# The Naked Scientists Forum

### Author Topic: How fast is a black hole spinning?  (Read 1052 times)

#### thedoc

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##### How fast is a black hole spinning?
« on: 05/04/2014 09:30:01 »
Søren Ragsdale asked the Naked Scientists:

I've heard that black holes spin, which causes frame dragging. I'm not sure what that means, though.

If we talk about a bicycle wheel, its "rotations per minute" measure how fast the tire travels around its axle. But a singularity inside the event horizon of a black hole is a point, it has zero radius.

How can we say how fast a black hole's singularity is spinning if it has zero radius?
What do you think?
« Last Edit: 05/04/2014 09:30:01 by _system »

#### Pmb

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##### Re: How fast is a black hole spinning?
« Reply #1 on: 05/04/2014 10:53:14 »
Søren Ragsdale asked the Naked Scientists:

I've heard that black holes spin, which causes frame dragging. I'm not sure what that means, though.

If we talk about a bicycle wheel, its "rotations per minute" measure how fast the tire travels around its axle. But a singularity inside the event horizon of a black hole is a point, it has zero radius.

How can we say how fast a black hole's singularity is spinning if it has zero radius?
What do you think?
I'm not 100% sure but I believe that black hole rotation isn't quantified in terms of angular velocity. It's quantified in terms of angular momentum.

#### evan_au

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##### Re: How fast is a black hole spinning?
« Reply #2 on: 06/04/2014 12:18:52 »
It is not possible (or necessary) to measure the angular velocity of the singularity.
It is possible to monitor the angular momentum of objects falling into a black hole (before they pass the event horizon).
The angular momentum affects the shape of the ergosphere around the black hole. In principle, this should allow measurement of the angular momentum of a black hole without having to observe all the material that has fallen into it.

There is an upper limit to the angular momentum of a black hole, based on it's mass and electrical charge. Since electrical charge is likely to be near-neutral, the angular momentum would be mainly limited by the mass.

#### Pmb

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##### Re: How fast is a black hole spinning?
« Reply #3 on: 06/04/2014 12:27:23 »
Quote from: evan_au
It is not possible (or necessary) to measure the angular velocity of the singularity.
It's more than just that. It's not even possible to define it. One would have to give meaning to such a thing and that can't be done with a singularity.

#### The Naked Scientists Forum

##### Re: How fast is a black hole spinning?
« Reply #3 on: 06/04/2014 12:27:23 »

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