<P> Schrödinger wrote: </P> <P> 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 . </P> <P> 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 . </P> <P> 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?" (More technically, when does the actual quantum state stop being a linear combination of states, each of which resembles different classical states, and instead begin to have a unique classical description?) If the cat survives, it remembers only being alive . But explanations of the EPR experiments that are consistent with standard microscopic quantum mechanics require that macroscopic objects, such as cats and notebooks, do not always have unique classical descriptions . The thought experiment illustrates this apparent paradox . Our intuition says that no observer can be in a mixture of states--yet the cat, it seems from the thought experiment, can be such a mixture . Is the cat required to be an observer, or does its existence in a single well - defined classical state require another external observer? Each alternative seemed absurd to Einstein, who was impressed by the ability of the thought experiment to highlight these issues . In a letter to Schrödinger dated 1950, he wrote: </P>

If you put a cat in a box with poison