Gravity around a near maximal black hole

[dendros]

It's been a long time since I posted here, but I've read on the forum from time to time. And I have a curiosity and some questions about these objects, namely the naked singularities.
From what I understand, a naked singularity is a singularity without an event horizon, so they are visible. Questions:
1. Are they real, or their existence is just theoretical?
2. How can they be visible if the gravity in their vicinity is so strong?
3. A black hole may lose its own event horizon, in certain conditions, thus becoming a naked singularity?
4. If the naked singularities do exist, what would be the consequences on the current understanding of universe?

Thanks for any insights.

 

[nikkkom]

From what I understand, a naked singularity is a singularity without an event horizon, so they are visible. Questions:
1. Are they real, or their existence is just theoretical?

Singularities (naked or not) were not observed.

In most cases, a non-removable singularity in a theory indicates breakdown of this theory. It would be hard to propose a point of view that such a prediction is actually valid - that it describes something physically existing.

Most physicists assume that there will be a quantum gravity theory which will describe black hole structure without singularities.

 

[Haelfix]

This is what is known as the cosmic censorship hypothesis. Whether or not in nature, there exists the possibility of a naked singularity (that is one without the corresponding existence of an absolute horizon) visible to an observer at future null infinity.

From the point of view of the classical theory, it was believed for a very long time that cosmic censorship holds (with a suitable set of technical assumptions). However there now exists counterexamples to this claim in spacetime dimensions greater than 4 (obviously unphysical counterexamples, but nevertheless it shows that the argument is less robust than previously thought).

When you introduce quantum mechanics into the picture, it is widely believed that the singularity will be regulated in some fashion and the infinity removed (and therefore a naked singularity would constitute a direct physical window into the realm of quantum gravity, which would be extremely cool). However this argument is actually somewhat vacuous, as the existence or nonexistence of a horizon seems to be quite independant of the details of quantum gravity by spacetime locality arguments and therefore the answer seems to get kicked right back into a statement of (semi) classical physics..

Needless to say, the theoretical situation remains opaque.

 

[stevebd1]

2. How can they be visible if the gravity in their vicinity is so strong?
3. A black hole may lose its own event horizon, in certain conditions, thus becoming a naked singularity?

For a black hole to become a naked singularity, it has to become maximal where $a^2+Q^2\geq M^2$ where $a$ represents spin, $Q$ represents charge and $M$ represents mass. The more maximal a black hole becomes, the more gravity reduces at the event horizon until at $(a^2+Q^2)/M^2=1$ it becomes zero. Within the event horizon region (though technically, there would no longer be an event horizon), gravity would be negative until reaching the naked singularity where gravity would become positive again. But all this is very speculative. For a black hole to become maximal, as stated, the surface gravity would have to become zero which would be a violation of the third law in black hole thermodynamics that states 'The limit $k=0$ cannot be reached within a finite time' (see https://www.physicsforums.com/threads/black-hole-thermodynamics.762982/). For more information regarding gravity around a near maximal black hole, see this post.