Black holes have two energy sources that counter each source. The outer spin energy is slowed by the countered inner energy. Both energies can produce rotations at the speed of light. The energy on the inside of a black hole is slowed by the inner gravitational pull. So the counter balance is only enough to reduce the outside rotation by half. Thus 1/2 the speed of light is the answer.

if you approached the event horizon to observe this you wouldn't he able to tell us about it because the huge gravitational forces would be warping time itself, stretching yourself until you die while an outside observer would just see yourself hanging there indefinitely...

You can't discuss falling into a black hole without using the term spaghettification, one of the coolest terms in science.

A black hole should be a geometrical point, since all the matter falls into the center and there's nothing to prevent it from all getting there. There is an event horizon, which is the radius from the infinite-density center where the escape velocity is the speed of light, but this isn't a substance or "thing," it's just a place where spacetime curvature reaches a critical value. So then, what could be measured to be having a speed? The core doesn't have any volume, so no surface is moving. The event horizon isn't a "thing" that moves.

I wonder if this is like quantum spin. Electrons and all other particles (I'm just picking on electrons since they're so negative) have spin, but this isn't the same thing as geometrical spin - it's purely angular momentum. Electron spin is either clockwise or counterclockwise, no matter what direction one measures it in. Hence, when electrons are deflected by magnetic fields, their paths are either bent one way or the opposite: nothing in between, nothing in a different direction.

But we can all probably agree on this: black holes suck. Spinning black holes suck in new and interesting ways.

I'm guessing the angular momentum and moment of inertia were (somehow) determined, making it possible to measure the angular velocity. Then, a "characteristic length" was used to determine the rotational speed. The length could well have been the radius of the event horizon.

But I'm not an astronomer. I only play one on TV.

In any case, that's pretty freaking fast.

I specifically included that excerpt in the OP:

"Measuring the radius of the disk allows the astronomers to calculate the black hole's spin speed."

I think that the speed of the material falling into the black hole is qualitatively different than the spin of the actual black hole. When black holes are spinning, they do really weird things to spacetime, which the material falling in very much does NOT do. Hoever, given that the actual black hole is, theoretically, of zero size, it seems like the "spinning" measurement would have to be that of angular momentum and not of an object with finite size rotating around like a basketball. Hence, I wonder if the angular momentum is somehow dissociated from the normal, Newonian concept of spin, similarly to the way that quantum particles are not spinning in the traditional sense, yet have angular momentum.

The more fundamental question is, what is angular momentum?

the article is paywalled, so it's unclear exactly what they mean.

What I did see were references to angular momentum. Here's my reasoning: say there's a star, it's big, but has burned its fuel and is collapsing. If the start is rotating - most are - it has angular momentum. That the rotational inertia multiplied by the rotational velocity, where rotational inertia is the integral over volume of (differential) mass times distance from the center. Anyway, as the star collapses, the distance-to-center decreases but since angular momentum is conserved, it has to spin faster. A black hole collapses to, supposedly zero radius, which means the matter has to be moving infinitely fast. Of our physics breaks down.

In the end, I think that just the angular momentum is conserved - it has to be - but our normal definition doesn't make sense. There is no volume that the mass is inhabiting. It can't be measured from the event horizon because there is no stuff at the event horizon that can be rotating around the black hole, at any speed. Any stuff there won't be there for long! Rotating black holes drag space around with them, in the direction of rotation, but that begs the question, is space itself "stuff?" Can it have mass and, hence, angular momentum as the black hole "drags it around?" I kind of don't think so.

So the thing that the article says about SOMETHING moving at half the speed of light doesn't yet make sense, but I believe there is some logical and real thing that they are measuring. I just don't know what that is yet.

n/t

all they have to do is come to my house and look into my bedroom closet...