Quantum Mechanics
Interference: Conceptual experiment

Schematic set-up of two-slit experiment with electronsConsider the experiment illustrated on the right. The source at the top emits electrons that have the same energy but not the same direction. An electrical potential on the wire and plates repels the electrons. The electrons impinge on the screen (gray area) one at a time. Initially there is no interference pattern on the screen: Each electron shows up as a white dot. With the passage of time typical interference fringes appear. The electrons are most numerous at the interference maxima. The following cartoon gives an artists impression of the formation of these interference fringesFOOTNOTE.

Cartoon of two-slit experiment with electrons

It turns out that in a real experiment it is impossible to follow the trajectories of the individual electrons. The systematic creation of a coherent microscopic interference pattern by non-interacting particles arriving randomly has been demonstrated in genuine experiments that use photons, electrons or atoms. This interference is one manifestation of "particle-wave duality". It is a property of a single quantum particle and does not involve an interaction between particles

FOOTNOTE: From a detailed analysis of this problem it follows that the interference pattern that emerges after collecting many electrons is not the same as the interference pattern of the double slit. However, on a conceptual level there is no difference between this experiment and the experiment with a double slit.