The idea, called Schrödinger's Cat after the physicist,Erwin Schrödinger, who proposed it in 1935, goes like this: Put a cat in a box with a vial of poison gas. The vial opens when a tiny piece of radioactive metal emits an alpha particle (the nucleus of a helium atom) as it decays. Emitting an alpha particle is a quantum-mechanical process, which means that whether it happens in any given stretch of time is basically random.