Turn the source down until there is only ever one particle in the apparatus at a time. Waves can interfere with each other. But a single electron has nothing to interfere with, so the fringes should vanish. They don't. Each particle lands at a single point, apparently at random; the fringe pattern emerges only in the statistics of thousands of arrivals. This is the version of the experiment Tonomura's team filmed with single electrons in 1989, and the reason Feynman called the double slit the only mystery: each electron, alone, behaves as if it took both paths.
Each dot below is one arrival, drawn at random from the probability distribution |ψ|² that the wave engine computes for this geometry (λ = 550 nm, d = 60 µm, a = 10 µm, L = 1 m). No dot knows where any other dot landed.
No individual arrival hints at a pattern: quantum mechanics predicts only probabilities. Watch at one per second for a while; it is oddly persuasive in a way the mathematics is not.
How random is randomTurn the rate up and fringes condense out of the statistics. There are dark bands where no particle ever lands, though each slit alone would happily deliver particles there.
Why opening a slit removes placesDetector on: interference gone. What's left is one smooth band, not two neat stripes. And nothing was blocked; the particles were merely watched.
Why looking kills it