One of the most famous gedankens (thought experiments) is that of schrodinger and his cat. The experiment is motivated by the intriguing characteristics of observation in quantum mechanics. What makes the process of observation worth pondering is that experimental results have shown that when observations, or more precisely measurements are taken of a system, the system is affected. For instance, a second measurement immediately following the measurement of spin angular moment along any direction, will always have exactly the same spin. Incredible right!!! Actually it is rather surprising as theory would lead us to believe that there ought to be an equal probability of it being in any of its possible spin states. So what in our measurement causes such a drastic change in the probabilities? Now that we have our motivation, let’s look at the experiment:
A cat is placed in a steel chamber, together with the following hellish contraption… In a Geiger counter (measures radiation) there is a tiny amount of a radioactive substance, so tiny that maybe within an hour one of the atoms decays, but equally probable is that none of them decay. If one decays the counter triggers a hammer to break a bottle of cyanide, killing the cat. If one leaves this system for an hour, then one would say the cat is alive with 50% probability and dead with the same likelihood. Thus the “wave function” (state) of the entire system would express this by containing equal part of the living and dead cat.
Thus we might say the cat is neither dead nor alive until we observe the system and at that point the system “collapses” into one of these states, the cat is alive or the cat is dead. The thought experiment itself is for the most part considered nonsense since a macro system such as a cat is by definition not a quantum system (such as a single electron); however the idea of the superposition of states is the important point to take away. The basic structure of quantum mechanics hinges upon the idea that there is an uncertainty in everything and consequently we can only consider the probability of things happening. The fact that our observation changes this probability is known but not understood and that is why it is worth a gendanken.
Some credit for this article must be given to David J. Griffiths’ second edition of Quantum Mechanics which I would highly recommend for any looking for some light reading this summer.