It is shown how relativistic and gauge invariance come into play as basic ingredients to get the Green’s function for a Dirac particle interacting with a classical plane wave field in the vacuum. This point of view is also illustrated in the case of two electromagnetic plane waves having orthogonal propagating directions in the vacuum.

中文翻译：

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狄拉克粒子与经典平面波场相互作用的相对论不变性，规范不变性和格林函数

它显示了相对论和尺度不变性如何作为基本成分发挥作用，从而获得狄拉克粒子在真空中与经典平面波场相互作用的格林函数。在两个电磁平面波在真空中具有正交传播方向的情况下，也说明了这种观点。