First, what waves do at two slits
Richard Feynman called this one experiment the heart of quantum mechanics — the place where all the mystery lives. To feel it, start with plain water waves, where nothing is strange at all. Send straight ripples toward a barrier with two narrow gaps. Each gap becomes a new source of circular ripples spreading out behind the barrier. Now those two sets of ripples overlap, and something lovely happens.
Where a crest from one slit meets a crest from the other, they reinforce into a taller crest. Where a crest meets a trough, they cancel into flat calm. So on a screen beyond, you get an alternating pattern: tall waviness, dead calm, tall waviness, dead calm — a row of bands. This is the interference pattern, and it is the unmistakable signature of two waves combining. No collection of little balls flying through two holes could ever produce it; balls just pile up in two clumps behind the two gaps.
Now fire particles, one at a time
Here is the move that makes the double-slit experiment legendary. Replace the water with electrons (or photons) and fire them at two slits — but turn the source down so low that only one electron is in flight at any moment. Each electron arrives at the screen as a single tiny dot, a clean particle hit, exactly where one ball would land. So far, so sensible: they come one at a time, they land as points, they are particles.
But watch the dots pile up over minutes and hours. You might expect two simple clumps, one behind each slit — the particle answer. Instead, the dots slowly, dot by dot, assemble themselves into the banded interference pattern. The very pattern that requires two overlapping waves is being built up by particles that arrived one lonely dot at a time, never sharing the apparatus with another. Each electron, it seems, went through both slits and interfered with itself. This is single-particle interference, and it is the cleanest jolt of the quantum world there is.
- Send electrons one at a time. Each leaves the source alone, with no companion in the apparatus.
- Each lands as a single sharp dot. Individually, every electron behaves like a particle.
- Let thousands accumulate. The dots quietly arrange into bright and dark bands — an interference pattern.
- Conclude: each lone electron explored both paths as a wave, yet registered as one particle on arrival.
Wave on the way, particle on arrival
So which is it, wave or particle? The experiment's honest answer is: a wave between source and screen, a particle at the moment it lands. While in flight, each electron is described by a spread-out wave that passes through both slits and overlaps with itself, building the same crest-and-trough pattern as water. But you never detect that wave directly. When the electron is finally registered, it shows up as a single point — and where it is likely to show up is governed by the wave. Tall waviness means a high chance of a dot landing there; dead-calm cancellation means almost no chance.
This is the first whisper of one of the deepest ideas in all of physics: the quantum wave is a wave of probability. Its height at a place tells you how likely the particle is to be found there — a quantity called the probability density. A single electron's landing spot is genuinely unpredictable; only the pattern built from many is certain. The wave does not tell you where the electron is. It tells you the odds.
Why this experiment is the whole story
Every theme of this track is already sitting inside this one setup. Wave-particle duality is right there: wave in transit, particle on detection. The probability wave is there, governing where dots land. And the next deep twist — what happens when you try to peek at which slit the electron used — lives here too, waiting. The double slit is not one experiment among many; it is a single, endlessly re-runnable demonstration that the quantum world plays by rules our marble-and-ripple intuitions simply do not cover.