We’ve been studying rotating black holes in my class this week, which has been fun. We get to apply the techniques we’ve been honing in the context of the Schwarzschild solution (link to recent posts below) to a bit more complicated solution, the Kerr solution, which includes rotation. Some equations follow, although you needn’t be put off by them. Most of this will make sense without really understanding them much. Just so you can see the shape of the things we scribble, I’ll show you the equation that captures this curved spacetime geometry, with no real explanation (sorry):
Here and The parameter is the ratio of the solution’s spin or angular momentum to its total mass , measured in appropriate units. It’s a very important solution to get to grips with, since it’s not just fantasy physics, but highly relevant for astrophysics since black holes that are “out there” are unlikely to be non-rotating, and in fact, one can expect them to be rotating at quite a clip in many cases. A good many black holes – including some of the supermassive ones that power active galactic nuclei making some of the most spectacularly energetic objects known in the universe – are likely to be rotating almost as much as they can.
As much as they can? A key thing about black holes is that they can only rotate so much. If exceeds , their horizon (see earlier post), located at , disappears and the singularity contained within would be visible for all to see – A Naked Singularity!! As far as we know, this can’t happen in Nature. That this can’t happen is called “Cosmic Censorship”. It is a conjecture. Black hole singularities always seem to come clothed inside an horizon, shielding them from our gaze (and protecting our physics from the inherent unpredictability that seems to arise at these points of infinite spacetime curvature). (There’s a lot more to the issue than this, which you can read about elsewhere, but this’ll do for our purposes.)
Happily, there was the appropriate giggle at this point, and one of the students in the class raised his hand and asked something like “Is it kind of like the FCC?”, to which I replied “It’s bigger than the FCC”.
That was on Tuesday, and today we studied the motion of objects in the geometry around the rotating black hole. A cute and simple computation that supports the Cosmic Censorship conjecture is as follows. You might imagine that you can start with a black hole that has as much angular momentum as it possibly can – the “extremal” case it is called. Now take an object that has the right amount of spin compared to mass and throw it toward the black hole. If it falls into the black hole it could increase by an amount more than the amount increases – the horizon would disappear and so the singularity would be naked! This would seem to be a natural way to create naked singularities in Nature. You can imagine the right sort of matter falling into an extremal or near-extremal black hole in a real astrophysics context and making it turn into a naked singularity.
Well, the real test of this is to see from the equations themselves whether it is actually possible to throw such an object into the hole at all, and the analysis actually yields something rather cute – it’s impossible. (In slightly technical terms, the “scattering problem” produces an effective potential whose shape depends upon the angular momentum and mass of the object being thrown in. For just the type of object that can over-spin the hole and make it into a naked singularity, the effective potential has a barrier that reflects the object away from the black hole before it can cross the horizon and merge with it!)
We don’t know why, but Nature seems to naturally want us to be protected from singularities, throwing up horizons to clothe them. Horizons and singularities seem to go together in this way, horizons being the tool by which the cosmic censors express their modesty – In Einstein’s General Relativity, anyway.
While typing this, I just had a thought. If string theory goes where General Relativity cannot go and repairs singularities, supplying new physics that restores predictability (this is still being worked on – see earlier post), then it would seem that horizons are totally unnecessary. The natural conclusion would be that they themselves are simply figments of the limitations of General Relativity. So by this chain of logic (perhaps I’m stretching the word here) they probably are also removed by string theory. Horizons are then, one would conclude, just General Relativity’s way of covering its embarrassment at being incomplete. They are approximations to something else. Ah. Perhaps this is what Sumir Mathur’s conjecture* is all about. Huh. I never thought of that before.
* As you may not know what that is I’ll try to find a good link to further information about it later. Too tired to type more.