In several posting in Special & General Relativity it is suggested that photons influence the curvature of space-time. Now, if I understand QM correctly, a photon does not posses a definite momentum, unless it interacts. So it appears that between the times of interactions it evolves in state space but not in real space. So then how could we possibly say that photons influence the curvature of space-time while traveling? What am I missing?