Matter-Wave Interaction Continued, Introduction to Quantum Mechanics, Scattering Theory, Perturbation Theory, Methods & Problems in QED (July 1968 - June 1969)

Feynman went on in greater detail to complete his lectures on wave-matter interaction. From there he started into quantum mechanics and his path history formulation. He extended his lectures to include Lie Group theory and the SU 2&3 “Stuff”. Feynman diagrams are discussed in Volume 3 at some length as he went deep into QED theory including such topics as quantum scattering. As better understood today, his diagrams represent a visual language of the complex physical processes at the particle interaction level. I have noted recently that with the power of new computers and new concepts the Feynman diagrams have, arguably, run their course. While this is possibly the case, I would assert that bypassing a fundamental understanding of the Feynman diagram concept makes it hard to understand what replaces them. This is like hand held calculators replacing the need to know the fundamental multiplication tables and being able to check what the calculator is telling you. I personally observed in a number of lectures where Feynman would self-check himself as he was working out the math because he could sense that if he kept going he would not get the right physics. This was his true genius at work. That was truly amazing to both watch and try to absorb in real time.