Feynman's thought experiment from The Character of Physical Law, lecture 6, "Probability and Uncertainty". The same apparatus, run three times: with bullets, with water waves, and with electrons. Then we try to watch the electrons.
| Bullets | Water waves | Electrons (photons) | |
|---|---|---|---|
| What arrives | Lumps | Any amount at all | Lumps |
| What we measure | Probability of arrival | Intensity of the waves | Probability of arrival |
| Both holes open | N₁₂ = N₁ + N₂ no interference | I₁₂ ≠ I₁ + I₂ interference | N₁₂ ≠ N₁ + N₂ interference |
Keys: 1–4 switch experiment, Space pause, A/B toggle hole 1/2, F fire one, C clear, P predicted curve, S single-hole curves. In the water experiment, drag along the backstop to move the cork.