If black holes spin how can we tell?

As I understand it only three things define a black hole:

  1. Mass
  2. Spin
  3. Charge (and IIRC they are neutral so charge can be ignored as a practical matter)

Mass we get. Kinda what a BH is all about.

I also understand that the star a BH forms from is spinning and conservation of angular momentum demands that it keeps spinning and spins faster and faster as it collapses.

All well and good so far.

But what is spinning and how can we tell from the outside? Put another way how can a spinning black hole affect its environment?

It would seem to me that the only thing that can be spinning is the singularity. The event horizon is not a “thing” as such. There is nothing there to “spin”.

The singularity, being a point “particle” (or mass), I would think would have to be spinning at just barely under light speed. As such a little black hole basically spins as fast as a big black hole. There would be no discerning a difference.

Matter falling in, adding its angular momentum, can’t really speed it up…there is no more to be sped up (unless we are talking some deep decimal increase like 0.9999999997C to 0.9999999998C).

But even if the singularity is spinning like crazy how does that change things outside the black hole? Nothing can escape, the spinning point is an infinitesimally small point so why would spinning matter?

Are they really talking about things falling in to the BH that spiral in and we measure that spinning stuff?

If you could stop the singularity spinning (cuz magic) would we notice anything different from outside the BH?

The shape of the event horizon is markedly different for spinning black holes and non-spinning ones. There are gravitational effects of a black hole’s spin (frame-dragging) that are in principle detectable. But the most obvious effect is that a non-spinning black hole has no magnetic field, while a spinning one does.

It’s a relativistic effect: the rotating black hole (or other massive object) actually drags space-time around it, so that inertial reference frames are forced to rotate in the same direction.

How does a BH form a magnetic field from the singularity?

I can see a magnetic field if matter is spiraling around and falling into the BH but a nice, quiet BH is still generating a magnetic field? How? Nothing can escape the BH.

And what about charge. We have a nice black hole sitting there in space. I point the exhaust of by Bussard ram jet at it and pump in n knows how many protons. (I seem to recall reading this in an SF novel now that I think of it.) So it’s neutral charge has now become n known how may positive electron charge units. How do we know? Can the electrostatic charge field get out of the black hole?

We know there is frame dragging around the earth.

How do we know it happens around a black hole?

I mean, this goes to the OP. What is spinning?

What does an infinitely small thing (the singularity) spinning even mean? And how is it spinning (if it is spinning) transmit outside the black hole to affect the outside universe?

Yes, a charged black hole has an electrostatic field, just like a black hole with mass (i.e., all of them) has a gravitational field. In principle, a black hole could have a ludicrously large charge (equal to its mass, in appropriate units), but you really have to contrive it to come up with a way to get to that charge: If you just try shoving in electrons, you’ll quickly get to a point where, due to overcoming the existing electrostatic force, you’re adding more mass than charge, and so the charge-to-mass ratio remains low. And in practice, we expect that real black holes in the Universe would, on average, have a total charge only a few times the charge of the electron (IIRC, the RMS average charge would be around 6).

Which brings us to magnetic fields. In principle, a black hole with both electric charge and spin will also have a magnetic field (unsurprising, since that’s what you get from any other sort of charged spinning object). But with a charge of only a few electrons, that magnetic field is not going to be anything to write home about. When we observe insanely-strong magnetic fields around black holes (which we do), it’s due to the material in the disk around the hole, not directly due to the hole itself.

Alternately, it’s also possible for a black hole to have magnetic charge. How does a black hole get a magnetic charge? By eating something else with a magnetic charge, such as… um… let me get back to you on that. Heck, maybe you had a pair of very small black holes produced via pair production, with opposite magnetic charges. That’d take ludicrous amounts of energy, of course, and there’s no process we know of in the present Universe that could pull it off, but heck, in the very early Universe, who knows.

Didn’t notice that Whack-a-Mole added a question there. Up until recently, we hadn’t actually measured the frame dragging around a black hole, but our calculations (the same ones that we used to calculate it around the Earth, which we have measured and which matches very well) said that it had to be there (this is, really, how we know most of what we know about black holes). Now, though, we’ve detected gravitational waves from colliding black holes, and the details of those waves depend on the angular momenta of both holes, as well as precisely how they were moving relative to each other before the collision. So, yes, we have now directly observed the effects.

So the electric field ‘escapes’ the BH in much the same way as the gravitational field, the latter bending spacetime at the speed of light in all directions. By what mechanism could we detect the EM field of a black hole?


Would you get an accretion disk for a non-spinning black hole? It seems to me that you wouldn’t but I could be wrong. At any rate, if you see a black hole with an accretion disk, there’s angular momentum in the system - which the black hole will eventually ingest some of, spinning it up.

Niven’s Borderlands of Space (a novella, I think, not a novel, but it has someone charging up a black hole like that)

If you’re talking about naturally-occurring real-world black holes, we couldn’t. A few electrons worth of charge on something kilometers across is far below the limit of detectability if we were right there, much less many lightyears away, and especially against the background of the ludicrously-strong fields of the accretion disk.

If you’re talking about a “tame”, artificially-made black hole that we built in a super-tech laboratory to have whatever properties were convenient, we’d detect the electric field in the same ways we detect any other electric field.

It’s hard to imagine any real-world situation which would naturally result in a black hole without significant spin, and similarly hard to imagine a situation which would result in a hole without an accretion disk. Stars tend to have angular momentum much, much higher than is allowed for a black hole, and in fact shedding all of that excess angular momentum is a bit problematic (in the sense that it makes it harder for us to simulate supernovas, not in the sense that the stars themselves have a hard time figuring it out).

EDIT: A nitpick, but the Niven story is “The Borderlands of Sol”, not “…of Space”.

So, we could detect a light field without detecting its light? I guess what I am saying is that by definition black holes are black and allow no light to escape, yet the field itself ‘escapes’ and is detectable. Interesting.

The static field escapes and is (at least in principle) detectable. That’s a very different matter from a dynamic field like light escaping.

Can a black hole have “spin” in the quantum mechanical sense? Or are classical spin and quantum spin identical in the case of black holes?


The static electric field doesn’t have to escape the black hole. A cloud of charge around a hole has already set up a static field outside the cloud. As the cloud falls into the hole, the field outside where the cloud used to be wouldn’t look any different. Information, including changes in static field, can’t escape the hole, so that if a magician could destroy charge inside the hole I don’t think we could tell from the outside. But the static field outside the hole never had to change – the charge falling in would already have established it before becoming hidden.

Right? I mean, I actually am a physicist, but not necessarily a very good one, and I mostly work with heat and temperature anyway.

Again I ask, what is spinning?

In the OP I mentioned that I get the conservation of angular momentum and since the star that was collapsing was certainly spinning it would speed up as it collapsed ever smaller.

But we are now talking about a singularity which, I think, has no dimensions. It is an infinitely small point. How is it “spinning”. How does that make sense?

A rotating black hole has a ring singularity, not a point.

The angular momentum is in the fabric of spacetime itself. If it helps, you can think of it as being spread out over the entire region inside the horizon, instead of all being at the singularity: It doesn’t make any difference what you do, as long as it’s inside the horizon.

I suppose they would have to be identical: If you took a hole with exactly zero angular momentum, and fed it a single electron, the hole would then have to have an angular momentum of hbar/2, and it couldn’t under any circumstance have an angular momentum less than that. But that’s all just “I suppose”, because we’d need a quantum theory of gravity (which we don’t have and don’t expect to have any time soon) in order to be able to answer that properly.