Feynman Diagrams

A full lecture reconstruction introducing perturbative quantum electrodynamics from the Coulomb-gauge Hamiltonian, identifying the fine-structure constant as the small expansion parameter, and using time-independent perturbation theory to analyze the QED vacuum and one-electron states.

A full lecture reconstruction showing why naive low-order perturbative QED corrections to particle energies are problematic, why Compton scattering is the first clean finite order-α process, and how Lorentz-invariant scattering amplitudes emerge only after coherent addition of all electron and positron intermediate-state processes.

A full lecture reconstruction showing how Lorentz-invariant perturbative QED emerges from the QED path integral as variational calculus on a Gaussian generating functional, and how the surviving terms organize into Feynman diagrams and Feynman rules.

Introduce the electron-photon vertex as a perturbative building block. See how incoming and outgoing particles meet at a point where charge and energy-momentum are exchanged through the coupling.

Animate incoming and outgoing states, the two leading diagrams, and the kinematic shift.

Introduce a source J(x), differentiate the generating functional to pull down field insertions, and watch correlators and diagrams emerge from one master object.