The Double-Slit Experiment
quantum mechanics · exhibit ii
Fire a single electron — or photon, or even a molecule — at a barrier with two narrow slits. You'd expect it to go through one or the other, land in a predictable spot, behave like a particle. Instead, it produces an interference pattern on the detector wall: the characteristic striped signature of a wave passing through both slits simultaneously.
The experiment's deeper provocation arrives when you try to find out which slit the particle used. The moment you place a detector at either slit — the moment the system is observed — the interference pattern vanishes. The particle stops behaving like a wave and lands in one of two clumps, exactly where you'd expect a classical particle to land. Observation itself changes the outcome.
Wave mode (unobserved). The particle's quantum state is a superposition — it exists in multiple states at once, described by the wavefunction ψ. When both paths are open and unmonitored, ψ passes through both slits and the two wavefronts interfere. Constructive interference (peaks meeting peaks) produces bright bands. Destructive interference (peaks meeting troughs) produces dark gaps.
Particle mode (observed). The act of measurement forces the wavefunction to collapse — the probability distribution resolves into a single definite outcome. The particle is found at one specific location, and the interference pattern disappears. The math still works; only the phenomenology changes.
The deep strangeness. The particle doesn't secretly travel one path while we're not looking. Bell's theorem and subsequent experiments confirm: the superposition is real. Before measurement, the particle has no definite position. Quantum mechanics is not incomplete — the universe is genuinely non-classical at this scale.
toggle observe to collapse the wavefunction · switch modes to compare · single slit removes interference