The Double-Slit Experiment Explained

The Setup

The double-slit experiment uses a source that emits particles (photons, electrons, atoms, or even large molecules), a barrier with two narrow parallel slits, and a detection screen behind the barrier. In the original version performed by Thomas Young in 1801 with light, the experiment demonstrated the wave nature of light by producing an interference pattern on the screen. In the quantum mechanical version, the same experiment is performed with individual particles sent one at a time, revealing the deepest mysteries of quantum physics.

The apparatus is conceptually simple but the results are profound. The key controls are whether the slits are open or closed and whether a detector is present at the slits to determine which path each particle takes. The results change dramatically depending on these choices, in ways that have no classical explanation.

The Interference Pattern

When both slits are open and no detector is watching the slits, particles gradually build up an interference pattern on the detection screen. This pattern consists of alternating bright and dark bands. The bright bands appear where the waves from the two slits arrive in phase (constructive interference), and the dark bands appear where they arrive out of phase (destructive interference). The spacing of the bands depends on the wavelength of the particles and the distance between the slits.

When only one slit is open, no interference pattern appears. Instead, you see a single broad band behind the open slit, exactly as you would expect for particles passing through a single opening. The interference pattern requires both slits to be open, meaning both paths must be available for the particle to travel.

When both slits are open and particles are sent one at a time, each particle arrives at a single point on the screen, like a tiny dot. There is no interference pattern from a single particle. But after thousands of particles have been sent through, the dots collectively form the same interference pattern seen with a continuous beam. Each particle, arriving individually, somehow knows about both slits and contributes to the pattern as if it had traveled through both slits as a wave.

The Which-Path Problem

If you add a detector at one of the slits to determine which slit each particle passes through, the interference pattern completely disappears. With the detector active, particles behave like classical particles, producing two simple bands behind the two slits with no interference. The act of detecting which slit the particle uses forces it to behave as a particle rather than a wave.

This is not caused by the physical disturbance of the detector, at least not primarily. Even extremely gentle measurement schemes that barely interact with the particle still destroy the interference pattern. The key factor is whether which-path information exists, regardless of whether anyone looks at it. If the information is recorded but never examined, the interference pattern is still destroyed. If the information is recorded and then erased (as in quantum eraser experiments), the interference pattern can be recovered.

This suggests that what matters is the existence of a correlation between the particle path and some physical record, not the consciousness of an observer. When which-path information is entangled with a detector state, the quantum coherence between the two paths is destroyed, and the interference vanishes. This is a specific instance of decoherence.

Delayed-Choice and Quantum Eraser Experiments

John Archibald Wheeler proposed the delayed-choice experiment in 1978. In this version, the decision to observe or not observe which-path information is made after the particle has already passed through the slits. If you decide to look for interference, you see interference. If you decide to determine which path, you see particle behavior. The particle seems to retroactively choose whether to be a wave or a particle based on a future measurement decision.

Quantum eraser experiments, first demonstrated in the 1990s, extend this further. Which-path information is recorded in an auxiliary system (an idler photon), and then this information is either preserved or erased. When the information is erased, the interference pattern is recovered, but only in the subset of events correlated with the erased idler photons. The full pattern, summing over all events, shows no interference. The interference is hidden in the correlations and can only be revealed by post-selection.

These experiments do not allow faster-than-light signaling or backwards-in-time communication. The interference pattern recovered in a quantum eraser experiment is only visible after correlating the signal and idler photon data using a classical communication channel. No observer at the detection screen alone can determine whether the eraser was activated.

What It Means

Richard Feynman called the double-slit experiment the central mystery of quantum mechanics and said that it contains the only mystery. He meant that every other quantum phenomenon, entanglement, tunneling, uncertainty, can be understood as a variation of the same fundamental puzzle demonstrated by the double slit: quantum objects are neither waves nor particles, and the outcome depends on the experimental arrangement in ways that defy classical explanation.

The experiment has been performed with photons, electrons, neutrons, atoms, and molecules of increasing size. In every case, the results match quantum mechanical predictions exactly. It has been repeated with individual particles separated by days, with the interference pattern building up over long accumulation times. The results are always the same, confirming that quantum mechanics operates identically regardless of when or how the experiment is performed.

Interpretations of the Double-Slit Experiment

Different interpretations of quantum mechanics offer different accounts of what happens at the double slit. Copenhagen says the particle has no definite path until measured and that asking which slit it went through when no detector is present is meaningless. Many-worlds says the particle goes through both slits in different branches of the universe. Pilot wave theory says the particle goes through one slit while the pilot wave goes through both, guiding the particle to locations consistent with the interference pattern. All interpretations reproduce the same experimental predictions.

Modern Variations and Extreme Tests

Modern technology has enabled increasingly sophisticated versions of the double-slit experiment. Electron biprism experiments use electrostatic fields rather than physical slits to split electron beams, achieving cleaner interference patterns and better control over experimental parameters. These experiments have been performed with individual electrons, demonstrating that single-particle quantum interference is a robust, repeatable phenomenon and not an artifact of beam statistics.

Matter-wave interferometry with large molecules pushes the boundaries of what exhibits quantum behavior. Experiments with fullerene (C60) molecules demonstrated interference in 1999, followed by increasingly larger molecules. The current record holders are molecules with masses exceeding 25,000 atomic mass units, consisting of thousands of atoms. These experiments place constraints on proposed modifications to quantum mechanics that would cause superposition to break down above a certain mass or size threshold. So far, no such breakdown has been observed.

In 2012, researchers performed a double-slit experiment with antimatter for the first time, using positrons (anti-electrons). The positrons produced the same interference pattern as electrons, confirming that antimatter obeys the same quantum mechanical rules as ordinary matter. This result, while expected from theory, provided important experimental verification of the universality of quantum mechanics.