Schroedinger's Cat

Schroedinger's Cat was a thought experiment developed by Erwin Schroedinger to illustrate a paradox that can be encountered when quantum mechanics are applied to the world on something other than the sub-atomic scale. The existence of this paradox highlighted the incompleteness of quantum theory.

The thought experiment has to do with the decoherence of a quantum system. Because quantum mechanics is a statistical method of describing the physical world, there are multiple states that a system can exist in at the same time; this is the principle of superposition. A quantum system is said to always be in a state of superposition until the system is measured, at which point the system decoheres and assumes a unique value. Such behavior of a system is acceptable on the subatomic scale for individual particles, and has even been the basis of revolutionary research into computing technologies (see quantum computer), but is a foreign concept in the classical scale, as nothing in a person's daily life goes unmeasured.

The Paradox

In a paper entitled The Present Situation in Quantum Mechanics, Schroedinger wrote the following:

One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Geiger counter there is a tiny bit of radioactive substance, so small, that perhaps in the course of the hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer which shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The psi-function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts.