How and why would black holes repel each other?

Question

This item was in today's Space.com news.
http://news.yahoo.com/s/space/20070103/sc_space/scientistsspotblackholeinunlikelyplace
In part, it says:
"Astronomers have spied a small black hole nestled in the middle of a packed star cluster...
The discovery...is surprising because some theories predict that gravitational interactions among black holes inside a cluster would simply kick most or all of the black holes out of the cluster.
Scientists think [that] because black holes are usually among the most massive objects in a globular cluster, they sink to the cluster's center. There, they typically pair up with either a star or with another black hole that has also [sunk] into the cluster's middle. A binary system made up of a black hole and a star can be stable, but when two black holes pair up, strong gravity interactions between them are thought to give one or both the boot."
MQI, why wouldn't all the black holes inside a star cluster, or anywhere else, simply merge???
Thanks.

Answer 1

This interaction is inside of a galactic cluster which has different traits than a free standing black hole. Two black holes spinning around each other create enough centripetal force to send one of them hurtling back out. This is because of their extreme gravitational force. As they get closer to one another they spin faster much like an accretion disk in a rotating black hole. Since a star has much less gravity than a black hole, it is possible for a star and a black hole to reach a stable rotation around each other. Black holes just keep falling into one another until conservation of angular momentum forces one out. If they had no angular momentum to start with they would just simply fall into each other.

Answer 2

Gravity Assist or Gravitational Slingshot

There is a phenomenom such that you can exchange net momentum from one body of mass to another when their paths intersect in certain ways. Many spacecraft are launched into the outer solar system using this method. They may be gravity assisted with Mercury, Venus, Earth, our moon, then Mars many times before being sent on a path to the outer reaches of our solar system. It takes a long time but requires minimal fuel.

I'm guessing that a cluster of black holes is unstable and as such will gravity assist each other away.

They do not merge because they are not travelling towards each other. For them to merge they would have to be within their respective event horizons. The space in the cluster is much much larger than the black holes themselves. It may be comparable to expecting a marble in the U.S. to merge with a marble in China. Their gravity is enough to influence each other but not so much to actually combine them.

Answer 3

It's not that they repel each other, but they begin to orbit each other around a common center of mass. Due to the great mass of the black holes, they orbit very fast and one gets 'flung' out like a rock spun around in a sling. If two black holes hit dead on and collide, they will combine into a larger black hole.

Answer 4

Black holes repel each other using a very simple product by RONCO called Black Hole Repelent. You can buy it from many different sources. Such as TV infomercials