The Science of Discworld Revised Edition

do actually behave in the bizarre way that they do. Fine … but it’s got nothing to do with cats. The wizards of Unseen University, who know nothing about electrons but have an intimate familiarity with cats, wouldn’t be fooled for an instant. Neither would the witch Gytha Ogg, whose cat Greebo is shut in a box in
Lords and Ladies
. Greebo is the sort of cat that would take on a ferocious wolf and eat it. 2 In
Witches Abroad
he eats a vampire by accident, and the witches can’t understand why the local villagers are so ecstatic.
    Greebo has his own way of handling quantum paradoxes: ‘Greebo had spent an irritating two minutes in that box. Technically, a cat locked in a box may be alive or it may be dead. You never know until you look. In fact, the mere act of opening the box will determine the state of the cat, although in this case there were three determinate states the cat could be in: these being Alive, Dead, and Bloody Furious.’
    Schrödinger would have applauded. He wasn’t talking about quantum states: he wanted to know how they led to ordinary, classical physics in the large, and he could see that the Copenhagen interpretation didn’t have anything to say about that. So how
do
classical yes/no answers emerge from quantum Ant Country? The closest we have to an answer is something called ‘decoherence’, which has been studied by a number of physicists, among them Anthony Leggett, Roland Omnés, Serge Haroche and Luis Davidovich. If you have a big collection of quantum waves and you leave it to its own devices, then the component waves get out of step and fuzz out. This is what a classical object is ‘really’ like from the quantum standpoint, and it means that cats do, in fact, behave like cats. Experiments show that the same is true even when the role of the detector is played by a microscopic quantum object: a photon’s wave function can collapse without any observers being aware, at the time, that it has done so. Even with a quantum cat, death occurs at the instant that the
detector
notices that the atom has decayed. It doesn’t require a mind.
    In short, Archchancellor, the universe always notices the cat. And a tree in a forest does make a sound when it falls, even if no one is around. The forest is always there.
    1 This rule does require some special assumptions, such as the chronic and irreversible stupidity of humanity.
    2 As Nanny Ogg always says, ‘He’ just a big softy.’

THIRTEEN
NO, IT CAN’T DO THAT

    ARCHCHANCELLOR RIDCULLY LOOKED around at his colleagues. They’d chosen the long table in the Great Hall for the meeting, since the HEM was getting too crowded.
    ‘All here? Good,’ he said. ‘Carry on, Mister Stibbons.’
    Ponder sifted through his papers.
    ‘I’ve, er, asked for this meeting,’ he said, ‘because I’m afraid we’re doing things wrong.’
    ‘How can that be?’ said the Dean. ‘It’s
our
universe!’
    ‘Yes, Dean. And, er, no. It’s made up its own rules.’
    ‘No, no, it can’t do that,’ said the Archchancellor. ‘We’re intelligent creatures. We make the rules. Lumps of rock don’t make rules.’
    ‘Not
exactly
, sir,’ said Ponder, employing the phrase in its traditional sense of ‘absolutely wrong’. ‘There are some rules in the Project.’
    ‘How? Is someone else meddling with it?’ the Dean demanded. ‘Has a Creator turned up?’
    ‘An interesting thought, sir. I’m not qualified to answer that one. The point I’m trying to make is that if we want to do anything constructive, we’ve got to obey the rules.’
    The Lecturer in Recent Runes looked down at the table in front of him. It had been laid for lunch.
    ‘I don’t see why,’ he said. ‘This knife and fork don’t tell me how to eat.’
    ‘Er … in fact, sir, they do. In a roundabout way.’
    ‘Are you trying to tell us that the rules are built in?’ said Ridcully.
    ‘Yes, sir. Like: big rocks are heavier than small rocks.’
    ‘That’s not a rule, man, that’s just common sense!’
    ‘Yes, sir. It’s just that the more I look into the Project, the more I’m not sure any more what common sense is. Sir, if we’re going to build a world it has to be a ball. A big ball.’
    ‘That’s a lot of outmoded religious nonsense, Mister Stibbons.’ 1
    ‘Yes, sir. But in the Project universe, it’s real. Some of the ba … the spheres the students have made are huge.’
    ‘Yes, I’ve seen them. Showy, to my mind.’
    ‘I was thinking of something smaller,