How do black holes collide?

[levass1o]

Hi space gang,

What is weird for me is that my thoughts on the black hole collision matter crush in the very early stages:

As per special relativity theory the body gains mass with the speed and notably near the speed of light.
As per the Escape Velocity, two objects bound in a gravitational rotation -- the closer they come the more speed should they have to retain their orbits. Basically we all know the gas accreting into black hole speeds up to almost light speeds and emits...whatever possible in a process.

Having the above 2 rules in mind it is hard to imagine how can the pair of black holes collide.....especially for me as for a side observer. Because of their infinitely small sizes we can assume holes can come infinitely close to each other which will mean huge speed gains for both while at close distances. Huge speeds mean huge mass gains. Up to infinite number of times. So....according to Einstein, colliding black holes should swallow the entire galaxy if not the Universe in their infinite mass gain.
Which we don't see happening. :roll: Much more likely I have a simple mistake somewhere. Could you please guys point where and explain how the collision goes?

 

[theridane]

Ithink it goes something like this: even though singularities are - as far as we're concerned - infinitely small, the black hole itself isn't: see Schwarzschild radius. When one crosses the event horizon of the other, it's not getting out, ever. So as far as we can tell they collided.

 

[levass1o]

Ithink it goes something like this: even though singularities are - as far as we're concerned - infinitely small, the black hole itself isn't: see Schwarzschild radius. When one crosses the event horizon of the other, it's not getting out, ever. So as far as we can tell they collided.

Hi Theridane,

According to my prejudice the event horison is some radious being measured from the center of the singularity. It doesn't change things by itself. And gravitational waves are able to spread from inside the horison. So what concers me are two singularity points spiralling into each other on tremendous speeds. Not sure where will they gain relativity speeds -- inside the horison or earlier ouside but once gained the system should become extremmely massive for side observer.

Also shouldn't they blow out half of each other's horisons on approach?

 

[ramparts]

Theridane is exactly right. Even though the singularities are presumed to be infinitely small, gravity will draw them together; once they're in each other's Schwarzschild horizons, there's nowhere to go but down.

While they will be drawn to very high speeds - and this will cause a huge release of gravitational waves - they won't be drawn to move at c, so there's no reason that there would be any "infinite mass".

 

[theridane]

Would this death spin be detectable (beyond the horizon)? Are those gravity waves gonna make it out?

 

[ramparts]

Would this death spin be detectable (beyond the horizon)? Are those gravity waves gonna make it out?

Yeah, that's a really good question. Gravity waves don't actually escape from inside the horizon, but rather are a property of the changing gravitational field. The event horizon, remember, is a property of what's inside, so if the black hole changes in a way that isn't spherically symmetric (such as if another black hole enters), then the horizon changes accordingly. This is a sign of the entire gravitational field changing, and the gravity waves follow from there.

 

[MeteorWayne]

Would this death spin be detectable (beyond the horizon)? Are those gravity waves gonna make it out?

They should, that is the purpose of the current generation of gravity wave detectors.

 

[theridane]

So if anything slower than or as fast as c can't make it out, does that imply that gravity waves propagate FTL? Or more generally, that the statement that information (in this case the orbital period of the two spinning singularities below the horizon) is limited to travel at c is false?

 

[MeteorWayne]

I think the idea is that the gravity waves are created by the effect on the space time of the mass surrounding the black hole mass (singularity) but I am NO expert on this subject.

 

[amshak]

The result is often a powerful recoil. Instead of coming together nicely, the forces are so extreme that one black holes is kicked away at a tremendous velocity.As one black hole is given a kick, the other receives a tremendous amount of energy, injected into the disk of gas and dust surrounding it. The accretion disk will blaze with a soft X-ray flare that should last thousands of years.

http://www.universetoday.com/2008/02/29 ... s-collide/

 

[ramparts]

So if anything slower than or as fast as c can't make it out, does that imply that gravity waves propagate FTL? Or more generally, that the statement that information (in this case the orbital period of the two spinning singularities below the horizon) is limited to travel at c is false?

Well, it's subtle. Signals can't make it out of the horizon, but the horizon itself is a function of the gravitational field, so when the mass distribution inside the horizon changes, the horizon changes as well.