A question about black holes (and others)

[LotusExcelle]

Okay so i re-watched an older Nova program on PBS last night about black holes. ("older" as in I've seen it before). And I suppose I never really thought about this before - and I'm quite sure no one has ever mentioned it to me before...

but what happens when two black holes collide? It seems to me there must be something mind bogglingly complex going on there. It also seems to me like they shouldn't be *able* to combine into a larger black hole. But that's my untrained brain telling me that. I'm sure there's math out there that explains it.

Also I had another question that relates: Are there such things as magnetic black holes? i.e. a well of electromagnetism that is an analog to a gravity black hole? How about other types of "black holes" (pick your force/particle)?

[Sock puppet]

Black holes do indeed combine to form a larger one upon collision. General Relativity indicates that this is so, and we can observe (indirectly) many supermassive black holes which, to the best of our knowledge could not have been formed without black hole mergers. The universe is not old enough for them to be so big otherwise.

As for a 'magnetic black hole' or some such thing, there are a few problems:

Electric black hole: Electric charges come in two opposite flavours, so if you have an enormous concentration of (for example) negative charge the charge itself will act to break your object up. It will also act to attract any nearby positive charges and neutralise the excess.

Magnetic black hole: We don't know if you can get magnetic monopoles yet, and without them, your magnetic black hole will eventually be neutralised by the addition of more magnetic dipoles. I have no idea how a magnetic monopole would behave.

Strong black hole: Well, the strong force is attractive but short ranged, and you only get it from baryons, which limits you to protons (stable, but positively charged- giving rise to the electric charge problems), neutrons (neutral, but only last ~15 minutes free - they need proximity to other strong charges to be stable) and other particles which have a lifetime shorter than a blink.

Weak black hole: I have no idea.

Colour charge black hole:similar problems to the electric one, even assuming there exists anything with colour charge which is stable when unbound.

The key point for most of them is that forces for which there are opposite charges which repel their own kind and attract the other work to cancel out any imbalance. The reason we can have gravitational black holes is that as gravity attracts more matter, that adds to the attractive force rather than reducing it.

[NEOWatcher]

There's quite a bit of discussion on this board about black hole mergers, but I won't point you to the latest one, since it's rather argumentative.


As for a 'magnetic black hole' or some such thing, there are a few problems:
Even simpler (maybe, I'm not sure). How does light react with the other charges? Would it even fit the definition of a black hole?

I know there's some strong EM in a black hole jet (saw it on the same Nova last night). So, somehow it is involved anyway.

[Frog march]

It could be that they just appear to merge, and that they are really still separate but squashed together- I'm not sure really.

I wonder what test could be done to determine if a giant black hole was one or many.
Maybe there would be more hawking radiation where they squash together.

If, whilst two black holes were merging and the event horizon formed the surface shape of a dumb bell, what is the matter that is falling into the black hole, at the neck, being attracted to?

[LotusExcelle]

Frog - See that's kind of what I was getting at. How *can* they combine? I mean I'm sure they can but I can also imagine that they just physically can't ever combine.

[NEOWatcher]

I wonder what test could be done to determine if a giant black hole was one or many.
Maybe there would be more hawking radiation where they squash together.
Wouldn't there be some gravitational imbalances outside the EH in this case? or would it just be too weak to see the effect on orbiting bodies.

[grant hutchison]

Frog - See that's kind of what I was getting at. How *can* they combine? I mean I'm sure they can but I can also imagine that they just physically can't ever combine.This problem with imagination seems to come about because people tend to think of event horizons as if they were material objects. But they're just significant coordinates in the curved spacetime surrounding each black hole. As two black holes approach each other their gravitational effects combine, and the event horizon surfaces simply reach out towards each other and merge into one distorted event horizon, which then settles into a spherical shape.
The general relativity for this is well worked out, and the situation of black hole mergers has been simulated numerically.
Frog march's "squashed together" scenario is just speculation in complete opposition to the mathematics, I'm afraid.

Grant Hutchison

[grant hutchison]

Here (http://www.bautforum.com/1315882-post25.html) is one of my posts from a previous black hole thread, linking to some relevant science and animations.

Grant Hutchison

[Frog march]

This problem with imagination seems to come about because people tend to think of event horizons as if they were material objects. But they're just significant coordinates in the curved spacetime surrounding each black hole. As two black holes approach each other their gravitational effects combine, and the event horizon surfaces simply reach out towards each other and merge into one distorted event horizon, which then settles into a spherical shape.
The general relativity for this is well worked out, and the situation of black hole mergers has been simulated numerically.
Frog march's "squashed together" scenario is just speculation in complete opposition to the mathematics, I'm afraid.

Grant Hutchison

but can you rely on a coordinate system that contains a singularity?

[Lepton]

This problem with imagination seems to come about because people tend to think of event horizons as if they were material objects. But they're just significant coordinates in the curved spacetime surrounding each black hole. As two black holes approach each other their gravitational effects combine, and the event horizon surfaces simply reach out towards each other and merge into one distorted event horizon, which then settles into a spherical shape.
The general relativity for this is well worked out, and the situation of black hole mergers has been simulated numerically.
Frog march's "squashed together" scenario is just speculation in complete opposition to the mathematics, I'm afraid.

Grant Hutchison

I tend to look at an event horizon as a simple point of no return.

[Tensor]

but can you rely on a coordinate system that contains a singularity?

Yes, the coordinate system and the equations are valid everywhere except the singularity. Since the EH are not at the singularity(in the proper coordinates, of course), the analysis of what happens is valid.

[LotusExcelle]

Honestly I had always pictured a kind of legrange point setup occurring of high complexity. In particular I'm picturing, say, several black holes at once doing a kind of Kemplerer Rosette. Not with their event horizons but with the singularities - so the even horizons can essentially be any intermediate shape from torus to sphere but the singularities themselves never touch.

I'm not claiming ATM on my thoughts - it is just something I had pictured last night watching Nova. And I really appreciate Grant's links - that is basically what I was looking for.

[Frog march]

Yes, the coordinate system and the equations are valid everywhere except the singularity. Since the EH are not at the singularity(in the proper coordinates, of course), the analysis of what happens is valid.


but isn't that [dodgy analogy] saying that all denomination of a currency is valid, like 20pence, £1, £20 £50 etc, but a penny is worthless, even as a theoretical amount of money?[/dodgy analogy]

[Lepton]

but isn't that [dodgy analogy] saying that all denomination of a currency is valid, like 20pence, £1, £20 £50 etc, but a penny is worthless, even as a theoretical amount of money?[/dodgy analogy]

No. The equations were not written to deal with extreme space-time curvature and near infinite gravity much like the penny in your example is not meant to deal with pounds.

[grant hutchison]

but isn't that [dodgy analogy] saying that all denomination of a currency is valid, like 20pence, £1, £20 £50 etc, but a penny is worthless, even as a theoretical amount of money?[/dodgy analogy]More like saying that there's nothing sensible you can do with a bank-note for an infinite number of pounds.

Grant Hutchison

[grant hutchison]

Honestly I had always pictured a kind of legrange point setup occurring of high complexity. In particular I'm picturing, say, several black holes at once doing a kind of Kemplerer Rosette. Not with their event horizons but with the singularities - so the even horizons can essentially be any intermediate shape from torus to sphere but the singularities themselves never touch.Yes, it seems like it should work in Newtonian terms, but orbits in the near vicinity of the event horizon are unstable, so you can't produce a stable system of black holes with "touching" event horizons. There's also the matter of gravitational radiation, which makes the orbits of black hole binaries decay very quickly indeed as they get closer to each other.
So even if you could momentarily create such an arrangement, it would quickly "ring down" into one big stable black hole, with a massive release of energy in the form of gravitational waves.

Grant Hutchison

[Grey]

Honestly I had always pictured a kind of legrange point setup occurring of high complexity. In particular I'm picturing, say, several black holes at once doing a kind of Kemplerer Rosette. Not with their event horizons but with the singularities - so the even horizons can essentially be any intermediate shape from torus to sphere but the singularities themselves never touch.Grant is right about this. But also, it's good to remember that the radius of an event horizon scales linearly with the mass contained therein. That means that (if we ignore the distortion in the event horizons caused by the close proximity of the other black hole) if two black holes get close enough that their event horizons would be touching, they have already merged into a single larger black hole.

[mugaliens]

So even if you could momentarily create such an arrangement, it would quickly "ring down" into one big stable black hole, with a massive release of energy in the form of gravitational waves.

Grant Hutchison

Now that would make for an interesng scene in a Star Trek flick, with the ever-increasing crescendo and frequency of gravimental pertubations as their orbits decayed until they became one.

That's the sort of event that Deep Note (http://en.wikipedia.org/wiki/Deep_Note)was made for!

[tommac]

Is there any way that two black holes that came close to a collision could escape afterwards as seperate entities?



Yes, it seems like it should work in Newtonian terms, but orbits in the near vicinity of the event horizon are unstable, so you can't produce a stable system of black holes with "touching" event horizons. There's also the matter of gravitational radiation, which makes the orbits of black hole binaries decay very quickly indeed as they get closer to each other.
So even if you could momentarily create such an arrangement, it would quickly "ring down" into one big stable black hole, with a massive release of energy in the form of gravitational waves.

Grant Hutchison

[Grey]

This thread is over two years old, tommac, and grant hasn't been around for a while. I linked to it to provide some background discussion, but I'm not sure there's any point to asking any of the thread participants to respond. As for an answer, the answer is "it depends on what you mean by close". If the event horizons touch, not a chance. Even well before that, I expect that collision is inevitable. But certainly there's some closest point that they could come and still avoid a merger. It might take a fair amount of calculation to determine that, and it might be pretty sensitive to the kinematic details.

[Tensor]

As for an answer, the answer is "it depends on what you mean by close".

Ya know, I saw the question, was going to answer "Define close", but then my wife needed the computer. When I got back, you had already answered.

[tusenfem]

Let't not move this discussion into two threads, keep it in the S&T thread please.
closed