others comment:

https://www.youtube.com/watch?v=Tr1zDVbSjTM

https://www.youtube.com/watch?v=1agm33iEAuo

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1. ars ◴[15 Jul 25 02:49 UTC] No.44567416{6}[source]▶
The second one is much more correct, notice how it freezes before they actually merge because they time dilate out to infinity.
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2. gus_massa ◴[15 Jul 25 11:09 UTC] No.44569918[source]▶
If they actually froze just before the merge, they will be peanut shaped like in the video.

I'm not sure if we can measure the shape of black holes, but I'm sure everyone think they are spheres with a slight deformation due to rotation.

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3. gus_massa ◴[15 Jul 25 18:37 UTC] No.44574428[source]▶
The algorithm choose this for me https://www.youtube.com/watch?v=fKgQYOlpxmo