Gravitons are still theoretical, of course, but I'm not quite sure how

[Bussani]

Gravitons are still theoretical, of course, but I'm not quite sure how they're supposed to fit into the current picture. If gravity is, in basic terms, caused by the curvature of spacetime around objects with mass, then what role would the theoretical graviton be playing? What comes first, the curve or the graviton? I just have no idea how the two fit together.

Thanks for any help.

 

[JesseM]

The bottom section of the virtual particles FAQ has some good info on this:

I hear physicists saying that the "quantum of the gravitational force" is something called a graviton. Doesn't general relativity say that gravity isn't a force at all?

You don't have to accept that gravity is a "force" in order to believe that gravitons might exist. According to QM, anything that behaves like a harmonic oscillator has discrete energy levels, as I said in part 1. General relativity allows gravitational waves, ripples in the geometry of spacetime which travel at the speed of light. Under a certain definition of gravitational energy (a tricky subject), the wave can be said to carry energy. If QM is ever successfully applied to GR, it seems sensible to expect that these oscillations will also possesses discrete "gravitational energies," corresponding to different numbers of gravitons.

Quantum gravity is not yet a complete, established theory, so gravitons are still speculative. It is also unlikely that individual gravitons will be detected any time in the near future.

Furthermore, it is not at all clear that it will be useful to think of gravitational "forces," such as the one that sticks you to the Earth's surface, as mediated by virtual gravitons. The notion of virtual particles mediating static forces comes from perturbation theory, and if there is one thing we know about quantum gravity, it's that the usual way of doing perturbation theory doesn't work.

Quantum field theory is plagued with infinities, which show up in diagrams in which virtual particles go in closed loops. Normally these infinities can be gotten rid of by "renormalization," in which infinite "counterterms" cancel the infinite parts of the diagrams, leaving finite results for experimentally observable quantities. Renormalization works for QED and the other field theories used to describe particle interactions, but it fails when applied to gravity. Graviton loops generate an infinite family of counterterms. The theory ends up with an infinite number of free parameters, and it's no theory at all. Other approaches to quantum gravity are needed, and they might not describe static fields with virtual gravitons.

 

[cfrogue]

The bottom section of the virtual particles FAQ has some good info on this:

How do gravitons cause light to bend?

Also, could you clear out you PM?

I wanted to ask you something and I do not want to hijack a thread.

 

[Bussani]

How do gravitons cause light to bend?

Well, I can understand light following a straight path through curved time, thus bending. So it's the curve that causes it to bend, if I'm understanding it right.

I guess me asking, "what comes first, the curve or the graviton?" is sort of like asking, "what comes first, the photon or the light"? Anyway, thanks for pointing me to the FAQ. That clears it up a little.

 

[Mentz114]

Gravitons are still theoretical, of course, but I'm not quite sure how they're supposed to fit into the current picture. If gravity is, in basic terms, caused by the curvature of spacetime around objects with mass, then what role would the theoretical graviton be playing? What comes first, the curve or the graviton? I just have no idea how the two fit together.

Thanks for any help.

You shouldn't be too literal about interpreting curvature. Technically, curvature is a mathematical defect in space-time which is useful in creating gauge theories of gravity like GR ( which can be made by fixing a global->local invariance breaking under a transformation from a symmetry group). It's possible to construct theories theories that give the same predictions as GR using different symmetries and defects.

However, if you insist on the reality of curvature, and you manage to quantise this approach, then gravitons will carry curvature, not momentum as in energy based gauge theories.

 

[Bussani]

You shouldn't be too literal about interpreting curvature. Technically, curvature is a mathematical defect in space-time which is useful in creating gauge theories of gravity like GR ( which can be made by fixing a global->local invariance breaking under a transformation from a symmetry group). It's possible to construct theories theories that give the same predictions as GR using different symmetries and defects.

Ahh, I get you. That's a helpful explanation.

However, if you insist on the reality of curvature, and you manage to quantise this approach, then gravitons will carry curvature, not momentum as in energy based gauge theories.

Right, that makes sense. I sort of figured that when I made my last post. Thanks for the reply.