Physicists like to convert their technically most useful mathematical concepts into real things: like Discworld, they reify the abstract. It does no physical harm to 'project' the mathematics back into the universe like this, but it may do philosophical harm if you take the result literally.

Thanks to a similar process, for example, entirely sane physicists today insist that our universe is merely one of trillions that coexist in a quantum superposition. In one of them you left your house this morning and were hit by a meteorite; in the one in which you're reading this book, that didn't happen. 'Oh, yes,' they urge: 'those other universes really do exist. We can do experiments to prove it.'

Not so.

Consistency with an experimental result is not a proof, not even a demonstration, that an explanation is valid. The 'many-worlds' concept, as it is called, is an interpretation of the experiments, within its own framework. But any experiment has many interpretations, not all of which can be 'how the universe really does it'. For example, all experiments can be interpreted as

'God made that happen', but those selfsame physicists would reject their experiment as a proof of the existence of God. In that they are correct: it's just one interpretation. But then, so are a trillion coexisting universes.

Quantum states do superpose. Quantum universes can also superpose. But separating them out into classical worlds in which real-life people do real-life things, and saying that those superpose, is nonsense. There isn't a quantum physicist anywhere in the world that can write down the quantum-mechanical description of a person. How, then, can they claim that their experiment

(usually done with a couple of electrons or photons) 'proves' that an alternate you was hit by a meteorite in another universe?

'Information' began its existence as a human construct, a concept that described certain processes in communication. This was 'bit from it', the abstraction of a metaphor from reality, rather than 'it from bit', the reconstruction of reality from the metaphor. The metaphor of information has since been extended far beyond its original bounds, often unwisely. Reifying information into the basic substance of the universe is probably even more unwise. Mathematically, it probably does no harm, but Reification Can Damage Your Philosophy.

LETTER FROM LANCRE

Granny Weatherwax, known to all and not least to herself as Discworld's most competent witch, was gathering wood in the forests of Lancre, high in the mountains and far from any university at all.

Wood gathering was a task fraught with danger for an old lady so attractive to narrativium. It was quite hard these days, when gathering firewood, to avoid third sons of kings, young swineherds seeking their destiny and others whose unfolding adventure demanded that they be kind to an old lady who would with a certainty turn out to be a witch, thus proving that smug virtue is its own reward.

There is only a limited number of times even a kindly disposed person wishes to be carried across a stream that they had, in fact, not particularly desired to cross. These days, she kept a pocket full of small stones and pine cones to discourage that kind of thing.

She heard the soft sound of hooves behind her and turned with a pine cone raised.

'I warn you, I'm fed up with you lads always on the ear'ole for three wishes—' she began.

Shawn Ogg, astride his official donkey, waved his hands desperately.52

'It's me, Mistress Weatherwax! I wish you'd stop doing this!' 'See?' said Granny. 'You ain't havin'

another two!' 'No, no, I've just come up to deliver this for you ... ' Shawn waved quite a thick wad of paper. 'What is it?'

'Tis a clacks for you, Mistress Weatherwax! It's only the third one we've ever had!' Shawn beamed at the thought of being so close to the cutting edge of technology.

'What's one of them things?' Granny demanded. 'It's like a letter that's taken to bits and sent through the air,' said Sean.

'By them towers I keep flyin' into?' 'That's right, Mistress Weatherwax.'

'They move 'em around at night, you know,' said Granny. She took the paper.

'Er ... I don't think they do ...' Shawn ventured. 'Oh, so I don't know how to fly a broomstick right, do I?' said Granny, her eyes glinting.

'Actually, yes, I've remembered,' said Shawn quickly. 'They move them around all the time. On carts. Big, big carts. They ...'

'Yes, yes,' said Granny, sitting on a stump. 'Be quiet now, I'm readin'

The forest went silent, except for the occasional shuffling of paper.

Finally, Granny Weatherwax finished. She sniffed. Birdsong came back into the forest.

'Silly old fools think they can't see the wood for the trees, and the trees are the wood,' she muttered. 'Cost a lot, does it, sendin' messages like this?'

'That message,' said Shawn, in awe, 'cost more than 600 dollars! I counted the words! Wizards must be made of money!'

'Well, I ain't,' said the witch. 'How much is one word?'

'Five pence for the sending and five pence the first word,' said Shawn, promptly.

'Ah,' said Granny. She frowned in concentration, and her lips moved silently. 'I've never been one for numbers,' she said, 'but I reckon that comes to ... sixpence and one half-penny?'

Shawn knew his witches. It was best to give in right at the start.

'That's right,' he said.

'You have a pencil?' said Granny. Shawn handed it over. With great care, the witch printed some block capitals on the back of one of the pages, and gave it to him.

'That's all?' he said.

'Long question, short answer,' said Granny, as it if was some universal truth. 'Was there anything else?'

Well, there might be the money, Shawn thought. But in her own localised way, Granny Weatherwax had an academic position in these matters. Witches took the view that they helped society in all kinds of ways which couldn't easily be explained but would become obvious if they stopped doing them, and that it was worth six pence and one half-penny not to find out what these were.

He didn't get his pencil back.

The hole into L-space was quite obvious now. It fascinated Dr.Dee, who was confidently expecting angels to come out of it, although all it had produced so far was an ape.

The wizards' automatic response to any problem was to see if there was a book about it. L-space was providing plenty of books. The difficulty, however, was finding the ones that applied to the current history; when you potentially know everything, it's hard to find anything you want to know.

'So let's see where we are now, shall we?' said Ridcully, after a while. 'The last known books in this leg of the trousers of time are due to be written in—?'

'About a hundred years' time,' said the Lecturer in Recent Runes, looking at his notes. 'Just before the collapse of civilisation, such as it is. Then there's fire, famine, war ... all the usual stuff.'

'Hex says people here are back to living in villages when the asteroid hits,' said Ponder. 'Things are rather better on one or two other continents, but no one even sees it coming.'

'There have been periods like it before,' said the Dean. 'But as far as we can tell, in the area where we are now there were always small isolated groups of people who preserved what books there were.'

'Ah. Our kind of people,' said Ridcully.

'Afraid not,' said the Dean. 'Religious.'

'Oh dear,' said Ridcully.

'It's hard to follow, but there appear to be about four main gods on this continent,' said the Dean.

'Loosely associated.'

'Big beards in the sky?' said Ridcully.

'A couple, yes.'

'Clearly a morphic memory of ourselves, then,' said Ridcully.

'It's hard to tell, with religions,' said the Dean. 'But at least they preserved the idea that books were important and that reading and writing were more than just a skive for people too weedy to hack at one another with swords.'