The Science of Discworld Revised Edition

narrativium goes a long way: the simpler the story, the better you understand it. Storytelling is the opposite of reductionism; 26 letters and some rules of grammar are no story at all.
    One set of modern physical rules poses more philosophical questions than all the others combined: Quantum Mechanics. Newton’s rules explained the universe in terms of force, position, speed, and the like – things that make intuitive sense to human beings and let us tell good stories. A century or so ago, however, it became clear that the universe’s hidden wiring has other, less intuitive layers. Concepts such as position and speed not only ceased to be fundamental – they ceased to have a well defined meaning at all.
    This new layer of explanation, quantum theory, tells us that on small scales the rules are random. Instead of something happening or not, it may do a bit of both. Empty space is a seething mass of potentialities, and time is something you can borrow and pay back again if you do it quickly enough for the universe not to notice. And the Heisenberg Uncertainty Principle says that if you know where something is then you can’t also know how fast it’s going. Ponder Stibbons would consider himself lucky if he did not have to explain this to his Archchancellor.
    A thorough discussion of the quantum world would need a book all to itself, but there’s one topic that benefits from some Discworld insights. This is the notorious case of the cat in the box. Quantum objects obey Schrödinger’s Equation, a rule named after Erwin Schrödinger which describes how ‘wave functions’ – waves of quantum existence – propagate through space and time. Atoms and their sub-atomic components aren’t really particles: they’re quantum wave functions.
    The early pioneers of quantum mechanics had enough problems
solving
Schrödinger’s equation: they didn’t want to worry about what it
meant
. So they spatchcocked together a cop-out clause, the ‘Copenhagen interpretation’ of quantum observations. This says that whenever you try to observe a quantum wave function it immediately ‘collapses’ to give a single particle-like answer. 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 replaces a complex physical process (which we don’t understand) by a piece of magic: the wave collapses as soon as you try to observe it.
    Most of the time this problem is discussed, physicists manage to turn Schrödinger’s point on its head. ‘No, quantum waves really
are
like that!’ And they’ve done lots of experiments to prove they’re right. Except … those experiments have no box, no poison gas, no alive, no dead, and no cat. What they have is quantum-scale analogues – an electron for a cat, positive spin for alive and negative for dead, and a box with Chinese walls, through which anything
can
be observed, but you take great care not to notice.
    These discussions and experiments are lies-to-children: their aim is to convince the next generation of physicists that
quantum
-level systems