- #1

- 99

- 0

ISTM unlikely that there could be any non-rotating black holes, but I don't really know, hence the question. Do we have the means to determine with any certainty what the answer is?

You are using an out of date browser. It may not display this or other websites correctly.

You should upgrade or use an alternative browser.

You should upgrade or use an alternative browser.

- Thread starter EskWIRED
- Start date

- #1

- 99

- 0

ISTM unlikely that there could be any non-rotating black holes, but I don't really know, hence the question. Do we have the means to determine with any certainty what the answer is?

- #2

tiny-tim

Science Advisor

Homework Helper

- 25,836

- 252

A non-rotating black hole is extremely unlikely …

even if one existed, it would only take one photon to hit the event horizon off-centre to give it angular momentum, ie start it rotating.

- #3

- 99

- 0

A non-rotating black hole is extremely unlikely …

even if one existed, it would only take one photon to hit the event horizon off-centre to give it angular momentum, ie start it rotating.

I never thought about it in quite so dramatic a manner as a single photon being able to disrupt things, but yeah, that's kind of what I expected.

So are pretty much all black holes thought to be Kerr black holes? Are we able to estimate their rate of rotation?

- #4

Nabeshin

Science Advisor

- 2,207

- 16

I never thought about it in quite so dramatic a manner as a single photon being able to disrupt things, but yeah, that's kind of what I expected.

So are pretty much all black holes thought to be Kerr black holes? Are we able to estimate their rate of rotation?

I'm fairly certain we have no direct observational evidence for the spin of ANY black holes. Nevertheless, computer models of how black holes form from stellar collapse in general predict that you can get very high spins, a~0.9 or even more.

- #5

Chronos

Science Advisor

Gold Member

- 11,429

- 745

- #6

- 487

- 3

Instead of spin it might be better to think of angular momentum. I don't know any of the details, but would think that this can be measured by measuring frame dragging, about which I know very little.

- #7

- 99

- 0

That is fascinating. I was half-wondering about how a point could be said to spin, without ever really thinking about it clearly.

But what I understand even less is how a point can be a torus, which is said to be the shape of the singularity in a Kerr black hole. Isn't there an inconsistency between the concept of a singularity and the concept of a three-dimensional torus?

(More likely, the apparent inconsistency is a result of my limited understanding of both black holes and geometry...)

- #8

- 5,607

- 40

The concept of spin applied to a black hole is not easily grasped. What is it that spins? The spin of a dimensionless point [the putative singularity] does not appear to be physically meaningful.

Somehow particles manage 'spin; Why would it not BH spin be as 'physically meangingfull' as the spin of a particle. [/QUOTE]

The difference I'm aware of is that a BH singularity is a 'point' in GR; point particles are described via QM. Oh yes, and a BH singularity is a point in time, not space.

Are we able to estimate their rate of rotation?

From an earlier forums discussion:

The event horizon is rotating as though it were a solid body, so can be characterized by an angular velocity or rotation rate in RPM. For a 10 solar mass black hole rotating at the maximal rate, it is rotating at about 10^4 radians/second or about 10^5 RPM. Larger black holes will rotate at lower RPM rates, but at the maximal rotation rate, a point on the equator of the event horizon is always rotating at 1/2 the speed of light.

don't have a source....maybe someone can confirm or refute....

edit: I skimmed Wikipedia Kerr BH...I did not see a spin rate....

- #9

Chronos

Science Advisor

Gold Member

- 11,429

- 745

- #10

- 487

- 3

That is fascinating. I was half-wondering about how a point could be said to spin, without ever really thinking about it clearly.

But what I understand even less is how a point can be a torus, which is said to be the shape of the singularity in a Kerr black hole. Isn't there an inconsistency between the concept of a singularity and the concept of a three-dimensional torus?

(More likely, the apparent inconsistency is a result of my limited understanding of both black holes and geometry...)

A singularity is a place where the math breaks down and you get an undefined answer, usually an infinity. I doesn't have to be any particular shape. There is a "coordinate singularity" at the event horizon. That more or less means you get infinity if you look at it one way, but can get rid of it if you look at the problem another way.

- #11

stevebd1

Gold Member

- 749

- 41

The spin of a black hole can be pretty much established by were the location of the marginally stable orbit is. For a static black hole, the MSO is at 6M and for a spinning bh, this can range from between 6M to 1M, 1M being maximal. The equations used are on page 258 of the following link-

http://www.lsw.uni-heidelberg.de/users/mcamenzi/CObjects_06.pdf [Broken]

if we have spin (a) and mass (M) then we can establish the MSO, if we already have the MSO and mass then we can establish spin. For a bh with a MSO of 2.5M, the spin parameter would be a/M≈0.87

http://www.lsw.uni-heidelberg.de/users/mcamenzi/CObjects_06.pdf [Broken]

if we have spin (a) and mass (M) then we can establish the MSO, if we already have the MSO and mass then we can establish spin. For a bh with a MSO of 2.5M, the spin parameter would be a/M≈0.87

Last edited by a moderator:

- #12

- 33

- 0

As stated by great astrophysisists it can never be known and only infered.

Share: