In 1926, physicist Erwin Schrödinger proposed one of the most famous thought experiments of twentieth-century-science. Schrödinger intended to illustrate the difficulties involved in extending the insights of quantum mechanics from the probabilistic world of the vanishingly small to the macroscopic world of everyday life, where classical physics applies. Schrödinger's thought experiment takes its name from the central role played by a hypothetical cat, distinguished by the unusual property of being dead and alive at the same time.

Schrödinger's experiment was simple. Put a cat and a radioactive atom with a half life of one hour into a box with a vial of hydrocyanic acid connected to a Geiger counter. In the course of an hour, there is a 50:50 chance the atom will decay. If it does, the detector will register the emission of an alpha...