Schrödinger Cat: At once dead and alive! 

"Schrödinger cat" is a quantum paradox suggested by Schrödinger in 1935. Its purpose is to illustrate a highly counterintuitive nature of the quantum theory.
The quantum theory, or quantum mechanics, was created by Werner Heisenberg who received Nobel Prize in 1932 for this achievement. This theory describes the behavior of objects at the atomic scale, with unsurpassed precision. The question of applicability of quantum mechanics to macroscopic objects remains a subject of debate and some controversy, as is illustrated by the "Schrödinger cat" thought experiment. 
According to quantum mechanics, any physical system can be in a quantum superposition of any number of possible states. "Superposition of states" means that the system is not in any one of these states but in all states at once (with a certain probability, given by the square of its "wavefunction"). Only when the system is observed (i.e. "measured"), it "collapses" into one of the states involved in the superposition. This states becomes the "observed" state.
The original Schrödinger experiment involves a perfectly isolated box, which contains a cat, and an initially sealed ampule of poisonous gas. Attached to the gas ampule is a mechanism containing a radioactive nucleus. When the nucleus decays, it emits a particle that triggers a mechanism which opens the ampule. In this case the gas is released and the cat get killed.
Since the nucleus, as long as it remains unobserved, is in a superposition of the "decayed" and "not decayed" states, then it drives the cat into a quantum superposition of "alive" and "dead" states. One interesting question, of course, is how the cat perceives such unusual state of affairs. According to quantum theory the cat remains in such state till the moment somebody looks into the box. The idea that the cat fate is decided only when the box is opened seems to defy common sense. It is unknown whether there are additional laws of physics, which drive the cat into a definite state (dead or alive) without anybody looking at it from outside. Such process is called "collapse of the wavefunction". The search for such additional phenomena continues. Schrodinger cats have recently been made from superconducting materials. 

Related Links: 
Werner Karl Heisenberg (Germany)Erwin Schrödinger (Austria)Paul Adrien Maurice Dirac (United Kingdom)The first superconducting Schrödinger catWho Was Werner Heisenberg ? 




