If gravitation is mediated via particle doesn't it mean thatthis

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vlad777
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If gravitation is mediated via particle doesn't it mean that
this particle would have to escape the black hole to transfer
it's gravitation?
 

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Photon is massless boson but is affected by gravity.

From Wikipedia:
For example, if gravitational waves were observed to propagate slower than c (the speed of light in a vacuum), that would imply that the graviton has mass.

So if gravitons have mass there would be a second event horison?
 


vlad777 said:
If gravitation is mediated via particle doesn't it mean that
this particle would have to escape the black hole to transfer
it's gravitation?

There's no consistent accepted quantum theory of gravity yet, but I believe the general consensus is that particles would only be involved in communicating changes in the field (like gravitational waves), but the field of a black hole is created before the black hole itself, being left behind as it collapses.
 


If there were a quantum theory of gravity then the graviton would be gauge particle that carries the gravitational "charge". It would probably have to be spin 2 (only one charge type) and massless (long range). Similarly in Electrodynamics we have a photon which carries the EM charge, spin 1 (two charge types), and massless (long range). Real photons are not exchanged in a static electric field, here we employ the use of virtual photons which live a fairly like existence. Real photons only appear in propagating EM waves.

The gravity field around a block hole is static like the electric field around a charged particle. This implies that like the EM case a virtual graviton would need to be employed. The real graviton would carry the gravity "charge" in a propagating gravity wave.

The black hole is static has been there for a long time and the gravitational field is stationary and is the same as the gravitational field of the original body (a star maybe), that was there before.

There is nothing to propagate. Hence no need for gravitons or anything to escape.
 
bcrowell said:
Nice post, cosmik_debris. I would only add that this is really a classical puzzle, not a quantum-mechanical one. But in any case, the resolution is the same. A static field doesn't transmit information.

It does tell you how many charges or how many mass units are present. Is this considered information?
 


vlad777 said:
So if gravitons have mass there would be a second event horison?

You've just hit on one of the big reasons that no one has come up with a workable theory of quantum gravity. It's called the renormalization problem.

A photon just sits there and doesn't naturally create photons, so it's not that hard to come up with a theory that that talks about electromagnetism as photons moving back and forth. What's more is that if you think about particles transmitting *one* photon and then maybe *two* photons, you come pretty close to the number in which describes how particles interact.

The trouble with doing this with gravitons is that gravitons have mass, which means that they should produce more gravitons, which have mass which produce more gravitons. What you end up with is this giant mathematical mess that no one knows quite what to do with.
 


twofish-quant said:
The trouble with doing this with gravitons is that gravitons have mass, which means that they should produce more gravitons, which have mass which produce more gravitons. What you end up with is this giant mathematical mess that no one knows quite what to do with.

Until a full quantum theory of gravitation is proposed we cannot say much about the properties of a graviton, but in the low energy effective field theories and various string theories, the graviton is thought to be massless. It is true however that the gravitational field has energy which itself gravitates, that makes GR non-linear and non-renormalizable.
 


cosmik debris said:
If there were a quantum theory of gravity then the graviton would be gauge particle that carries the gravitational "charge". It would probably have to be spin 2 (only one charge type) and massless (long range). Similarly in Electrodynamics we have a photon which carries the EM charge, spin 1 (two charge types), and massless (long range). Real photons are not exchanged in a static electric field, here we employ the use of virtual photons which live a fairly like existence. Real photons only appear in propagating EM waves.

The gravity field around a block hole is static like the electric field around a charged particle. This implies that like the EM case a virtual graviton would need to be employed. The real graviton would carry the gravity "charge" in a propagating gravity wave.

The black hole is static has been there for a long time and the gravitational field is stationary and is the same as the gravitational field of the original body (a star maybe), that was there before.

There is nothing to propagate. Hence no need for gravitons or anything to escape.

To clarify this, the interaction between the mass inside the black hole and outside objects is transmitted by a virtual graviton, which is a spin 2 gauge boson of a hypothetical quantum theory of gravity. Virtual means that it only appears in interaction vertices of the respective theory and therefore is not affected by the constraint that "nothing can leave a black hole", it is what defines the gravitational field. The key point is not staticity of the black-hole, but rather the nature of virtual particles, since possible oscillations of the black hole would only produce another kind of gravitons, gravitational waves (just as you mentioned, analogous to EM gauge photons vs real photons as EM waves).

Renormalizability due to a non-dimensional coupling constant is the big issue that keeps us from finding a consistent theory of quantum gravity in a traditional quantum field theoretical sense, although there has been some progress in this area recently. There's the so called "Asymptotic Safety" scenario which roughly predicts that if one can find a non-trivial fixed point in the renormalization group flow, then the gravity is renormalizable. It's just that nobody has found such a fixed point so far.

String theory deals with quantum gravity through 2 main aspects:
1.) It contains a spin 2 particle (graviton) in a natural way
2.) Renormalizability is less troublesome due to the shift from zero- to one-dimensional objects as fundamental entities.
 
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Polyrhythmic (post 10), in a word, yes.
 


vlad777 said:
If gravitation is mediated via particle doesn't it mean that
this particle would have to escape the black hole to transfer
it's gravitation?

The idea that gravity is caused by a small gravity particle takes an enormous leap of faith considering the lack of evidence supporting such a claim and the unsuccessful billions wasted trying to prove such fairy tell.
 


ttmark said:
The idea that gravity is caused by a small gravity particle takes an enormous leap of faith considering the lack of evidence supporting such a claim and the unsuccessful billions wasted trying to prove such fairy tell.

Well, if it was possible to find a working theory of perturbative quantum gravity, a graviton would just be natural, as virtual particles show up in theories like Quantum Electrodynamics and Quantum Chromodynamics (which are usually treated perturbatively). But the key to realizing what the significance of this really is would be realizing what a virtual particle actually is. It is something which shows up mathematically, but it doesn't have any actual physical significance. Therefore the graviton would only be a side product of what is actually interesting: a quantized theory of gravity.
 


Interesting, i see that what we often call sub atomic particles themselves are not actually known to exist as individual pieces of matter in space. As in the case of electrons they are a statistical distribution. The same could be said that a graviton does not actually exists but what we can do instead if claim it in an theoretical abstract model so that the model will work.
 


ttmark said:
Interesting, i see that what we often call sub atomic particles themselves are not actually known to exist as individual pieces of matter in space. As in the case of electrons they are a statistical distribution. The same could be said that a graviton does not actually exists but what we can do instead if claim it in an theoretical abstract model so that the model will work.

I think you missed my point. The electron is something which is actually there, which we can measure. It is quantum mechanically described by a probability density, but it's nevertheless something that is real in the sense that we can measure it. A virtual particle however is something entirely different. It is something we cannot measure, and its "existence" as a particle is due to the interpretation of perturbative quantum field theory in terms of Feynman diagrams, which is definitely useful but completely arbitrary. It is not needed to talk about particles being exchanged in any way.
 


Polyrhythmic said:
I think you missed my point. The electron is something which is actually there, which we can measure. It is quantum mechanically described by a probability density, but it's nevertheless something that is real in the sense that we can measure it. A virtual particle however is something entirely different. It is something we cannot measure, and its "existence" as a particle is due to the interpretation of perturbative quantum field theory in terms of Feynman diagrams, which is definitely useful but completely arbitrary. It is not needed to talk about particles being exchanged in any way.

Yeah, that's right, virtual particles are a product of a theory. Lattice QFT for example doesn't use virtual particles. Read Arnold Neumaier's FAQ about virtual particles.