Fire single particles, one at a time, at a barrier with two slits — and they build up an interference pattern as if each one went through both slits at once. Try to watch which slit they take, and the pattern vanishes. Richard Feynman called it the one mystery that contains 'the heart of quantum mechanics.' It has never stopped being strange.
The best 9-minute orientation to why this experiment matters. It lays out the wave-particle paradox cleanly, then walks straight into the part where merely observing changes the outcome.
The full lineage: Thomas Young's 1801 light experiment, its reinvention with electrons, atoms, and even large molecules, and the formal statement of wave-particle duality it forced physics to accept.
Derek Muller runs the actual experiment with a real laser and a human hair — no animation, no hand-waving. Seeing the interference fringes appear in physical reality hits differently than a diagram.
The moment it turns genuinely eerie: send light so faint that only one photon is in the apparatus at a time, and the interference pattern still builds up. Each particle somehow interferes with itself.
A friendly deep dive into the interpretations — Copenhagen, many-worlds, pilot-wave — and the delayed-choice quantum eraser, the variant that seems to let a measurement affect the past.
Modern confirmation from Wolfgang Ketterle's group: an idealized version using single atoms and single photons that settles the old Einstein–Bohr argument about whether you can ever see both natures at once.