刘教授:
请你注意英捷特数据库平台(ingentaconnect) 中physics today68卷第6期以封面文章发表(31日前)名为 A new approach to the correction of Galilean transformation的文章,其属下期刊physics Essays已在线发表。我认为这文章的内容值得人们关注,故作此留言。如果你们能细读这文章,也许会给你们带来一些重要的课题和机遇。文章作者——刘圣。
PS:
Spatially structured photons that travel in free space slower than the speed of light
DOI: 10.1126/science.aaa3035
Abstract: That the speed of light in free space is constant is a cornerstone of modern physics. However, light beams have finite transverse size, which leads to a modification of their wavevectors resulting in a change to their phase and group velocities. We study the group velocity of single photons by measuring a change in their arrival time that results from changing the beam’s transverse spatial structure. Using time-correlated photon pairs we show a reduction of the group velocity of photons in both a Bessel beam and photons in a focused Gaussian beam. In both cases, the delay is several microns over a propagation distance of the order of 1 m. Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves. Introducing spatial structure to an optical beam, even for a single photon, reduces the group velocity of the light by a readily measurable amount.
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。