# Spin of a black hole

1. Feb 6, 2006

### tony873004

I was just discussing a paper about the spin of a black hole and it made me wonder.

If the object that collapsed into a black hole had some spin to begin with, and a collapsing rotating object spins faster like the ice scater pulling her arms towards her body, then as the size approached 0 (singularity), shouldn't the spin approach infinity?

2. Feb 7, 2006

### Soul Surfer

Look up Kerr black holes. These are the ones with spin. they do not have point singularities but ring or torodal ones the angular momentum is in the spin. See also the topic "view from a ring singularity" on this topics page

3. Feb 8, 2006

### varsha

i've heard that spining bhs have wormholes. is it right? i know that this worm hole stuff hasen't been proved. i think this is just a hypothesis. is this true?

4. Feb 8, 2006

### Entropy

No, there is no sciencific basis for that at all. In fact, I think it is straight out of some sci-fi movie from the 60's.

5. Feb 8, 2006

### Haitham

I am not very well informed on Kerr black holes. Why don't they have point singularities? And even if they don't I still expect the angular momentum to increase as they collapse. Can someone elaborate?

Thanks!

6. Feb 8, 2006

### AlphaNumeric

When you're working with the Kerr metric and you compute $$R^{abcd}R_{abcd}$$ you end up with (assuming no charge, otherwise it's horrific according to my lecturer) $$R^{abcd}R_{abcd} = \frac{48m^{2}}{(r^{2}+a^{2}\cos^{2}\theta)^{2}}$$. This is never singular unless you approach the black hole along the equatorial plane $$\theta = \frac{\pi}{2}$$.

As such, if you fall into the black hole off this plane, then you can actually go to a region with r<0, because from your point of view no singularity exists at r=0. Hence, you end up with a toroidal singularity which exists in the equatorial plane.

If you're familiar withg Penrose diagrams, you can show that moving in such a fashion takes you into another asymptotically flat space-time, but seperate from your original space time.

The toroidal singularity also have a region around it where causality is broken too.

I'm just going on my lecture notes on my desk, so not terribly familiar with it myself.

7. Feb 8, 2006

### tony873004

It's hard to imagine any objects in the universe whose spin rate is 0.00000...
So why aren't all black holes Kerrs?

8. Feb 8, 2006

### SpaceTiger

Staff Emeritus
They probably are, technically, but if the spin is sufficiently small, the Schwarzschild solution will be a good approximation. The metric and orbits of a Schwarzschild black hole are much simpler, so that's often the only type of black hole you'll hear about from people interested in black hole phenomenology.