A thought experiment designed to illustrate the counterintuitive
and strange notions of reality that come along with quantum
mechanics.
A cat is sealed inside a closed box; the cat has ample air,
food, and water to survive an extended period. This box is
designed so that no information (i.e., sight, sound, etc.) can
pass into or out of the box -- the cat is totally cut off from
your observations. Also inside the box with the poor kitty
(apparently Schroedinger was not too fond of felines) is a phial
of a gaseous poison, and an automatic hammer to break it, flooding
the box and killing the cat. The hammer is hooked up to a Geiger
counter; this counter is monitoring a radioactive sample and is
designed to trigger the hammer -- killing the cat -- should a
radioactive decay be detected. The sample is chosen so that
after, say, one hour, there stands a fifty-fifty chance of a decay
occurring.
The question is, what is the state of the cat after that one
hour has elapsed? The intuitive answer is that the cat is either
alive or dead, but you don't know which until you look. But it
is one of them. Quantum mechanics, on the other hands, says
that the wavefunction describing the cat is in a superposition of
states: the cat is, in fact, fifty per cent alive and fifty per
cent dead; it is both. Not until one looks and "collapses the
wavefunction" is the Universe forced to choose either a live cat
or a dead cat and not something in between.
This indicates that observation also seems to be an important
part of the scientific process -- quite a departure from the
absolutely objective, deterministic way things used to be with
Newton.