A quantum mechanical system supposedly exists in a superposition of states until a measurement or observation is made, whereupon the system will be found to exist in just one of those states - though it is impossible to predict with certainty which state that will be. As long as this picture is only held to apply to microphysical states, it may not appear unacceptably paradoxical. But the following thought experiment suggests that the threat is not so easily contained.

Imagine a cat confined to a box containing a bottle of poisonous gas which will break, killing the cat, if and only if a device connected to it registers the radioactive decay of a radium atom. If the atom, device, and cat together constitute a quantum system, then it seems that this system will exist in a superposition of states unless and until an observer tries to determine which state it is in, by seeing whether or not the cat is dead. But this implies that in the absence of such an observation the cat is neither determinately dead nor determinately alive, which seems absurd.

This thought experiment was conceived by Erwin Schrödinger (1887-1961), an eminent Austrian physicist who was one of the founders of quantum mechanics. He also had philosophical interests.

(from xrefer.com)

http://www.lassp.cornell.edu/ardlouis/dissipative/Schrcat.html