Can two black holes cancel each other out?

[Aroddo]

I have a question:

The earth and the moon are both close enough so that the gravity field of the moon causes tides on earth.

So, can two black holes orbit each other close enough that their gravity fields influence each other strong enough for light to escape, effectively removing the event horizon on the sides facing each other?

And further, since black holes are ultra dense compressed matter, will the matter try to expand and escape, resulting in some kind of black hole nova?
And if so, what kind of matter will be expelled by such an explosion?

 

[uppi]

Now I haven't done the General Relativity calculations (and I don't think I'd be able to), but I would say that the only thing that could happen would be light (or matter) escaping from one black hole to the other black hole. But I don't think that light could escape from the system. So you would get a shared event horizon, which would mean that it would behave like a single (strangely shaped) black hole.

The matter in a black hole should be far too compressed for any expansion to happen. If light cannot escape, why should anything else? In a black hole gravity dominates everything.

 

[Aroddo]

Now I haven't done the General Relativity calculations (and I don't think I'd be able to), but I would say that the only thing that could happen would be light (or matter) escaping from one black hole to the other black hole. But I don't think that light could escape from the system. So you would get a shared event horizon, which would mean that it would behave like a single (strangely shaped) black hole.

The matter in a black hole should be far too compressed for any expansion to happen. If light cannot escape, why should anything else? In a black hole gravity dominates everything.

yes, but if two black holes orbit each other close enough (and at a breakneck speed) to remove the event horizon between them, then gravity might even be low enough to allow matter to escape. the bh will probably be rotating as well as orbiting very fast and the coriolis force must be incredible.
now locally reduce the gravity that held everything in check and i'd imagine matter would move.

 

[Manco Capac]

Hmmm...I vaguely remember what I read, but in a nutshell...

The two black holes move like a binary system of stars...along a ellipse trajectory.

But they tend to get close by time and this creates gravitational waves until they colludes and this creates a gigantic gravitational wave.

Aftermath, bigger black hole.

Matter escapes from black holes? I think yes. It has to do Hawking rays I think. Those diametrical opposive beams each side consisting of X-rays and mainly gamma rays.

My, my mind is blurry.

 

[Cutlass]

I would think it would be unstable and the weaker would rip apart and merge with the stronger.

 

[Gigaz]

Thats roughly true. When two black holes are close enough to each other, no more stable orbit exists. The black holes usually have deformed event horizons before they merge but the idea that there might be a stable path in between where the gravitation cancels out is definitely not what I would expect from general relativity.

 

[uppi]

yes, but if two black holes orbit each other close enough (and at a breakneck speed) to remove the event horizon between them, then gravity might even be low enough to allow matter to escape. the bh will probably be rotating as well as orbiting very fast and the coriolis force must be incredible.
now locally reduce the gravity that held everything in check and i'd imagine matter would move.

No. Locally the force might be weaker, but in total you have to consider the energy needed to escape the black holes. And if we ignore GR and rely on classical arguments, this energy is the sum of the energies needed to escape the single black holes. So putting two black holes close together (if we assume for the moment that this would be stable long enough) would actually make matter less likely to escape.

On the axis between the two black holes there will be a (unstable) point where the force is zero. But when you try to move away from that point towards freedom you have to overcome the attraction of both black holes.

 

[kix27145]

i doubt it.

 

[GoodGame]

Wouldn't two black holes adjacent, effectively merge into one?

What would cause them enjoy an orbit, or what not?

 

[Aroddo]

Wouldn't two black holes adjacent, effectively merge into one?

What would cause them enjoy an orbit, or what not?

in theory, given enough time and force, you could accelerate a black hole and move it on a desired orbit.

and assuming that's possible, i wonder if you could break a black hole by accelerating it to the speed of light and crashing it into a likewise accelerated black hole.

 

[wilycoyote]

1) Yes, two black holes can orbit each other. When galaxies merge, which is very frequently, their central black holes (there is some evidence that most if not all large galaxies have large black holes at their centers) fall into each other. However, if they were ever close enough to share an event horizon they would merge, as by definition, anything within their event horizons can't escape the black hole.

2) You can't accelerate anything to the speed of light that has mass, let alone something with the astronomical mass of a black hole.

3) The gravitational force of a black hole within its event horizon is strong enough to pull in mass-less photons (it is effectively an infinite force)! Thus, no known force will negate that. Matter can't be taken out of a black hole, except in the case of Hawking radiation, which is really a quantum phenomenon and not a gravitational one.

Neutron stars are very dense, and still not as dense as black holes, and they spin VERY fast, yet matter isn't just thrown off of these dead stars. That is because you are talking about degenerate matter. The rules of physics change for these objects. If electrons can't resist being crushed into a neutron star (and they can't, the electromagnetic force that normally keeps atoms apart, partially collapses, the electrons fail to repulse each other, leaving the nuclei of each atom to stop the collapse into a black hole), centrifugal forces can't possibly do anything. Well, they can do somethings, such as create ridges on the surface of the neutron star that periodically break and give off a very distinctive pattern when one observes pulsars (which are neutron stars that are positioned in such a way as to be visible from Earth.

4) Gravity falls as the reciprocal of radius squared. One black hole can never steal matter from another as anything within one is closer to the center of its black hole than to the center of the other black hole. Of course, this argument relies on classical reasoning. And black holes don't behave that way. However, the conclusion is still true.

5) Looking at a black hole would be like looking at nothingness with a ring of light around it (due to gravitational lensing). Thus, before two black holes merge, there is a short-lived stable path of cancelled out gravity between the event horizons of the black holes.

6) Black holes merge, they don't rip apart. When you have finite mass in an infinitely small radius, nothing will tear that apart (since this implies infinite gravitational attraction as well). You would see a distortion of the event horizons as the black holes came together, though.

Remember, the event horizon of a black hole doesn't represent the radius at which matter is held. The matter is in an infinity small area at the center of the black hole. The event horizon is just the radius at which light can no longer escape the attraction of the black hole.

 

[Aroddo]

are there any sings for an upper limit to black hole "size" or could a single black hole gobble up the entire universe?

 

[wilycoyote]

are there any sings for an upper limit to black hole "size" or could a single black hole gobble up the entire universe?

I have never heard of one. A black hole has already overcome all known forces that would oppose the gravitational collapse of matter. Thus, the only limit that I know of is the amount of matter (and rate of consumption of that matter to a small degree) that the black hole can consume. Also, I don't know what you mean by "size." The matter is contained in an infinitely small point of zero size within the black hole. However, if you are talking about the event horizon, it does grow with the mass of the black hole and...

1) There is only a finite amount of matter in the universe. The black hole could never be more massive than that.

2) The universe is expanding. Consequently, the matter in the universe is becoming more spread out, making the possibility of it all being pulled into one black hole impossible. Though, it was already too spread out to begin with. Also, the gravitational reach of black holes really isn't that far, not even on the scale of a galaxy. And there are an arbitrarily large number of galaxies in the universe. Furthermore, there is a large black hole at the center of the Milky Way and we are not being sucked in, rather we are orbiting the dense area in which the black hole is found.

3) Black holes "evaporate" due to Hawking radiation. Though, this is a VERY slow process.

4) The universe is basically infinitely larger than a black hole, so it could never be swallowed. Current theories predict eternal expansion of the universe, while black holes are much more compact than anything else that had as much mass.