Said cat is placed in a sealed box. Attached to the box is an apparatus containing a radioactive nucleus and a canister of poison gas. When the nucleus decays, it emits a particle that triggers the apparatus, which opens the canister and kills the cat.
While the box is closed, no observation has taken place. According to the Copenhagen Interpretation, the system exists in a mixed superposition of the states "decayed nucleus/dead cat" and "undecayed nucleus/living cat." Only when the box is opened and an observation performed does the wave function collapse into one of the two states. In the Everett many-worlds interpretation of quantum mechanics, which does not single out observation as a special process, both states persist, but decohere?. When an observer opens the box, he becomes entangled with the cat, so observer-states corresponding to the cat being alive and dead are formed, and each can have no interaction with the other.
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