Description of diffraction-grating experiments for photons and electrons in Feynman's spacetime formulation of quantum mechanics: the quantum origins of classical wave theories of light and massive particles

The five laws of relativistic quantum mechanics, according to Feynman's path integral formulation, are concisely stated and applied to experiments. Reflection–diffraction-grating experiments for both photons and electrons are analysed, in particular, the Davisson–Germer experiment in which the wave-like property of electrons was first established. It is shown how classical, purely spatial, effective wave theories for both photons and electrons are predicted by the path integral formulation of quantum mechanics. The standard Copenhagen interpretation of wave mechanics is critically discussed in the light of the described experimental applications of the path integral formulation.