Relativistic Dressing of a Free Particle

An application of Feynman-type diagrams in a simple one-particle problem is offered as a pedagogical prop for setting the stage for the diagrammatic representation of serious perturbation calculations. This example is useful to the student, since it brings out several features that will be involved in the applications of diagrams to more sophisticated problems. Important aspects of this application are: (1) It is shown how the rules for combining probability amplitudes can be used; (2) a technique for introducing an external field (which can cause transition among states) is described; and (3) some of the language used in diagrammatic representations is introduced. The concept of dressing a particle is only partially developed.