The four vector J_μ is the source of the Electromagnetic potential four vector A_μ. If I wanted a source term for the Dirac equation (not that I think there is such a thing) could it be (must it be) a spinor source term (what ever that means)? Related question, consider the two Feynman diagrams below. In the first diagram the current is a source of a virtual photon. In the second a photon is destroyed and a electron positron pair is produced. Can I think of the virtual photon as being a source for the Dirac equation? Diagram from, http://www.google.com/imgres?hl=en&...49&start=12&ndsp=15&ved=1t:429,r:6,s:12,i:116 Thanks for any help!