1. Feb 13, 2013

### swemy

I have some basic questions about gravitons.

If you have two particles with mass, do they "exchange" gravitons, if so what is the frequency?

2. Feb 13, 2013

### Simon Bridge

3. Feb 13, 2013

### swemy

How frequently do two particles with mass exchange gravitons, or if they are "broadcast" how frequently does a particle with mass emit gravitons?

Last edited: Feb 13, 2013
4. Feb 13, 2013

### Simon Bridge

5. Feb 13, 2013

### swemy

Sorry Simon, I'm not framing the question coherently. Am I right in thinking that in a simple 1 star 1 planet model, the GR solution gives space as curved and the planet's orbit is the natural or straight path thru it. The QM solution uses gravitons as a messenger particle to tell the planet how to move thru space?
So in the GM model the larger the star, the greater the number of particles with mass, the greater the number of gravitons the planet receives?

Last edited: Feb 13, 2013
6. Feb 13, 2013

### bapowell

The 2nd of Simon's links above answers this (towards the bottom.) Virtual particles arise in perturbation theory, but since gravity is not renormalizable, there is no well-behaved perturbation expansion to speak of. The role of virtual gravitons in establishing static gravitational fields is not implied by current theories.

7. Feb 13, 2013

### negru

That's a pretty strong statement. It is implied by standard QFT. Gravity isn't the only non-renormalizable theory with a low energy perturbation expansion which is consistent. The low energy side has to be described by an effective field theory, regardless of the UV completion.

This discussion shows up at least every week or so on PF. I'm not sure why all the graviton hating :)

8. Feb 13, 2013

### swemy

Gravitons seems a complex solution, compared to curved space. Consider two galaxies in a local cluster if each particle with mass is interacting with every other via gravitons?

As gravity is rangeless, does that imply that every particle with mass in the universe is in a graviton exchange relation with every other mass particle?

9. Feb 13, 2013

### Staff: Mentor

If they are causally connected, I think.
That is true for all charged particles as well.

10. Feb 14, 2013

### andrien

infinite range of gravity implies that graviton must be massless.

11. Feb 15, 2013

### torquil

Shouldn't this be in "Beyond the Standard Model"?

12. Feb 15, 2013

### Simon Bridge

Others have picked up on this too:
(my emphasis) - the boldface part implies the possibility that gravitons break relativity constraints doesn't it? In fact, the kind of formulation of QM being used to generate that statement is non-relativistic.

Related:
I've been trying to find a reference for the rate of EM interactions ... a charged particle may move in a curved path in an electromagnetic field, which implies many successive interactions... seems legitimate to ask after the rate (possibly "mean rate") of the interactions which would be expected to depend on the field strength.

I suspect I'm momentarily forgetting something obvious.

13. Feb 15, 2013

### Bill_K

Yes, Simon, the thing you're forgetting is -- Quantum Machanics! The interaction of a charged particle with the electromagnetic field is not a succession of bumps, it's ongoing and continuous. There's an amplitude that the particle interacts with the field at time t, for each and every t, and the overall result is a superposition of all of them. To ask at what time t the interaction occurs, or the interval Δt between two successive interactions, is meaningless.

14. Feb 15, 2013

### Simon Bridge

Thanks Bill_K - a suspicion like that was behind my reply in post #4 and I suspect it is the core OPs original question.

The Feynman diagrams do make them look a bit like there should be a series of bumps don't they?
But they are actually representations of some mathematical process? Which leads back into the nature of virtual particles.

15. Feb 16, 2013

### negru

Feynman diagrams are just a way to draw the Taylor series of e^S, where S is the action (lagrangian) of the system. The action has parts that represent propagators and interactions. Expanding e^S gives a whole mess of combinations of such ingredients. Feynman diagrams are just a way of systematically accounting for all possible terms.