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Letters to Nature

Nature 425, 695-698 (16 October 2003) | doi:10.1038/nature02016; Received 11 July 2003; Accepted 20 August 2003

There is a Brief Communications Arising (6 May 2004) associated with this document.

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The speed of information in a 'fast-light' optical medium

Michael D. Stenner1, Daniel J. Gauthier1 & Mark A. Neifeld2

  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

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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 upsilong exceeding the vacuum speed of light c or taking on negative values3—have led to renewed debate about the definition of the information velocity upsiloni. One view is that upsiloni = upsilong (ref. 4), which would violate causality, while another is that upsiloni = 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 upsilong in the medium vastly exceeds c. Our observations are therefore consistent with relativistic causality and help to resolve the controversies surrounding superluminal pulse propagation.