# Gravitons corresponding to flat spacetime.

1. Mar 9, 2009

### arroy_0205

Is it possible to define gravitons corresponding to flat spacetime? If so, how? Flat spacetme has zero curvature. How does one mathematically describe gravitons for flat spacetime?

2. Mar 9, 2009

### Haelfix

I'll tell you how I picture them (and I warn you, there are problems with this picture, but it is the best I can do interpretating rather complicated mathematics)

Gravitons, by themselves are no different than any other gauge particle. Taken by themselves, if you could put them on mass shell, they'd propogate just like any other free particle in whatever ambient spacetime with whatever curvature you want with some appropriate equation of motions/geodesics for a massless, spin2 particle.

Now they won't ordinarily arise in that sort of free field case, usually the situation we have is two point masses that interact through the exchange of a virtual graviton. Here again, by itself the graviton is again no different than any other gauge particle. What is however different, is that the *interaction* between the graviton and the point particles, as well as the graviton on graviton interaction (arising from higher order diagrams) will (if you could sum up all the diagrams) induce a backreaction on the actual metric tensor itself, causing a curvature change. Thats exactly what you want. A coherent state of gravitons can and will create dynamics for the gravitational field or alternatively in the more familiar GR language, create the dynamics for how Geometry changes and evolves.

In fact, to answer your original question, the only case where we actually know how to solve anything, is exactly the flat space case. Here you take the original ambient space to also be flat, and we require the point masses to be completely tiny (or far away) such that the interaction is completely negligable, leaving the flat space intact to good approximation. Or in other words, the interaction to preserve zero curvature must be trivial, just like in normal GR.

3. Mar 10, 2009

### arroy_0205

Thanks, Haelfix. You gave an interesting answer but I am yet to understand a few points. Please clarify if you have time.
First, though I have not tried to do any calculations, I guess from the last line of your answer, that gravitons corresponding to flat spacetime will be constants in time and space. Is that correct?
Second, why should the graviton-graviton and graviton-particle interactions would back-react on the metric? I think these are related to cross-section of the corresponding process. Also why should one bother whether the perturbation series is summable or not? Do you mean to say in case of divergent series, the back-reaction will be infinite? But the connection is not clear to me. Moreover I guess the exact nature how metric will be changed will be a coplicated issue. May be you can tell me if there are any good elementary articles in the archive related to this. Thanks.