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Schrödinger's cat

Schrödinger's cat is a thought experiment devised by Austrian physicist Erwin Schrödinger in 1935. It illustrates what he saw as the problem of the Copenhagen interpretation of quantum mechanics applied to everyday objects. The scenario presents a cat which may be simultaneously both alive and dead a state known as a quantum superposition, as a result of being linked to a random subatomic event that may or may not occur. The thought experiment is also often featured in theoretical discussions of the interpretations of quantum mechanics. Schrödinger coined the term Verschränkung (entanglement) in the course of developing the thought experiment.

Here's Schrödinger's thought experiment: We place a living cat into a steel chamber, along with a device containing a vial of hydrocyanic acid. There is, in the chamber, a very small amount of hydrocyanic acid, a radioactive substance. If even a single atom of the substance decays during the test period, a relay mechanism will trip a hammer, which will, in turn, break the vial and kill the cat.

The observer can not know whether an atom of the substance has decayed, and consequently, he can not know whether the vial has been broken, the hydrocyanic acid released, and the cat killed. Since we can not know, according to quantum law, the cat is both dead and alive, in what is called a superposition of states. It is only when we open the box and we get to know the condition of the cat that the superposition is lost, and the cat becomes one or the other (dead or alive). This situation is sometimes called quantum indeterminacy or the observer's paradox: the observation or measurement itself affects an outcome, so that the outcome does not exist unless the measurement is made. (That is, there is no single outcome unless it is observed.)