One of Feynman's greatest contributions to physics was the interpretation of
negative energies as antimatter in quantum field theory. A key component of
this interpretation is the Feynman propagator, which seeks to describe the
behavior of antimatter at the virtual particle level. Ironically, it turns out
that one can dispense with the Feynman propagator in a direct-action theory of
fields, while still retaining the interpretation of negative energy solutions
as antiparticles.Comment: Forthcoming in a special issue of Quanta Magazine honoring Richard P.
Feynman; ed. Eliahu Cohe
