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View Full Version here: : Black hole; to spin or not to spin? That is the question.

[1ponders]
06-04-2011, 04:00 PM
In 1979 I read a book on black holes ( haven't been able to find the book since) and it was describing the various types of black holes; with/without charge, with/without magnetic fields, spinning and non-spinning and various combinations. And something just occurred to me. Yes my friends do call me a bit slow at times. :lol:

When star goes supernova and it is massive enough to form a neutron star, much of angular momentum is conserved and the neutron star that forms is spun up, often to incredible speeds. Yet does the same happen with black holes. Is angular momentum conserved and if it is what is it conserved to?

The "black" hole that is "observed' is only the event horizon, not the actual black hole, or more correctly the singularity. And this is my problem. How can a mathematical point, that has no size, retain the angular momentum that should be conserved as increase in rotational rate. I.e how can a point source 'spin'?

I can understand a gravity well due to the mass present at that point source, but how can momentum be conserved???

And if it is conserved in a point source couldn't that mathematical point's angular momentum be spun up to somewhere near to, or heaven forbid faster than the speed of light (angularly ;). I mean look at a neutron star. Something the diameter of a moderate city spinning dozens if not hundreds of times a sec. What rotational rate a black hole/singularity.

Please keep this simple for a simple man. I'm sure there are lengthy mathematical dissertations out there on exactly this topic, but a laypersons explanation would be appreciated.

Thanks

bartman
06-04-2011, 04:10 PM
Ditto.....I am also interested in this.....
Bartman

sjastro
06-04-2011, 07:29 PM
Rotating black holes do not have a point like singularity.
The singularity is ring like, the ring is in the "equatorial" plane perpendicular to the axis of rotation.
The diameter of the ring is a function of the angular momentum of the black hole. The greater the angular momentum the larger the diameter of the ring. Angular momentum is conserved since the black hole cannot shrink to diameters smaller than the ring.

Due to the ring like property an object falling into the black hole along the axis of rotation can pass through the ring.

Regards

Steven

[1ponders]
06-04-2011, 07:35 PM
Thanks Steve.

Yes now that you bring that up I remember now reading something like that in that book, but that leads me to further wonder why a ring and not a flattened disk like a pancake or even an egg with a bulge in the center?

sjastro
07-04-2011, 07:40 AM
Hello Paul,

The ring is not an "object" which is shaped by centrifugal forces but is a property of spacetime.

A non rotating black hole is defined as spherically symmetrical spacetime with a point singularity and a single event horizon to an observer outside the black hole.

When you rotate the black hole, the spacetime doesn't flatten out. Instead you have an inner and outer horizon and a ring singularity.

The ring singularity can be thought of as an "orbit" of a point singularity.

Regards

Steven

[1ponders]
07-04-2011, 07:56 AM
Ok I think I can visualize that.

Maybe you can explain to me then what the momentum is 'conserved' to. I mean when in our 'normal' spacetime I can get it. Like a spinning skater pulling in their arms. But is there actually anything there with a singularity? Is the singularity 'real' as I would understand it? If its not then where does the momentum get conserved to?

sjastro
07-04-2011, 12:59 PM
Rotating black holes can only carry a maximum angular momentum which is related to the mass of the black hole. If the angular momentum exceeds this limit, the two event horizons merge and become smaller than the ring singularity.
This is known as a "naked singularity" as the singularity should be observable as it is no longer hidden inside an event horizon.
Unfortunately this is where the esoteric mathematics of Messrs. Hawking and Penrose comes into play. Directly observing a naked singularity results in violation of causality. Hence black holes with exposed singularities don't exist. Not all scientists however agree with this theory. So at this stage we can't make any conclusions on the "reality" of a singularities as they cannot be observed due to the event horizons.

With regards to the second part of your question consider a spinning black hole with maximum angular momentum. If it interacts with another spinning black hole the excess angular momentum is converted into gravitational waves.

The search for gravitational waves is one of the ongoing activities in physics.

The angular momentum of a black hole can also be conserved (transferred) to particles in the accretion disk.

Regards

Steven

[1ponders]
07-04-2011, 01:55 PM
Thanks for that.

Its a pity I can't remember the name of the book I read, but its slowly coming back. I recall reading about the possibility of a naked singularity but not how it was formed.