Photons that travel in free space slower than the speed of light
DOI: 10.1126/science.aaa3035
One sentence summary: The group velocity of light in free space is reduced by controlling the transverse spatial structure of the light beam.
---- 这种 One sentence summary 的结论都算新?真空中传播的 Gaussian 光速的 group velocity 小于光速 c,依赖于 transverse spatial structure,是众所周知的,教科书上到处有介绍,也能把它当成创新结果放在 Science 上?真是欺天下无人。当然如果改成 “实验证实自由空间中单个光子的传播速度可以小于光速 c”,的确是推翻传统观念的创新结果。
博主回复(2015-1-31 22:46):尽管这个工作的确细化了对光速不变的理解,但是正好证实了电动力学的预言,也是论文作者自己承认的那样:“Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves.”
(1)刘教授,按照 Einstein 1905 狭义相对论论文的假定,平面波是把光源放在无穷远处。再根据能量动量守恒定律,光子速度是不变的。因此光子的速度和产生光子的物理光源是无关的。
(2)根据狭义相对论,自由空间中的光子的能量和动量必须构成 Lorentz 4-矢量。如果光子 speed 小于光速 c,如何构成 Lorentz 4-矢量?
(3)这篇文章的概念非常模糊,什么是光子速度(speed),什么是群速?按照他们的意思,光子的速度就是群速;实验中光脉冲的每个光子都有相同速度。而实际上,如果一个光脉冲中每个光子的速度都相等,光脉冲就没有发散和色散,那就是平面波光脉冲。但作者却又说 “our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves”,实在有蒙外行之嫌。
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.
我的回复(2015-1-31 22:46):尽管这个工作的确细化了对光速不变的理解,但是正好证实了电动力学的预言,也是论文作者自己承认的那样:“Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves.”