Terry Pratchett - The Science of Discworld I

The Moon has been important in another way. The Babylonians and Greeks knew that the Moon is a sphere; the phases are obvious, and there is also a slight wobble which means that, over time, humans see rather more than one half of the Moon's surface. There it was, hanging in the sky, a big ball, not a disc like the sun, and a hint that perhaps 'big balls in space' is a much better way of think­ing about the Earth and its neighbours than 'lights in the sky'.

All this is a long way from lance-constable Angua, even a long way from the female menstrual cycle. But it shows how much we are creatures of the universe. Things Up There really do affect us Down Here, every day of our lives.

THERE WAS NO DARK. This came as such a shock to Ponder Stibbons that he made HEX look again. There had to be Dark, surely? Otherwise, what was there for the light to show up against?

Eventually, he reported this lack to the other wizards.

'There should be lots of Dark and there isn't,' he said flatly. There's just Light and ... no light. And it's a pretty strange light, too.'

'In what way?' said the Archchancellor.

'Well, sir, as you know [25] A phrase meaning 'I'm not sure you know this.' , there's ordinary light, which travels at about the same speed as sound... '

'That's right. You've only got to watch shadows across a land­scape to realize that.'

'Quite, sir ... and then there's meta-light, which doesn't really travel at all because it is already everywhere.'

'Otherwise we wouldn't even be able to see darkness,' said the Senior Wrangler.

'Exactly. But the Project universe has just got the one sort of light. HEX thinks it moves at hundreds of thousands of miles a sec­ond.'

'What use is that?'

'Er... in this universe, that's as fast as you can go.'

'That's nonsense, because...' Ridcully began, but Ponder held up a hand. He had not been looking forward to this one.

'Please, Archchancellor. It's doing the best it can. Just trust me on this one. Please? Yes, I can see all the reasons why it's impossi­ble. But, in there, it seems to work. HEX has written pages of stuff about it, if anyone's interested. Just don't ask me about any of it. Please, gentlemen? It's all supposed to be logical but you'll find your brain squeaking around until the ends point out of your ears.'

He placed his hands together and tried to look wise.

'It really is almost as if the Project is aping the real universe...'

'Ook.'

'I beg your pardon,' said Ponder. 'A figure of speech.'

The Librarian nodded at him and knuckled his way across the floor. The wizards watched him carefully.

'You really believe that that thing,' said the Dean, pointing, 'with its moon-hating water and worlds that go around suns...'

'As far as I can see from this,' interrupted the Senior Wrangler, who'd been reading HEX's write-out on the more complex physics of the Project, 'if you were travelling in a cart at the speed of light, and threw a ball ahead of you ...' he turned over the page, read on silently for a moment, creased his brows, turned the page over to see if any enlightenment was to be found on the other side, and went on '... your twin brother would ... be fifty years older than you when you got home ... I think.'

'Twins are the same age,' said the Dean, coldly. 'That's why they are twins.'

'Look at the world we're working on, sir,' said Ponder. 'It could be thought of as two turtle shells tied together. It's got no top and bottom but if you think of it as two worlds, bent around, with one sun and moon doing the work of two ... it's similar.'

He fried in their gaze.

'In a way, anyway,' he said.

Unnoticed by the others, the Bursar picked up the write-out on the physics of the Roundworld universe. After making himself a paper hat out of the title page, he began to read ...

LIGHT HAS A SPEED, SO WHY NOT DARK?

It's a reasonable question. Let's see where it leads. In the 1960s a biological supply company advertised a device for scientists who used micro­scopes. In order to see things under a microscope, it's often a good idea to make a very thin slice of whatever it is you're going to look at. Then you put the slice on a glass slide, stick it under the microscope lens, and peer in at the other end to see what it looks like. How do you make the slice? Not like slicing bread. The thing you want to cut, let's assume it's a piece of liver for the sake of argument, is too floppy to be sliced on its own.

Come to think of it, so is a lot of bread.

You have to hold the liver firmly while you're cutting it, so you embed it in a block of wax. Then you use a gadget called a micro­tome, something like a miniature bacon-slicer, to cut off a series of very thin slices. You drop them on the surface of warm water, stick some on to a microscope slide, dissolve away the wax, and prepare the slide for viewing. Simple...

But the device that the company was selling wasn't a microtome: it was something to keep the wax block cool while the microtome was slicing it, so that the heat generated by the friction would not make the wax difficult to slice and damage delicate details of the specimen.

Their solution to this problem was a large concave (dish-shaped) mirror. You were supposed to build a little pile of ice cubes and 'focus the cold' on to your specimen.

Perhaps you don't see anything remarkable here. In that case you probably speak of the 'spread of ignorance', and draw the curtains in the evening to 'keep the cold out', and the darkness [26] And if so: congratulations! You are a human being, thinking narratively. .

In Discworld, such things make sense. Lots of things are real in Discworld while being mere abstractions in ours. Death, for exam­ple. And Dark. On Discworld you can worry about the speed of Dark, and how it can get out of the way of the light that is plough­ing into it at 600 mph [27] Light on the Disc travels at about the same speed as sound. This does not appear to cause problems. . In our world such a concept is called a 'privative', an absence of something. And in our world, privatives don't have their own existence. Knowledge does exist, but igno­rance doesn't; heat and light exist, but cold and darkness don't. Not as things.

We can see the Archchancellor looking puzzled, and we realize that here is something that runs quite deep in the human psyche. Yes, you can freeze to death, and 'cold' is a good word for describ­ing the absence of heat. Without privatives, we would end up talking like the pod people from the Planet Zog. But we run into trouble, though, if we forget that we're using them as an easy short­hand.

In our world there are plenty of borderline cases. Is 'drunk' or 'sober' the privative? In Discworld you can get 'knurd', which is as far on the other side of sober as drunk is on the inebriated side [28] And a terrible thing it is, akin to a state of horrible depression. Hence the affliction of Captain Vimes in Guards! Guards! who needs a couple of drinks simply to become sober. , but on planet Earth there's no such thing. By and large, we think we know which member of such a pairing has an existence, and which is merely an absence. (We vote for 'sober' as the privative. It is the absence of drink, and, usually, the normal state of a person [29] Well ... most people. . In fact that normal state is only called sobriety when the subject of drink is at hand. There's nothing strange about this. 'Cold' is the normal state of the universe, after all, even though as a thing it does not exist. Er ... we're not going to get past you on this one, are we, Archchancellor?)