double-slit experiment

The double-slit experiment is the single demonstration that lays the strangeness of quantum mechanics bare. Particles — photons, electrons, even whole molecules — are sent toward a barrier with two narrow openings, and where they land on a screen behind is recorded. If they were simple little balls, you would expect two bright bands, one behind each slit. Instead a striped pattern of many bands appears, the unmistakable signature of waves passing through both slits and interfering.

The truly astonishing part comes when the particles are fired one at a time, so slowly that only one is ever in the apparatus at once. Each arrives as a single dot, perfectly particle-like. Yet as thousands of dots accumulate, the interference stripes build up anyway. A lone electron, with nothing to interfere with, somehow goes through both slits at once and interferes with itself, in the precise sense that its wavefunction does.

Richard Feynman called this the heart of quantum mechanics, holding the only mystery. Try to learn which slit each particle went through, and the stripes vanish, replaced by the two plain bands you would expect of pellets. The experiment forces the lesson that a quantum object explores all available paths as a wave, but is found at one place as a particle, with the pattern dictated by probability.