光速在自由空间中的不变性
Non-invariance of the speed of light in free-space
摘要：
平面单色波在真空中以速度c传播。然而，在空间和时间上有限的波包被用于传输能量和信息。在这里，它已被证明基于波的方法，轴上的部分的脉冲光束在自由空间中传播以可变的速度，表现出在靠近源的区域中的亚光速和超光速的行为，和他们的速度接近的值c与距离。虽然脉冲可以在比真空中的光速更快的小距离上行进，但是通过脉冲在距离z>> ld（ld是瑞利衍射范围）和z> ct（t是脉冲宽度）处的到达时间估计的平均轴上速度小于c。总脉冲束以恒定的亚光速传播通过整个距离。研究了真空非傍轴传输中辐射空间分布和脉冲时间形状的相互影响。研究发现，入射光束宽度的减小和脉冲中心波长的增大导致波包的传播速度变小。结果表明，贝塞尔-高斯和拉盖尔-高斯脉冲光束的速度随着轨道角动量的增加而减小。所得结果与自由空间中的亚光速和超光速测量结果符合得很好。
英文摘要：
A plane monochromatic wave propagates in vacuum at the velocity of c. However, wave packets limited in space and time are used to transmit energy and information. Here it has been shown based on the wave approach that the on-axis part of the pulsed beams propagates in free space at a variable speed, exhibiting both subluminal and superluminal behaviours in the region close to the source, and their velocity approaches the value of c with distance. Although the pulse can travel over small distances faster than the speed of light in vacuum, the average on-axis velocity, which is estimated by the arrival time of the pulse at distances z >> ld (ld is the Rayleigh diffraction range) and z > ct (t is the pulse width) is less than c. The total pulse beam propagates at a constant subluminal velocity through the whole distance. The mutual influence of the spatial distribution of radiation and temporal shape of the pulse during nonparaxial propagation in vacuum is studied. It is found that the decrease in the width of the incident beam and the increase in the central wavelength of the pulse lead to a decrease in the propagation velocity of the wave packet. It is shown that the velocities of the Bessel-Gauss and Laguerre-Gauss pulsed beams decrease with increasing orbital angular momentum (OAM). The results obtained are in good agreement with subluminal and superluminal measurements in free space.
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