Black hole spin

If a black hole is (in the classic model) a zero-dimensional point and it’s gravity, then how do black holes spin? Spin (with the exception of the particle quantum property) is usually thought of as masses revolving around each other. If a black hole is essentially nothing but a singularity and an event horizon, then what is spinning? Or more precisely how do you define its spin?

I am certainly not a physicist, but I don’t think a black hole consists of just a singularity and an event horizon. There’s also a lot of stuff inside the event horizon heading towards whatever is in the center (and I suspect that with the gravitational time dilation involved, reaching the center can take a long time from the point of view of one who is outside that event horizon.)

Finally, we really don’t know that there’s a singularity at the center of a black hole. A singularity is just what our current knowledge of physics predicts is there, and we don’t have a lot of practical experience with the realities of that situation. It’s quite possible that there’s a certain level of compression that can’t be exceeded because of practicalities that we don’t currently understand.

The answer to “what’s spinning?” is the same as the answer to “where’s the mass?”. If it helps, the singularity of a rotating black hole isn’t a point, but a one-dimensional ring.

Actually, no. Black holes are purely vacuum. From an external viewpoint, it takes matter an infinite amount of time to even reach the event horizon, but that doesn’t matter. The structure of the hole is all vacuum.

Pun intended? Two questions here. Is it a vacuum like space itself; i.e., containing virtual particles? I assume it contains virtual particles as this is Hawking’s explanation for the evaporation of black holes. Also, if it is a vacuum, how does it contain a singularity? Isn’t the singularity where all the mass and its info are concentrated? (Well, that’s 3 questions.)

Not having any mass there to spin would not stop a physicist from attributing spin to it. The neutrino was first proposed as a massless particle with a spin of 1/2. It was postulated to allow angular momentum to be concerved in beta decay. As Isaac Asimov wrote: “The books gotta balance.”

You see, there you go again. Ruining my perfectly good explanation with mere facts. :slight_smile:

The actual spin on the event horizon in a black hole is opposite each other depending on its relative hemisphere, being south or north.

For example, South Carolina always goes left to right.

As I understand it (poorly) there are virtual particles between the event horizon and the singularity. (What happens at the singularity is … well … undefined.) A fact I find interesting (if I have this right) is that the radiation from a black hole is caused by virtual particle pairs created at the event horizon. If one goes down, the other has to go up, in order to preserve some kind of bookkeeping.

Hopefully someone with a clue will demuddle this a bit.

I think the statement that black holes have spin is equivalent to stating that angular momentum can’t be destroyed.

Angular momentum can not be made nor destroyed in a black hole, but at the event horizon angular momentum can run rampant.

I don’t doubt it, but where exactly is the angular momentum located- in the singularity itself or in the black hole’s overall gravity field?

If you like the angular momentum of a spinning black hole is a parameter that expresses the strength of how much it differs from a non-spinning black hole in a particular way. The effect of which is most simply illustrated on free-falling particles of negligible mass, whose motion will tend to follow the rotation of the black hole, with the effect increasing the closer they are to the event horizon.

Is relativistic frame dragging apart from the angular momentum of a spinning black hole?

Is the quantity particle physicists call ‘particle spin’ the same as black hole’s spin? I am no physicist, but I think the two concepts are unrelated. A black hole’s spin is an actual physical rotation, that causes frame-dragging and results in the formation of the ergosphere, an area around the hole where nothing can remain stationary, because spacetime itself is being dragged around.

The two concepts are not unrelated, in that both are actual genuine angular momentum. If an electron falls into a black hole, the spin of the black hole changes accordingly.

By “vacuum”, I mean the same thing that I mean by “vacuum” outside of a black hole. If speaking in a quantum or semiclassical context, that includes virtual particles.

And it’s incorrect to say that the singularity is where the mass or angular momentum is concentrated, because it’s incorrect to say that the singularity is where. The singularity is a hole in space, in the topological sense: To wax poetic, it’s not a location, but a lack of location.