Schrodinger's cat

In November 1935, the Austrian physicist Erwin Schrodinger published a three-part paper in the German journal Die Naturwissenschaften, titled roughly The Present Situation in Quantum Mechanics. In it, he described a scenario that he called ridiculous. A cat is shut in a steel chamber with a device holding a tiny amount of radioactive substance, so little that within an hour one atom may decay, with equal odds of decay or no decay. If an atom decays, a Geiger counter triggers a hammer that breaks a flask of hydrocyanic acid, killing the cat. If quantum mechanics describes the whole system, he wrote, the wave function would express the living and dead cat mixed or smeared in equal parts.¹

Schrodinger was among the creators of quantum mechanics, but he disliked the Copenhagen interpretation of Bohr and Heisenberg, in which a system is described by a superposition of states until it is measured. The cat was meant to show where that view goes wrong: superposition is acceptable for atoms, but it cannot reasonably extend to a visible cat. He wrote the paper after Einstein, Podolsky and Rosen published their famous paper on entanglement in May 1935 and after exchanging letters with Einstein, who had himself described an unstable keg of gunpowder that is both exploded and unexploded.¹²

The puzzle has produced a family of answers. In Copenhagen thinking, observation collapses the wave function. In Hugh Everett's many-worlds theory of 1957, both outcomes occur in branching worlds. Decoherence theory, developed from the 1970s, shows how interaction with the environment makes superpositions of large objects vanish almost at once. Experiments have created cat states with photons and atoms. The cat remains the most common image of quantum strangeness in popular culture.¹