Gravitons

[Enola_Straight]

In Quantum Electrodynamics, the Electromagnetic force is transmitted via virtual photon exchange.

Regular, real photons are percieved as light.

If a Quantum theory of Gravity is found to behave the same way, with the exchange of virtual gravitons, what are REAL gravitons?

[DarrenS]

I found this by searching Google for “real gravitons”:

when a massive particle is at rest, it is surrounded by a static gravitational field (a static curvature of spacetime, no waves) . If another massive particle encounters this field, it experiences a force that can be described in quantum terms as an exchange of virtual gravitons by the two masses. On the other hand, if a massive particle is accelerated, its gravitational field is ‘shaken’ to produce a gravitational wave that spreads out through spacetime from the particle. The energy and momentum of that gravitational wave are carried by real gravitons.

It was on the Scientific American website (“Ask the experts”) but the link doesn’t seem to be working and the site itself is currently down (I got this from Google’s cache).

Hopefully Chronos or someone will be along soon to give us more of a “feel” for what real gravitons are and how they behave. (AFAIK we haven’t even successfully detected gravity waves yet?)

[TVAA]

Gravitons are merely hypothetical – they’re not even predicted (directly) by any of our current theories.

Although a highly sensitive gravity-wave detector has recently been reaching the final stages of its construction and testing, we’ve never actually detected gravitational waves yet.

[g8rguy]

While it’s true that we’ve not detected gravitational waves at present (and I’m not convinced we will any time soon, because we’re looking for a very weak effect), we have seen evidence of them, in that certain (IIRC; my notes are back at the office) binary systems are showing energy loss at exactly the rate predicted by the gravitational radiation they would emit. So while we haven’t seen gravitational radiation yet, I think it’s safe to say that few physicists doubt their existence.

There are quantum theories of gravity out there, but they suffer from the great drawback that we don’t know how to solve them (QG is, in physicist’s parlance, not “perturbatively renormalizable,” which means that the techniques we’ve developed to solve QED can’t be applied to QG). These theories do indeed assume gravitons, but they’re somewhat speculative. Again, though, I think it’s safe to say that most physicists believe that the graviton should exist.

Lastly, the distinction between “real” and “virtual” photons (and gravitons) is in some sense rather arbitrary. In a way, a real photon is a virtual photon that’s lived long enough to interact with something far from where it’s produced, and a virtual photon is one that hasn’t. The theory itself makes no other distinction between the two.