© David Kaplan.

Feynman diagrams like the ones shown here facilitate the calculation of scattering cross sections. The diagrams are drawn according to strict rules about what particles are allowed to interact. To calculate, for example, the scattering cross section for the two electrons in Unit 2, Figure 5, you need to draw all the possible diagrams that start and end with two electrons (represented by the blue lines) that exchange force carriers of the electromagnetic force (red lines). The diagram shown at left is the simplest of these, and is called a "tree-level" diagram because it has branches but not loops like the diagram on the right. Each diagram represents instructions on how to do a calculation that contributes to the final answer. The diagrams are organized by how many loops there are (zero and one above), with the smallest number of loops normally giving the biggest contribution. The more accurately one wants an answer, the higher the number of loops that must be calculated. (Unit: 2)