How do Gravitons escape from a Black Hole?

  • #1
1,685
1
1 : Am I right in thinking gravitons are the mediators of the gravitational force, just as photons are the mediators of the electrostatic force?

If this is correct, then presumably the gravitational force between A and B can only be manifest (detected) in circumstances where gravitons can travel from A to B?

2 : If A is in the interior of a Black Hole (ie inside the event horizon) and B is on the exterior of a Black Hole (ie outside the event horizon), then is it possible for gravitons to travel from A to B?

If the answer to (2) is "yes", how is this possible? Surely gravitons cannot travel faster than light, and therefore cannot escape the Black Hole?

If the answer to (2) is "no", then how is it possible for any matter outside a Black Hole to "feel" the gravitational attraction of matter inside the Black Hole (ie the Black Hole should have no gravitational field)?
 

Answers and Replies

  • #2
Janus
Staff Emeritus
Science Advisor
Insights Author
Gold Member
3,706
1,626
From: http://sciastro.astronomy.net/sci.astro.4.FAQ
Subject: D.09 How can gravity escape from a black hole?
Author: Matthew P Wiener <weemba@sagi.wistar.upenn.edu>,
Steve Carlip <carlip@dirac.ucdavis.edu>

In a classical point of view, this question is based on an incorrect
picture of gravity. Gravity is just the manifestation of spacetime
curvature, and a black hole is just a certain very steep puckering
that captures anything that comes too closely. Ripples in the
curvature travel along in small undulatory packs (radiation---see
D.05), but these are an optional addition to the gravitation that is
already around. In particular, black holes don't need to radiate to
have the fields that they do. Once formed, they and their gravity
just are.

In a quantum point of view, though, it's a good question. We don't
yet have a good quantum theory of gravity, and it's risky to predict
what such a theory will look like. But we do have a good theory of
quantum electrodynamics, so let's ask the same question for a charged
black hole: how can a such an object attract or repel other charged
objects if photons can't escape from the event horizon?

The key point is that electromagnetic interactions (and gravity, if
quantum gravity ends up looking like quantum electrodynamics) are
mediated by the exchange of *virtual* particles. This allows a
standard loophole: virtual particles can pretty much "do" whatever they
like, including travelling faster than light, so long as they disappear
before they violate the Heisenberg uncertainty principle.

The black hole event horizon is where normal matter (and forces) must
exceed the speed of light in order to escape, and thus are trapped.
The horizon is meaningless to a virtual particle with enough speed.
In particular, a charged black hole is a source of virtual photons
that can then do their usual virtual business with the rest of the
universe. Once again, we don't know for sure that quantum gravity
will have a description in terms of gravitons, but if it does, the
same loophole will apply---gravitational attraction will be mediated
by virtual gravitons, which are free to ignore a black hole event
horizon.

See R Feynman QED (Princeton, ???) for the best nontechnical account
of how virtual photon exchange manifests itself as long range
electrical forces.
 
  • #3
1,685
1
Janus said:
Thank you Janus!
I'm not sure I fully understand however. What is being said is that (in the quantum explanation) fields are mediated by virtual photons and gravitons, and not real ones. OK. But this requires that these virtual particles travel "faster than light" in order to escape the black hole. If they can travel faster than light, then (since they are the mediators of the forces involved) surely this means they violate one of the basic principles of relativity, which is that no information/signal can be transmitted faster than light?
 
  • #4
turbo
Gold Member
3,147
51
moving finger said:
...But this requires that these virtual particles travel "faster than light" in order to escape the black hole. If they can travel faster than light, then (since they are the mediators of the forces involved) surely this means they violate one of the basic principles of relativity, which is that no information/signal can be transmitted faster than light?
In quantum theory, the virtual particles can also appear to travel backward in time. Here is a nice series of non-technical lectures by Richard Feynman explaining in layman's terms just how odd the quantum world is.

http://www.vega.org.uk/series/lectures/feynman/index.php [Broken]
 
Last edited by a moderator:
  • #5
Phobos
Staff Emeritus
Science Advisor
Gold Member
1,954
6
moving If they can travel faster than light said:
Also note this caveat from teh quote Janus provided...

"... so long as they disappear before they violate the Heisenberg uncertainty principle."

So, they don't exist long enough for any real info transfer.
 
  • #6
1,685
1
Phobos said:
Also note this caveat from teh quote Janus provided...

"... so long as they disappear before they violate the Heisenberg uncertainty principle."

So, they don't exist long enough for any real info transfer.
but then this explains nothing? in order for an outside observer to detect the charge or mass of the black hole, information must be transferred from the black hole to the observer. the mediators of the electrostatic and gravitational forces (photons & gravitons) are the agents of this information transfer. if these virtual particles disappear before any information is transferred then they do not mediate the force involved?

in other words - these virtual mediators must transfer information from the interior of the black hole to the exterior, and to cross the event horizon they must also be travelling faster than light. correct? Hence information is being transmitted faster than light - against the basic assumptions of relativity?
 
  • #7
162
0
im not positive if this will help or not,

but try doing some research into the "Theory of Everything," string theory, about how gravitons can be created and how they can move between places.
 
  • #8
Chronos
Science Advisor
Gold Member
11,429
743
I sense some confusion here [and I'm not ruling it might be me]. I was under the impression gravitons are not emissions. Unlike photons, they do not travel from point A to point B. They appear out of the gravitational field potential when called upon - but obey relativity by respecting the distance to the caller before answering.
 
  • #9
1,685
1
Chronos said:
I sense some confusion here [and I'm not ruling it might be me]. I was under the impression gravitons are not emissions. Unlike photons, they do not travel from point A to point B. They appear out of the gravitational field potential when called upon - but obey relativity by respecting the distance to the caller before answering.
I think it depends on whether you prefer to adopt the relativisitic or the quantum mechanical "explanation" of what's happening. General Relativity has no need for gravitons, instead it postulates the mass/energy induced curvature of space-time (whatever that might mean in reality) to explain the effect of gravity. In QM however, all forces are mediated by force carriers - and the force-carrier of the gravitational force is the graviton. I'm not sure why we would need to postulate any force-carriers at all if the "gravitational field potential" (whatever that is) could do the job in their absence? Why not also have an electrostatic field potential which mediates the electrostatic force (instead of those pesky virtual photons)?
 
  • #10
Chronos
Science Advisor
Gold Member
11,429
743
Indeed, that is why a black hole can have an electromagnetic field without eating it.
 
  • #11
ohwilleke
Gold Member
1,731
572
In a number of loop quantum gravity theories, neither the Big Bang nor black holes are singularities in the way they are in classical gravity theories.
 
  • #12
hellfire
Science Advisor
1,051
1
moving finger said:
but then this explains nothing? in order for an outside observer to detect the charge or mass of the black hole, information must be transferred from the black hole to the observer. the mediators of the electrostatic and gravitational forces (photons & gravitons) are the agents of this information transfer. if these virtual particles disappear before any information is transferred then they do not mediate the force involved?
I think that in this sense vitual particles model forces providing a mechanism for transfer of momentum between interacting particles. It is not possible however, to detect them "in between" the interacting particles, as they are emitted and absorbed in an time interval which is less than the interval determined by the uncertainty principle.

moving finger said:
I'm not sure why we would need to postulate any force-carriers at all if the "gravitational field potential" (whatever that is) could do the job in their absence? Why not also have an electrostatic field potential which mediates the electrostatic force (instead of those pesky virtual photons)?
Virtual particles as force mediators arise naturally when quantizing fields and their interactions in quantum field theory. It exists a quantum theory of electrodynamics, but not a quantum theory of gravitation.

(This is my heuristic understanding).
 

Related Threads on How do Gravitons escape from a Black Hole?

  • Last Post
Replies
18
Views
4K
  • Last Post
Replies
4
Views
2K
Replies
5
Views
3K
Replies
4
Views
3K
  • Last Post
Replies
21
Views
4K
Replies
52
Views
5K
Replies
3
Views
2K
  • Last Post
Replies
8
Views
2K
  • Last Post
Replies
8
Views
6K
Top