# Superextreme rotating black hole

1. Feb 10, 2009

### Dmitry67

Can we gradually create one from an ordinary BH, firing objects into the BH so they add more and more momentum?

2. Feb 10, 2009

### lightarrow

I'm not an expert of BH, but I'de say: why not? Also, you could send spinning bodies to it .

3. Feb 10, 2009

### Dmitry67

I hope it is true, because I dont believe in 'cosmic censorship'. If fact, I dont understand why Penrose and Hawking believe in it. It would be really nice to see a NAKED RING.

4. Feb 10, 2009

### xantox

While there is no conclusive proof, it is generally believed that it is not possible. R. Wald has given an example of how this cannot happen in "Gedanken experiments to destroy a black hole", Ann. Phys., 82, 548-556 (1974). A related result is Israel's proof of 3rd law of black hole dynamics (which requires the weak energy condition to hold). A possible counterexample of cosmic censorship in a very special case was shown by Brill and Horowitz for extremally charged black holes in de Sitter background, and by Hertog and Horowitz in anti de Sitter, but their status is controversial. Christodoulou has found solutions of the Einstein-Klein-Gordon equations evolving to naked singularities for some choices of initial data, but they were later shown to be non-generic, thus reinstating the cosmic censorship.

Last edited: Feb 10, 2009
5. Feb 11, 2009

### Dmitry67

I see...

Do you know why?
(the money they want for the article I would rather spend for girls :) )

6. Feb 11, 2009

### stevebd1

One possible explanation is that the Killing surface gravity for a rotating black hole is-

$$\tag{1}\kappa_\pm=c^2\frac{r_+-r_-}{2(r_\pm^2+a^2)}$$

where $r_+$ is the (outer) event horizon, $r_-$ is the (inner) Cauchy horizon and $a$ is the spin parameter

The above reduces to zero at a/M=1, if a/M>1, then this would imply the possibility of negative gravity or some form of repulsive force which might explain why the extremal black hole would not capture any more mass (unless the mass had enough momentum to overcome the 'anti-gravity')

(1)-
'Quantum Temperature Near the Ring Singularity in the Kerr Space-Time' by Zhao Zheng
http://cpl.iphy.ac.cn/qikan/manage/wenzhang/0090390.pdf
page 392

Last edited: Feb 11, 2009
7. Feb 11, 2009

### xantox

Wald's argument for the purely spinning case (another is given for the charged case) is that spinning bodies do not move on geodesics but are subject to an additional spin-spin force. After determining the equations of motion, it is found that for sufficient spin the trajectory has a turning point and the infalling body is repelled to infinity. It is then derived that the upper limit of the spin to mass ratio of particles which enter the black hole is the one saturating the extremal case.

Bekenstein's generalized second law of thermodynamics also provides another heuristic argument: since horizons have a specific entropy, their removal would need to include a way to compensate for the entropy loss.

Last edited: Feb 11, 2009
8. Feb 11, 2009

### JesseM

Here's an interesting article from Scientific American about how the results of various numerical simulations seem to suggest that cosmic censorship doesn't hold up, and that some realistic stellar collapse scenarios can actually lead to naked singularities (or whatever the equivalent is in quantum gravity):

http://www.sciam.com/article.cfm?id=naked-singularities

9. Feb 11, 2009

### Dmitry67

Interesting
Also, what if a body is approaching very quickly - is it repelled no matter how fast it is falling?

I hope the article referenced about is correct. Otherwise I dont understand how jets can form. It is obvious that any process which can explain jet formation becomes less and less efficient for the super massive BH. I would rather believe that quasars are naked BH

10. Feb 11, 2009