Schrödinger's cat. Image: Dhatfield.
One interpretation of the strange theory of quantum mechanics is that tiny particles can simultaneously exist in states that we would usually deem mutually exclusive. For example, an electron can be in two places at once, or a radioactive atom can be both decayed an non-decayed at the same time. It's only when we go to measure a system in superposition, as this strange state is called, that reality somehow "collapses" to one of the possibilities.
In 1935 the physicist Erwin Schrödinger, who made major contributions to the theory of quantum mechanics, developed a thought experiment in order to demonstrate just how counter-intuitive the idea of superposition is. We let him describe it in his own words, taken from a translation of his 1935 paper:
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 which shatters a small flask of hydrocyanic acid.
Thus, when an atom decays, poison will be released from the flask and the cat killed. And here's the main point. If it is true that, as long as we don't look, the system can evolve into a superposition state of atoms being simultaneously decayed and not decayed, then it follows that, as long as we don't look, the cat will be simultaneously dead and alive. Poor cat. Or should we say lucky cat?
You can find out more about quantum mechanics in our package Who killed Schrödinger's cat?.