The Double-Slit Experiment

The double-slit experiment was first performed by Thomas Young in 1801 to demonstrate the wave behavior of visible light. In 1927, at the advent of quantum mechanics, the experiment was reproduced with electrons, and since then, with atoms and ever larger molecules. The observations are qualitatively the same.

A beam of particles is directed at a plate with two small, parallel slits. On a detector screen behind, one observes an interference pattern, alternating bands of high and low intensity. Crucially, this interference pattern is created by the statistical distribution of individual impact points. If one sends in a single particle at a time, each produces a small dot on the screen in a seemingly random position. A large number of particles build up the interference pattern: alternating regions where many particles are detected and regions where few or none are.

Interference is a typical wave phenomenon: when two waves overlap, their amplitudes combine, so that in some places, the waves reinforce each other (constructive interference), while in others, they reduce or cancel each other (destructive interference). On the other hand, the individual dots appearing on the screen suggest the impact of localized particles rather than extended waves.

The double-slit experiment is therefore a key demonstration of the so-called wave-particle duality. The microscopic constituents of light and matter, photons, electrons, atoms, etc., exhibit both wave-like and particle-like properties.

Different representational approaches motivate different explanatory narratives about the same interference pattern. This demo includes (i) trajectory-based visualizations, (ii) measurement-centered renderings, and (iii) wave-only views. These modes are intended to support comparison of explanatory strategies rather than to endorse a single interpretation.

Note: The screen must interact with the particles in order to detect them. The simulation ignores the effect of this interaction on the particle‘s wave function and trajectories. In most cases, this effect is small, but in principle, it could alter the trajectories, especially in situations where the detector screen is placed very close to the slits.