We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery. (Richard Feynman)

In this section we describe in a reader-friendly fashion the 2-slit experiment. We derived the inspiration from the Nobel Laureate Richard Feynman's presentation, as contained in the first chapter of his “The Feynman Lectures on Physics Part III", the volume devoted to Quantum Mechanics. We are thus moving through the steps shown in the following scheme. We will analyze and compare the behaviour exhibited by different systems, i.e.

1) material particles (little balls)

2) waves (in particular waves produced in water)

3) light

4) electrons

in the two-slit experiment, devised by Thomas Young with the purpose of demonstrating the wavelike behaviour of light (an experiment which also appears in the list of the 10 most beautiful experiments according to the Physics World poll). Other descriptions of the experiment, also Feynman inspired, are to be found in:

• wikipedia
• animation from YouTube
• excerpts from R. Gilmore's Alice in Quantumland

Further descriptions can be found in the link section and in the references section.

The rest of the content entails a more in-depth educational treatment of the theme waves and interference. We advise to read this before approaching the part concerning waves, since it deals with concepts necessary for the comprehension of the different steps of the experiment.

Image above (taken from the 1976 Merli, Missiroli and Pozzi article) and right-hand scheme: the light dots on the detector screen DO NOT coalesce as in the case of little balls (that is in two strips only) but form a multi-strip, similar to the one of interference between waves. Every electron, as well as a ball, hits the screen in a precise spot, as revealed by the light signal it produces. Yet the cumulative behaviour of many electrons (even when they are transferred one by one from emitter to receiver) shows aspects similar to those of waves.