# Black Hole and Electric Charge

1. Oct 14, 2013

### Philosophaie

A galaxy is many many light years away. All Electric Charge, Q, must be dissipated. How do you tell if charge exists and what magnitude.

2. Oct 14, 2013

### Simon Bridge

How do you mean "dissipated"?

Note: we can tell there are charged particles in distant galaxies by examining the light.
For instance, we discover that there are hydrogen spectral lines in the light from distant galaxies ... this suggests the presence of hydrogen which is composed of a positive and a negative charge.

Either that or something else we don't know about gives rise to the spectra... apply Occam's Razor.

3. Oct 14, 2013

### Philosophaie

Can you tell the mass, charge and spin thru the Hydrogen Spectral Lines? Does the band on the hydrogen spectrum get distorted to reveal these things? Can a Radio Telescope find these quantities also? What are found with Radio waves? Also how do you find the mass, charge and spin of a black hole?

4. Oct 14, 2013

### Simon Bridge

So that was what you meant:
The presence of hydrogen spectral lines says there is hydrogen present... which says their must be an electron and a proton in a bound state. Can you find any other situation where such a spectra is found in nature?

Thus - masses, charges, and spins.
The hydrogen that gave that spectra must have the same masses and charges and spins as the local stuff or that spectra won't happen.

But we don't need to go just by hydrogen spectra alone - we also see the spectra for other atoms and molecules in starlight and we can compare them with local sources. So we don't just see evidence of hydrogen, we se evidence of the kinds of chemistry that we get here too and so on. Its like doing a crossword puzzle -several words may fit the clue and the available spaces, but, when you consider the rest of the puzzle, it is very unlikely there is more than one set of words that satisfy the clues, and interlock in the right way to fit the puzzle.

Now we could just assume that physics is very different in distant parts of the Universe ... different in just the right sort of way that makes it look exactly like the same processes are happening there that happen here even though they don't.

That would be like doing a crossword but assuming a different language and different grammar and different meanings of words and still getting the same letters filling the same squares.

....... but what has all this got to do with Black Holes?

Last edited: Oct 14, 2013
5. Oct 14, 2013

### atyy

Astrophysical black holes are thought to be Kerr black holes, which are uncharged, because astrophysical black holes are surrounded by matter which would quickly neutalize the charge. I think there hasn't yet been definitive observational evidence that the spacetime around astrophysical black holes is described by the Kerr metric. How one might get such data is discussed in
http://arxiv.org/abs/0903.0100
http://www.slac.stanford.edu/econf/C0507252/papers/L006.PDF

6. Oct 14, 2013

### Simon Bridge

Hah ... I was just coming from there...

7. Oct 16, 2013

### stevebd1

Here are a couple of links that relate to measuring charge and spin for a black hole-

Black holes may gain extra charge when swallowing matter

New method to measure black hole spin rate raises questions

Normally the amount of spin can be calculated by the location of the inner edge of the accretion disk (the marginally stable orbit or MSO) which is at r=6M for a static black hole and at r=1M for a black hole at maximum spin.

Also, if you could observe directly, the coordinate radius of the event horizon reduces with spin and charge-

$$r_+=M+\sqrt{M^2-a^2-Q^2}$$
where r+ is the event horizon, M, a and Q are mass, spin and charge in geometric units where M=Gm/c2, a=J/mc and Q=C(G/(4∏ε0))1/2/c2

http://en.wikipedia.org/wiki/Geometrized_unit_system

Last edited: Oct 16, 2013
8. Oct 16, 2013

### Philosophaie

Cutting edge stuff.

The Hydrogen Spectrum shows a Red Shift in most Galaxies at the 1420.8Hz or so point. How does it show charge? Is it calculate from the velocity of the stars around the Black Hole?

The Charge is $Q=\sqrt{\frac{C^2*G}{(4\piε_0)*c^4}}$.

Is there some examples of values for C, even though it may be exploding Star matter,
of galaxies that have a charge, C, near the center and maybe a range of what the charges, C, can be.

Last edited: Oct 16, 2013
9. Oct 17, 2013

### stevebd1

The range of charge for a BH is Q≤M and if spin is included then Q2+a2≤M2.

http://www.fysik.su.se/~narit/bh.pdf [Broken]

Last edited by a moderator: May 6, 2017