The Science of Discworld (Science of Discworld, #1)

by Terry Pratchett

This seems to promote the human mind to a special status – it has even been suggested that our purpose in the universe is to observe it, thereby ensuring its existence, an idea that the wizards of UU consider to be simple common sense. Schrödinger, however, thought this was silly, and in support he introduced a thought experiment now called Schrödinger’s Cat. Imagine a box, with a lid that can be sealed so tightly that nothing, not even the barest hint of a quantum wavelet, can leak out. The box contains a radioactive atom, which at some random moment will decay and emit a particle, and a particle detector that releases poison gas when it detects the atom decaying. Put the cat in the box and close the lid. Wait a bit. Is the cat alive or dead? If the atom has decayed, then the cat’s dead. If not, it’s alive. However, the box is sealed, so you can’t observe what’s inside. Since unobserved quantum systems are waves, the quantum rules tell us that the atom must be in a ‘mixed’ state – half decayed and half not. Therefore the cat, which is a collection of atoms and so can be considered as a gigantic quantum system, is also in a mixed state: half alive, half dead. In 1935 Schrödinger pointed out that cats aren’t like that. Cats are macroscopic systems with classical yes/no physics. His point was that the Copenhagen interpretation does not explain – or even address – the link from microscopic quantum physics to macroscopic classical physics. The Copenhagen interpretation replace...