[A] cat, a flask of poison and a radioactive source are placed in a sealed box. If an internal monitor (e.g. Geiger counter) detects radioactivity (i.e. a single atom decaying), the flask is shattered, releasing the poison, which kills the cat. The Copenhagen interpretation of quantum mechanics implies that, after a while, the cat is simultaneously alive and dead. Yet, when one looks in the box, one sees the cat either alive or dead, not both alive and dead. This poses the question of when, exactly, quantum superposition ends and reality collapses into one possibility or the other.
[Schrödinger] intended the example [above] to illustrate the absurdity of the existing view of quantum mechanics:
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 that 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 first atomic decay would have poisoned it. 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.
It is typical of these cases that an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation. That prevents us from so naively accepting as valid a “blurred model” for representing reality. In itself, it would not embody anything unclear or contradictory. There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks.
Schrödinger’s famous thought experiment poses the question, “when does a quantum system stop existing as a superposition of states and become one or the other?”
The EPR Paradox was a thought experiment proposed by physicists Albert Einstein, Boris Podolsky and Nathan Rosen (EPR) that they interpreted as indicating that the explanation of physical reality provided by quantum mechanics was incomplete (PDF).