The speed of information in a ‘fast-light’ optical medium

One consequence of the special theory of relativity is that no signal can cause an effect outside the source light cone, the space-time surface on which light rays emanate from the source 1 . Violation of this principle of relativistic causality leads to paradoxes, such as that of an effect preceding its cause 2 . Recent experiments on optical pulse propagation in so-called ‘fast-light’ media—which are characterized by a wave group velocity υ g exceeding the vacuum speed of light c or taking on negative values 3 —have led to renewed debate about the definition of the information velocity υ i . One view is that υ i = υ g (ref. 4 ), which would violate causality, while another is that υ i = c in all situations 5 , which would preserve causality. Here we find that the time to detect information propagating through a fast-light medium is slightly longer than the time required to detect the same information travelling through a vacuum, even though υ g in the medium vastly exceeds c . Our observations are therefore consistent with relativistic causality and help to resolve the controversies surrounding superluminal pulse propagation.