Why does black hole spin? Only three numbers define a black hole: Mass, Charge and Angular Momentum. This is known popularly by the statement “Black Holes have no hair”. But actually, Black Holes aren’t that bald. They have three hairs! But that is a discussion for some other time.
Consider the mass of star that came before the Black Hole… It was formed from a solar nebula, gaining it’s rotation by averaging out the momentum of all the individual particles in the cloud.
As mutual gravity pulled the star together, through the conservation of angular momentum, it rotated more rapidly!
When a star becomes a Black Hole, it still harbors all that mass, but now compressed down into an infinitesimally smaller space. And to conserve that angular momentum, the Black Hole’s rate of rotation speeds up! A LOT!!!
The entire history of everything, the black hole ever consumed, averages down into a single number… Spin Rate!
If the black hole could shrink down to an infinitely small size, common man’s logic dictates that it’s spin rate would blow up to infinity!
But here’s where things get interesting!
Black holes have a Speed Limit!!!
It is sort of set by the fact that the faster a black hole spins, the smaller is it’s Event Horizon. I’ll get to that in a second…
There is this region called the Ergosphere that exists between the Event Horizon and another boundary outside of that. The Ergosphere is a region outside the event horizon, where gravitational forces start to influence objects movements. Objects here can no longer remain stationary in space. Depending on the distance between object and to the event horizon, the influence can be extremely strong or very weak.
General relativity theory predicts that any rotating mass drags surrounding space-time with it. This makes the Ergosphere not just a characteristic of black holes, but it is present with all regular cosmic objects of mass, including Earth, planets or the Sun.
The Ergosphere is worth a discussion because that’s where a variety of interesting effects can occur!
Imagine the Event Horizon as a sphere in space. And then surrounding this black hole, is the Ergosphere. The faster the black hole spins, the more this Ergosphere FLATTENS OUT!
The speed limit is set by the fact that the Event Horizon eventually gets smaller and smaller at a high enough spin and reaches the Singularity. And you can’t have a naked singularity sitting in free space, exposed to the rest of the Cosmos! It would mean that the Singularity, all by itself, could emmit energy or light and someone from the outside could actually see it! And we certainly know that can’t happen. That’s the limit to how fast a black hole can spin.
Just think about it… The Black hole spins so fast that it is just about to reveal itself! But that’s impossible! The laws of Physics, like a devil, won’t let it spin any faster!
It is even more fascinating to know that Astronomers have, actually detected super massive black holes spinning at the limits predicted by these theories!
One Black Hole, at the heart of Galaxy NGC 1365 is turning at 84% of the speed of light! It has reached the cosmic speed limit and can’t turn any faster without revealing it’s Singularity.
The Cosmos is a crazy place to be in!
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