传统上,一般认为反常色散区的超光速群速没有物理意义【1】。但有人提出批评,认为这是长期以来的一个错误概念【2】。理由是 L.J. Wang 的实验中观察到光脉冲几乎无变形地超光速传播【3】。如果超光速脉冲物理上是可行的,那么就可以把超光速脉冲用于数字通讯,实现超光速通信。但这样一来又和相对论相矛盾。错在哪儿呢?
1. J.D. Jackson, Classical Electrodynamics, John Wiley & Sons, New Jersey, 1999, Sec. 7.8.
书中说 “group velocity is generally not a useful concept in regions of anomalous dispersion”.
2. P. W. Milonni, Fast Light, Slow Light, and Left-Handed Light, IOP Publishing, London, 2005.
书中Milonni 批评说, “group velocity ceases to have physical significance in the case of anomalous dispersion, when it can exceed c” is a long-standing misconception, because the group velocity in such a case “still retains its meaning as the velocity of nearly undistorted pulse propagation, as experiments have shown”. 这儿的实验指的是L.J. Wang的experiments.
3. L.J. Wang, A. Kuzmich, A. Dogariu, Gain-assisted superluminal light propagation, Nature 406 (2000) 277-279.
俺不清楚实验中测量的相继出现的两个脉冲是否同一个脉冲。