The Science of Discworld IV Judgement Da - By Terry Pratchett, Ian Stewart

There is a sensible way to make a world.

It should be flat, so that no one falls off accidentallyfn1 unless they get too near the edge, in which case it’s their own fault.

It should be circular, so that it can revolve sedately to create the slow progression of the seasons.

It should have strong supports, so that it doesn’t fall down.

The supports should rest on firm foundations.

To avoid an infinite regression, the foundations should do what foundations are supposed to do, and stay up of their own accord.

It should have a sun, to provide light. This sun should be small and not too hot, to save energy, and it should revolve around the disc to separate day from night.

The world should be populated by people, since there is no point in making it if no one is going to live there.

Everything should happen because people want it to (magic) or because the power of story (narrativium) demands it.

This sensible world is Discworld – flat, circular, held up by four world-bearing elephants standing firmly on the back of a giant space-faring turtle and inhabited by ordinary humans, wizards, witches, trolls, dwarves, vampires, golems, elves, the tooth fairy and the Hogfather.


There is also a stupid way to make a world. And sometimes, that is necessary.

When an experiment in fundamental thaumaturgy on the squash court of Unseen University ran wild and threatened to destroy the universe, the computer Hex had to use up a huge quantity of magic in an instant. The only option was to activate the Roundworld Project, a magical force field that – paradoxically – keeps magic out. When the Dean of Unseen University poked his finger in to see what would happen, Roundworld switched on.

Roundworld isn’t entirely sure which bit of itself its name applies to. Sometimes the name refers to the planet, sometimes to the entire universe. There have been a few mishaps along the way, but the Roundworld universe has now been running fairly successfully for thirteen and a half billion years; all of it started by an old man with a beard.

In the absence of magic, and lacking natural narrativium, the Roundworld universe runs on rules. Not rules made by people, but rules made by Roundworld itself; which is weird, because Roundworld has no idea what its rules ought to be. It seems to make them up as it goes along, but it’s hard to be sure.

Certainly, it doesn’t know what size it ought to be. From outside, as it gathers dust on a shelf in Rincewind’s office, the Roundworld universe – a globe about 20 centimetres in diameter – resembles a cross between a foot-the-ball and a child’s snowstorm toy. From inside, it appears to be somewhat larger: a sphere whose radius is about 400 sextillion kilometres. As far as its only knownfn2 inhabitants can tell, it may be much larger still; perhaps even infinite.

Such a huge universe seems to be cosmic overkill, because those inhabitants occupy only the tiniest part of its awe-inspiring volume, namely the surface of an approximate sphere a mere twelve thousand kilometres across.

The wizards call this sphere Roundworld too. Its inhabitants call it Earth, because that’s what the surface is usually made of (except for the wet, rocky, sandy and icy bits): a typically parochial attitude. Until a few centuries ago they thought that Earth was fixed at the centre of the universe; the rest, which revolved around it or wandered crazily across the sky, was of minor importance since it didn’t contain them.

Roundworld the planet, as the name suggests, is round. Not round like a disc, but round like a foot-the-ball. It is younger than Roundworld the universe: about one third of its age. Though cosmically minuscule, the planet is fairly big compared to its inhabitants, so that if you live there, and you’re stupid, you can be fooled into imagining that it’s flat.

To prevent the planet’s inhabitants falling off, the rules state that a mysterious force glues them on. Thankfully, there are no world-bearing elephants. If there were, the inhabitants would be able to walk round their world to the point where it meets an elephant. This world-bearing beast of immense power would appear to be lying on its back, its feet in the air. (Paint the soles yellow and you wouldn’t be able to see it floating in a bowl of custard …)

Roundworld’s rules are democratic. Not only does this mysterious force glue people to their world: it glues everything to everything else. But the glue is weak, and