Superluminal speed of information?: Optical media

Arising from: M. D. Stenner, D. J. Gauthier & M. A. Neifeld Nature 425 , 695–698 (2003); Stenner et al . reply The theory of special relativity limits signal velocity to the velocity of light in a vacuum. A faster-than-light (superluminal) signal velocity would violate causality 1 , 2 . However, there are some experimental data and theoretical arguments that a causality violation does not necessarily happen if a signal velocity becomes superluminal 3 . Stenner et al . 4 claim to have measured the speed of information in a fast-light optical medium by using a novel experimental set-up. The measured information (its front) travelled at a speed that did not exceed c (the velocity of light in vacuum). Their experimental result is correct but the interpretation is misleading because the information did not travel in the range of fast-light frequencies, as I explain here.