Superluminal censorship
Abstract
We argue that "effective" superluminal travel, potentially caused by the tipping over of light cones in Einstein gravity, is always associated with violations of the null energy condition (NEC). This is most easily seen by working perturbatively around Minkowski spacetime, where we use linearized Einstein gravity to show that the NEC forces the light cones to contract (narrow). Given the NEC, the Shapiro time delay in any weak gravitational field is always a delay relative to the Minkowski background, and never an advance. Furthermore, any object travelling within the lightcones of the weak gravitational field is similarly delayed with respect to the minimum traversal time possible in the background Minkowski geometry.
 Publication:

Nuclear Physics B Proceedings Supplements
 Pub Date:
 June 2000
 DOI:
 10.1016/S09205632(00)007829
 arXiv:
 arXiv:grqc/9810026
 Bibcode:
 2000NuPhS..88..267V
 Keywords:

 General Relativity and Quantum Cosmology;
 High Energy Physics  Theory
 EPrint:
 4 pages, uses espcrc2.sty. Significant revisions: (1) the perturbative analysis is extended