Science › Quantum world
The double-slit experiment
1801· Discovery / experiment
ProvenTested many times; part of working physics.
Send light or particles through two narrow slits and a striped pattern appears, the signature of overlapping waves.
See it on the Science map →How it works
Thomas Young did it with light around 1801, showing light is a wave. The quantum twist: send electrons one at a time. Each lands as a single dot, yet over thousands of dots the stripes build up, as if each electron passed through both slits and interfered with itself. Add a detector to see which slit it used and the stripes vanish, leaving two plain bands.
Why it matters
Richard Feynman said it holds the heart of quantum mechanics. It shows superposition, wave–particle duality and the role of measurement in one tabletop setup.
How we know
Young's light experiments (about 1801–1803); single-electron versions by Merli, Missiroli and Pozzi (1974) and Tonomura at Hitachi (1989); since repeated with atoms and large molecules.
Still a mystery
How a single particle 'goes through both' slits, and why looking changes the outcome, is the measurement problem, still unresolved.
Connections (3)
▸Wave–particle duality is shown most clearly by The double-slit experimentProven
The relationship
Send particles one by one through two slits. Each lands as a single dot, its particle side. But the dots build up a striped interference pattern, which requires waves passing through both slits and overlapping. Both natures appear in one experiment, making duality visible to the eye.
How the link was found
Young showed the pattern with light around 1801. G. I. Taylor did it with very dim light in 1909. Single-electron versions came from Merli and colleagues in 1974 and Tonomura in 1989.
Why it's proven
The pattern has been reproduced with photons, electrons, neutrons, atoms and molecules of about 2,000 atoms, always matching quantum predictions.
Open questions and mysteries
Which path, if any, the particle takes is unanswerable in standard quantum mechanics. Pilot-wave theory says it takes one path guided by a wave; that remains a minority view.
▸The double-slit experiment demonstrates Superposition: many states at onceProven
The relationship
The stripes appear only if each particle's two possibilities, through the left slit and through the right, coexist and interfere. If each particle secretly took one slit, the result would be just two bands. So the experiment directly shows a particle in a superposition of two paths.
How the link was found
Implicit in quantum mechanics from 1926; Feynman made the double slit the standard teaching example in his 1960s lectures.
Why it's proven
Interference vanishes exactly when which-path information becomes available, even in principle, as quantum theory predicts. Shown with electrons, atoms and molecules, and in 'quantum eraser' experiments.
Open questions and mysteries
Whether the superposition is physically real or just bookkeeping for knowledge depends on the interpretation.
▸The double-slit experiment shows watching changes the result, posing The measurement problemProven
The relationship
Put a detector at the slits to learn which one each particle uses and the stripes disappear: particles behave as if they took one path. Remove the detector and the stripes return. Learning the path destroys the superposition. Why gaining information has this effect, and what counts as 'watching', is the measurement problem.
How the link was found
Debated by Bohr and Einstein from 1927; real which-path experiments became possible from the 1990s, for example with atoms by Dürr, Nonn and Rempe in 1998.
Why it's proven
Experiments confirm that path information, not a human looking, kills the interference. Any physical record of the path does it, so 'watching' means any recording interaction.
Open questions and mysteries
Why one definite outcome appears is unexplained. 'Quantum eraser' experiments, where erasing path data restores stripes, are often misread as backward-in-time effects; standard analysis shows they are not.