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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?

chasil 1y ago
What happens inside cannot be known. As I understand it, black holes are defined by three quantities: mass, spin, and charge. I'm assuming that these quantities will be additive post-merger. Edit: "The black holes appear to be spinning very rapidly—near the limit allowed by Einstein's theory of general relativity." Perhaps the additive spin becomes asymptotic. Alternately, the gravitational waves might have departed with the energy of the excess spin.
jameskilton 1y ago
My basic understanding is that they combine, basically you just add the masses together. That increased mass then means more gravity, so the event horizon is pushed further out.
__MatrixMan__ 1y ago
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?
- a012 1y ago
  I think the GP meant “merge”
- dataflow 1y ago
  > 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?
  - gus_massa 1y ago
    It's the https://en.wikipedia.org/wiki/No-hair_theorem 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.) --- > 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.
    - ars 1y ago
      > 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.
      - gus_massa 1y ago
        Simulations linked in others comment: https://www.youtube.com/watch?v=Tr1zDVbSjTM https://www.youtube.com/watch?v=Tr1zDVbSjTM https://www.youtube.com/watch?v=1agm33iEAuo https://www.youtube.com/watch?v=1agm33iEAuo
        ars 1y ago
        The second one is much more correct, notice how it freezes before they actually merge because they time dilate out to infinity.
        gus_massa 1y ago
        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.
- gus_massa 1y ago
  > the "consume" question My guess is that in some popular depictions black holes are like holes, and things fall in the holes, and even a small black hole can possible fall inside a bigger hole. A better image is too drops of water on a glass, add some black ink for bonus realism. They merge into a bigger drop. Except, obviously black holes are not filled with water. And the "average density" of the new black hole is smaller then the "average density" of both original black holes, unlike the density of water drops on a glass. So don't take this image too literaly. (There are some problems to define the "density" of a black hole, but let's ignore all of them.)
- Iwan-Zotow 1y ago
  "The volume of a black hole" and how you define it?
hnuser123456 1y ago
They become a larger black hole, mostly conserving mass, minus a few percent to gravitational waves. However, their mass is proportional to their radius, not volume, so it gets LESS dense. If you laid out a bunch of black holes in a line, just barely not touching, and let them merge, suddenly, the whole sphere of space enclosing the line becomes black hole. It also turns out that a black hole with the mass of the universe would have a volume about the size of the universe.
- JumpCrisscross 1y ago
  > turns out that a black hole with the mass of the universe would have a volume about the size of the universe Mass and energy.
  - gjm11 1y ago
    Is that intended to be a correction? (I don't think the original statement needs correcting, other than by replacing "universe" with "observable universe" in both places.)
    - hnuser123456 1y ago
      Up until the universe was around a few billion years old, its Schwarzchild radius would have been larger than even the co-moving (not just observable) universe's radius, but the initial momentum from the big bang was high enough to prevent collapse.
      - AnimalMuppet 1y ago
        That sounds suspiciously like "they were inside a region with enough mass to form an event horizon, but they escaped because they had enough momentum", which in turn sounds like "we can escape from inside an event horizon if we just move fast enough". Can you explain how that's not what you're saying?
        hnuser123456 1y ago
ars 1y ago
My understanding is they just spiral into each other forever. From our point of view nothing can actually fall into a black hole, instead it time dilates into nothing. "It is true that objects that encounter the event horizon of a black hole would appear “frozen” in time"[1] So we would never actually see the black holes merge. In fact I'm not clear how a black hole can even form in the first place, since it would take an infinite amount of time to do so (again, from our POV). (And yes, I know that from the POV of the falling object, they just fall in like normal. But that doesn't help us, because we'll never see it.) [1] https://public.nrao.edu/ask/does-an-observer-see-objects-frozen-in-time-at-the-event-horizon-of-a-black-hole/ https://public.nrao.edu/ask/does-an-observer-see-objects-fro...
- ajross 1y ago
  This is true, but it's not an observable distinction. It's true that in some sense those two black holes "haven't yet collided", but at this point they're well past the point of last observability and have now red shifted and time dilated into the invisible background. All the interesting stuff happens before that.

chasil 1y ago

What happens inside cannot be known. As I understand it, black holes are defined by three quantities: mass, spin, and charge. I'm assuming that these quantities will be additive post-merger. Edit: "The black holes appear to be spinning very rapidly—near the limit allowed by Einstein's theory of general relativity." Perhaps the additive spin becomes asymptotic. Alternately, the gravitational waves might have departed with the energy of the excess spin.

jameskilton 1y ago

My basic understanding is that they combine, basically you just add the masses together. That increased mass then means more gravity, so the event horizon is pushed further out.

__MatrixMan__ 1y ago

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?

a012 1y ago
I think the GP meant “merge”
dataflow 1y ago
> 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?
- gus_massa 1y ago
  It's the https://en.wikipedia.org/wiki/No-hair_theorem 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.) --- > 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.
  - ars 1y ago
    > 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.
    - gus_massa 1y ago
      Simulations linked in others comment: https://www.youtube.com/watch?v=Tr1zDVbSjTM https://www.youtube.com/watch?v=Tr1zDVbSjTM https://www.youtube.com/watch?v=1agm33iEAuo https://www.youtube.com/watch?v=1agm33iEAuo
      - ars 1y ago
        The second one is much more correct, notice how it freezes before they actually merge because they time dilate out to infinity.
        gus_massa 1y ago
        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.
gus_massa 1y ago
> the "consume" question My guess is that in some popular depictions black holes are like holes, and things fall in the holes, and even a small black hole can possible fall inside a bigger hole. A better image is too drops of water on a glass, add some black ink for bonus realism. They merge into a bigger drop. Except, obviously black holes are not filled with water. And the "average density" of the new black hole is smaller then the "average density" of both original black holes, unlike the density of water drops on a glass. So don't take this image too literaly. (There are some problems to define the "density" of a black hole, but let's ignore all of them.)
Iwan-Zotow 1y ago
"The volume of a black hole" and how you define it?

hnuser123456 1y ago

They become a larger black hole, mostly conserving mass, minus a few percent to gravitational waves. However, their mass is proportional to their radius, not volume, so it gets LESS dense. If you laid out a bunch of black holes in a line, just barely not touching, and let them merge, suddenly, the whole sphere of space enclosing the line becomes black hole. It also turns out that a black hole with the mass of the universe would have a volume about the size of the universe.

JumpCrisscross 1y ago
> turns out that a black hole with the mass of the universe would have a volume about the size of the universe Mass and energy.
- gjm11 1y ago
  Is that intended to be a correction? (I don't think the original statement needs correcting, other than by replacing "universe" with "observable universe" in both places.)
  - hnuser123456 1y ago
    Up until the universe was around a few billion years old, its Schwarzchild radius would have been larger than even the co-moving (not just observable) universe's radius, but the initial momentum from the big bang was high enough to prevent collapse.
    - AnimalMuppet 1y ago
      That sounds suspiciously like "they were inside a region with enough mass to form an event horizon, but they escaped because they had enough momentum", which in turn sounds like "we can escape from inside an event horizon if we just move fast enough". Can you explain how that's not what you're saying?
      - hnuser123456 1y ago

ars 1y ago

My understanding is they just spiral into each other forever. From our point of view nothing can actually fall into a black hole, instead it time dilates into nothing. "It is true that objects that encounter the event horizon of a black hole would appear “frozen” in time"[1] So we would never actually see the black holes merge. In fact I'm not clear how a black hole can even form in the first place, since it would take an infinite amount of time to do so (again, from our POV). (And yes, I know that from the POV of the falling object, they just fall in like normal. But that doesn't help us, because we'll never see it.) [1] https://public.nrao.edu/ask/does-an-observer-see-objects-frozen-in-time-at-the-event-horizon-of-a-black-hole/ https://public.nrao.edu/ask/does-an-observer-see-objects-fro...

ajross 1y ago
This is true, but it's not an observable distinction. It's true that in some sense those two black holes "haven't yet collided", but at this point they're well past the point of last observability and have now red shifted and time dilated into the invisible background. All the interesting stuff happens before that.