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Double-Slit Experiment

Reading time: 9 minConfidence: HighLast verified: July 2026

The double-slit experiment demonstrates that particles sent one at a time through two narrow slits accumulate, over many trials, into a wave-like interference pattern on a detection screen — direct evidence of wave-particle duality and quantum superposition.

Curious

One sentence that captures itFire particles one at a time through two slits, and each one still somehow "knows" about both slits — producing a wave pattern that only makes sense if each individual particle passed through both paths at once.

Cut two narrow slits in a barrier and shine light through them onto a screen. Since the 1801 work of Thomas Young, this has been the classic demonstration that light behaves as a wave: overlapping ripples from the two slits reinforce in some places and cancel in others, producing alternating bright and dark bands — an interference pattern.

The unsettling part came when physicists performed the same experiment with electrons, sent through one at a time, so slowly that only one electron could possibly be in the apparatus at any moment. Logically, with no other electron around to "interfere with," you'd expect two simple bands, matching the two slits. Instead, given enough time, the same full interference pattern builds up from individual electron impacts — as if each single electron were somehow interfering with itself.

Exploring

Real-world exampleThe experiment has been successfully repeated with progressively larger objects — electrons, neutrons, atoms, and even large organic molecules containing hundreds of atoms — each time showing the same interference pattern, confirming that wave-particle duality applies to matter at increasingly macroscopic scales, limited mainly by how well isolated the system can be kept from environmental disturbance.

See the interactive double-slit simulation to adjust slit separation, wavelength, and screen distance and observe the resulting fringe pattern directly.

The critical twist is what happens if a detector is placed at the slits to determine which slit each particle actually goes through. The moment "which-path" information becomes available — even in principle, even if no one looks at the data — the interference pattern disappears entirely, replaced by the simple two-band pattern expected of classical particles. Gaining particle-like (which-path) information necessarily destroys the wave-like (interference) behaviour.

Common mistake — Commonly ConfusedThe pattern doesn't disappear because "the act of human observation collapses reality" in some mystical sense. It disappears because any physical interaction capable of recording which-path information — a detector, a stray photon, any environmental disturbance — necessarily entangles the particle's path with that recording system, destroying the delicate phase relationship needed for interference. The effect is physical, not a matter of conscious awareness.

Worked example: For monochromatic light of wavelength λ passing through two slits separated by distance d, onto a screen a distance L away, the spacing between adjacent bright fringes is Δy = λL/d. For λ = 500 nm, d = 0.1 mm, L = 1 m: Δy = (500×10⁻⁹ × 1)/(0.1×10⁻³) = 5×10⁻³ m = 5 mm — comfortably visible fringe spacing for a benchtop setup.

Deep Dive

Primary sourceYoung, T. (1804). "The Bakerian Lecture: Experiments and Calculations Relative to Physical Optics." Philosophical Transactions of the Royal Society of London, 94, 1–16.

Thomas Young's original 1801–1804 experiments used light and were intended to settle the wave-versus-particle debate about light's nature in Young's favour, against Newton's earlier corpuscular theory — for over a century this was considered decisive evidence that light is purely a wave. The reintroduction of particle behaviour via Einstein's 1905 photoelectric work, and the subsequent extension of the double-slit result to matter particles (first performed conceptually by Davisson and Germer's 1927 crystal diffraction, and later literally with electron double-slit setups by Claus Jönsson in 1961), transformed the experiment's meaning: it now demonstrates duality rather than settling it in favour of one side.

The quantum mechanical account: In the formalism, each particle is described by a wavefunction that, absent which-path information, passes through both slits as a coherent superposition. The two resulting wave components interfere according to their relative phase when they recombine at the screen, and the Born rule (|Ψ|²) gives the probability of detection at each point — reproducing the classical interference intensity pattern exactly, but built up one probabilistic particle detection at a time rather than as a continuous physical wave.

Delayed-choice and quantum eraser variants: Wheeler's delayed-choice thought experiment (1978), later realised experimentally, showed that the decision to measure or not measure which-path information can be made even after a particle has already passed the slits, with the same effect on the resulting pattern — reinforcing that "collapse" is not tied to a moment in time in any simple, classical sense, and highlighting how counterintuitive the standard interpretation's account of measurement remains.

Concept Relationship Map

Requires: wave-particle duality, quantum superposition
Enables: quantum measurement [PENDING FULL ENTRY], decoherence [PENDING FULL ENTRY]
Commonly confused with: the idea that conscious observation itself causes wavefunction collapse (the effect is physical decoherence, not consciousness-dependent)

Sources

Young, T. (1804). Philosophical Transactions of the Royal Society of London, 94, 1–16.
Jönsson, C. (1961). "Elektroneninterferenzen an mehreren künstlich hergestellten Feinspalten." Zeitschrift für Physik, 161(4), 454–474.
Feynman, R.P. et al. The Feynman Lectures on Physics, Vol. III, Ch. 1. feynmanlectures.caltech.edu

Last verified: July 2026 · Source: Young (1804); Jönsson (1961) · Confidence: high

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