Gravitons

The thread on gravitons reminded me of a question i’ve been thinking about for a while. For a long time physicist have been looking for a particle that mediates the gravitational force. Most recently at the LHC, but without much luck.
I understand that the math of GR and QM do not agree at the limits which suggest that one or the other needs work.

Do we know what properties the graviton will have? Or in other words, can we explain gravity(as modelled by spacetime curvature) in terms of particle interactions, even in theory?

Will the discovery of the graviton make the idea of curved space obsolete (as an ontological concept)?

Thanks

The LHC is getting close: The Large Hadron Collider is tantalizingly close to finding the Higgs Boson

Yes and no. If the influence of gravity isn’t very strong, we can do pretty well by describing it via the exchange of virtual gravitons, just like electric and magnetic forces can be described using virtual photons. The main problem is that gravity gravitates: every graviton has its own energy, and all forms of energy induce gravity, so each graviton will cause its own gravity. (This is to be contrasted with the situation in electromagnetism: virtual gravitons have energy, but virtual photons don’t have charge.) Normally this “self-gravitation” of the gravitons can be ignored, but if the fields get strong enough, you either have to include an intractably large number of terms or revert to the curved spacetime description to accurately describe what’s going on.

If I knew that I’d be booking my flight to Stockholm. Certainly it’ll still be a useful concept, just as the Newtonian gravitational potential is still a useful concept even if we now know about general relativity. Beyond that, it’ll depend a lot on how gravity and quantum mechanics eventually get reconciled with each other.

First of all, the Higgs boson is completely unrelated to the graviton. The confusion is due to the Higgs’ association with mass, but even that is much overplayed: You can have mass without the Higgs mechanism, the Higgs boson itself isn’t so much a cause of the mass as a side effect of the same mechanism, and even once you have mass, that’s unrelated to the manner in which masses interact with each other.

We do know a few things about the graviton: Since gravity falls off as 1/r^2 over very large distances, it must be massless, or extremely close to it. And based on the polarization states available to gravitational waves, it must be spin 2. And since everything has energy, and energy gravitates, the graviton must couple to all particles. For any more details than that, we’d need a quantum theory of gravity, which we don’t yet have. Since the best description of gravity we currently have is based on curved spacetime, it’s widely suspected that a quantum theory of gravity will involve quantization of spacetime, but that’s really just an educated guess.

In the same sense that a light wave can be regarded as a stream of many photons, so too a gravitational wave can be regarded as a stream of many gravitons. And we’re only a few years away from being able to detect gravitational waves, at LIGO or via pulsar timing. So in that sense, we expect to detect gravitons soon. But that’s only for an extremely large number of them. Detecting an individual graviton is far, far larger, such that I think it very likely that we will literally never be able to detect them individually.

It has been argued that no conceivable experiment could detect a single graviton. See for example:

The authors devise an appartus to detect gravitons and conclude that in order for it to be sensitive enough to detect a single graviton, it will be so massive that it will collapse to a black hole. That’s pretty cool!