Superluminal censorship

We argue that "effective" superluminal travel, potentially caused by the ti pping over of light cones in Einstein gravity, is always associated with vi olations of the null energy condition (NEC). This is most easily seen by wo rking perturbatively around Minkowski spacetime, where we use linearized Ei nstein gravity to show that the NEC forces the light cones to contract (nar row). Given the NEC, the Shapiro time delay in any weak gravitational field is always a delay relative to the Minkowski background, and never an advan ce. Furthermore, any object travelling within the lightcones of the weak gr avitational field is similarly delayed with respect to the minimum traversa l time possible in the background Minkowski geometry.