🔗Video 68. "Deriving The QED Lagrangian | Quantum Electrodynamics" by dietterich Labs (2018) Source. As mentioned at the start of the video, he starts with the Dirac equation Lagrangian derived in an earlier video. He notes that that Dirac Lagrangian has a global U(1) invariance, and he upgrades it into a local transformation whatever that means, likely related to local symmetry.