““This elementary process is symbolically illustrated in figure 59c: A photon becomes an electron-positron pair; inversely, an electron and a positron can combine into a photon (or as we said above, annihilate into a photon). But for the process to take place, we need a fourth particle as a catalyst. In the cloud chamber picture in figure 12, the catalyst was an electron that was knocked out of its atomic configuration in the process. Whereas the process in figure 59c cannot proceed without such a catalyst, the one in figure 59d can: Two photons can materialize into an electron-positron pair; and, of course, the inverse process, electron-positron annihilation into two photons, is equally possible.””
