# Spin (or not) of a Kerr Black Hole

• I
• gerald V
In summary: But let's consider instead a rotating shell of test particles with no angular momentum, and a central ball of dust which is spherically symmetric. (We're talking about dust, so the particles are moving radially.)One of the many ways you can solve for the trajectory of a test particle in a given spacetime is to find a conserved quantity (energy, angular momentum, etc.) and then use that to find some trajectory. The test particle's worldline will be constrained to that trajectory, and it will be easiest to find in the particle's proper time. So we want to find the conserved quantity for a test particle inside the central ball.The energy is easy: we can just use conservation of energy. The angular
gerald V
The Kerr solution describes the gravitational field of a rotating black hole. Oftenly, the hole is said to be „spinning“, what appears as misleading to me. My questions:

1.) Is it correct to say that angular momentum in this way is treated like orbital angular momentum, not like spin?

2.) Can a black hole have spin like an elementary particle (say, a massive gauge Boson or a Fermion, or even an entire atomic nucleus)? By which equations would this have to be described?

3.) If an elementary particle with spin falls radially into a Schwarzschild black hole, what is the result? Will a Kerr hole be generated? Or will this hole have spin in the sense the infalling elementary particle had?

Thank you very much in advance

gerald V said:
The Kerr solution describes the gravitational field of a rotating black hole. Oftenly, the hole is said to be „spinning“, what appears as misleading to me. My questions:

1.) Is it correct to say that angular momentum in this way is treated like orbital angular momentum, not like spin?
GR is a classical field theory, so it's definitely classical angular momentum.
gerald V said:
2.) Can a black hole have spin like an elementary particle (say, a massive gauge Boson or a Fermion, or even an entire atomic nucleus)? By which equations would this have to be described?
It's not quantum spin. A black hole is not a (quantised) elementary particle.
gerald V said:
3.) If an elementary particle with spin falls radially into a Schwarzschild black hole, what is the result? Will a Kerr hole be generated? Or will this hole have spin in the sense the infalling elementary particle had?

Thank you very much in advance
All black holes must have a small angular momentum - but in some cases it's negligible. An elementary particle falling into a black hole would have negligible angular momentum. And, not just spin, but from its trajectory into the black hole, which is unlikely to be perfectly radial.

vanhees71
gerald V said:
1.) Is it correct to say that angular momentum in this way is treated like orbital angular momentum, not like spin?
Strictly speaking, it's neither. It's a global property of the spacetime geometry. See further comments below.

If you are viewing the hole from far away, and don't have to worry about its internal structure or things falling into it, its angular momentum will act like a classical spin (the same as the spin of the Earth or any other astronomical body). But of course not all scenarios can be handled using that approximation.

gerald V said:
2.) Can a black hole have spin like an elementary particle (say, a massive gauge Boson or a Fermion, or even an entire atomic nucleus)?
Black holes are classical objects, not quantum objects.

gerald V said:
3.) If an elementary particle with spin falls radially into a Schwarzschild black hole, what is the result?
Black holes are classical objects, not quantum objects. So your thought experiment here would be better formulated as a classical object with spin (such as a spinning ball) falling into a black hole.

Assuming that re-formulation has been done, angular momentum in GR has to be viewed, as I noted above, as a global property of the spacetime geometry. So it's not really possible to have a spacetime with nonzero angular momentum in it that contains a Schwarzschild black hole. At the very least, one has to be very careful about how the scenario is constructed: you would need a spacetime with multiple regions having different geometries and you would need to make sure things match at the boundaries between the different regions. It's not going to be anything simple like "Schwarzschild hole turns into Kerr hole"--there is no way to connect those two geometries and have conditions match at a boundary.

vanhees71
gerald V said:
3.) If an elementary particle with spin falls radially into a Schwarzschild black hole, what is the result? Will a Kerr hole be generated? Or will this hole have spin in the sense the infalling elementary particle had?
Remember that both Kerr and Schwarzschild black holes are eternal, unchanging solutions. So if a non-rotating hole gains angular momentum by some process it was never a Schwarzschild nor Kerr black hole, strictly speaking. That said, we would expect the hole to look (externally) very like a Schwarzschild black hole in the past and very like a Kerr black hole in the future, with a messy bit in the middle with gravitational waves and whatnot as the mass and angular momentum is absorbed. Not sure what the interior would look like.

We don't have a quantum theory of gravity, so you have to hedge a bit if you're talking about absorbing quantum objects. For stellar mass black holes a single electron will make no measurable difference to anything. Presumably the hole would spin up to conserve angular momentum, but it's such a tiny change how would you ever know? For holes that are small enough that a single electron would matter you'd need a quantum theory of gravity - but we'd expect it to evaporate almost instantly, so I don't think there'd be much opportunity to study it.

vanhees71
PeterDonis said:
It's not going to be anything simple like "Schwarzschild hole turns into Kerr hole"--there is no way to connect those two geometries and have conditions match at a boundary.
Perhaps a related question would be, suppose that we started with a central spherical ball of dust, sufficiently large to collapse, and an outer spherical shell of dust, with a large spherical shell of vacuum between them. Suppose further that the outer shell is initially rotating uniformly about some axis with some given total angular momentum. How does the spacetime evolve?

Even though the central ball has a well known analytical solution, and even though there are rotating black hole solutions, I suspect that this combination would require a numerical solution.

vanhees71
Dale said:
Suppose further that the outer shell is initially rotating uniformly about some axis with some given total angular momentum.
If it is, then I don't think it's possible for the inner shell to be spherically symmetric. Indeed, I don't think it's possible for the spacetime geometry in general inside a rotating shell to be spherically symmetric. The shell theorem only applies to a non-rotating, spherically symmetric shell.

Dale and vanhees71
PeterDonis said:
If it is, then I don't think it's possible for the inner shell to be spherically symmetric. Indeed, I don't think it's possible for the spacetime geometry in general inside a rotating shell to be spherically symmetric.
You can, for example, match an exterior Kerr solution onto an interior (spherically symmetric) Minkowski solution, with the constraint equations at the boundary being enforced by a thin, slowly-rotating shell (see e.g. E. Poisson's treatment of the thin-shell formalism). But I don't know about Schwarzschild?

Dale and vanhees71
ergospherical said:
You can, for example, match an exterior Kerr solution onto an interior (spherically symmetric) Minkowski solution, with the constraint equations at the boundary being enforced by a thin, slowly-rotating shell (see e.g. E. Poisson's treatment of the thin-shell formalism).
Do you have a reference for the Poisson treatment? I dimly remember seeing something like this, but it's been quite some time.

It's chapter 3.10 in his textbook "A relativist's toolkit"/"An advanced course in GR". The stuff on junction conditions is the most difficult section of the whole book, IMO.

vanhees71
ergospherical said:
It's chapter 3.10 in his textbook "A relativist's toolkit"/"An advanced course in GR".
Ok, got it. But, as you noted, this solution is only valid for a slowly rotating shell, where "slowly rotating" ends up meaning "rotating slowly enough that we can ignore the centrifugal force on the shell due to the rotation, which would make the shell non-spherical". In other words, the matching to the Minkowski geometry inside only works because the shell itself is being approximated as spherical, which means the shell theorem is taken to be approximately valid for this case.

Also note that the "Minkowski" geometry inside the shell is "rotating" with respect to infinity, just as the shell itself is. In other words, observers inside the shell who are inertial will find that they are not at rest with respect to observers at infinity; they are slowly rotating with the shell (because of the frame dragging of the shell). This effect I would expect to still be there for a shell that was not slowly rotating, but I would not necessarily expect the geometry inside it to be Minkowski.

ergospherical

## 1. What is the spin of a Kerr Black Hole?

The spin of a Kerr Black Hole refers to its angular momentum, which is a measure of how fast it is rotating.

## 2. How is the spin of a Kerr Black Hole measured?

The spin of a Kerr Black Hole can be measured using various methods, such as observing the motion of matter around the black hole or analyzing the gravitational waves it emits.

## 3. Can a Kerr Black Hole have no spin?

Yes, a Kerr Black Hole can have no spin, also known as a non-rotating or Schwarzschild black hole. This type of black hole has a singularity at its center and does not exhibit any frame-dragging effects.

## 4. What is the significance of the spin of a Kerr Black Hole?

The spin of a Kerr Black Hole plays a crucial role in determining its properties, such as the shape of its event horizon and the strength of its gravitational pull. It also affects the behavior of matter and light around the black hole.

## 5. Can the spin of a Kerr Black Hole change over time?

Yes, the spin of a Kerr Black Hole can change over time due to various factors, such as absorbing matter or merging with another black hole. This process is known as black hole spin evolution.

• Special and General Relativity
Replies
7
Views
2K
• Special and General Relativity
Replies
12
Views
689
• Special and General Relativity
Replies
18
Views
2K
• Special and General Relativity
Replies
4
Views
610
• Special and General Relativity
Replies
43
Views
2K
• Special and General Relativity
Replies
1
Views
1K
• Special and General Relativity
Replies
24
Views
406
• Special and General Relativity
Replies
19
Views
1K
• Special and General Relativity
Replies
11
Views
905
• Special and General Relativity
Replies
67
Views
3K