Superluminal censorship

TL;DR

This paper demonstrates that effective superluminal travel scenarios involving light cone tipping are fundamentally linked to violations of the null energy condition, ensuring delays rather than advances in weak gravitational fields.

Contribution

It provides a perturbative analysis showing NEC enforces light cone contraction and rules out superluminal travel or time advances in weak gravity regimes.

Findings

NEC enforces light cone narrowing in Minkowski spacetime.

Superluminal travel implies NEC violation.

Weak gravitational fields always produce delays, not time advances.

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.