  It has been demonstrated in a recent paper (Mod.Phys.Lett. A13, 1265 (1998);
hep-th/9902020) that the existence of a non-thermodynamic arrow of time at the
atomic level is a fundamental requirement for conservation of energy in
matter-radiation interaction. Since the universe consists of two things only
--- energy and massive matter --- we argue that as a consequence of this
earlier result, particles and antiparticles must necessarily move in opposite
directions in time. Our result further indicates that the CPT theorem can be
extended to cover non-local gauge fields.
