# Rotating black hole?

1. Feb 11, 2010

### cbd1

I would like to ask if there is any direct evidence of black holes with spin. I understand that there are formulas that would describe a black hole if it were to have spin, but this does not prove that they actually can. I also know there are accretion disks and relativistic jets around black holes. However, those only prove that matter revolves around black holes and that sometimes, if the conditions are right, matter and energy can be directed away from them in jets. In simpler terms, accretion disks and relativistic jets only decisively prove that there are accretion disks and relativistic jets. Is there any solid physical evidence of a black hole rotating?

2. Feb 12, 2010

### FrankPlanck

Black holes without spin curve spacetime, black holes with spin also distort it. This causes spacetime vibration which in turn causes thermal emission modulation (oscillations) from the inner part of the accretion disk, for more information see thirring-lense effect.
Observing these oscillations it's possible to evaluate black hole's spin (obviously this isn't a simple method).

If you are trying to connect relativistic jets and disk I have to quote Krolik (it's old but it's still pretty funny):
“In principle one could imagine that accretion onto a black hole occurs without any outflow at any point. Put another way, we don't know why jets exist.”

3. Feb 12, 2010

### Chronos

Spin is one of three theoretically possible variables for a black hole - mass, charge and angular momentum. Spin is the angular momentum component. One with spin is called a Kerr black hole, without spin it is called a Schwarzchild black hole.

4. Feb 12, 2010

### stevebd1

According to the static solution (Schwarzschild metric) the marginally stable orbit (MSO) which normally also represents the inner edge of the accretion disk is at 6M, anything inside this radius quickly falls into the BH. For a rotating BH, due to frame dragging, there are two MSO, retrograde and prograde, the prograde MSO can reduce to simply M for an extremal BH where a/M=1. X-rays/gamma rays have been detected around BH candidates that hint at a MSO inside 6M, in some cases, close to where the event horizon would be, implying a very high spin (approx. a/M=0.98). Below are a couple of articles-

http://www.nytimes.com/2001/05/01/s...usive-spin-of-a-black-hole.html?pagewanted=1" NY Times

http://www.newscientist.com/article/dn10611-spinning-black-hole-is-fastest-on-record.html" New Scientist

Last edited by a moderator: Apr 24, 2017
5. Feb 13, 2010

### qraal

That's what I love about Physics Forums! Learn something new every day!

Last edited by a moderator: Apr 24, 2017
6. Feb 13, 2010

### Frame Dragger

There is also a theoretical upper limit of rotation past which a Black Hole couldn't have an Event Horizon. No such object has been observed, but it's another limit on the system to consider.

7. Feb 13, 2010

### qraal

An extremal Kerr-Newman 'black hole'... quite an exotic beastie, if it could evolve naturally. A naked ring singularity would be quite a sight to see. Stephen Baxter imagines such in his Xeelee stories - the Xeelee create a vast spinning ring of cosmic-string to open up a ring singularity to escape the Photino birds' destruction of the stars.

8. Feb 14, 2010

### Frame Dragger

Hmmm, sounds like a book I need to read. A Kerr-Newmann BH that isn't 'Black. From my understanding it would be both inhaling matter, and blowing energy out of broad polar cones. Needless to say, a naked ring-singularity would be the ultimate aphrodesiac for sci-fi writers... other side of the galaxy here we come *sphagettification... pain*.

9. Feb 14, 2010

### qraal

A large black hole or singularity has low tidal forces, thus 'spaghettification' is a minor hazard. We're talking an astrophysical object as large as a solar system or bigger, thus negligible tidal forces except right up close to the singularity itself. The Galactic Core black-holes are sufficiently large to be navigable too - the Milky Way's masses c. 4 million solar masses, thus has an event horizon 12 million km in radius. To go through it's Kerr-Newman wormhole you'd need to enter it via the poles and hope the dynamical tidal forces are forgiving.

10. Feb 14, 2010

### Frame Dragger

Fair enough, but you're going to run into the singularity, not go through in all likliehood. Like an ERB, I think the whole mess wouldn't be traversable. It's fascinating to imagine however...

11. Feb 14, 2010

### qraal

A polar trajectory takes you through the middle of the singularity ring, which is where the wormhole forms, thus you dodge the singularity - but that's for massless infalling trajectories. Any real object's gravity interacts with the singularity and causes dynamical tides... but it's hard to compute exactly how bad or for how long those tides occur. Novikov's own work on it suggests the wormhole is traversable, though where it exits is anyone's guess.

12. Feb 14, 2010

### Frame Dragger

Whichever physicist said that we'd find green slime and lost socks there is probably right.

Of course, Novikov also predicted that history would be consistant regardless of potential paradoxes arising from his traversable wormholes. An interesting notion, but it would seem to indicate that at some point history becomes deterministic once measured. I don't know about that.