Double-slit experiment

In the classical arrangement associated with Thomas Young, light from one source illuminates two narrow slits. The emerging light waves overlap on a screen. Where their phase differences make the waves reinforce one another, bright fringes appear; where they cancel, dark fringes form. Young's 1801 investigation of interference provided important evidence for the wave theory of light.

The arrangement can also be used with electrons and other quantum objects. A detector records each electron as a localized event, but many detections build up a distribution with interference fringes when both paths remain available and the setup preserves coherence. The pattern is predicted by adding quantum probability amplitudes for the alternatives, rather than by treating the electron as a classical particle that secretly follows one ordinary path and produces the same result.

If the apparatus is changed so that reliable information about which slit an electron passed through is available, the interference contrast is reduced or lost. In quantum physics, measurement devices interact with the system; the observed result depends on the entire arrangement, including how path information is recorded. It is not necessary to invoke a conscious observer. The detailed effects depend on the experiment and on how well the two paths remain distinguishable and coherent.

The double-slit arrangement makes the difference between classical expectations and quantum predictions visible in a simple geometry. It illustrates how wave-like interference and localized detection appear together in quantum experiments, and why experimental context matters when describing what can be measured. Variants using electrons and other particles continue to test interference, coherence and measurement in increasingly controlled settings.

Thomas Young performed his influential light-interference experiment in 1801. Electron interference at artificial slits was demonstrated by Claus Jönsson in 1961. Later work made it possible to build an interference pattern from individual electron detections and to control the slit arrangement more precisely, extending the classical optical demonstration into tests of quantum behavior.