Abstract
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 causality1,2. However, there are some experimental data and theoretical arguments that a causality violation does not necessarily happen if a signal velocity becomes superluminal3. 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.
Similar content being viewed by others
Main
The authors transmitted a pulse with a carrier frequency of 389 THz through an optical medium having near this very frequency a narrow fast-light frequency band of 23 MHz. (The precise numbers of the relevant frequencies are not given.) The group refractive index ng in this narrow band is said to be negative, with ng = −19.6 resulting in a negative pulse speed. The information (generated by a sudden positive change of unity or a negative change of zero in the pulse amplitude, representing points of non-analyticity) was modulated at the pulse maximum. The modulation had a broad frequency band up to some gigahertz (that is, some nanoseconds in the time domain).
The complete frequency band of the signal is given by the carrier frequency plus the modulation frequency. It exceeds the extremely narrow fast-light frequency regime of 389 THz ± 11.5 MHz. Thus, the front of the information experienced a positive group refractive index ng = 3.38 outside the small fast-light window according to its 3.3-nanosecond front delay. The front of the modulation, defined as information, travelled more slowly than light in a vacuum. The pulse without modulation has travelled with superluminal speed through the fast-light frequency regime of the optical medium.
The new experiment does not provide an answer to the fundamental question of whether faster-than-light transport of information is possible.
References
Sexl, R. & Urbantke, H. Relativity, Groups, Particles (Springer, Wien/New York, 2001).
Fayngold, M. Special Relativity and Motions Faster than Light (Wiley, Weinheim, 2002).
Nimtz, G. Progr. Quant. Electr. 27, 417–450 (2003).
Stenner, M. D., Gauthier, D. J. & Neifeld, M. A. Nature 425, 695–698 (2003).
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Nimtz, G. Superluminal speed of information?. Nature 429, 40 (2004). https://doi.org/10.1038/nature02586
Issue Date:
DOI: https://doi.org/10.1038/nature02586
This article is cited by
-
Superluminal speed of information? (reply)
Nature (2004)
Comments
By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.
