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Author Topic: Does a charged black hole emit an electric field?  (Read 8956 times)

Offline JP

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There have been a few questions about charged black holes lately, but I think this very basic question about them hasn't been answered yet.  Will a charged black hole (let's say non-rotating for simplicity) emit an electric field and why/why not?  (I'm talking about the black hole itself, not any field that might be emitted by infalling matter).

By the way, I found the site below on the gravitational forces in a charged black hole (but not the field) and it's fascinating.  The charge makes it behave quite differently than an uncharged black hole.  http://jila.colorado.edu/~ajsh/insidebh/rn.html


 

Offline imatfaal

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Re: Does a charged black hole emit an electric field?
« Reply #1 on: 18/01/2012 17:50:25 »
Quote
Warning: If you fall into a black hole, you will die. You will not go through a wormhole to another time and place.

Nice site - will have to give it a good read
 

Offline MikeS

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Re: Does a charged black hole emit an electric field?
« Reply #2 on: 20/01/2012 09:18:47 »
Would anyone who believes that the internal charge of a black hole can be felt outside the event horizon please suggest by what mechanism they believe this can happen. 

Personally I don't see any mechanism that could allow it to happen.
 

Offline MikeS

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Re: Does a charged black hole emit an electric field?
« Reply #3 on: 20/01/2012 10:28:14 »
JP

Nice site.

It does raise a question in my mind.
It seems the gravitational effects of a black hole can be explained in two ways.
1) Light can not escape because space itself is falling into the black hole at the speed of light (similar to a light-speed waterfall.
2) The geometry of space-time around the black hole is so highly curved (warped) that light follows the curvature and can not escape.
Both scenarios have the same result but presumably only one can be correct.  Which one?
Or may be they are the same thing but we just can't visualise it.

Either way I don't see how electrical charge, that presumably can not travel faster than light, could break free.  The same goes for magnetism.
 

Offline JP

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Re: Does a charged black hole emit an electric field?
« Reply #4 on: 20/01/2012 10:58:13 »
Would anyone who believes that the internal charge of a black hole can be felt outside the event horizon please suggest by what mechanism they believe this can happen. 

Personally I don't see any mechanism that could allow it to happen.

I've heard of 2 methods, but I haven't seen them stated rigorously or in a scientific text, so I'm unsure.  The first is that matter falling into a black hole will appear to take forever to cross the event horizon, so the field from that matter will last forever outside of the black hole.  The problem with this is that the field will get weaker and weaker and this doesn't account for black holes that aren't sucking in charged matter.

The second is that on a more rigorous, quantum level, virtual particles carry the electrostatic force and virtual particles can escape the black hole. 
 

Offline imatfaal

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Re: Does a charged black hole emit an electric field?
« Reply #5 on: 20/01/2012 11:05:15 »
JP

Nice site.

It does raise a question in my mind.
It seems the gravitational effects of a black hole can be explained in two ways.
1) Light can not escape because space itself is falling into the black hole at the speed of light (similar to a light-speed waterfall.
2) The geometry of space-time around the black hole is so highly curved (warped) that light follows the curvature and can not escape.
Both scenarios have the same result but presumably only one can be correct.  Which one?
Or may be they are the same thing but we just can't visualise it.

Either way I don't see how electrical charge, that presumably can not travel faster than light, could break free.  The same goes for magnetism.

Mike - I think, and hopefully JP can correct me, that neither the EM field nor virtual particles are affected by the extreme conditions of the black hole.  Mathematically, it is way beyond my abilities to explain (or even understand this; even theoretically/heuristically I struggle to comprehend it.  CROSSPOSTED WITH JP and he explains it better.

one thought - if we feed a blackhole an electron diet, which it can continue to take cos of the intransmissibility to EM force of  the EH then we have removed a certain amount of negative charge from the universe to beyond the EH.  When the universe cools enough for the BH to evaporate, the radiation that slowly denudes the mass of the blackhole is clearly chargeless; this evaporation will continue at a higher and higher rate till the blackhole melts away in a flash of radiation.  Where has all the charge gone?  We have a firm position on conservation of charge - yet we seem to have lost a bunch.  (I guess this might be the reason behind the white hole theory on the site JP mentioned - which I still havent got around to reading)
 

Offline yor_on

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Re: Does a charged black hole emit an electric field?
« Reply #6 on: 20/01/2012 12:09:29 »
? "As you fall deeper into the black hole, the inward flow of space is slowed by the gravitational repulsion produced by the negative pressure of the radial electric field."

Really?

So (anti) gravity as a "negative pressure of the radial electric field?"

That hurts me head.
 

Offline yor_on

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Re: Does a charged black hole emit an electric field?
« Reply #7 on: 20/01/2012 12:20:01 »
And the idea of space and time changing is only restricted to some conjectures, isn't it (Schwarzschild?) You can use a different geometry as Kerr and get none of that, as I remember it? Also, 'space' doesn't 'propagate' as far as I know. An expansion is not a propagation, although the effect will be equivalent to those observing distant galaxies receding.
==

If I assume that it is a 'motion' then we have FTL as that is one of the postulates following a expansion. And then we also define a aether instead of a 'classical nuthin', as it seems to me?
===

Eh, doesn't mean that it isn't a nice site :)
« Last Edit: 20/01/2012 12:33:19 by yor_on »
 

Offline yor_on

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Re: Does a charged black hole emit an electric field?
« Reply #8 on: 20/01/2012 12:41:05 »
Here we have another approach defining it as Black holes can't be, using the same definition of 'vacuum energy'. I don't see why it needs to be gravitationally 'repulsive'. As far as I know we don't have a description for 'gravity' yet, only hypothesis's, and the best description, if so, will still be Einsteins.
=

ahem here, and reaching quite another conclusion.
« Last Edit: 20/01/2012 12:46:08 by yor_on »
 

Offline MikeS

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Re: Does a charged black hole emit an electric field?
« Reply #9 on: 21/01/2012 13:07:50 »
Would anyone who believes that the internal charge of a black hole can be felt outside the event horizon please suggest by what mechanism they believe this can happen. 

Personally I don't see any mechanism that could allow it to happen.

I've heard of 2 methods, but I haven't seen them stated rigorously or in a scientific text, so I'm unsure.  The first is that matter falling into a black hole will appear to take forever to cross the event horizon, so the field from that matter will last forever outside of the black hole. The problem with this is that the field will get weaker and weaker and this doesn't account for black holes that aren't sucking in charged matter.

The second is that on a more rigorous, quantum level, virtual particles carry the electrostatic force and virtual particles can escape the black hole. 

I don't see as that is addressing the question (below)
There have been a few questions about charged black holes lately, but I think this very basic question about them hasn't been answered yet.  Will a charged black hole (let's say non-rotating for simplicity) emit an electric field and why/why not?  (I'm talking about the black hole itself, not any field that might be emitted by infalling matter).
The second is that on a more rigorous, quantum level, virtual particles carry the electrostatic force and virtual particles can escape the black hole.



If a virtual particle does not become a real particle it will ultimately vanish, where does the charge go?  Or is it a virtual charge?
« Last Edit: 21/01/2012 13:09:30 by MikeS »
 

Offline MikeS

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Re: Does a charged black hole emit an electric field?
« Reply #10 on: 21/01/2012 13:37:51 »
JP


Clip
one thought - if we feed a blackhole an electron diet, which it can continue to take cos of the intransmissibility to EM force of  the EH then we have removed a certain amount of negative charge from the universe to beyond the EH.  When the universe cools enough for the BH to evaporate, the radiation that slowly denudes the mass of the blackhole is clearly chargeless; this evaporation will continue at a higher and higher rate till the blackhole melts away in a flash of radiation.  Where has all the charge gone?  We have a firm position on conservation of charge - yet we seem to have lost a bunch.  (I guess this might be the reason behind the white hole theory on the site JP mentioned - which I still havent got around to reading)

Good point.
The black hole is slowly radiating away energy via Hawkin Radiation.  Of every pair of particles produced just outside the event horizon lets assume that one is sucked in and the other escapes.  It is supposedly a random process as to which particle is sucked in.  Let's assume that all the particles that get sucked in (or a majority) are electrons.  The black hole will gradually acquire a negative charge.  When it eventually evaporates where did the charge go?
 

Offline Soul Surfer

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Re: Does a charged black hole emit an electric field?
« Reply #11 on: 21/01/2012 19:16:01 »
The original question contains a totally fundamental error that is very common in questions of this type and may even be in the minds of some of those who are replying.  Particles do not emit fields they possess them.  An electron has an electrical field, a magnetic field and also a gravitational one.  it is only when a great many particles are lumped together to form a planet or a star that the gravitational field is noticeable.  Now matter in large quantities is mostly electrically neutral but can be electrically charged and in some cases magnetically polarised and these fields are well known and obvious in all the simple science experiments.

A black hole has a strong gravitational field because it contains a lot of material in a small volume.  It is very likely to be electrically neutral but if it had an electrical field or a magnetic one (more likely) it would  exist outside the event horizon because it does NOT emit it it possesses it as an inherent property of the object.  To that extent it is nothing more or less than a very large heavy particle no different from an electron.
 

Offline JP

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Re: Does a charged black hole emit an electric field?
« Reply #12 on: 21/01/2012 22:58:26 »
SS, I see your point, but I don't completely agree with it.  A particle "possesses" mass, since that mass is an intrinsic property of the particle.  A particle does not possess an electric field in that sense, since that field extends beyond the particle, can act at a distance and any perturbations to the particle propagate outwards at the speed of light.  Certainly if I cause the particle to oscillate, it does emit photons due to disturbances to its electric field.

I think there it is good point that the field isn't being emitted and propagating outward from the singularity.  The black hole's event horizon contains charge information somehow, so that a field can extend out from the event horizon. 
« Last Edit: 21/01/2012 23:00:16 by JP »
 

Offline JP

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Re: Does a charged black hole emit an electric field?
« Reply #13 on: 21/01/2012 23:07:06 »
Also, as usual for GR questions, I found a page on Baez's site that answers it pretty satisfactorily:

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_gravity.html

The one sentence summary is that the electric field (and gravitational field) are "frozen" into the event horizon of a black hole, which is sufficient to create a field outside the black hole. 
 

Offline Soul Surfer

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Re: Does a charged black hole emit an electric field?
« Reply #14 on: 21/01/2012 23:12:34 »
JP you are wrong every individual particle possesses a gravitational field in just the same way as it may have an electrical field and this field from each individual particle extends out indefinitely.  It is just that this field is so small that is only obvious when vast numbers of particles congregate together reasonably closely.

How else would you expect it to be possible that vast volumes of cold gas with about one atom of hydrogen in every cubic metre could collapse gravitationally to form a star.
« Last Edit: 21/01/2012 23:14:42 by Soul Surfer »
 

Offline JP

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Re: Does a charged black hole emit an electric field?
« Reply #15 on: 22/01/2012 03:28:19 »
A field can only extend out indefinitely if the particle has been around indefinitely.  Otherwise you violate causality since the front of the field propagates faster than c.  Of course, you can argue that all matter/energy has existed in some form since the big bang, which may essentially be indefinite in terms of how far the fields can go.  I don't know enough cosmology to address that.  What I do know is that if you move a charged particle, that motion generates a "kink" in the EM field, which propagates away at the speed of light, which seems to argue to me that the field is being emitted from the particle. 

At any rate, whether you like the word "emit" or not, I think I understand how a charged black hole can imprint its charge onto the event horizon, since the electric field associated with matter creating the black hole is "frozen" on the event horizon, and the electric field is going to extend continuously out from the horizon.
« Last Edit: 22/01/2012 03:41:18 by JP »
 

Offline MikeS

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Re: Does a charged black hole emit an electric field?
« Reply #16 on: 22/01/2012 08:02:47 »
The original question contains a totally fundamental error that is very common in questions of this type and may even be in the minds of some of those who are replying.  Particles do not emit fields they possess them.  An electron has an electrical field, a magnetic field and also a gravitational one.  it is only when a great many particles are lumped together to form a planet or a star that the gravitational field is noticeable.  Now matter in large quantities is mostly electrically neutral but can be electrically charged and in some cases magnetically polarised and these fields are well known and obvious in all the simple science experiments.

A black hole has a strong gravitational field because it contains a lot of material in a small volume.  It is very likely to be electrically neutral but if it had an electrical field or a magnetic one (more likely) it would  exist outside the event horizon because it does NOT emit it it possesses it as an inherent property of the object. To that extent it is nothing more or less than a very large heavy particle no different from an electron.

Whether it is emitted or possessed, it still has to be felt outside the EH.  If space is flowing into the EH at the speed of light or space-time is highly curved I still can't see how this can happen.
 

Offline MikeS

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Re: Does a charged black hole emit an electric field?
« Reply #17 on: 22/01/2012 08:15:36 »
A field can only extend out indefinitely if the particle has been around indefinitely.  Otherwise you violate causality since the front of the field propagates faster than c.  Of course, you can argue that all matter/energy has existed in some form since the big bang, which may essentially be indefinite in terms of how far the fields can go.  I don't know enough cosmology to address that.  What I do know is that if you move a charged particle, that motion generates a "kink" in the EM field, which propagates away at the speed of light, which seems to argue to me that the field is being emitted from the particle. 

At any rate, whether you like the word "emit" or not, I think I understand how a charged black hole can imprint its charge onto the event horizon, since the electric field associated with matter creating the black hole is "frozen" on the event horizon, and the electric field is going to extend continuously out from the horizon.

That sounds like it might be possible.
The way I understand it is, as a memory of what was sucked in frozen in time.  That way a black hole could have electrical charge or magnetism felt on the outside of the EH.  But I don't see how it could be any more than that.  It is still not propagating from inside the EH.

It does raise another question in my mind.  If the electrical charge or magnetism is frozen in time, can it be experienced, measured?  In other words will it extend continuously out from the horizon?

The only place it will be frozen in time is at the EH.  If it were light then this would be the place where it vanishes from view.  Presumably charge and magnetism would also vanish from 'view' at the same place.
In other words charge or magnetism from inside the black hole or from the EH itself would not be felt outside the EH.  Only matter approaching but not in the event horizon could have any noticeable effect outside of the EH.
« Last Edit: 22/01/2012 15:55:55 by MikeS »
 

Offline yor_on

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Re: Does a charged black hole emit an electric field?
« Reply #18 on: 23/01/2012 15:41:11 »
Would anyone who believes that the internal charge of a black hole can be felt outside the event horizon please suggest by what mechanism they believe this can happen. 

Personally I don't see any mechanism that could allow it to happen.

I've heard of 2 methods, but I haven't seen them stated rigorously or in a scientific text, so I'm unsure.  The first is that matter falling into a black hole will appear to take forever to cross the event horizon, so the field from that matter will last forever outside of the black hole.  The problem with this is that the field will get weaker and weaker and this doesn't account for black holes that aren't sucking in charged matter.

The second is that on a more rigorous, quantum level, virtual particles carry the electrostatic force and virtual particles can escape the black hole. 

Interesting, and both have to do with how you think of 'time' or its arrow. From locality I expect everything to 'move' in its own time, only 'frames of reference'  becoming 'displaced' relative each other.  'Time', or its arrow, is a very elusive participant in that it is communicated by the constant 'c', also between those 'frames of reference', well, as I see it.

It makes a lot of sense defining a black hole as 'frozen' from our frame of reference, but not from itself.
 

Offline JP

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Re: Does a charged black hole emit an electric field?
« Reply #19 on: 24/01/2012 13:36:01 »
I don't believe any of it has to do with how you see time's arrow (but we probably need a general relativity expert to really understand all this in detail!)  What I've gathered from the Baez page and my own knowledge of GR is that electromagnetism is a local phenomena, meaning that the field at one point in space-time is going to be related to the fields at all the points around it in space-time.  If you have an (admittedly unlikely) charged, non-rotating star, for example, you can find an electric field outside of it by looking at the field right at the surface of it, which in turn is related to the charges on the star. 

As the star collapses into a black hole, the field doesn't disappear.  We can't see the charges anymore, since they've fallen into the black hole, but the electric field lines still lead to the event horizon.  Since that's the case, since the field as you move away from the event horizon is related to the field at the event horizon, you can get a field coming out of the black hole.

This seems pretty much the same as how a black hole can generate a gravitational field.  As a star collapses into a black hole, its gravitational field remains at the event horizon, even though we can't peek inside.  Since gravity at a point is related to gravity at neighboring points, the event horizon field is sufficient to describe the field away from the event horizon.

I see how this related to SS's argument about a black hole "possessing" rather than emitting a field as well.  The fact that there was a field before matter fell into a black hole is sufficient to maintain a field after it became a black hole.  The field doesn't disappear just because we can't see the source anymore. 
 

Offline MikeS

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Re: Does a charged black hole emit an electric field?
« Reply #20 on: 25/01/2012 11:55:56 »

As the star collapses into a black hole, the field doesn't disappear.  We can't see the charges anymore, since they've fallen into the black hole, but the electric field lines still lead to the event horizon.  Since that's the case, since the field as you move away from the event horizon is related to the field at the event horizon, you can get a field coming out of the black hole.

This seems pretty much the same as how a black hole can generate a gravitational field.  As a star collapses into a black hole, its gravitational field remains at the event horizon, even though we can't peek inside.  Since gravity at a point is related to gravity at neighboring points, the event horizon field is sufficient to describe the field away from the event horizon.

I see how this related to SS's argument about a black hole "possessing" rather than emitting a field as well.  The fact that there was a field before matter fell into a black hole is sufficient to maintain a field after it became a black hole.  The field doesn't disappear just because we can't see the source anymore. 

I still can't see it as I said previously
"The only place it will be frozen in time is at the EH.  If it were light then this would be the place where it vanishes from view.  Presumably charge and magnetism would also vanish from 'view' at the same place.
In other words charge or magnetism from inside the black hole or from the EH itself would not be felt outside the EH."  Only matter approaching but not in the event horizon could have any noticeable effect outside of the EH.
Anything at the event horizon is going to 'vanish from view' leaving no outside trace or memory.  Without something new falling  into the EH the frozen memory will vanish. 

This need not apply to gravity if we consider gravity to be the curvature of space-time.  The curvature is not a frozen memory and therefore will not fade with time.  Likewise if the gravity of a black hole is considered as a c velocity 'waterfall' it is continuous, not a frozen memory.
 

Offline JP

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Re: Does a charged black hole emit an electric field?
« Reply #21 on: 25/01/2012 15:46:07 »
Ah, but the way I've read the Baez page, the charge isn't being "felt," the field is, and if the field exists on the event horizon in the proper way, a field will be felt further away, also in the proper way.  The properties of the event horizon "freeze" the field there in such a way that is it felt far from the black hole.

Similarly, if I had some charged particles inside a sphere here on the earth, I could predict the field outside based on just knowing the charges, but I could also predict it by knowing the field distribution over the sphere.
 

Offline MikeS

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Re: Does a charged black hole emit an electric field?
« Reply #22 on: 26/01/2012 07:02:22 »
JP
You say "The properties of the event horizon "freeze" the field there in such a way that is it felt far from the black hole."

But the field can only be frozen right at the EH, not approaching it.  If it is at the EH then it is also at the vanishing point of light (charge and magnetism).

 A field would have to propagate faster than light for it to be felt outside the EH.  I'm not at all convinced.

To my mind a black hole should be a black hole, not just a black hole as regards light.
 

Offline imatfaal

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Re: Does a charged black hole emit an electric field?
« Reply #23 on: 26/01/2012 10:16:28 »
IF you look at it from a qft point of view - which you must be able to do - then you have no problem.  virtual photons - the gauge bosons of electromagnetic force are quite capable of escaping.  jp can correct me but I believe all gauge bosons are non-information bearing and are not restricted to light speed.  the event horizon just isn't for gauge bosons. 

how this is unified with the classical view is difficult - baez's idea of the field and charge emitting particles being frozen is cool, but at the event horizon itself there is no (or much much less) time dilation for two free falling particles.  if you had two test particles falling almost together, one +ve and one -ve , as the +ve crosses the point of no return, the -ve a few metres back is still attracted, but it cannot be due to a "frozen field" because to a free falling particle close to the EH the time dilation difference is small.

Personally I do not understand why the electromagnetic field must be doing strange things at the EH - true the gravity is high, and through GR the curvature is high - but the EM field does not require the spacetime continuum as some form of aether to propagate through.

 

Offline JP

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Re: Does a charged black hole emit an electric field?
« Reply #24 on: 26/01/2012 13:32:51 »
Matthew, I think you're right that quantum field theory and virtual particles has to be another way of explaining all this.  I bet it also goes deeper than the classical theory, since we know Hawking radiation and the evaporation of black holes goes beyond classical theories.  I have some vague idea of how GR works and a much better idea of how quantum field theory works, but very little idea of how the two work together, so I can't really comment on it much beyond that.  :p

On the Baez idea of "frozen" field on the event horizon--I'm not sure frozen is the best word for it, though I used it above.  Maxwell's equations can be formulated on curved space-time in a straightforward way.  Since they all involve first derivatives in space and time, they basically relate the field at one point in space and time to the field at another point.  Ultimately, these fields can be traced back to charges somewhere in space-time.  But if you're not next to a charge, the equations still relate the field at one point in space-time to neighboring points (because of the first derivatives).  So if you're outside the event horizon, you can't actually "see" the charges inside the black hole, but if a field exists on the event horizon, a field can exist outside due to Maxwell's equations, even if you can't see the charges directly. 
 

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Re: Does a charged black hole emit an electric field?
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