PDA

View Full Version : How would electrical charge radiated from a black hole?



WaxRubiks
2008-Sep-12, 06:04 AM
I think it is said that a black hole can have electrical charge.

So how could a positively, or negatively charged black hole radiate away its charge, if Hawking radiation is just neutral EM radiation?

grant hutchison
2008-Sep-12, 10:49 AM
I think it is said that a black hole can have electrical charge.

So how could a positively, or negatively charged black hole radiate away its charge, if Hawking radiation is just neutral EM radiation?There may be some subtle way it could manage it under acceleration; I dunno about that.
But they don't really need to emit charged particles to get rid of their charge: they'll preferentially attract and absorb opposite charges until they get to electrical neutrality.

Grant Hutchison

korjik
2008-Sep-12, 03:33 PM
Hawking radiation is particle radiation is how

phunk
2008-Sep-12, 06:07 PM
I don't think he's asking how it could be neutralized, but rather how a charged black hole could have an electric field around it in the first place. As in, why isn't the electric field pulled in also.

mugaliens
2008-Sep-12, 06:15 PM
Given that both charge (electric field) and gravity vary proportional to the inverse of the square of the distance, the ratio between charge and gravitation would remain constant as distance changes, and it's well proven that charge can easily overcome gravity for particles.

Theophage
2008-Sep-13, 03:57 PM
I think it is said that a black hole can have electrical charge.

So how could a positively, or negatively charged black hole radiate away its charge, if Hawking radiation is just neutral EM radiation?

I don't think Hawking radiation is neutral EM radiation. From what I understand, it goes like this:

Virtual particles, which are being created and destroyed all the time everywhere, are also being created and destroyed around the event horizon of a black hole. They are created in pairs, which together are electrically neutral. What happens is that sometimes one of the pair falls into the black hole and cannot escape to annhilate the other one before the time alotted by conservation of energy. The net effect of this is that the mass of the particle which escapes is now subtracted from the mass of the hole, since it was the hole which "promoted" it from temporary virtual particle to actual particle with mass.

Now, since sometimes the escaping particles are charged, this would mean that the black hole would constantly be gaining and losing small amounts of charge through this process. Overall this would average out, and leave the hole's net charge unchanged. But consider a black hole that has some net charge on it. Such a hole would be more likely to push like charged particles away, and so those particles would more likely be the ones that escaped. Just like the fact that a massive particle which escapes subtracts mass from the hole, so it is that if a charged particle escapes, the hole is considered to have lost that amount of charge. If this is the case, then the net charge of the hole over time would decrease down to neutral, at which point there would again be no preference for particles of a certain charge to escape.

mugaliens
2008-Sep-14, 09:05 PM
I don't think he's asking how it could be neutralized, but rather how a charged black hole could have an electric field around it in the first place. As in, why isn't the electric field pulled in also.

Given that the electric field effect on most charged particles is significantly stronger than gravitation, Grant brings up a good point - no matter how positively or negatively charged a black hole may be initially, over time it'll tend towards a charge of zero by attracting more opposite particles than like ones.