# Black holes collide

1. Sep 29, 2009

### Pjpic

When two black holes collide, do the singularities orbit each other, does one evaporate into the other, or is something else happening.

2. Sep 29, 2009

### DaveC426913

I do believe that, until they are in close contact, they act like normal massive bodies, i.e. they orbit each other. I don't see why, when they coaleasce, it would be any different than any other bodies coaleascing.

3. Sep 29, 2009

### Pjpic

If two singularities coaleasced wouldn't you end up with only one singularity? But I was under the impression (quite likely wrong) that, once the singularity formed, it was permanent (nothing can escape from it).

4. Sep 29, 2009

### DaveC426913

That is correct. Was there anything that said otherwise?

5. Sep 29, 2009

### Pjpic

If you start out with two singularities (niether of which can be destroyed), how can they coaleasce into one singularitiy without one of them no longer being a singularity? Whereas it is easy to see how two drops of water can coaleasce into one drop.

6. Sep 29, 2009

### DaveC426913

Your logic doesn't make sense; it is full of preconceptions.

- Who, other than you, said BHs cannot be destroyed? A BH can evaporate. That is not the same thing as "nothing can escape from it".

- What does being destroyed or not have to do with coalescing?

You have two objects, attracted by gravity. They coalesce until their masses are now one mass. What is confusing you?

If you want, you can think of two BHs of greatly differing sizes. a tiny BH falls into a supermassive BH, just like any other mass does.

7. Sep 29, 2009

### Pjpic

I must have misunderstood the posting that began with "that is correct". This misunderstanding makes the issue, about how an inviolate object could lose its individuality in merging with another object, moot and the second option of the original post to be the, relatively, most accurate.

8. Sep 29, 2009

### James Leighe

Sounds like someone needs a little lesson on black holes.

*Black holes evaporate (slowly unless they are very small) through what is called hawking radiation, so singularities are NOT permanent.

*When two black holes COLLIDE (not orbit or anything) the two singularities will eventually merge into one.

*Nothing can escape the EVENT HORIZON, as for a singularity, we don't know the first thing about them, or even if they exist, if a singularity has (any) finite size it would not have infinite curvature and therefor not be a singularity. But we can say that there is probably some pretty strong gravity going on.

*Black holes are not super mega special (well, ok, a little special). They are areas where the force of their own massive gravity can overcome matters ability to keep away from other matter. That's about it.

9. Sep 29, 2009

### DaveC426913

One does not evaporate into the other. Evaporate is a term with a particular meaning when it comes to BHs. A lone BH, if given enough time (a LOT of time) can, in theory, literally evaporate - it loses mass until it disappears. How that happens is another story, which we can address momentarily.

For now, let's just establish that two BHs can combine to form one larger BH. You would not say that one star 'evaporates' into another would you?

10. Sep 30, 2009

### Pjpic

*When two black holes COLLIDE (not orbit or anything) the two singularities will eventually merge into one.

This sounds to me like 1 +1 =1. Is one of the black holes being absorbed by the other? As if they were two hurricanes colliding and one eye gets disorganized (destroyed) while the other eye just gets stronger. Or is it as if both eyes lose their round shape before forming one new center. In either case wouldn't at least one of them be losing their curvature (which I thought I heard was impossible - except perhaps through evaporation).

11. Sep 30, 2009

### James Leighe

Pretty much, 1 black hole + 1 black hole = 1 LARGER black hole. This should not be difficult to grasp if you are reading what Dave is saying.

This does not work like you think hurricanes work. One black hole does not lose it's 'curvature', but rather the 'curvatures' of the two holes is summed to create a stronger black hole. Black holes do not behave any different than any other massive object in this sense. When two black holes collide you can think of it as two dense stars colliding, they just come together to form one... If you don't understand this then there is no chance of you understanding anything we say apparently.

12. Sep 30, 2009

### Pjpic

I think the misunderstanding comes from the "nothing can excape the event horizon" and the "singularity" descriptions. I must be conflating these into the 'singularitiy can't excape' , which it apparently can if two central gravity points are changed into one central point. As when a meteorite lands on earth.

13. Sep 30, 2009

### James Leighe

The reason people say you cant escape is because there is so much gravity at the point that we call the 'event horizon' that light becomes inexorably pulled toward the center. This is not some special unbreakable shell, if you send matter into a black hole it will grow. If you send a black hole into another black hole it will grow. The singularity from one black hole does not leave the 'event horizon' to go into the other black hole, since the event horizon is an effect of the 'singularity' itself, rather, they simply get sucked in together and coalesce into one singularity. Think of two spheres merging to become one.

If you are having trouble with the 'singularity' idea, then imagine there is no singularity but rather two highly dense objects. This is how it works as far as we know.

14. Sep 30, 2009

### pzona

Astrophysics really isn't my strong area, but this caught my interest. When you say the two black holes merge to create a larger black hole, I assume you mean larger in terms of mass, since black hole refers to a singularity (right so far?). Does this mean that the event horizon expands? If so, I would imagine that more objects become subject to the black hole's gravity, which increases the mass further, which in turn expands the event horizon even more, which creates a cycle. This cycle (as I see it anyway), should continue until the universe is reduced to a singularity. This doesn't make sense to me (for good reason I'm sure). Is my flaw in my understanding of the expansion of the event horizon (if it expands at all), or is it in my lack of consideration for the relativistic effects inside the event horizon?

I apologize in advance if these are dumb questions. If they are, let me know and I'll read more on black holes before posting in this section again.

15. Sep 30, 2009

### Arch2008

16. Sep 30, 2009

### sylas

Nothing can pass out of the event horizon. But when two holes collide, each one is moving INTO the event horizon of the other. Nothing escapes from either event horizon.

Suppose you have a black hole, with an event horizon with a 3km radius. (That's a small black hole, with about one solar mass.) Anything within 3km of the hole, therefore, cannot ever escape, and is bound to end up rapidly at the singularity in the center of the hole. Physics runs into strife describing the singularity. Describing the event horizon is well within existing physics.

Have two such holes approach each other. Once the distance between them is less than 3km, they are both inside the horizon of the other; and most likely before that, as the horizons merge. And they'll never get out. The two singular central concentrations of mass will rapidly approach and become a single singularity. You now have one hole, and it will end up with a single event horizon with radius about 6km, since it now has twice the mass at the central singularity.

On the other hand, they might not actually collide. The merger is inevitable once they get within each others' event horizon, but if they remain at a safe distance, there can be ways to get a kick as described in one of the links above. This apparently requires rotating black holes, which would have some energy from the rotation. This can be transformed into a kinetic energy sufficient for the orbits to be kicked up into a hyperbolic escape orbit. There is still nothing passing out of an event horizon in this case.

Felicitations -- sylas

17. Sep 30, 2009

### James Leighe

Snap, good point about the not always merging! I'm assuming this is because of the ergo-sphere (frame dragging) interactions keeping them from getting TOO close, but if one of the singularities crosses into the event horizon of another they will definitely merge AFAIK.

Yes, the event horizons expansion is the only significant effect of adding mass (or another black hole, as it has allot of mass).

And here are a few reasons why black holes don't devour the whole universe even though they get 'stronger' with more mass:

Gravity drops off at an exponential rate, but GR tells us that it never becomes exactly zero even at a ridiculous distance (as long as it has had time to get that far in the first place as gravity travels at the speed of light). So increasing the mass of a black hole will not extend its reach (it's reach is infinite, just as the gravitational attraction of a pea on earth is felt on the sun, the stars, etc.), it's just that it becomes stronger! But not so much stronger that everything stops what it's doing and changes course toward the black hole, since closer objects with less mass (by virtue that they are much closer) are still the dominant force of gravity. Just as the black hole in the center of our galaxy does not pull the earth from the sun (and so on) as the suns gravity is much stronger on the earth since the black hole is sooo far away.

Starting to get it?

EDIT: somewhat beaten

18. Sep 30, 2009

### Pjpic

Could you tear apart a black hole if you put it between two other larger black holes?

19. Sep 30, 2009

### sylas

No. You can't tear apart black holes with anything.

20. Sep 30, 2009

### Pjpic

When a massive black hole and a microscopic black hole merge event horizons, are both central masses immediately considered to be combined into one or does the smaller one have to fall for a while?