Double Slit Experiment

Young's Double Slit: The Most Beautiful Experiment in Physics

In 1801, Thomas Young shone light through two narrow slits and observed alternating bright and dark bands on a screen — conclusive proof that light behaves as a wave. The same experiment later performed with single electrons (Jönsson, 1961), neutrons, and even molecules like C₆₀ buckminsterfullerene (Vienna, 1999) revealed an astonishing truth: all matter exhibits wave behaviour.

Richard Feynman called this experiment "the only mystery of quantum mechanics" — the idea that a single particle interferes with itself, appearing to pass through both slits simultaneously if its path is not observed.

The bright fringes (constructive interference) appear where waves from the two slits are in phase — where the path difference equals an integer multiple of the wavelength:

d · sin θ = mλ   (m = 0, ±1, ±2, ...)

Dark fringes (destructive interference) occur where the path difference is a half-integer wavelength, causing the waves to perfectly cancel. The fringe spacing on a screen at distance L is Δy = λL/d.

Wave-Particle Duality and de Broglie

In 1924, Louis de Broglie proposed that every particle of matter has an associated wave, described by its wavelength:

λ = h / p = h / (mv)

where h is Planck's constant and p is the particle's momentum. For everyday objects, λ is so tiny it is undetectable. For electrons with kinetic energy ~150 eV, λ ≈ 0.1 nm — comparable to atomic spacings, enabling electron diffraction used in electron microscopy and crystal structure analysis.

The double slit experiment demonstrates this duality directly: when no detector records which slit a particle passes through, an interference pattern builds up particle by particle on the screen. When a detector is placed at the slits, the pattern disappears — as if the particle "knows" it is being watched.

This is not a mechanical disturbance from the measuring device. It is a consequence of quantum entanglement: once the particle's path is correlated with any other system (the detector), its quantum superposition collapses.

Single-Slit Diffraction

A single slit also produces a diffraction pattern — a wide central maximum flanked by narrower secondary maxima. This arises from Huygens' principle: every point within the slit acts as a source of spherical wavelets. The intensity pattern is described by the sinc² function:

I(θ) = I₀ [sin(α)/α]²   where α = πa·sinθ/λ

Here a is the slit width. Minima occur at a·sin θ = mλ (m ≠ 0). A narrower slit diffracts light more broadly (the uncertainty principle at work: confining the photon's y-position increases its y-momentum uncertainty).

In the double slit case, the interference fringes are modulated by this single-slit diffraction envelope. Fringes that fall at diffraction minima are "missing orders" — visible as dark gaps in the interference pattern.

The Observer Effect and Quantum Erasure

The collapse of the interference pattern when "which-path information" is available is one of quantum mechanics' most striking predictions. Crucially, the physical act of measurement need not disturb the particle's momentum at all — it is the information itself that destroys the interference.

In the quantum eraser experiment (Scully & Drühl, 1982), which-path information is first recorded, destroying interference — then the information is erased (while the particle is in flight), and the interference pattern reappears in a subset of particles correlated with the erasure. This "delayed-choice" erasure demonstrates that it is information, not physical disturbance, that collapses quantum superposition.

The double slit experiment remains central to quantum foundations research. Its modern descendants — entangled-photon experiments, quantum teleportation, and Bell inequality tests — all grapple with the same fundamental mystery: measurement, information, and the nature of quantum reality.