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I guess nobody really knows the true nature of black holes, however, based on everything I know about black holes, there is a "singularity" at their center, which has finite mass but is infinitely small and thus infinitely dense. So a black hole is really just a point mass that dramatically warps the space-time around it so that an event horizon is achieved.

What doesn't make sense to me is how a true mathematical "point" can rotate, or at least how it is even possible to determine if a point is rotating (or if it should even make a difference about whether or not it is rotating, for the same reason it would be impossible to tell if a perfect, featureless sphere were rotating (I know it would have higher rotational energy but lets assume you can't measure that because a true "point" can't attain rotational energy since it has no radius)).

The only conceptual solution I have to describe a rotating black hole would be to compare it to a rotating whirlpool, where the surface of the water is the fabric of space-time... but this is different because there is no "singularity" in a whirlpool that causes the water around it to rotate, whirlpools form from flow of water around a point (put into motion perhaps by some kid swirling water in a 2-liter bottle... it's due to an external cause), but the only thing that could cause rotation around a black hole would be the singularity itself, which doesn't make sense because, once again, how can we know if a true point is actually rotating?

Aside from everything I've said, here is what I am actually asking: Why/how do some black holes rotate... what is the mechanism of their rotation (is analogous to a spinning point or a 3-d whirlpool, or something different)?

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Somewhat related: – kleingordon Mar 13 '14 at 0:23
I don't think either of those other questions are duplicates. The question is essentially about how to make sense of rotation in classical setting when the thing rotating is infinitesimally small. – user27578 Mar 13 '14 at 0:35
This is a good question. I had the same question some time ago. You don't have to worry though -- Kerr metric black holes don't have point singularities. – Brandon Enright Mar 13 '14 at 1:04
I am no physicist, but I was under the impression that the mass of a black hole was simply contained in a smaller volume than that of a sphere with its Schwarzschild radius, not a mathematical point. The Earth (with its present mass) would then be a black hole if it were compressed to ~17mm diameter. Not only is 17mm not a mathematical point, it's visible to the naked eye (for something with a small enough mass to not be a black hole, of course) – Brian S Mar 13 '14 at 18:37
@BrianS, once something is smaller than its Schwarzschild radius, there is no force strong enough to hold it up against its own gravity. Hence, it will continue to collapse, at least until such a time as gravity as we understand it no longer applies. So, yes, a Schwarschild black hole's singularity is indeed an infinitesimal point. – user27578 Mar 13 '14 at 19:27

2 Answers 2

They rotate because they are produced by matter that has net angular momentum, and angular momentum is conserved in axially symmetric space-time. So, there's nothing unusual making them rotate that's different from any other physics.

However, you are absolutely right to object that rotation of an infinitesimally small point wouldn't make much sense. In quantum mechanics, we talk about infinitesimally small particles having intrinsic angular momentum ("spin") but this is a uniquely quantum effect and General Relativity is a classical theory. So, your question is a good one. Fortunately, it has a simple answer: the singularity of a rotating black hole in GR is not a point, it's a ring around the black hole's axis of rotation. A rotating ring - even an infinitesimally small one - is sensible because it's topologically distinct from a zero dimensional point.

As a side note, physicists don't tend to believe singularities are real. The general feeling is that quantum gravity will turn them into something more physical. However, it is at least reassuring that GR still makes sense regardless.

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Relevant: – Brandon Enright Mar 13 '14 at 1:02
+1 for first paragraph. – Russell Borogove Mar 13 '14 at 4:20
I have to disagree with this providing an answer to the OP's concerns. While true that the Kerr singularity is a "ring" in some convenient coordinate system, at best this just picks out a direction. If the OP is unwilling to accept angular momentum as intrinsic to spacetime, then a hoop is no better than a point: which way does it rotate? The point vs. ring distinction is a red herring here. Besides, classical mechanics self-consistently has point particles with nonzero angular momentum. – Chris White Mar 13 '14 at 6:30
@ChrisWhite: "Besides, classical mechanics self-consistently has point particles with nonzero angular momentum." What? No, it does not - unless you mean point particles with nonzero angular momentum about a point different from the one occupied by the particle. Classical mechanics does not have point particles with nonzero angular momentum about an axis that passes through them. The point vs. ring distinction is precisely what resolves this problem for the Kerr black hole, so I fail to see how it's a red herring. – user27578 Mar 14 '14 at 1:20
Why a ring and not a disc? Even a disc can have angular momentum and the velocity of a rotating disc need not be as high as that of a rotating ring to keep the same angular momentum. In this regard, a disc is more optimal than a ring. (assuming optimal implies lowest angular velocity). – Jus12 Sep 7 at 9:02

An ideal black hole with non-zero angular momentum is described by the Kerr metric. The singularity of such a black hole is not a point.

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