# Two black holes and the event horizon

1. Apr 21, 2004

### ExecNight

[SOLVED] Two black holes and the event horizon..

What happens if a blackhole enters another blackhole's event horizon??

I really wonder can matter escape by the gravitational chaos??

I asked this question to the scientist in NASA haven't recieved the answer yet..But this is a question we can only answer by maths which we can't use inside a black hole..So this means we can never know??

2. Apr 21, 2004

### Labguy

Your first sentence implies that one BH is larger than the other, so the smaller one "enters". In this case the two would merge into a more massive BH with somewhat less than the combined masses. Same would be true for any two colliding BH's, similar size or not. There have been several recent observations indicating merging Black Holes.

There would be a large gravitational effect by an output of "gravity waves" like the LIGO & LISA experiments are set to detect, and there would be a large Gravitomagnetism effect that would affect any nearby objects.

Matter would not (directly) escape the original or newly-formed, and larger, event horizon, but the combined angular momentum along with the larger magnetic field would certainly produce more "virtual particle" pairs which could, and do, become real particles by way of energy obtained from the magnetic field. See: https://www.physicsforums.com/showthread.php?t=17594 below for more on this effect.

But, all the detectable effects, and mass loss, would be observable from outside the event horizon as with any BH now, so the "unknown math" of the state(s) inside the event horizon would be no more or less necessary to understand than we do today. That said, two solitary and non-accreting black holes would be rare, and a collision of two even more rare. About all you can find published today about BH mergers deals with colliding galaxies and with binary (accreting) BH's.

3. Apr 21, 2004

### Labguy

Ok, I just watched some TV and pondered my last post, and rather than do an "edit" decided to post a seperate message here.

My post above has one significant mistake, and I wanted to see (know) who would catch it and make a correction or contest that particular point. Someone point out the obvious mistake. Marcus? Anyone?

4. Apr 21, 2004

### chroot

Staff Emeritus
The mass of the resulting black hole must be greater than or equal to the sum of the masses of its progenitors, not less than.

- Warren

5. Apr 22, 2004

### ExecNight

I am not so sure about "they would just combine.." statement..

Especially when you think about a specific situation where the masses of two black holes are equal with different vectoral speed..There is no reason for the black holes to stay inside one of their event horizon..At least i see no mathematical reasons at all..

Thats where "can i trust math at that point?" comes out..

Besides the masses don't need to be equal anyway as the angular momentum would have a great effect in that situation..

Besides i don't care about virtual particles..No offense i just want to talk about matter that can leave black holes..

We even know sometimes a black hole releases matter into space..Do we know why does it happen?

6. Apr 22, 2004

### Labguy

That's about it. The virtual particle production process I mentioned does happen, but all of the energy comes from the magnetic field and not from a mass-loss of the BH as it does by Hawking radiation.

However, the original example (ExecNight's) seemed to be about two single, non-binary and non-accreting black holes, so the only mass available would be the original mass of the two BH's.

First of all, the vectoral speed would, for sure, be huge if two BH's were on a collision course. But, you did say "entering the event horizon" and not just a near encounter. By definition, the EH is that distance from which nothing can escape, where the gravitational attraction is the same as the escape velocity of light. So, if event horizons were "breeched" any part of one BH would merge with the other with nothing escaping. That "nothing" would be whatever we would consider the "interior" of a BH whether we can define it or not. And, with two black holes, the merging would, in effect, go "both ways" as one EH encountered the other. Also, the new and larger combined mass would accellerate the process to something near instantanious.

The angular momentum would increase the frame dragging and the magnetic field for sure, but that shouldn't have any affect on the mass that I can see.

If you want to talk about matter leaving a black hole, then you have to "care" about virtual particles, because that's how it is done. Hawking radiation can account for that, and recent past threads cover Hawking radiation to death and back. Check back just a month or two and you'll find about all you would want to read about Hawking radiation and alternate Hawking "processes". All that matter detected leaving BH's as jets at velocities near c are just accretion matter being ejected from the poles by the magnetic field and are not a mass loss to the BH at all.

7. Apr 22, 2004

### ExecNight

The Theory suggests that noting escapes the event horizon for sure.. However i m talking about maths not theories,Therefore I don't understand why one of the black holes can not escape the others event horizon..

8. Apr 22, 2004

### Labguy

Rs = 2GM/c2.

That's about all I've got. Nothing can exceed c and the event horizon, defined by the simple formula, is that imaginary or "mathematical" area of influence. Your questions seem (I could be wrong) to indicate that a black hole is something more mysterious than just another form of gravitationally collapsed matter. Some people believe in a point singularity, others believe in a larger area of "quark plasma" or whatever, but the fact is that the total mass of the BH is in there somewhere in some as-yet unknown form, but it is still there and there is no process I have heard of that could cause it (matter) to "escape".

Your question about why a BH can't escape an event horizon is no different than asking why a 2000 pound rock wouldn't escape. The black hole and the rock both have a defined mass.

9. Apr 22, 2004

### Tail

That nothing escapes the event horizon is PRECISELY the reason why one of the black holes can't escape another's event horizon. Nothing can.

The only way for a black hole to get smaller is Hawking radiation. Essentially, nothing comes out of the black hole just virtual particles with negative energy go in.

10. Apr 22, 2004

### ExecNight

I see that you are suggesting it would only have an effect like adding up two vectors noting more..Well according to that formula(Thanks) ../c^2 that looks true...

I guess there is a difference between a Rock and a black hole while both black holes inside the event horizon will have the same mechanics which we don't know exactly..Whatever is inside the event horizon have to affect each other in some way..

The momentum and the total energy of the black hole can't just dissappear..First of all (talking with logic) there needs to be a gravitational chaos inside event horizon which would screw the formula you gave(Not so sure just trying to make a debate here =) ) I am not suggesting an everlasting chaos..I am just wondering if any matter can escape while this chaos occurs until a balance is set..

You tell me that something will change for both blackholes..While the energy we talk about here is massive and we know that the change won't happen in nanoseconds there might something unpredicted happen..

Besides i know we are talking about things that can never be proven or against current theories..Just asking in the name of Brainstorming who knows maybe the theories are wrong (Thats a very low probability but who cares :P )

11. Apr 25, 2004

### Stingray

Nobody knows exactly what would happen here. The problem is very hard, and many people are working on it, but they are rather far from a complete solution (special cases have been done though).

That being said, the event horizon is a terrible measure in a dynamic process like this one. I think you are conceptualizing it incorrectly. The event horizon is not a fixed object around a black hole, nor would it be given by a superposition of two spheres in this case. It is moved and distorted in an extremely unintuitive way (event horizons can form in flat space!). If you think about it, it is very hard to even formulate your question properly.

If you still want to talk about event horizons, then by definition an inside one will never leave (if it could, then it wouldn't be an event horizon). It is a little unclear exactly what that would mean for a black hole.

Also, don't think of energy or mass conservation in GR. Its a very subtle subject which does not work the way you would expect. For example, you can't say that each hole has a given fixed mass.

Sorry this post will probably confuse you more, but it is a difficult question, so you have to be careful in how its asked.

12. Apr 30, 2004

### ExecNight

Why everyone is so firm on " Noting leaves event horizon"..I don't understand that..

First is it formulated that inside event horizon the gravitational force is infinite? if not why is it not possible to leave?

13. Apr 30, 2004

### chroot

Staff Emeritus
The escape velocity at the event horizon is c, the speed of light. An object would have to go the speed of light to escape at the event horizon. Inside the event horizon, an object would have to travel faster than light. Modern physical models preclude anything ever going faster than light, thus nothing can ever get out of a black hole.

The gravitational force inside a black hole's event horizon is not infinite; it's just a matter of the velocity that would be needed to escape it.

- Warren

14. Apr 30, 2004

### Njorl

Consider 2 BHs of equal mass and opposite angular momentum. They approach eachother. As they become close, their event horizons become deformed, flattenting on the sides that will eventually contact. The gravitational field between them decreases, indeed, there is a point bisecting their seperation with zero gravity.

So, while matter can't escape the event horizon, can the event horizon deform inward toward the center of the BH, releasing matter?

My first impression, based on intuition, is that it should. Think of it. The event horizons are being pushed toward the center of the BH while the matter/light is being pulled toward the other BH more strongly.

I thought this was just all wrong until I considered another possibility. The matter/light that escapes in this way is probably doomed to recapture as the two halves collide.

So the the sequence would look like this:

1. Two BHs of equal mass, approach eachother. Their event horizons are barely deformed from spherical.

2. They become very close, their event horizons are deformed significantly, flattening on the side of their approach, elongating on their outer sides.

3. Matter and light is liberated in the region between the two event horizons.

4. The two event horizons meet, recapturing all the matter and light that was temporarily liberated.

Just idle musing.

Njorl

15. Apr 30, 2004

### jcsd

As Chroot says the force is finite within the vent horizon but the force required to have a 'staionery' (using the word very cautiously asin this situation it doesn't mean alot, though I think you know what I mean) object outside the event horizon tends to infinity as r -> Rs and inside the event horizon actually becomes imaginery, the obvious conclusion being that you can't be 'stationey' at or inside the event horizon.

When two black holes coalesce there is more than one factor governing the mass of the resulting black hole. The most obvious factor is the mass of the two original black holes which blantantly the sum of which it cannot exceed, the other main factor though is the surface area of the event horizons of the two black holes which the sum of which it must exceed (due to thermodynamics)

So if you have two black holes of masses M1 and M2 which coalesce the mass of the new black hole M3 must lie some where in the range of:

$$\frac{16\pi G^2}{c^4}({M_1}^2 + {M_2} ^2) < M_3 < M_1 + M_2$$

Tgis does allow in many situations a fair percentage of the mass to be shed, this is lost in the form of a gravitational wave.

Last edited: Apr 30, 2004
16. Apr 30, 2004

### LURCH

Will they? And if so, why?

The remainder of your post seems dependent upon this assumption, but I would think that the event horizons would deform toward one another.

17. Apr 30, 2004

### Njorl

I am assuming event horizons are dependent on gravitational fields. Within the event horizon, the field is strong enough to prevent light's escape, outside the horizon, it is not strong enough. The fields between the two BHs weaken as they approach each other. They tend to canel each other.

Should I be thinking in terms of gravitational potentials rather than fields?

Njorl

18. Apr 30, 2004

### jcsd

It has been worked out what happens when two black holes coalesce, but beyond the basics as posted in my last post I wouldn't like to work out what's going on as it's guarenteed to be horrendously complex. Anyqwya here's and animation of two black holes merging (there are loadfs of simlair animations all over the web):

http://chandra.harvard.edu/photo/2002/0192/animations.html

19. Apr 30, 2004

### ExecNight

Very nice animation but not long ago a star passed close to a black hole's event horizon..But it only lost a little of its mass and had some degrees of change in its course noting more..It was barely affected by the black holes gravitation..

I am gonna sleep now,gonna try to find the link tomorrow....

I guess that showed the scientists how important the momentum is..And how they exaggerate the gravitational force of the black holes..

20. Apr 30, 2004

### jcsd

I don't get yor point, it's entirely dependent on the trajectory it's mass, the mass of the black hole, etc. Your referring to a specific not a general situation.