The Science of Discworld Revised Edition

–’
    ‘As far as I can see from this,’ interrupted the Senior Wrangler, who’d been reading H EX ’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 …
    1 A phrase meaning ‘I’m not sure you know this’

TWENTY-TWO
THINGS THAT AREN’T

    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. 1
    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. 2 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, 3 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. 4 In fact that normal state is only
called
sobriety when the subject of drink is at hand. There’s nothing