homas Young showed light making bands of light and dark, the signature of waves, in work he presented between 1801 and 1803. The two-slit version is how it gets taught. In 1927 electrons were caught diffracting like waves too (Davisson and Germer at Bell Labs off a nickel crystal, and separately G. P. Thomson with Alexander Reid through thin films). Claus Jönsson sent electrons through real slits in 1961. Send them one at a time and the bands still build up.

Max Born, in the summer of 1926, gave the rule that the wavefunction tells you probabilities: square its size and you get the chance of finding the particle there. Nobody has fully agreed since then on what happens at the moment you look.

A few of the older, odder results:

• Schrödinger's cat, 1935. He meant it as an objection, a reductio. It became the mascot instead.
• John Bell, 1964. If the world works by local hidden variables, certain correlations have a ceiling. Measurements since then go over it, again and again.
• Eugene Wigner, 1961: put a friend in the lab, let the friend measure, and ask when the outcome becomes definite. For the friend? For you outside the door?
• The quantum Zeno effect: observe a system often enough and it can be held back from changing.

The last one bothers us the most. A thing that won't move while it's being watched.