Superextreme rotating black hole

[Dmitry67]

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

 

[lightarrow]

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

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

 

[Dmitry67]

I hope it is true, because I don't believe in 'cosmic censorship'. If fact, I don't understand why Penrose and Hawking believe in it. It would be really nice to see a NAKED RING.

 

[xantox]

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

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.

 

[Dmitry67]

I see...

It is widely believed that the complete gravitational collapse of a body always results in a black hole (i.e., “naked singularities” can never be produced) and that all black holes eventually “settled down” to Kerr-Newman solutions. An important feature of the Kerr-Newman black holes is that they satisfy relation m2 a2 + e2 where m is the mass of the black hole, e is its charge, a = J/m is its angular momentum per unit mass and geometrized units G = C = 1 are used. (For m2 < a2 + e2 the Kerr-Newman solutions describe naked singularities.) In this paper, we test the validity of the above conjectures on gravitational collapse by attempting to create a spacetime with m2 < a2 + e2 starting with a Kerr-Newman black hole with m2 = a2 + e2. Such a spacetime would either have to be a new black hole solution or a “naked singularity,” in violation of the above conjectures. In the first gedanken experiment we attempt to make the black hole capture a test particle having large charge and orbital angular momentum compared with energy. In the second gedanken experiment we attempt to drop into the black hole a spinning test body having large spin to mass ratio. In both cases we find that bodies which would cause violation of m2 a2 + e2 will not be captured by the black hole, and, thus, we cannot obtain m2 < a2 + e2, although we can come arbitrarily close in the sense that m2 = a2 + e2 can be maintained in these processes.

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

 

[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

 

[xantox]

Do you know why?

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.

 

[JesseM]

I hope it is true, because I don't believe in 'cosmic censorship'. If fact, I don't understand why Penrose and Hawking believe in it. It would be really nice to see a NAKED RING.

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

 

[Dmitry67]

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.

Interesting
Whats about particles with spin?
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 don't 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

 

[lightarrow]

It would be really nice to see a NAKED RING.

Are you thinking about strings?

 

[Dmitry67]

Are you thinking about strings?

Well, at least I am not afraid of the naked singularity
It must be something on the planks scale, so density is not infinite