Instead of the wave equation for light, we use the quantum mechanical wave equation for particles that have spin half—electrons and protons are spin half particles and fit the bill here—and ...

András Vasy, A correction to "Propagation of singularities for the wave equation on manifolds with corners", Annals of Mathematics, Second Series, Vol. 177, No. 2 (March, 2013), pp. 783-785 ...