# Rotating Charged Black-Hole?

1. Jul 29, 2009

### Chatt

I'm sorry if this is not the right place to ask such question and sorry for misspelling, but here goes

I wonder how Black-Holes can rotate and be charged..
As far as I know a black-hole consists of a singularity which is a point with unlimited density and the event horizon. The singurality is just a point with no dimensions so how can it rotate and since nothing can escape its gravitational pull including electromagnetic radiation how can it have a charge.

I dont know if the answer lies in the event horizon, but as i see it the event horizon is not "made of anything".. Its just the point in space in which the escape velocity becomes higher than the speed of light, so how can it rotate or have a charge?

Thanks in advance, its a great forum :)

Last edited: Jul 29, 2009
2. Jul 29, 2009

### jpreed

Good question,

The answer I believe lies in the conservation of quantum numbers. Even though the black hole vacuums up everything within the event horizon, it still cannot overcome the fact that if it absorbs 3 more electrons than protons, it WILL have a net charge of -3e. Many of the standard rules of physics break down when discussing black holes, but the conservation laws still must hold true.

This includes conservation of charge and conservation of angular momentum.

3. Jul 29, 2009

### tiny-tim

Hi Chatt!
You might as well say that an electron is just a point with no dimensions so how can it have spin?
There is an electric field outside the black hole. A particle moves in a field because of the field where it is, not because of whatever is at the "centre" of the field.
The ergosphere rotates. And the ergosphere is outside the event horizons of a rotating black hole.

4. Jul 29, 2009

### Chatt

The wikipedia page on black-holes (http://en.wikipedia.org/wiki/Black_hole#Properties_and_features) tells us that the effects of charge and rotation of black-holes have both been observed. Guass's law tells us that "the total electric flux going out of a big sphere always stays the same, and measures the total charge inside the sphere" and therefore the magnetic field of the black-hole reach outside the event horizont.

Though at the same time GR tells us that the electromagnetic radiation, which is responsible for the electric and magnetic forces, can never escape the black hole. This must imply (as far as my logic goes) that the electromagnetic radiation is in fact not responsible for the electric and magnetic forces, but is "coincidently" following the path of the magnetic field (at least when you are outside the event horizon). To me it seems that one of the following statements must be wrong:

1. Black-Holes have an electric charge
2. The electric and magnetic forces are the result of electromagnetic radiation
3. Electromagnetic radiation can never escape the event horizon

5. Jul 29, 2009

### Chatt

hmm, sorry if I misunderstand, but isn't the spin in electrons different from what we know as rotation. I mean the electrons have up and down spin although planets and alike is spinning around and axis that can point in any direction.
Im not really into field-theory. But I thought that at least the electric and magnetic forces were the product of electromagnetic radiation.
But the ergosphere is a product of the black-hole rotating in the first place dragging space-time with it.

6. Jul 29, 2009

### v2kkim

I remember that people say some black holes have finite size like a disk so it can rotate with angular momentum, but the disk thickness is zero so still zero volume.

Last edited: Jul 29, 2009
7. Jul 29, 2009

### humanino

Note that classically the electric charge never crosses the horizon for an outside observer. It approaches it infinitely close without ever reaching it. May it be that the outside influence of what has "really fallen" is stuck on the horizon ?

From a quantum point of view, electromagnetic interaction is carried by virtual photons which are not restricted inside a light cone, so they can escape the BH.

8. Jul 30, 2009

### Chatt

Cool thanks, i'll look into it :D