ABSTRACT
As noted earlier in this book we will discuss three of the four fundamental interactions: the strong, the weak and the electromagnetic. Among the three, there are many reasons why the electromagnetic interaction should be discussed first. First, this is the only one among the three for which there was a classical theory, and the insight from the classical theory might be helpful in discussing the quantum theory. Second, the quantum theory of the electromagnetic interactions, called quantum electrodynamics or QED for short, was developed earlier than the quantum theories of strong and weak interactions, so historically also this interaction comes first. And lastly, the theory is based on a local internal symmetry. This idea was taken over from QED and generalized to develop the present theories of weak and strong interactions. Because of this reason, and because the internal symmetry is much simpler for QED than for the theories strong and the weak interactions, it would be appropriate to study electromagnetic interactions first. This is what we do in this chapter.
