Black hole charge?

[Gilded]

"there are no features that distinguish one black hole from another, other than mass, charge, and angular momentum." (http://en.wikipedia.org/wiki/No_hair_theorem)

Electromagnetic energy transfer happens through transfer of photons, right? And even photons can't escape from a massive black hole. How is it possible to measure the black hole's charge? Are my fundamentals all wrong again or am I missing something very important here?

[[Tycho?]]

I always wondered about this as well, charge seemed like an odd property for a black hole to have.

[Bluenoise]

Hmmm I don't really see how one can have charge... It's like the next step past a neutron star...

[Severian]

Clearly it must have a charge. If an electron-positron pair is created out of the vacuum, and the electron is sucked in while the positron escapes, then the black hole will gain -e in charge. (Also, because U(1) electromagnetic is a local symmetry, one cannot violate charge conservation.)

 

However, this is in the formal sense. Since positrons are just as likely to be sucked in as electrons, and (traditional astrophysical) black holes are made up of lots of particles, you would expect them to be approximately neutral. It is just that they will most likely not be exactly neutral, so they have charge.

[Gilded]

Yes but my point was if the mediating particles of a field (in this case photons) are somehow able to escape the black hole and if so, how?

[Guest_Jim*]

Perhaps the accretion disk around the black hole has a charge that can be detected. What I find odd is how can something that dense can have a difference in charge from side to another. Unless of course it's like an atom where some of the mass with one charge orbits the rest of the mass which has a different charge. In any case they just do and I don't think I've ever seen a reason why they have a charge, unless it's caused by spinning.

It's like the next step past a neutron star...

Neutron stars actually do have charges, because if one has a strong enough magnetic field light will only escape from the poles. If such a neutron star spins you get a pulsar. (I really hope I'm remembering this correctly.)

[Edtharan]

One way that a black hole could aquire a charge is because of collisions in the accretion disk.

 

As matter swirls around in the accretion disk it emits high energy photons (x-rays). These high energy photons could knock electrons form atoms, ionizing the accretion disk. This ionized accretion disk will still have the particles collide.

 

As lighter particles in the disk (electrons) will end up with a higher velocity than heavier particles (protons and ionized atoms), these electrons will more likely have an velocity higher than the escape velocity of the accretion disk (not the black hole its self).

 

This will leave a net positive charge in the accretion disk, that will eventually be transferred to the black hole as the matter in the disk falls into the hole. This should mean that black holes will generally have a positive charge to them.

[insane_alien]

isn't the accretion disk generally a plasma anyways? the collisions betwen atoms(ions if i'm right) would knock electrons off anyway at the temps found in accretion disks before the x-rays did.

[swansont]

Neutron stars actually do have charges, because if one has a strong enough magnetic field light will only escape from the poles. If such a neutron star spins you get a pulsar. (I really hope I'm remembering this correctly.)

 

Magnetic fields do not require a net charge. Neutrons have a magnetic moment even though they are neutral, and a neutron star could have a charge distribution even though it is globally neutral. You get a pulsar when the magnetic field axis is different from the rotation axis. Light doesn't only escape from the poles, though.

[D H]

Yes but my point was if the mediating particles of a field (in this case photons) are somehow able to escape the black hole and if so, how?

 

By exchange of virtual particles across the Schwarzschild radius.

 

See question D.09 "How can gravity escape from a black hole?" in these FAQ.

[[Tycho?]]

isn't the accretion disk generally a plasma anyways? the collisions betwen atoms(ions if i'm right) would knock electrons off anyway at the temps found in accretion disks before the x-rays did.

 

Yeah, if something is hot enough to be emiting x-rays its safe to say its a plasma.

[Edtharan]

isn't the accretion disk generally a plasma anyways? the collisions betwen atoms(ions if i'm right) would knock electrons off anyway at the temps found in accretion disks before the x-rays did.

Yes that would be true. But the collisions in the plasma will still accelerate the electrons faster than protons or neutrons (or charged atoms) and therefore will loose electrons more than protons. The magnetic fields created from the charged plasma orbiting the blac khole might further accelerate the electrons.

[Gilded]

Thanks D H, that answered the question pretty well.

[Guest_Jim*]

Magnetic fields do not require a net charge. Neutrons have a magnetic moment even though they are neutral, and a neutron star could have a charge distribution even though it is globally neutral. You get a pulsar when the magnetic field axis is different from the rotation axis. Light doesn't only escape from the poles, though.

Thank you for correcting me.