Perturbative superluminal censorship and the null energy condition

TL;DR

This paper demonstrates that effective superluminal travel in Einstein gravity is inherently linked to violations of the null energy condition, ensuring causality is preserved by preventing true superluminal signaling.

Contribution

It establishes a perturbative connection between superluminal effects and NEC violations, showing NEC enforces light cone contraction and prevents superluminal travel in weak gravitational fields.

Findings

NEC enforces light cone contraction in linearized gravity.

Superluminal travel implies NEC violation.

Shapiro delay is always positive under NEC.

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.