Are Gravitons the Key to Understanding Gravity?

[humanino]

For the case of a photon - photon interaction with the exchange of a graviton, I think the solution is done explicitly in 'QFT in a Nutshell' by Zee, and its a real half a page quicky b/c all the indices contract.

I realize what's going on now. I have read Zee's paragraph and need to find this Tolman Ehrenfest and Podolsky reference. I see that you can formally calculate photon-photon scattering in terms of one virtual graviton scattering, but then the question is what happens for real graviton / real photon scattering.

Thank you very much for the precision

 

[peter0302]

But my point is that I think it is ! The traditional way of defining the graviton field as the difference between the metric and the metric of the vacuum for instance does not allow you to couple to fermions, that is, no matter in your universe. So people come up with other prescriptions, and nobody knows which one is the best one, and then I see you come around and claim "there is no conceptual difficulty". I don't think that's fair.

Jesse's right, what I mean are the OP's philosophical problems with the idea of a particle that reaches across galaxies or seemingly unbelieveable numbers of them. I certainly do not mean to say that the numerous scientific/mathematical difficulties, many of which you mention, aren't legitimate.

When you sayLike what difficulties ? Lately people have been doubting whether N=8 supergravity is finite or not, and if it is finite it would not be due to supersymmetry but to gravitational symmetry, hinted towards the fact that, maybe, a correct QFT of gravity would be finite. So it is not clear to me what is difficult when you come around and claim "one run into difficulties". What are the difficulties faced by the various approaches, or at least, one ? Choose your favorite.

The main difficulty I was referring to was the inability to renormalize.

 

[humanino]

The main difficulty I was referring to was the inability to renormalize.

In the part you just quoted, that is exactly what I question : do we need to renormalize or not ? We are not even sure ! PF is not a peer-reviewed reference, but this discussion contains interesting stuff :
[thread=242473]Hints gravity finite[/thread]

 

[D H]

Perhaps the problem is that you and Peter are using "conceptual" differently.

Humanino is thinking in terms of conceptual as meaning "an accepted mathematical definition". Peter is using conceptual in the sense of a rather vague and not very-well-defined idea; i.e., lots of hand-waving (sorry for the idiom, Humanino).

 

[peter0302]

I should have said philosophical or metaphysical. :)

 

[JesseM]

Humanino is thinking in terms of conceptual as meaning "an accepted mathematical definition". Peter is using conceptual in the sense of a rather vague and not very-well-defined idea; i.e., lots of hand-waving (sorry for the idiom, Humanino).

Although I agree with humanino that only in mathematics do we really have totally clear concepts of anything, if you google the phrase "conceptual argument" along with "physics", you can see that in a physics context this phrase is usually used to mean an argument that relies mostly on words and mental images and not much on mathematics (this doesn't necessarily mean it's totally handwavey; for example, if someone proposes there could be a strange form matter which falls upwards in a gravitational field, a good conceptual argument for why this should be impossible in GR is that it would conflict with the equivalence principle, since this object should still move inertially if placed in the middle of an accelerating elevator, and so appear to fall down from the perspective of the person in the elevator).