Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: cowlinator on 09/12/2016 04:31:28

Title: What limits the angular momentum of a black hole?
Post by: cowlinator on 09/12/2016 04:31:28

What limits the angular momentum of a black hole?

I've seen articles state that a rotating black hole has a theoretical maximum angular momentum.

Obviously, any object with volume would be limited by the speed of light.  The surface at the equator of a sphere cannot travel faster than light, which limits the spin.

But a black hole is supposed to contain a singularity with no volume.  And the surface of the event horizon isn't a physical phenomenon, so it isn't limited by the speed of light, right?

Title: Re: What limits the angular momentum of a black hole?
Post by: Novaflipps on 09/12/2016 05:53:04

According to schwarzschild radius, it has a voulme and mass. Singularity would not exist in a black whole, and a event horizon is just a phenomen we came up with because we dont understand the black hole, Its just a black sphere with huge gravity. I might be wrong, but we really don't understand the black hole imo. A singularity says to have infinite gravity, wich imo could not be, cause then it would suck up the entire universe. Infinite is imo also a therm that we invent cause we dont understand the universe fully.

rotating black hole has a theoretical maximum angular momentum and it probaly right. Thats another thing. If something has a spin, it would be the limit of how small it can be. If the earth would beome a black hole, shrink to schwarzschild radius, it would spin 1100 times faster than light wich is not possible accourding to relativistics laws

I know, im probably totaly out there.

Title: Re: What limits the angular momentum of a black hole?
Post by: jeffreyH on 09/12/2016 06:02:30

Quote from: cowlinator on 09/12/2016 04:31:28

What limits the angular momentum of a black hole?

I've seen articles state that a rotating black hole has a theoretical maximum angular momentum.

Obviously, any object with volume would be limited by the speed of light.  The surface at the equator of a sphere cannot travel faster than light, which limits the spin.

But a black hole is supposed to contain a singularity with no volume.  And the surface of the event horizon isn't a physical phenomenon, so it isn't limited by the speed of light, right?

The effects close to the event horizon are a physical phenomenon. If the black hole is rotating you have an ergosphere which bulges at the equator and meets the event horizon at the poles. Nothing can remain stationary within the ergosphere. Objects are subject to frame dragging and will automatically rotate about an axis in line with the poles of the black hole.

As far as a maximum angular momentum I cannot say. Maybe others can answer that.

Title: Re: What limits the angular momentum of a black hole?
Post by: evan_au on 09/12/2016 09:58:54

For a rotating (Kerr) black hole, there is a certain level of angular momentum for which the equations describing the event horizon don't have a real solution. This implies that the singularity may be exposed.

Some physicists are afraid of the paradoxes that might occur if it were possible to travel to a singularity (and return); perhaps they prefer to think that this angular momentum is a maximum limit?

See: https://en.wikipedia.org/wiki/Kerr_metric#Overextreme_Kerr_solutions

Title: Re: What limits the angular momentum of a black hole?
Post by: jeffreyH on 10/12/2016 05:22:29

The mathematics of the metric mentioned by Evan can be found at the following link.

https://en.m.wikipedia.org/wiki/Naked_singularity (https://en.m.wikipedia.org/wiki/Naked_singularity)