Where we start

Quantum electrodynamics — the quantum theory of electrons, positrons, and light — fits on a single line:

\mathcal{L}_{\mathrm{QED}} = \bar\psi\,(i\gamma^\mu D_\mu - m)\,\psi \;-\; \tfrac{1}{4}\,F_{\mu\nu}F^{\mu\nu}

This course begins where a first encounter with quantum field theory ends. The fields here are already quantized; we take for granted that a field's ripples are particles. What we will not take for granted is anything this line actually says.

Over three lessons we take the Lagrangian apart and reassemble it into a working rulebook — the Feynman rules. In Part 2, that rulebook computes numbers a collider can check. Every symbol above will earn its keep.

First: who are $\psi$ and $A_\mu$ ? Let's meet the cast.