ABSTRACT

This is the grand finale of this book – the Feynman diagram. But, before we can understand the Feynman diagram, we need to know about the principle of least action. This was introduced in Chapter 9 and again used in Chapter 15 for a field and is now applied to a particle like a photon or an electron. Using Feynman’s own examples, a graphical method of describing the action is provided, along with a mathematical explanation of what is actually being done.

The concepts in all the previous chapters are now brought together. A Feynman diagram is shown to be set in space–time and populated by events characterised by spinors, propagators and interaction terms. The way in which these are drawn on the diagram is then shown to be an equation involving probabilities associated with the principle of least action – the sum of the Lagrangians.

The meaning of the Feynman diagram is thus explained.