Here, pussy, pussy...

Oh dear, I was really hoping I wasn't going to have to explain Schrödinger's cat (see 23rd March), but here goes.  Quantum mechanics says that a subatomic particle can be in more than one state at the same time - until it is observed.  So - as a sort of joke - Schrödinger came up with the idea of putting a cat in a box with a very weak source of radioactivity, a geiger counter and a vial of poison gas.  There's a 50-50 chance that over the course of an hour the radioactive source will decay and emit a particle. If this happens, it will be picked up by the geiger counter and this will release the gas and the cat will die. The box is put away out of sight and sound for an hour. The question is - is the cat alive or dead?  Obviously if you open the box you will find out - but before you do?  Schrödinger maintained that, according to quantum theory, until you open the box, the cat is both alive and dead - in a so-called "superposition". Opening the box forces the superposition to collapse into one of the two possible outcomes.  In the same way, observing a subatomic particle forces it to adopt a particular state.  No, I don't really understand it either, but that's the best I can do.