Similarities between thermally equilibrated photons and an ideal gas composed of highly relativistic (ultra-light) particles have long been noted, as has the fact that the two systems appear to have different energy distribution functions. Here this connection is revisited to show that all the dynamical variable distribution functions of the two systems become identical when results obtained from ultra-light gas kinetic theory are required to be consistent with independently obtained results of thermodynamics and electromagnetic theory. Thus, classical ultra-light particles, whose rest mass approaches zero, are predicted to have equilibrium velocities that are practically indistinguishable from the speed of light, and the corresponding kinetic energy distribution functions are predicted to transform from a Wien distribution to a Planck distribution with increasing particle number density.

长期以来，人们一直注意到热平衡光子与由高相对论（超光）粒子组成的理想气体之间的相似性，因为这两个系统似乎具有不同的能量分布功能。在此重新讨论该连接，以表明当要求从超轻气体动力学理论获得的结果与独立获得的热力学和电磁理论的结果一致时，两个系统的所有动力学变量分布函数都变得相同。因此，预计其静止质量接近零的经典超轻粒子具有的平衡速度实际上与光速没有区别，