Revisiting a superluminal censorship

In an article by Visser, Basset & Liberati [Nucl.Phys.B Proc.Suppl. 88 (2000) 267-270] they argued that is possible to link the satisfaction of an energy condition with the absence of superluminal behaviours. This paper was soon criticised by Gao & Wald [Class.Quant.Grav. 17 (2000) 4999-5008] stressing that the result was not gauge invariant and thus had no physical significance. In this talk I will argue that although Gao-Wald's criticism is correct, it is possible to construct a proper bi-metric framework in which the VBL idea can be implemented rigorously.