# Gravitons and Black Holes

1. Jul 14, 2006

### eep

It is my understanding that gravitons are the elementary particles thought to transmist gravity. They must be massless and therefore travel at the speed of light. With this in mind, how can black holes gravitationally effect other bodies as the gravitons emitted by the black hole would not be able to escape?

2. Jul 14, 2006

### Brinx

3. Jul 14, 2006

### notknowing

Gravitons are noting more than an unproven hypothesis. It is very likely that they do not exist. If they would exist, one would encounter an absurd situation: Mass is composed of particles , held together by forces. The energy contained in the fields has also mass. So, every virtual particle (responsible for instance for the electromagnetic force) within the mass must contribute to the total mass. This means that there should for every virtual particle also exist a virtual graviton and since "the gravitational field" is also a source of gravitation (Einsteins Field Equations are nonlinear) it means that every virtual gravitaton must have a corresponding graviton to be able to feel the gravitational force. And these in turn must always have gravitons such that they are also subject to gravitation. So, one can continue this up to infinity, meaning that the whole space must be completely filled with particles, so that it would resemble more a solid than a vacuum.
In short, the concept of gravitons should best be abandoned.

4. Jul 14, 2006

### Born2Perform

I dont get this however to escape from the singularity virtual particles must have an infinite speed

5. Jul 14, 2006

### JohnnyTheFox

As far as I know gravity acts from the surface of a mass (assuming it's spherical) a bit like gauss's law of charge, everything inside cancells out.

Your graviton only then emerges from the surface of the blackhole so it doesn't need to escape from a singularity at infinite speed. What speed it needs to travel at I don't know. Anything within the event horizon can't get back out (unless you travel >c) but this isn't nessesarily the surface of the object so hmmm... This is more me thinking aloud rather than a proper answer!

After reading that page I'm still confused. I thought gravity was information and therefore limited to the speed of light. I guess it doesn't work like that then.

So using GR how does the information travel fast enough to escape and tell things outside the black hole to move towards it?

6. Jul 14, 2006

### Born2Perform

Unsing GR the field is alredy there
using QM....there are problems...

7. Jul 14, 2006

### exponent137

Are this mean one uncompatibility of general relativity and quantum mecahnics? How precisely?

8. Jul 14, 2006

### Born2Perform

Not if the graviton mass is = 0 ; because it would not attract ....or not?

9. Jul 14, 2006

### Brinx

As far as I have understood from my query on that other forum, changes in gravity (i.e. changes in curvature of spacetime), that are for example caused by changes in position of matter, are thought to propagate at the speed of light. The curvature that is already 'in place' doesn't need to be propagated, apparently: it's already a local property of spacetime. So during the collapse of an object to a black hole, all locations outside the (future) event horizon are 'updated' regarding the new situation up until the actual formation of that event horizon, after which the curvature of spacetime outside the event horizon doesn't change anymore - at least not as a consequence of the configuration of collapsing matter inside the event horizon. How spacetime curvature can remain in one configuration without it being 'in touch' with the source of that curvature (the matter making up the black hole) is beyond me.

10. Jul 14, 2006

### notknowing

No, the graviton (non existing) mass should be different from zero because otherwise it would not feel gravity and the Einstein field equations imply that gravity (curvature) can be the source of gravity (curvature). Of course, the gravitons rest mass could still be zero (similar to the zero rest mass of a photon but the non-zero energy of the photon).

11. Jul 14, 2006

### notknowing

No, they are not incompatible. The incompatibility arises just because of a wrong approach, there is no fundamental compatibility. Gravitation should not be treated in the same way as the electromagnetic force. The key to understanding gravity lies in the quantum vacuum which theoreticians unfortunately usually (try to) throw out of their calculations (using so-called renormalisation).

12. Jul 14, 2006

### pervect

Staff Emeritus
Yep - that's the standard FAQ answer.

Several important things to note:

1) "Gravitons" aren't the usual approach to GR at all, though some approaches can derive an equivalent theory to GR from "gravitions".

2) One must distinguish between virtual gravitons and real gravitons, just as one must distinguish between virtual photons and real photons. The question (and most people who ask the question) unfortunately does not / do not make this important distniction. One can use the same argument to ask "How can a black hole be charged, if virtual photons can't escape it?"

Last edited: Jul 14, 2006
13. Jul 14, 2006

### pervect

Staff Emeritus
Do you have a peer-reviewed reference for this statement?

Highly speculative personal theories generally aren't welcome here on PF, see the posting guidelines.

Last edited: Jul 14, 2006
14. Jul 14, 2006

### notknowing

Yes, I have a peer-reviewed reference for this statement.
You can find it on http://home.online.no/~avannieu/darkmatter/
The paper is :
R. Van Nieuwenhove, Quantum Gravity : a Hypothesis, Europhysics Letters, 17 (1), pp. 1-4 (1992)

PS : I have seen much more speculative theories on this site without any perr-reviewed reference (in fact maybe as much as 99 %)

Last edited by a moderator: Apr 22, 2017
15. Jul 14, 2006

### Farsight

I'm getting confused here. Surely there can be no messenger-particle hail of "gravitons" flooding through all space and matter to create the effect of gravity? Which means gravitons are better thought of as something like curvature calculation units? Can anybody correct this or give me a concept of what the graviton is supposed to be? Real or virtual?

16. Jul 14, 2006

### Born2Perform

Cornell's FAQ on black holes says the opposite thing of what you all say:
http://curious.astro.cornell.edu/question.php?number=264 [Broken]

this guy says that gravitons excapes from the black holes because their speed is >c ;
i know FAQs are made to let normal people understand but after this i am a bit confused..

Last edited by a moderator: May 2, 2017
17. Jul 14, 2006

### notknowing

This is really a difficult subject. One is really considering the graviton as a virtual particle with zero rest mass (to ensure a force with infinite range). The graviton (if it would exist) should however have some mass (since it not as rest) but some people think that this can not have an effect on the rest of the universe because it exists for a too short time. This argument would hold for virtual particles which are not massless (such as W's) but I don't see how the argument could hold for gravitons. If you think of the mass of an atom than you find that most of the particle (if not all) consists of fields or virtual particles interacting between its constituents (for instance virtual gluons between the quarks, virtual photons between quarks, electrons, etc.). Maybe quarks are composed of other sub-particles with again virtual particles between these, etc. So, in the end most of matter seems to be in the form of virtual particles. If one then claims that these virtual particles do not contribute to the mass of the particle than I ask, what is then making up its mass ? Thus, coming back to the graviton now, it looks plausible that if it would exist, that it would feel gravitation and hence one encounters the implausible situation I mentioned in a previous message (space completely filled with gravitons).

18. Jul 14, 2006

### pervect

Staff Emeritus
The sci.physics faq says much the same thing:

Of course, before that, the sci.physics.faq points out, as I also like to point out, that GR is not actually formulated in terms of a particle or field model. GR is formulated in terms of curvature, instead. (Certain non-standard formulations of GR in terms of spin 2 particles do exist, however - references on request, or search the forum).

There is no doubt that virtual particles are weird. I personally think that they are more trouble than they are worth, but many people seem to find the idea of forces carried by "particles" attractive, no matter how weird the "particles" act. I personally suspect that most people who are fond of virtual particles mistakenly think they act like normal particles, which is why I start out by pointing out that forces are not carried by real particles.

Since GR isn't actually formulated in terms of "particles" at all, I would suggest that the quantum physics forum is the best place to ask about the weird and counter-intuitive properties of virtual particles, or to complain about how weird they are :-).

Last edited by a moderator: May 2, 2017
19. Jul 14, 2006

### pervect

Staff Emeritus
Gravitons, if they exist, would come in both "real" and "virtual" flavors, just as photons do.

Real gravitons would be quantized packets of gravitational radiation, just like real photons are packets of quantized electromagnetic radiation. Real gravitions, like real photons would carry information. Real gravitions, like real photons, could not escape from a black hole.

Note that since we can't detect gravitational radiation at all (hopefully that will change soon)m, we are a VERY long way from being able to detect whether or not gravitational radiation is quantized. Qunatitiation of gravitational radiation is entirely theoretical at this point.

Virtual gravitons would be responsible for the attractive force between masses, just as virtual photons are responsible for the attractive and repulsive forces bedtween charges.

Virtual gravitons would share the many oddities of virtual photons.

Note that while neither real photons nor real gravitions could escape a black hole, both virtual photons and virtual gravitons could (in principle, anyway), as a black hole has both a mass and a charge.

20. Jul 14, 2006

### eep

Thanks for your answers, everyone. I don't really know anything about virtual particles besides the fact that virtual photons are responsible for the electromagnetic force. So the gravitons reponsible for the gravitational force would be virtual gravitons, and therefore can escape due to their ability to travel greater than the speed of light. Hrm.