Black Hole Collisions: What Happens?

[Pjpic]

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

 

[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.

 

[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).

 

[DaveC426913]

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).

That is correct. Was there anything that said otherwise?

 

[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.

 

[DaveC426913]

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.

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.

 

[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.

 

[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.

 

[DaveC426913]

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.

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?

 

[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).

 

[James Leighe]

*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).

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.

 

[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.

 

[James Leighe]

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.

The reason people say you can't 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.

 

[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.

 

[Arch2008]

Here's how they collide:
http://www.space.com/scienceastronomy/blackhole_collide_020801.html

Not a clear answer:
http://www.space.com/scienceastronomy/astronomy/blackhole_merges_020208.html

They don't always merge:
http://www.universetoday.com/2008/02/29/what-happens-when-supermassive-black-holes-collide/

 

[sylas]

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.

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

 

[James Leighe]

words

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.

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?

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

 

[Pjpic]

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

 

[sylas]

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

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

 

[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?

 

[sylas]

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?

Any particle takes a finite proper time to proceed from the event horizon to the singularity. It makes no difference whether the falling particle is a black hole or not; the time is the same.

 

[DaveC426913]

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?).

Yes. The mass of the final BH is equal to the sum of the masses of the two intial BHs.

Note that the gravitational attraction as experienced by the rest of the universe has not changed. The rest of the universe neither knows nor cares if it's two or suddenly one BH.

Does this mean that the event horizon expands? If so, I would imagine that more objects become subject to the black hole's gravity,

The event horizon expands, yes. But the event horizon is not a "thing". And it is definitely not the "thing" that is doing the attracting. It is the mass that is doing the attracting.


BHs do not have any special powers. They attract objects via gravity just like any other body.

If you had two massive stars and they coalesced, the final stars would not have any more attractive power than the sum of the two initial stars. There is no runaway effect. Same with BHs.

Is my flaw in my understanding of the expansion of the event horizon

Yes. You are thinking of the event horizon as if it is the boundary of the BH - as if things behave differently depending on which side of the boundary you are on.

That would be like saying the terminator line (the line between night and day on Earth) is a boundary that controls night and day. It isn't. It isn't a cause of anything, it is simply an effect, a label.

The key is to recognize that attractive force is caused by the presence of mass. Period. And the sum of the mass of two small BHs or one large BH does not change.

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.

These are extremely insightful and inquisitive questions. The kind PF members fight over to be the first to answer.

 

[Ich]

The mass of the final BH is equal to the sum of the masses of the two intial BHs.

Actually, it's somewhere between $$\sqrt{m_1^2+m_2^2} < M < m_1+m_2$$.

 

[George Jones]

Actually, it's somewhere between $$\sqrt{m_1^2+m_2^2} < M < m_1+m_2$$.

As two back holes merge, gravitational radiation is emitted that carries away energy; hence, the right inequality. The left inequality is a constraint imposed by the second law of thermodynamics.

 

[Pjpic]

Any particle takes a finite proper time to proceed from the event horizon to the singularity. It makes no difference whether the falling particle is a black hole or not; the time is the same.

As the microscopic BH falls into the massive BH, is the microscopic BH subject to the "spagetti effect"? Are the particles that were (before the horizons merged) falling into the microscopic BH still falling to its central mass as the micro BH falls to the center of the massive BH?

 

[DaveC426913]

As the microscopic BH falls into the massive BH, is the microscopic BH subject to the "spagetti effect"?

Well, the singularity is theoretically a point so it's not going to "stretch".

 

[Dmitry67]

Whats about a ring singularity?

P.S.
I strongly believe that there are much more "what if" questions than anybody can answer analytically, but it would be nice to have a good simulator (with light tracing feature so we can not only calculate what happens, but would be able to calculate what observer would see)

I would I've my right hand to know:
1. how it would look like if I orbited around the ring singularity in CTL zone.
2. if we can artificially construct a super-extreme BH

 

[pzona]

DaveC-
Thanks for the response. I have another question regarding the gravitational field of the black hole. If the mass increases (say, from merging with another black hole), doesn't that increase the strength of the gravitational field as a whole, relative to the distance from the black hole? For instance, if a star was x distance from the black hole, and the gravitational field was some arbitrarily small measure of force from overcoming the star's inertia and "sucking it in," and the BH's mass increased, wouldn't this mean that the gravitational strength at x distance would increase? And if so, wouldn't the star be drawn into the black hole, increasing its mass and thus expanding its gravitational field (relative to distance of course) in the same way again? This is what I meant by a runaway effect.

 

[Jonathan Scott]

DaveC-
Thanks for the response. I have another question regarding the gravitational field of the black hole. If the mass increases (say, from merging with another black hole), doesn't that increase the strength of the gravitational field as a whole, relative to the distance from the black hole? For instance, if a star was x distance from the black hole, and the gravitational field was some arbitrarily small measure of force from overcoming the star's inertia and "sucking it in," and the BH's mass increased, wouldn't this mean that the gravitational strength at x distance would increase? And if so, wouldn't the star be drawn into the black hole, increasing its mass and thus expanding its gravitational field (relative to distance of course) in the same way again? This is what I meant by a runaway effect.

From a sufficient distance to avoid tidal effects, the overall average gravitational field of any combination of massive objects is constant, regardless of how they are combined and whether any of them are black holes. If a new massive object arrives, then that increases the overall gravitational field when it arrives (and decreases it if it departs again), but no change occurs as a result of internal changes of configuration, such as collisions or objects falling into black holes.

 

[the pro]

There are many different proprosals for resolving this puzzle, but all of the suggestions have problems of their own. Some say that information is lost, but for some reason the effects are highly suppressed at scales we can observe. Another proprosal is that black hole evaporation ends with a massive remnant, and that all the information about how the black hole was formed is somehow stuffed into this single Planck scale object. Another suggestion is that when an object crosses the black hole's event horizon, information about it is somehow transferred to the outgoing Hawking radiation. The difficulty here is that to an object falling into the black hole, there is nothing special about the horizon - it looks just like everywhere else, aside from some tidal forces which will be small for a large enough black hole. Once it is inside the event horizon, there is no way that information about it could escape without going faster than light. Ultimately, no one has been able to come up with a completely satisfactory answer.

All the evidence suggests that physics is completely deterministic at some level. We may not be able to actually collect all the data we need to make accurate predictions, but the information is there nonetheless. Therefore, the loss of information in black hole evaporation presents a puzzle - is the information actually being lost? If it is, how can we reconcile this with the deterministic world we observe? If it is not, how does the information escape the black hole?



http://images.google.com/imgres?img...aporation&hl=en&rlz=1T4ADBF_enRS318&sa=X&um=1

 

[James Leighe]

DaveC-
Thanks for the response. I have another question regarding the gravitational field of the black hole. If the mass increases (say, from merging with another black hole), doesn't that increase the strength of the gravitational field as a whole, relative to the distance from the black hole? For instance, if a star was x distance from the black hole, and the gravitational field was some arbitrarily small measure of force from overcoming the star's inertia and "sucking it in," and the BH's mass increased, wouldn't this mean that the gravitational strength at x distance would increase? And if so, wouldn't the star be drawn into the black hole, increasing its mass and thus expanding its gravitational field (relative to distance of course) in the same way again? This is what I meant by a runaway effect.

First of all you have been answered in many different ways so make sure you read all of what we have said and ask specific questions as to what you did not understand about our explanations.

I'll answer it again, the answer is no.

Think about it like this, I have two peas next to each other and a meter away from an orange all sitting on a table. The orange feels some small gravitational effect from each pea, and since the peas are next to each other you can say, "The orange is feeling two peas worth of gravity in blah direction (the direction of the peas)". Now, mash the peas together, the orange still only feels the attraction of two peas. There is no extra gravity.

The peas can be distant black holes in our example and the orange can be any massive body.

Also, the field is not 'expanded' as I have explained in my previous post.

 

[pzona]

Alright I think I understand. I was completely ignoring the fact that each black hole exerts its own gravitational force; I was only considering one black hole, and ignoring the other one until the point at which they merge. A pretty big error, I admit. I haven't taken a physics course since junior year of high school, so I'm a little (actually a lot) rusty on it. Thanks to everyone for your patience.