kerr black hole question

[hoola]

does a black hole of slowing rotation with a shrinking event horizon equatorial diameter, reduce that diameter in discrete steps of multiples of plank units, or in a smoother curve of a classical object in plank unit steps?

Edited May 24, 2019 by hoola

[MigL]

What's making it slow down ?

( also, the 'outer' horizon will shrink as the BH's angular momentum increases )

[Strange]

1 hour ago, hoola said:

does a black hole of slowing rotation with a shrinking event horizon equatorial diameter, reduce that diameter in discrete steps of multiples of plank units, or in a smoother curve of a classical object in plank unit steps?

Black holes are classical (non-quantum) objects. So Planck units don't appear to be relevant.

If you are thinking of Hawking radiation, then the change in radius will be proportional to the mass lost. Again, Planck lengths don't appear to be relevant. So, for example, if the black hole lost the mass of one electron, the radius would decrease by a very, very small amount, about 10^-23 Planck lengths. 

[beecee]

2 hours ago, MigL said:

What's making it slow down ?

A BH can have three properties that we are aware of...mass, spin or angular momentum and charge.  The charge would I 'm pretty sure, be rather quickly negated. Which leaves spin. Why couldn't spin [angular momentum] be either increased or slowed down? Ignoring the fact that a BH's spin rate does have limitations [naked singularities just do not and cannot exist] why couldn't say magnetic field lines, matter/energy falling in, colliding BH's or BH's colliding with Neutron stars not have the opposite effect of slowing it down? Sure, over a much much longer time frame. The other possibility that just came to mind is Hawking Radiation.

 

correction: Doing a bit of research, I found that BH's do not really have any defined "spin velocity" but do have angular momentum.Perhaps a bit pedant but anyway.... https://www.google.com/search?q=can+black+holes+lose+angular+momentum%3F&oq=can+black+holes+lose+angular+momentum%3F&aqs=chrome..69

I also found this, which seems to support my contention....

https://academic.oup.com/mnras/article/305/3/654/983338

The evolution of black hole mass and angular momentum: 

Abstract: 

We show that neither accretion nor angular momentum extraction is likely to lead to significant changes in the mass M₁ or angular momentum parameter a_* of a black hole in a binary system with realistic parameters. Current values of M₁ and a_* therefore probably reflect those at formation. We show further that sufficiently energetic jet ejection powered by the rotational energy of a black hole can stabilize mass transfer in systems with large adverse mass ratios, and even reduce the mass transfer rate to the point where the binary becomes transient.

<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

 

Summing up my thoughts re BH properties, I see charge as being negated over short time frames,and angular momentum negated over much much longer time frames approaching the lifetime of the universe, though still just short of BH evaporation via Hawking Radiation. Any thoughts, errors or corrections from those that know better? 

Edited May 24, 2019 by beecee

[MigL]

My thoughts on the matter, BeeCee...

Once a rotating mass collapses to a BH and associated event horizon, classically, there is nothing that actually spins.
The event horizon is a mathematical construct, and ( at least classically ) everything else associated with the BH is in the singularity.
The conserved angular momentum can be considered intrinsic, similar to quantum particle spin.
All other effects would have to be external ( the accretion disc, maybe ) to the event horizon, as lines of force ( magnetic field lines ) cannot travel from inside the EH to the exterior.

I had thought that frame dragging might siphon off angular momentum, but I'm not so sure.
Any loss of mass would reduce angular momentum, but any existing BHs are gaining mass and it will be a long time before they radiate appreciable quantities of Hawking radiation to start evaporating.
( stellar BHs, the smallest we know of, are way colder than the CMB, and are gaining mass )

Any massive collisions ( other BHs or neutron stars ) that could reduce the angular momentum, would tend to INCREASE the BH's mass-energy and so, the extent of the event horizon. Certainly not reduce it.

Edited May 25, 2019 by MigL

[beecee]

2 hours ago, MigL said:

My thoughts on the matter, BeeCee...

Once a rotating mass collapses to a BH and associated event horizon, classically, there is nothing that actually spins.
The event horizon is a mathematical construct, and ( at least classically ) everything else associated with the BH is in the singularity.
The conserved angular momentum can be considered intrinsic, similar to quantum particle spin.
All other effects would have to be external ( the accretion disc, maybe ) to the event horizon, as lines of force ( magnetic field lines ) cannot travel from inside the EH to the exterior.

I had thought that frame dragging might siphon off angular momentum, but I'm not so sure.
Any loss of mass would reduce angular momentum, but any existing BHs are gaining mass and it will be a long time before they radiate appreciable quantities of Hawking radiation to start evaporating.
( stellar BHs, the smallest we know of, are way colder than the CMB, and are gaining mass )

Any massive collisions ( other BHs or neutron stars ) that could reduce the angular momentum, would tend to INCREASE the BH's mass-energy and so, the extent of the event horizon. Certainly not reduce it.

Thanks Migl....particularly re your comment on spin and angular momentum, hence my "correction" comment. Your comment on "frame dragging" is valid I believe and relevant to what my thoughts are.

[hoola]

I have  read that a spinning black hole has a central bulge and is not perfectly spherical. This is the shrinkage I am talking about, in a physical sense, as it spins down due to frame dragging or other effects and becomes more spherical. I am not referring to mass loss, and as a thought experiment, keep the mass constant and consider the matter as a change in shape. Does the hole approach the perfect spherical  shape  in units of multiples of plank units, with no steps between, exhibiting quantum behavior  and if so,  would that  be classically measurable ? Thanks for the responses.

[beecee]

13 minutes ago, hoola said:

I have  read that a spinning black hole has a central bulge and is not perfectly spherical. This is the shrinkage I am talking about, in a physical sense, as it spins down due to frame dragging or other effects and becomes more spherical. I am not referring to mass loss, and as a thought experiment, keep the mass constant and consider the matter as a change in shape. Does the hole approach the perfect spherical  shape  in units of multiples of plank units, with no steps between, exhibiting quantum behavior  and if so,  would that  be classically measurable ? Thanks for the responses.

The above is a diagram in line with the best knowledge we have, that may help.

Also I believe you will obtain some good info here....https://jila.colorado.edu/~ajsh/

[Strange]

3 hours ago, hoola said:

This is the shrinkage I am talking about, in a physical sense, as it spins down due to frame dragging or other effects and becomes more spherical.

I am not aware of any mechanism that could cause it to slow down.

3 hours ago, hoola said:

Does the hole approach the perfect spherical  shape  in units of multiples of plank units, with no steps between, exhibiting quantum behavior  and if so,  would that  be classically measurable ?

If the shape changed, then it would be continuous (no steps) as GR is a classical theory (so Planck units not relevant).

(That may change if/when we have a theory of quantum gravity.)

[beecee]

17 hours ago, beecee said:

The above is a diagram in line with the best knowledge we have, that may help.

Also I believe you will obtain some good info here....https://jila.colorado.edu/~ajsh/

The above diagram comes from https://www.google.com/search?q=diagram+of+kerr+black+hole&tbm=isch&source=iu&ictx=1&fir=4V9uShC-DtKQeM%3A%2Cbu-ap0qGAmS9qM%2C_&vet=1&usg=AI4_-kQgSMBliLO0AMivK3dyy8VV7CWSWg&sa=X&ved=2ahUKEwjTz56Rw7fiAhVQfisKHU9qCwkQ9QEwBHoECAkQDA#imgrc=4V9uShC-DtKQeM: