Lecture 21 of my Quantum Theory course at McGill University, Fall 2012. Relativistic Quantum Mechanics. The Need for Quantum Field Theory.
The course webpage, including links to other lectures and problem sets, is available at
http://www.physics.mcgill.ca/~maloney/551/
The written notes for this lecture are available at
http://www.physics.mcgill.ca/~maloney/551/551-21.pdf
