It turns out that accelerated charge produces electromagnetic radiation . Electromagnetic radiation is nothing more than one or more photons which have zero mass, and are therefore real, not virtual. Feynman diagrams for two processes which potentially might produce real photons and hence electromagnetic radiation. The process in the left panel turns out to be impossible if the masses of particles A and B are the same, for reasons discussed in the text. The process in the right panel occurs commonly.
Solid lines represent electrons while dashed lines represent photons.
Particles are taken to be real unless otherwise labeled. Acceleration of a charged particle is needed to produce radiation because of the conservation of energy and momentum. The left panel of figure 16.8 shows why. Since a photon carries off energy and momentum, conservation means that the energy and momentum of the emitting particle change due to the emission of a photon. This corresponds in classical mechanics to an acceleration.