How can a black hole have electric charge?

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Kasuha
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How can a black hole have electric charge?

Post by Kasuha » Fri Feb 19, 2010 1:46 pm

General relativity theory predicts that a black hole can have electric charge. That's fair, I can (theoretically) make a black hole out of protons only and that'd be a lot of electric charge concentrated in it. Let's assume I have created such a black hole and let's assume its charge did not get neutralized by some kind of relativistic effect so there's still only protons inside. If I place an electron and a positron in equal distance from it, will these be attracted differently because of the charge?

Because well, for two particles to interact electromagnetically, they need to exchange a photon. But the black hole has an event horizon and there are no photons coming out of it so I don't really see how could possibly an electrically charged particle interact electromagnetically with what's inside...

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photino
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Re: How can a black hole have electric charge?

Post by photino » Fri Feb 19, 2010 5:01 pm

If I place an electron and a positron in equal distance from it, will these be attracted differently because of the charge?
Yes because they have opposite charges. So one will be attracted by the charged black hole, the other will be repelled.
Because well, for two particles to interact electromagnetically, they need to exchange a photon. But the black hole has an event horizon and there are no photons coming out of it so I don't really see how could possibly an electrically charged particle interact electromagnetically with what's inside...
Wow good question! Shows how difficult things get when you put quantum mechanics and relativity together...

An (admittedly handwaving) answer is that while electromagnetic interactions can be understood as following from the exchange of virtual photons, the word "virtual" is important. Virtual photons do not have to obey quite the same rules as real ones such as those that make up a laser beam. In particular, they can in a sense "cross the horizon".

When you add up the effect of all virtual photons, you find that an electric field around the black hole is produced, if the matter that makes up the black hole is charged. Virtual photons are probably not the easiest or best way to think of this real electric field, though in the end it is equivalent. (Virtual photons are handy for describing interactions between elementary particles, but a black hole pretty much by definition is not an elementary particle - it's a macroscopic object.)

This is no different from the fact that the effect of the mass of the black hole is obviously still felt outside the black hole (the effect is the gravitational attraction). Mass is also a charge, just for the gravitational field.

Somewhere else on the forum people were recommending books, on black holes Kip Thorne's "Black Holes and Timewarps: Einstein's outrageous legacy" is a good place to find out more about this stuff.

Kasuha
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Re: How can a black hole have electric charge?

Post by Kasuha » Wed Feb 24, 2010 8:03 am

Well... if virtual particles don't have to obey laws of real particles, then I'd expect there to be no Hawking radiation - because it is based on assumption that they do.

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Re: How can a black hole have electric charge?

Post by photino » Wed Feb 24, 2010 11:15 am


reak21
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Re: How can a black hole have electric charge?

Post by reak21 » Wed Aug 03, 2011 6:45 am

hello! i'm in junior high school, so please explain it to me in a understandable way...
how can a black hole have an electric charge? since the electromagnetic force messengers are photons, and even they can't escape from behind the horizon, than if there was an electron just outside the horizon, it woludn't be attracted to the black hole because its magnetic charge...
and the other thing i can't get is: even if the black hole has a charge, than what happens to it once the black hole evaporates due to hawking's radiation??? it's whole electromagnetically charged interior would be emmited in form of non-charged photons...

one more question about the black holes: when we analyze them from the point of view of the string theory (if i got the theory right...) than there is no singularity. if i remeber it right brian greene wrote in his book that there is a minimun size an object can have in the string theory. if there is a minimum size an object can have, then the black hole has no singularity, since there can't be any point of infinite density...

it would be nice if someone could reply.

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Re: How can a black hole have electric charge?

Post by DCWhitworth » Thu Aug 04, 2011 9:22 pm

reak21 wrote:hello! i'm in junior high school, so please explain it to me in a understandable way...
how can a black hole have an electric charge? since the electromagnetic force messengers are photons, and even they can't escape from behind the horizon, than if there was an electron just outside the horizon, it woludn't be attracted to the black hole because its magnetic charge...
and the other thing i can't get is: even if the black hole has a charge, than what happens to it once the black hole evaporates due to hawking's radiation??? it's whole electromagnetically charged interior would be emmited in form of non-charged photons...

one more question about the black holes: when we analyze them from the point of view of the string theory (if i got the theory right...) than there is no singularity. if i remeber it right brian greene wrote in his book that there is a minimun size an object can have in the string theory. if there is a minimum size an object can have, then the black hole has no singularity, since there can't be any point of infinite density...

it would be nice if someone could reply.
Try looking at this forum - http://www.physicsforums.com/showthread.php?t=9141

Someone there seems to have asked the same question eight years ago. In fact *exactly* the same, right down to the number of question marks. :doh:
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simpsons
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Re: How can a black hole have electric charge?

Post by simpsons » Fri Feb 03, 2012 12:38 pm

If you want to adopt a particle description of the electromagnetic force, as being mediated by photons, then virtual photons can get out of a black hole because they can travel faster than light. However, we can't observe them, because they're virtual. Real photons, which we can observe, cannot escape.

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Re: How can a black hole have electric charge?

Post by Pantomas » Fri Feb 10, 2012 7:34 am

Hawking radiation can emit particles other than photons, including charged particles. However, this is a still problem, because presumably by the time the hole evaporates, its mass is smaller than the mass of the smallest charged particle; there are no massless charged particles. Nobody understands the final stages of black hole evaporation.

If a black hole evaporates, then presumably it will evaporate to the point where it's mass can no longer sustain the gravitational force necessary to remain a black hole, right? Shouldn't it explode outwards at that point?

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photino
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Re: How can a black hole have electric charge?

Post by photino » Fri Feb 10, 2012 7:29 pm

Pantomas wrote:Hawking radiation can emit particles other than photons, including charged particles. However, this is a still problem, because presumably by the time the hole evaporates, its mass is smaller than the mass of the smallest charged particle; there are no massless charged particles. Nobody understands the final stages of black hole evaporation. If a black hole evaporates, then presumably it will evaporate to the point where it's mass can no longer sustain the gravitational force necessary to remain a black hole, right?
Yes, once the mass of the black hole gets down to the order of the Planck mass, the description of the object as a black hole breaks down and you need a full theory of quantum gravity to understand what happens next.
Pantomas wrote:Shouldn't it explode outwards at that point?
Have a look at the title of Hawking's original paper ;)
http://adsabs.harvard.edu/abs/1974Natur.248...30H

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Re: How can a black hole have electric charge?

Post by Kasuha » Sat Feb 11, 2012 12:15 am

Pantomas is a spammer copy-pasting a post from one forum to another. The original post is from 2003, source is here: http://www.physicsforums.com/showthread.php?t=9141 (already linked on this page)

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Re: How can a black hole have electric charge?

Post by CharmQuark » Sat Feb 11, 2012 7:01 pm

I am trying to keep on top of things......But I do have a lot going on in real life at the minute.
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Lorane Jack
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Re: How can a black hole have electric charge?

Post by Lorane Jack » Fri Apr 27, 2012 6:54 am

Well Guys about this thing I don't have enough knowledge but I just can say about it that If you want to adopt a particle description of the electromagnetic force, as being mediated by photons, then virtual photons can get out of a black hole because they can travel faster than light. Anyways Thanks everyone for your opinions.
Lorane Jack

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