What would happen if two black holes collided?

[ocalhoun]

This problem is an interesting one (at least for me), and I wonder if ya'll will have any insights into it...

Suppose two black holes were on a collision course with each other. As they get closer, their combined gravitational force causes them to accelerate. As they accelerate, due to relativity, their mass increases, as their masses increase, their gravitational force increases (right?) As their gravitational force increases, they accelerate more. So, could they eventually get to the speed of light?

Their mass would then be infinite, so it would take an infinite force to accelerate them, but their mass being infinite, their gravity would also be infinite, causing the force accelerating them to be infinite as well...

 

[ray b]

.9999 light speed maybe but not over
I think the key to a super masive BH
is in collisions growing the mass

maybe a quasar too

 

[Chris Hillman]

Pair of approaching black holes?

Hi, ocalhoun,

Suppose two black holes were on a collision course with each other. As they get closer, their combined gravitational force causes them to accelerate. As they accelerate, due to relativity, their mass increases, as their masses increase, their gravitational force increases (right?) As their gravitational force increases, they accelerate more. So, could they eventually get to the speed of light?

The short answer is "no". Similar issues are addressed in the sci.physics FAQ; see http://www.math.ucr.edu/home/baez/physics/Relativity/SR/mass.html and http://www.math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_fast.html

It is often helpful to think of black holes as behaving pretty much like a "dark" object with the same mass and angular momentum. What makes a black hole different from an ordinary star in its interactions with other objects is that it is much more COMPACT, which means that you can get much closer without hitting it and thus experience much strong tidal forces than you can experience outside the surface of an ordinary star.

Chris Hillman