  An earlier forward and backward in time formalism developed by us to discuss
non-relativistic electron diffraction is generalized to the relativistic case
and here applied to photons. We show how naturally the zero-point energy
emerges in the Planck black-body spectrum once symmetric in time motion -
inherent in the Maxwell equations - is invoked for photons. Then, a detailed
study is made of two-slit experiments for photons and some novel phenomena,
amenable to experiments, are proposed, that arise due to the spin of the
photon.
