1. Duke University, Department of Physics, and The Fitzpatrick Center for Photonics and Communication Systems, Durham, North Carolina 27708, USA
2. Department of Electrical and Computer Engineering, The Optical Sciences Center, University of Arizona, Tucson, Arizona 85721, USA

Correspondence to: Daniel J. Gauthier¹ Email: gauthier@phy.duke.edu

Abstract

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 source¹. Violation of this principle of relativistic causality leads to paradoxes, such as that of an effect preceding its cause². Recent experiments on optical pulse propagation in so-called 'fast-light' media—which are characterized by a wave group velocity _g exceeding the vacuum speed of light c or taking on negative values³—have led to renewed debate about the definition of the information velocity _i. One view is that _i = _g (ref. 4), which would violate causality, while another is that _i = c in all situations⁵, 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 _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.