Virtual Photons and Black Holes

0 views
Skip to first unread message

vernonner3voltazim

unread,
Dec 14, 2004, 11:19:30 AM12/14/04
to

I've read that if you dump a bunch of protons (or electrons)
into a black hole, the hole as a whole will begin to express
a net electric charge. This bothers me because electric
fields are described as being represented by virtual
photons "in action", and as we all know, photons are not
allowed to escape a black hole.

Now I do know that ordinary photons and virtual photons
are not quite identical. ONE significant difference is
that virtual photons cannot constantly possess a certain
amount of energy, as do ordinary photons. The Uncertainty
Principle lets virtual photons pop into existence with
some random amount of energy, and as Time passes, they
have to "pay back" what they borrowed. Based on the way
that Energy/Time Uncertainty equation is expressed, it
can be shown that the "curve" of a virtual photon's energy
content (on Y-axis with Time on X-axis) is identical in
shape to that of the function 1/x. It's content of
borrowed can be enormous at T=tiny-fraction-of-second,
and it falls to zero only as "forever" is reached.
Which gives the Electromagnetic Force infinite range,
of course.

At least the preceding is true when the source of virtual
photons, like an electric charge, is not located inside
a black hole. I suppose the most important Question that
can be asked is whether or not virtual photons interact
with the virtual gravitons that are (in a Quantum theory
of Gravity) going to be so enormously abundant in the
vicinity of a black hole. If they interact, then I don't
see how virtual photons are at all more likely to escape
the hole than ordinary photons. Well, yes, I'm aware
that virtual photons have been claimed to be able to
travel at speeds different from c (especially can they go
FTL), BUT (1) in a prior discussion here, it was indicated
that Feynman's "sum of histories" method is indeed more
of a useful tool than a True Description of Reality, and
(2) if they don't travel at c, then why should we continue
to think of them as being photons? That is, why should
modifying a particle's NAME with "virtual" also let us
modify such an important thing about its Existence that
photons (not to mention other virtual particles, some of
which possess ordinary and not imaginary mass) can go
Faster-Than-Light?

Next, please recall those "cosmic jets" that we see here
and there in our telescopes. It is assumed that those jets
of high-speed matter and energy BECAME jets by interacting
with extremely intense magnetic fields. Yet black holes
are supposed to be involved, too. I'm pretty sure I've
read that a black hole cannot sustain a magnetic field, and
so the assumed jet-collimating-fields must be associated
with the accretion disks surrounding the holes. WELL!!!
If a BH cannot have a magnetic field (which is another form
of virtual photons in action), then why should we think that
the BH can exhibit an electric charge? On the other hand!
If the BH COULD sustain an electric charge, then it SHOULD
also be able to sustain a magnetic field! As for what the
Theorists have to say about this apparent discrepancy, I
don't know. Perhaps what I read was wrong?

Thanks in advance!

tes...@tum.bot

unread,
Jan 8, 2005, 5:05:17 PM1/8/05
to
vernonner3voltazim <vne...@pinn.net> wrote:

> I've read that if you dump a bunch of protons (or electrons)
> into a black hole, the hole as a whole will begin to express
> a net electric charge.

According to gtr (or any other reasonable theory), yes.

> This bothers me because electric
> fields are described as being represented by virtual
> photons "in action",

But that's not the classical field theory description, so you can't expect
to naively make sense of this in the context of a classical relativistic
field theory like gtr.

If you complain that it seems unnatural that our current gold standard
theory of gravitation is a relativistic -classical- field theory (gtr),
whereas our gold standard theory for electrodynamics is a relativistic
-quantum- field theory (QED), well, no argument from me, in fact, this is
a major motivation for all that work on quantum gravity and/or grand
unification.

> and as we all know, photons are not allowed to escape a black hole.

Gtr is a classical field theory, and it says that laser pulses fired by
some observer falling -inside- the horizon cannot escape to the exterior.
But this doesn't mean that the hole cannot continue to possess a
gravitational field after formation, or even that it cannot acquire an EM
field after formation (if you toss in some charged matter, thus increasing
its mass and giving it a charge). See the FAQ or look for numerous past
posts to this newsgroup, explaining how this can be.

<snip discussion based on a false premise>

> Next, please recall those "cosmic jets" that we see here and there in
> our telescopes.

Yes, jets are associated with many "quasars" (very distant galaxies,
observed here and now as they were when they were very young).

> It is assumed that those jets of high-speed matter and energy BECAME
> jets by interacting with extremely intense magnetic fields. Yet black
> holes are supposed to be involved, too.

Most galaxies now believed to harbour supermassive black holes, which are
thought to drive such jets. Various astrophysical models have been
proposed for exactly how they might be generated. These are currently the
subject of hot debate in astrophysics.

The best known model is called the "Blandford-Znajek mechanism". Very
roughly, this model postulates that in a young galaxy possessing a
supermassive black hole, dust near the hole is continually drawn into a
rotating "accretion disk" (think of the rings of Saturn). Friction heats
up and ionizes atoms inside the disk. Now we have orbiting charged
matter; this creates a more or less steady state EM field. Near the
horizon, the magnetic field lines are highly compressed. But ions
continually fall off the inner edge of the accretion disk and plunge
toward the hole. As they encounter the intense magnetic field near the
horizon, they are whipped out along the axis of rotation, forming two huge
jets of relativistic material. As each jet rams into the surrounding
dust, they form a shock wave which explosively heats the surrounding
material. This results in steady radiation which we see from Earth as a
brilliant glowing "double lobe" surrounding the hole.

Ultimately, the energy stoking this beacon is the rotational energy of the
hole itself. According to gtr, a sizeable fraction of the mass-energy of
a rapidly rotating hole can eventually be extracted in this way, which
explains why these double jets can apparently persist for millions of
year. Eventually, things quiet down as material near the hole is
gradually exhausted and/or as it spins down. Nowadays, in our own galaxy,
we still possess our very own supermassive black hole, but this is no
longer producing a brilliant double jet. But when the Milky Way was
young, it too may have been a quasar!

Some might complain that this scenario merely shoves the problem of
explaining how the jets were formed back onto the problem of explaining
how the supermassive black holes were formed. But that's precisely how
progress occurs in science!

Unfortunately, most gtr textbooks, and even many cosmology textbooks, do
not discuss this important topic, but I offer some references below.

> I'm pretty sure I've read that a black hole cannot sustain a magnetic
> field,

I can think of two ways you might have gotten that impression:

1. The classical "no hair" theorem is often stated: "an isolated
stationary black hole is characterized by its mass, angular momentum, and
electric charge". But of course, this does not mean that no observer
measures a magnetic field; for example Boyer-Lindquist observers orbiting
a Kerr-Newman hole measure a magnetic field; so do infalling inertial
observers ("Doran observers"). Also, the no hair theorem refers to the EM
field of the hole itself, and assumes the hole is -isolated- (far from any
other matter or EM fields). But the jets are not associated with
-isolated- holes, and the BZ model assumes that the EM field originates in
the surroundin gaccretion disk.

2. The "membrane paradigm" for "black hole electrodynamics" is a
convenient way of discussing the physics of a black hole immersed in an
external EM field. This "membrane" is an imaginary surface located just
-outside- the horizon (also an imaginary surface), and we pretend that it
is something like the physical surface of an object with certain
properties. This would not be appropriate for studying matter falling
under the horizon, obviously, but it has proven useful for studying things
like energy transport in jet models, etc. You might have seen a statement
to the effect that "the hole expels magnetic field lines". But you should
consider this part of the explanation for why the magnetic field becomes
intense near the horizon.

> and so the assumed jet-collimating-fields must be associated with the
> accretion disks surrounding the holes.

Yes.

> WELL!!! If a BH cannot have a magnetic field (which is another form of
> virtual photons in action), then why should we think that the BH can
> exhibit an electric charge? On the other hand! If the BH COULD sustain
> an electric charge, then it SHOULD also be able to sustain a magnetic
> field! As for what the Theorists have to say about this apparent
> discrepancy, I don't know. Perhaps what I read was wrong?

Did I clear up your confusion above?

References: for "no hair", the Kerr/Newman electrovacuum, and black hole
electrodynamics, see Frolov & Novikov, Black Hole Physics, Kluwer, 1998.
For Doran observers in Kerr/Newman, look for previous posts to this group.
For jets, try the ArXiV.

"T. Essel"

(spelunking somewhere in cyberspace)

tes...@tum.bot

unread,
Jan 8, 2005, 5:08:09 PM1/8/05
to
vernonner3voltazim <vne...@pinn.net> wrote:

> I've read that if you dump a bunch of protons (or electrons)
> into a black hole, the hole as a whole will begin to express
> a net electric charge.

According to gtr (or any other reasonable theory), yes.

> This bothers me because electric


> fields are described as being represented by virtual
> photons "in action",

But that's not the classical field theory description, so you can't expect


to naively make sense of this in the context of a classical relativistic
field theory like gtr.

If you complain that it seems unnatural that our current gold standard
theory of gravitation is a relativistic -classical- field theory (gtr),
whereas our gold standard theory for electrodynamics is a relativistic
-quantum- field theory (QED), well, no argument from me, in fact, this is
a major motivation for all that work on quantum gravity and/or grand
unification.

> and as we all know, photons are not allowed to escape a black hole.

Gtr is a classical field theory, and it says that laser pulses fired by


some observer falling -inside- the horizon cannot escape to the exterior.
But this doesn't mean that the hole cannot continue to possess a
gravitational field after formation, or even that it cannot acquire an EM
field after formation (if you toss in some charged matter, thus increasing
its mass and giving it a charge). See the FAQ or look for numerous past
posts to this newsgroup, explaining how this can be.

<snip discussion based on a false premise>

> Next, please recall those "cosmic jets" that we see here and there in
> our telescopes.

Yes, jets are associated with many "quasars" (very distant galaxies,


observed here and now as they were when they were very young).

> It is assumed that those jets of high-speed matter and energy BECAME


> jets by interacting with extremely intense magnetic fields. Yet black
> holes are supposed to be involved, too.

Most galaxies now believed to harbour supermassive black holes, which are

> I'm pretty sure I've read that a black hole cannot sustain a magnetic
> field,

I can think of two ways you might have gotten that impression:

1. The classical "no hair" theorem is often stated: "an isolated
stationary black hole is characterized by its mass, angular momentum, and
electric charge". But of course, this does not mean that no observer
measures a magnetic field; for example Boyer-Lindquist observers orbiting
a Kerr-Newman hole measure a magnetic field; so do infalling inertial
observers ("Doran observers"). Also, the no hair theorem refers to the EM
field of the hole itself, and assumes the hole is -isolated- (far from any
other matter or EM fields). But the jets are not associated with
-isolated- holes, and the BZ model assumes that the EM field originates in
the surroundin gaccretion disk.

2. The "membrane paradigm" for "black hole electrodynamics" is a
convenient way of discussing the physics of a black hole immersed in an
external EM field. This "membrane" is an imaginary surface located just
-outside- the horizon (also an imaginary surface), and we pretend that it
is something like the physical surface of an object with certain
properties. This would not be appropriate for studying matter falling
under the horizon, obviously, but it has proven useful for studying things
like energy transport in jet models, etc. You might have seen a statement
to the effect that "the hole expels magnetic field lines". But you should
consider this part of the explanation for why the magnetic field becomes
intense near the horizon.

> and so the assumed jet-collimating-fields must be associated with the


> accretion disks surrounding the holes.

Yes.

> WELL!!! If a BH cannot have a magnetic field (which is another form of
> virtual photons in action), then why should we think that the BH can
> exhibit an electric charge? On the other hand! If the BH COULD sustain
> an electric charge, then it SHOULD also be able to sustain a magnetic
> field! As for what the Theorists have to say about this apparent
> discrepancy, I don't know. Perhaps what I read was wrong?

Did I clear up your confusion above?

Message has been deleted
Message has been deleted

vernonner3voltazim

unread,
Jan 22, 2005, 6:33:21 AM1/22/05
to
Thanks for the reply; I'd just about given up looking to see if someone
had posted one.

I agree that I was sort-of mixing apples and oranges in my original
Question. However, if black holes truly exist (confidence in that is
quite high these days), then they MUST be describable in a theory of
quantized gravitation, as well as good old gtr manages. That should
lead, willy-nilly, to the scenario of virtual photons interacting with
virtual gravitons, and the consequences thereof. (why should virtual
photons be able to escape the hole?)

Regarding a gtr black hole sustaining a magnetic field, the thing I had
in mind when I mentioned it was something like this: Consider some
charged particles that do the spiralling thing into the hole. Despite
our not knowing what goes on inside the hole, it should be safe to
assume that those particles still have plenty of angular momentum, even
after crossing the event horizon. This means that the overall magnetic
field of those moving charged particles, as seen when outside the hole,
should still be noticeable after they fall in -- IF their static
charges can be seen, that is -- simply because, as you know, a magnetic
field is just an electric field seen from a relativistic viewpoint.
For this reason I was willing to assume that what I had read, about a
black hole not being able to sustain a magnetic field, was wrong.

On the other hand, getting back to the quantum-gravitation black hole,
it seemed to me that neither electric charge nor magnetic field could
escape. (I see you wrote something hinting that virtual gravitons
wouldn't escape the hole, either, but that's a non-issue. Like I
implied before, if BHs really exist, then a quantum gravity theory MUST
have some way for virtual gravitons to escape! Whether or not that
mechanism also works for virtual photons remains to be seen.) Which
is why I posted the Question. I do thank you again for the information
you provided; I hadn't heard about the "membrane paradigm" before.

tes...@tum.bot

unread,
Jan 25, 2005, 11:40:31 AM1/25/05
to
On Sat, 22 Jan 2005, vernonner3voltazim wrote:

> I agree that I was sort-of mixing apples and oranges in my original
> Question. However, if black holes truly exist (confidence in that is
> quite high these days), then they MUST be describable in a theory of
> quantized gravitation, as well as good old gtr manages. That should
> lead, willy-nilly, to the scenario of virtual photons interacting with
> virtual gravitons, and the consequences thereof. (why should virtual
> photons be able to escape the hole?)

[snip]

> Like I implied before, if BHs really exist, then a quantum gravity
> theory MUST have some way for virtual gravitons to escape! Whether or
> not that mechanism also works for virtual photons remains to be seen.)

You're -still- mixing apples and oranges, in fact now you're tossing in
lemons and grapefruits too!

Let me just mention a few very basic points which I think are universally
appreciated among professional physicists, but which seem not be
recognized by every contributor to this newsgroup:

1. Gtr is a relativistic classical field theory, not a quantum theory, but
there is no law stating that every physical theory must be quantum in
order to be interesting/useful/relevant! Quite the contrary. Geometric
optics, the Navier/Stokes equations, or for that matter the
Chandrasekhar/von Neumann equations for the motion of stars in stellar
clusters (treated according to Newtonian gravity) are no less interesting
because more fundamental theories are available! Indeed, geometric
optics, the EFE, and the Navier/Stokes equations are all likely to remain
relevant as long as anyone does physics, even though we may expect that
"better" theories will eventually supercede them as -fundamental theories-
(or indeed, already have, for two of these three examples). Reason: these
more advanced theories will presumably be harder to use, so EFE, NS, etc
will remain useful and even essential for obtaining results in situtations
where we can get away with using them.

2. When one talks of "virtual photons", "virtual gravitons", or other
quantum field theory notions in the context of gtr, since gtr is not a
quantum theory, one must expect the presence of some approximation to be
involved. Indeed, that is the case, as you presumably know.
Furthermore, experienced physicists know that approximations is just where
mistakes and misconceptions are most likely to creep into a discussion,
and in fact that is well known to happen here. In particular, the notion
of "virtual particles" is well known to be a frequent source of confusion
to beginners in quantum gravity, and you should be very cautious about
invoking it freely. See the excellent expository article by Jennie
Traschen on RWWW

http://math.ucr.edu/home/baez/relativity.html

and references therein. In fact, see the other articles listed there for
useful background on gravitation in general.

3. There is indeed much evidence that Nature adores the quantum, and this
does imply that there should be a more fundamental theory of gravitation
than gtr, which would comprise (or include) a truly -quantum- theory of
gravitation. But this expectation is not a mathematical theorem! And the
huge amount of work on quantum gravity in the past fifty years has tended
to suggest that to achieve this goal, conventional notions of quantum
field theory may need to be modified as much as the classical field theory
notions used in gtr.

There is, so far, no firm concensus on what any eventual quantum theory of
gravitation would look like (how it would be formulated mathematically,
and what its main conceptual features would be in comparision to classical
GTR and classical QFT), and what little there is tends to -contradict-
your rather dogmatic claims.

You also seem to be forgetting that just as gtr reduces to Newtonian
gravitity in a slow motion weak-field limit, aka "the Newtonian limit", so
too any eventual quantum theory of gravity should reduce to gtr in an
appropriate "Einstein limit".

> Regarding a gtr black hole sustaining a magnetic field, the thing I had
> in mind when I mentioned it was something like this: Consider some
> charged particles that do the spiralling thing into the hole.

Even in classical gtr, "spiraling infall" is too ambiguous to have meaning
unless you specify what you have in mind more carefully.

> For this reason I was willing to assume that what I had read, about a

> black hole not being able to sustain a magnetic field, was wrong.

Where -did- you read that, anyway? Again, without qualification this
statement is too ambiguous to have meaning (e.g. consider typical
observers immersed in the Kerr/Newman electrovacuum: do they measure a
magnetic field?).

> On the other hand, getting back to the quantum-gravitation black hole,
> it seemed to me that neither electric charge nor magnetic field could
> escape.

From inside the horizon? I was trying to explain that there is no need
for this to happen, in order for a hole to sustain an EM field.

> (I see you wrote something hinting that virtual gravitons wouldn't
> escape the hole, either, but that's a non-issue.

To the contrary, this fact -together with its classical explanation- is
one of those things which must be remain valid in any eventual theory of
quantum gravity, at least in the Einstein limit!

Reply all
Reply to author
Forward
0 new messages