Section 10.4 Feynman Diagrams
In putting the ideas of quantum field theory together in the 1940's, Richard Feynman developed a method of diagramming interactions. These diagrams can actually be translated directly to equations using a set of Feynman rules that associates different parts of the diagram to various factors and terms in the equations. We will not try to do that, but we will use Feynman-like diagrams to sketch reactions.
Let's diagram the interaction of two electrons repelling each other via the electromagnetic interaction. We've just seen that one electron emits a virtual photon as the messenger, and the other electron receives it. If we plot time upward and space horizontally (like a space-time diagram) we find the interaction looks like Figure 10.2.
The particles that interact are drawn as solid straight lines, indicating constant velocity before and after the interaction. The lines can change direction only at a vertex, the point where two solid lines and a squiggly photon line meet. The vertex represents the emission or absorption of the messenger particle, events which can change the properties of the interacting particles. In this case, the momentum of each electron changes at the vertex.
The weak and color interactions can also be illustrated in this way. The messenger particles for the weak interactions are the \(W^+\text{,}\) \(W^-\text{,}\) and \(Z^0\) bosons, which on a diagram are represented by closely spaced parallel lines. For the strong interaction between the quarks, the messengers are colored gluons, which appear as double squiggly curves. Figs. Figure 10.3 and [cross-reference to target(s) "fig_quark_gluon" missing or not unique] show examples of weak and strong interactions.
