In lowest order, the process e⁺ + e^- †’ γ + γ is given by the Feynman diagrams of Figure 1.9. Show that for electrons and positrons almost at rest, the distance between the two vertices is typically of order m^-1in natural units, where m is the electron mass. Convert this result to practical units and evaluate it in fermis.

Figure 1.9

ex (a) ex (b)