Schrödinger's cat

Celebrated animal introduced by the Austrian physicist Erwin Schrödinger (1887–1961) in 1935, in a thought experiment showing the strange nature of the world of quantum mechanics . The cat is thought of as locked in a box with a capsule of cyanide, which will break if a Geiger counter triggers. This will happen if an atom in a radioactive substance in the box decays, and there is a chance of 50% of such an event within an hour. Otherwise the cat is alive. The problem is that the system is in an indeterminate state. The wave function of the entire system is a ‘superposition’ of states, fully described by the probabilities of events occurring when it is eventually measured, and therefore ‘contains equal parts of the living and dead cat’. When we look and see we will find either an alive cat or a dead cat, but if it is only as we look that the wave packet collapses, quantum mechanics forces us to say that before we looked it was not true that the cat was dead and also not true that it was alive. The thought experiment makes vivid the difficulty of conceiving of quantum indeterminacies when these are translated to the familiar world of everyday objects.