Our friends have finished their paper about the constraints on the signs of various terms in the effective action that are required by causality, i.e. by the fact that the speed of light can't be exceeded, not even at non-trivial backgrounds.

These superluminality constraints are classically contained in the energy conditions, especially the dominant energy condition, but these Gentlemen start from a very satisfactory and fundamental principle, namely the condition of unitarity.

When it comes to causality, Lubos, I want you to consider this paradox:

ReplyDeleteConsider a photon going straight up in the earth's gravity field. According to GR, the photon continuously lose energy due to the gravity effect, which we can all agree.

Now, since you believe gravity is mediated by gravitons, which travel at light speed. The question is how do you suppose the gravitons emitted by the earth can CATCH UP with the photon, and so be absorbed by it? Or how come the photon manage to catch up with the gravitons in from of it, and absorbs them. Remember both gravitons and photons travel at the same light speed, not a bit slower or faster. So how can they catch each other. If they can't catch each other, then how does the photon feel the gravity and lose energy?