QED · Light & matter
Predict firstCan an uncharged photon set an electron moving?
Understand how light exchanges energy with matter, why annihilation respects conservation, and how real photons differ from internal propagators.
Prerequisites: Energy can change carrier; momentum describes motion; positive and negative electric charges have opposite signs. No field theory is assumed.Meet light and the electron
Predict firstCan an uncharged photon set an electron moving?
An electron has negative charge. A photon is uncharged but carries energy and momentum. Quantum electrodynamics, or QED, describes how light and charged matter interact. Begin with Compton scattering: a photon meets a free electron initially at rest; a scattered photon and a recoiling electron emerge. Gold wave packets identify photons; blue clouds mark electrons. These are teaching symbols, not pictures of particle shapes. Press play.
How to observe
- Play and scrub to follow energy moving between a photon and an electron.
- Compare angles and energies, then light up the detector’s relative probability.
- Separate real photons, virtual photons and conservation in annihilation and elastic scattering.
Model notes
Lowest-order QED with free particles. Unpolarized Klein–Nishina Compton scattering; selected centre-of-momentum annihilation and elastic electron events. Packets, colours and lengths are teaching symbols, not particle shapes or complete quantum states. No binding, radiative corrections, polarization or electron-scattering cross sections.