Technology Review - 11 June 2010
Astrophysicists think they know how to destroy a black hole. The puzzle is what such destruction would leave behind.
The idea of a body so massive that its escape velocity exceeds the speed of light dates back to the English geologist John Michell who first considered it in 1783. In his scenario, a beam of light would travel away from the massive body until it reached a certain height and then returned to the surface.
Modern thinking about black holes is somewhat different, not least because special relativity tells us that the speed of light is a universal constant. The critical concept that physicists focus on today is the event horizon: a theoretical boundary in space through which light and other objects can pass in one direction but not in the other. Since light cannot escape, the event horizon is what makes a black hole black.
The event horizon is somewhat of a disappointment to many astrophysicists because the interesting physics, the stuff beyond the known laws of the universe, all occurs inside it and is therefore hidden from us.
What physicists would like, therefore, is way to get rid of the event horizon and expose the inner workings to proper scrutiny. Doing this would destroy the black hole but reveal something far more bizarre and exotic.
...
In general relativity, the mathematical condition for the existence of a black hole with an event horizon is simple. It is given by the following inequality: M^2 > (J/M)^2 + Q^2, where M is the mass of the black hole, J is its angular momentum and Q is its charge.
Getting rid of the event horizon is simply a question of increasing the angular momentum and/or charge of this object until the inequality is reversed. When that happens the event horizon disappears and the exotic object beneath emerges.
...
But that hides a multitude of problems. For a start, things with angular momentum and charge also tend to have mass. And in any case, the equation above describes a steady state. Feeding a black hole creates a dynamic state and there is no guarantee that the object will settle back into a steady state again without shedding the angular momentum and charge that it has been fed.
...
What would a black hole without its event horizon reveal? That's where physics turns philosophical. The mathematics here indicates that spacetime becomes infinitely curved, creating what astrophysicists call a singularity.
To any ordinary physicist, a singularity is an indication that a theory has broken down and some new theory is needed to describe what is going on. It is a matter of principle that singularities are mathematical objects, not physical ones and that any 'hole' they suggest exists not in the fabric of the Universe but in our understanding of it.
Astrophysicists are different. ... For them, removing the event horizon around a black hole raises the exciting prospect of revealing a singularity in all its naked glory. When that happens, we will be able to gaze at infinity.
Destroying black holes with test bodies
- arXiv.org > gr-qc > arXiv:1006.1764 > 09 Jun 2010
If a black hole can accrete a body whose spin or charge would send the black hole parameters over the extremal limit, then a naked singularity would presumably form, in violation of the cosmic censorship conjecture. We review some previous results on testing cosmic censorship in this way using the test body approximation, focusing mostly on the case of neutral black holes. Under certain conditions a black hole can indeed be over-spun or over-charged in this approximation, hence radiative and self-force effects must be taken into account to further test cosmic censorship.