Superluminal Photons Possible?

Can superluminal photon propagation really be compatible with the principle of causality, or does it necessarily imply the existence of time machines? After all, such motion is genuinely backwards in time as viewed locally by a class of inertial observers. The question remains controversial, but the key is the SEP. In special relativity, a causal paradox requires both outward and return signals to be backwards in time in a global inertial frame. In general relativity, however, global Lorentz invariance is lost and the existence of a sufficiently superluminal return signal is not guaranteed. The quantum violation of the SEP certainly permits superluminal motion, but with photon velocities predetermined by the local curvature. Consistency with causality is therefore a global question. If the original space-time admits a global causal structure with respect to the geometrical light-cones, then causality will be respected even in the presence of superluminal photons if this structure is preserved with respect to the new light-cones. This rapidly leads to sophisticated issues of global topology in general relativity, but at this stage superluminal photons appear to be both consistent with causality and predicted by QED.
http://cerncourier.com/main/article/42/3/13