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

• Bussani
In summary: Thanks for clearing that up!In summary, it's still unclear how gravitons would fit into the current picture of gravity, but they might play a role in mediating static forces. It's also uncertain whether gravitons will be detected any time in the near future.
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.

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.

JesseM said:
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.

cfrogue said:
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.

Bussani said:
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.

Mentz114 said:
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.

## 1. What are gravitons and why are they considered theoretical?

Gravitons are hypothetical particles that are believed to be the carriers of the force of gravity. They have not yet been observed or detected, which is why they are still considered theoretical.

## 2. How do gravitons fit into the current understanding of gravity?

Gravitons are a part of the theory of quantum gravity, which attempts to reconcile the theories of general relativity and quantum mechanics. They are believed to play a crucial role in explaining how gravity works on a quantum level.

## 3. Can gravitons be proven to exist?

At this time, there is no definitive way to prove the existence of gravitons. Scientists are still working on ways to detect them and gather evidence for their existence.

## 4. How do gravitons interact with other particles?

Gravitons are thought to interact with other particles through the force of gravity. Just as photons are the carriers of the electromagnetic force, gravitons are believed to be the carriers of the gravitational force.

## 5. Could gravitons potentially lead to a unified theory of everything?

It is possible that gravitons could play a key role in a unified theory of everything, but at this time, there is no definitive evidence to support this idea. Scientists are still researching and testing various theories to try and understand the fundamental forces of the universe.

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