TB: QED (Feynman)
Core Thesis
QED — the quantum theory of light and matter — is the most precisely tested theory in the history of science. It rests on three basic actions and one rule: photons go from place to place, electrons go from place to place, electrons emit or absorb photons. Each action has an amplitude (an arrow). To find the probability of any event, add all the arrows for all the ways it can happen, and square the result. This rule — the path integral — explains everything from optics to chemistry to the partial reflection of glass.
Key Takeaways
The three basic actions
- A photon goes from place to place.
- An electron goes from place to place.
- An electron emits or absorbs a photon.
- All of optics, chemistry, atomic physics, and electromagnetism reduce to combinations of these three processes.
Partial reflection — QM's first deep mystery
- Newton couldn't explain why some photons reflect from glass and others pass through. Neither can anyone today in a "simple-minded" way.
- The resolution: every way a photon can travel has an amplitude (an arrow). To find the total probability, sum all arrows and square the result.
- Science has not collapsed: identical circumstances (same photons) can produce different results (reflect or transmit). We can only predict probabilities. This is QM.
- Diffraction grating proves we cannot ignore "irrelevant" paths: scrape away parts of a mirror to bias the arrow directions and light reflects at unexpected angles. Every path contributes.
Light appears to slow in glass — but doesn't
- Light appears to go slower in glass because electrons in the glass scatter it. Adding the scattering amplitudes produces a result equivalent to a slower wave. The index of refraction is a consequence of the three basic actions, not a separate law.
- The same scattering explains partial reflection: intimate connection between the index of refraction and reflectivity.
Bose statistics and lasers
- When two photons end up in the same state, their arrows always constructively interfere — we get four times the single-photon probability, not twice.
- This is stimulated emission: the chance of emitting a photon is enhanced when photons are already present. It is why lasers work.
QCD — the still-unsolved frontier
- We have solved 99% of everyday phenomena with electrons and photons. Explaining the remaining 1% requires 10–20 times more complication (quarks, gluons, the Standard Model).
- With QCD we have a definite theory and hundreds of experiments, but we cannot yet compare them quantitatively. This situation — definite theory, untestable prediction — has never existed before in physics.
Mental Models
- Complex Systems: Features from Path, Not Design — all of optics and chemistry emerge from three primitive actions; the complexity is in the combinations, not the rules
- The Map is Not the Territory — light "going slower" in glass is not literal; it is a useful map generated by summing scattering amplitudes