The Science of Discworld Read online

  '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.'*

  '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, sir. And ... and I'm pretty sure things will stay on it. I've been experimenting.'

  'Experimenting?' said the Dean. 'What good does that do?'

  The doors were flung open. Turnipseed, Ponder's assistant, hurried across to the table in a state of some agitation.

  'Mister Stibbons! HEX has found something!'

  The wizards turned to stare at him. He shrugged.

  'It's gold,' he said.

  'The Guild of Alchemists is not going to be happy about this,' said the Senior Wrangler, as the entire faculty clustered around the project. 'You know what they are for demarcation.'

  'Fair enough,' said Ridcully, steering the omniscope. 'We'll just give them a few minutes to turn up, otherwise we'll go on as we are, all right?'

  'How can we get it out?' said the Dean.

  Ponder looked horrified. 'Sir! This is a universe! It is not a piggy-bank! You can't just turn it upside down, stick a knife in the slot and rattle it around!'

  'I don't see why not,' said Ridcully, without looking up. 'It's what people do all the time.' He adjusted the focus. 'Personally I'm glad nothing can get out of the thing, though. Call me old fashioned, but I don't intend to occupy the same room as a million miles of exploding gas. What happened?'

  'HEX says one of the new stars exploded.'

  'They're too big to be stars, Ponder, We've been into this.'

  'Yes, sir,' Ponder disagreed.

  'They've only been around for five minutes.'

  'A few days, sir. But millions of years in Project time. People have been dumping rubbish into it, and I think some just drifted in and ... I don't think it was a very well-made st, furnace in the first place.'

  The exploding star was shrinking now, but flinging out a great halo of brilliant gases that even lit up one side of the rocky lumps the wizards had been making. Things want to come together and get big, Ponder thought. But when they're big enough, they want to explode. Another law.

  'There's lead and copper here, too,' said Ridcully. 'We're in the money now, gentlemen. Except that in this universe there's nothing to spend it on. Even so, it seems we're making progress. You're looking peaky, Mister Stibbons. You ought to get some sleep.'

  Progress, thought Ponder. Was that what they were making? But without narrativium, how did anything know?

  It was day four. Ponder had been awake all night. He wasn't sure, but he thought he'd probably been awake the previous night, too. He may have nodded off for a while, pillowing his head on the growing pile of screwed-up pieces of paper, with the Project winking and twinkling in front of him. If so, he'd dreamed of nothing.

  But he'd decided that Progress was what you made it.

  After breakfast, the wizards looked at the ball which currently occupied the centre of the omniscope.

  'Um, I used iron to start with,' said Ponder. 'Well, mostly iron. There's quite a lot of it about. Some of the ices are really nasty things, and rock by itself just sits there. See this one here?'

  A smaller ball of rock hung in space a little way away.

  'Yes, very dull,' said the Senior Wrangler. 'Why's it got holes all over it?'

  'I'm afraid that when I was dropping rocks on the ball of iron there were a few that went out of control.'

  'Could happen to anyone, Stibbons,' said the Archchancellor generously. 'Did you add gold?'

  'Oh yes, sir. And other metals,'

  'Gold does give a crust some style, I think. Are these volcanoes?'

  'Sort of, sir. They are the, er, acne of young worlds. Only unlike ours, where the rock is melted in the internal magical fields generated in the sub-strata, the magma is kept molten by the heat trapped inside the sphere.'

  'Very smoky atmosphere. I can hardly see anything.'

  'Yes, sir.'

  'Well, I don't call it much of a world,' said the Dean, sniffing. 'Practically red hot, smoke belching out everywhere ...'

  'The Dean does have a point, young man,' said Ridcully. He was extra kind, just to annoy the Dean. 'It's a brave attempt, but you just seem to have made another ball.'

  Ponder coughed. 'I just put this one together for demonstration purposes, sir.' He fiddled with the controls of the omniscope. The scene flickered, and changed. 'Now this,' he said, and there was a twinge of pride in his voice, 'is one I made earlier.'

  They stared into the lens.

  'Well? Just more smoke,' said the Dean.

  'Cloud, sir, in fact,' said Ponder.

  'Well, we can all make clouds of gas —’

  'Er ... it's water vapour, sir,' said Ponder.

  He reached over and adjusted the omniscope.

  The room was filled with the roar of the biggest rainstorm of all time.

  By lunchtime it was a world of ice.

  'And we were doing so well,' said Ridcully.

  'I can't think what went wrong,' said Ponder, wringing his hands. 'We were getting seas!'

  'Can't we just warm it up?' said the Senior Wrangler.

  Ponder sat down on his chair and put his head in his hands.

  'Bound to cool a world down, all that rain,' said the Lecturer in Recent Runes, slowly.

  'Very good ... er, rocks,' said the Dean. He patted Ponder on the back.

  'Poor chap looks a bit down,' hissed the Senior Wrangler to Ridcully. 'I don't think he's been eating properly.'

  'You mean ... not chewing right?'

  'No eating enough, Archchancellor.'

  The Dean picked up a piece of paper from Ponder's crowded desk.

  'I say, look at these,' he said.

  On the paper was written, in Ponder's very neat handwriting:

  THE RULES

  1 Things fall apart, but centres hold.

  2 Everything moves in curves.

  3 You get balls.

  4 Big balls tell space to bend.

  5 There are no turtles anywhere.

  6 ... It's so depressing.

  'Always been a bit of a one for rules, our Ponder,' said the Senior Wrangler.

  'Number Six doesn't sound incredibly well formulated,' said Ridcully.

  'You don't think he's going a bit bursar, do you?' said the Lecturer in Recent Runes.

  'He always thinks everything has to mean something,' said Ridcully, who generally took the view that trying to find any deep meaning to events was like trying to find reflections in a mirror: you always succeeded, but you didn't learn anything new.

  'I suppose we could simply heat the thing up,' said the Senior Wrangler.

  'A sun should be easy,' said Ridcully 'A big ball of fire should be no problem to a thinking wizard.' He cracked his knuckles. 'Get some of the students to put Mister Stibbons to bed. We'll soon have his little world all warm or my name's not Mustrum Ridcully.'

  Omnianism had taught for thousands of years that the Discworld was in fact a sphere, and violently persecuted those who preferred to believe the evidence of their own eyes. At the time of writing, Omnianism was teaching that there was something to be said for every point of view.

  FOURTEEN

  DISC WORLDS

  TO THE WIZARDS OF UNSEEN UNIVERSITY, the heavens include two obviously different types of body: stars, which are tiny pinpricks of light, and the sun, which is a hot ball, not too far away, and passes over the Disc during the day and under it at night. It's taken humanity a while to realize that in our universe it's not like that. Our Sun is a star, and like all stars it's huge,
so those tiny pinpricks must be a very long way off. Moreover, some of the pinpricks that seem to be stars aren't: they betray themselves by moving differently from the rest. These are the planets, which are a lot closer and a lot smaller, and together with the Earth, Moon, and Sun they form the solar system. Our solar system may look like a lot of balls whizzing around in some kind of cosmic game of pool, but that doesn't mean that it started out as balls or rock and ice. It is the outcome of a physical process, and the ingredients that went into that process are not obliged to resemble the result that comes out.

  The more we learn about the solar system, the more difficult it is to give a plausible answer to the question: how did it start? It is not the 'answer' part that gets harder — it's the plausibility. As we learn more and more about the solar system, the reality-check that our theories have to pass becomes more and more stringent. This is one reason why scientists have a habit of opening up old questions that everybody assumed were settled long ago, and deciding that they weren't. It doesn't mean that scientists are incompetent: it demonstrates their willingness to contemplate new evidence and re-examine old conclusions in its light. Science certainly does not claim to get things right, but it has a good record of ruling out ways to get things wrong.

  What must a theory of the formation of the solar system explain? Principally, of course, the planets — nine of them, dotted rather randomly in space; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto. It must explain their differences in size. Mercury is a mere 3,032 miles (4,878 km) in diameter, whereas Jupiter is 88,750 miles (142,800 km) in diameter — 29 times as big, 24,000 times the volume, an enormous discrepancy. It must explain their differences in chemical composition: Mercury is made of iron, nickel, and silicate rock; Jupiter is made from hydrogen and helium. It must explain why the planets near the Sun are generally smaller than those further out, with the exception of tiny Pluto, out in the cold and the dark. We don't know a great deal about Pluto, but most of what we do know is strange. For instance, all the other planets lie pretty close to a single plane through the centre of the Sun, but Pluto's orbit is inclined at a noticeable angle. All the other planets have orbits that are pretty close to circles, but Pluto's orbit is much more elongated — to the extent that some of the time it is closer to the Sun than Neptune is.

  But that's not all that a theory of the origin of the solar system has to get right. Most planets have smaller bodies in orbit around them — our own familiar Moon; Phobos and Deimos, the diminutive twin satellites of Mars; Jupiter's 16 satellites; Saturn's 17 ... Even Pluto has a satellite, called Charon, and that's weird too. Saturn goes one better and also has entire rings of smaller bodies surrounding it, a broad, thin band of encircling rocks that breaks up into a myriad distinct ringlets, with satellites mixed up among them as well as more conventional satellites elsewhere. Then there are the asteroids, thousands of small bodies, some spherical like planets, others irregular lumps of rock, most of which orbit between Mars and Jupiter — except for quite a few that don't. There are comets, which fall in towards the Sun from the huge 'Oort cloud' way out beyond the orbit of Pluto — a cloud that contains trillions of comets. There is the Kuiper belt, a bit like the asteroid belt but outside Pluto's orbit: we know over 30 bodies out there now, but we suspect there are hundreds of thousands.

  These bodies are known as 'Kuiper Belt Objects' or KBOs. A few years back there was a big fuss because some astronomers wanted to redefine Pluto as a KBO rather than a planet. Pluto probably wouldn’t have minded either way, but an awful lot of textbook publishers would have. The scientific case was strong: Pluto is weird in almost every respect, as we’ve just seen, and it could easily be a KBO that accidentally strayed into the outer reaches of the solar system when disturbed by other bodies. If so, that would explain why it’s so weird. It doesn’t look like a planet because it isn’t one. Other astronomers disagreed strongly with this proposal — for sentimental reasons, for historical ones, or because we don’t know for sure that Pluto is a wandering KBO. In the end, Pluto remained on the list of planets. But whether it can hang on to that status for much longer is unclear.

  Then there are meteorites, lumps of rock of various sizes that wander erratically through the whole thing ...

  Each of these celestial objects, moreover, is a one-off. Mercury is a blisteringly hot lump of cratered rock. Venus has a sulphuric acid atmosphere, rotates the wrong way compared to nearly everything else in the solar system, and is believed to resurface itself every hundred million years or so in a vast, planetwide surge of volcanic activity. Earth has oceans and supports life; since we live on it we find it the most congenial of the planets, but many aliens would probably be aghast at its deadly, poisonous, corrosive oxygen atmosphere. Mars has rock-strewn deserts and dry ice at its poles. Jupiter is a gas giant, with a core of hydrogen compressed so much that it has become metallic, and maybe a small rocky core inside that — 'small' compared to Jupiter, but about three times the diameter of the Earth. Saturn has its rings — but so do Jupiter, Uranus, and Neptune, though these are nowhere near as extensive or spectacular. Uranus has an icy mantle of methane and ammonia, and its axis of rotation is tilted so far that it is slightly upside down. Neptune is similar to Uranus but without that ridiculous axial tilt. Pluto, as we've said, is just crazy. We don't even know accurately how big it is or how massive it is, but it's a Lilliputian in the country of the Gas Giants.

  Right... all that is what a theory of the origins of the solar system has to explain. It was all a lot easier when we thought there were six planets, plus the Sun and the Moon, and that was it. As for the solar system being an act of special creation by a supernatural being — why would any self-respecting supernatural being make the thing so complicated?

  Because it makes itself complicated — that's why. We now think that the solar system was formed as a complete package, starting from quite complicated ingredients. But it us took a while to realize this.

  The first theory of planetary formation that makes any kind of sense by modern standards was thought up by the great German philosopher Immanuel Kant about 250 years ago. Kant envisaged it all starting as a vast cloud of matter — big lumps, small lumps, dust, gas — which attracted each other gravitationally and clumped together.

  About 40 years later the French mathematician Pierre-Simon de Laplace came up with an alternative theory of enormous intrinsic beauty, whose sole flaw is that it doesn't actually work. Laplace thought that the Sun formed before the planets did, perhaps by some cosmic aggregation process like Kant's. However, that ancient Sun was much bigger than today's, because it hadn't fully collected together, and the outer fringes of its atmosphere extended well beyond what is now the orbit of Pluto. Like the wizards of Unseen University, Lapkce thought of the Sun as a gigantic fire whose fuel must be slowly burning away. As the Sun aged, it would cool down. Cool gas contracts, so the Sun would shrink.

  Now comes a neat peculiarity of moving bodies, a consequence of another of Newton's laws, the Law(s) of Motion. Associated with any spinning body is a quantity called 'angular momentum' — a combination of how much mass it contains, how fast it is spinning, and how far out from the centre the spinning takes place. According to Newton, angular momentum is conserved — it can be redistributed, but it neither goes away nor appears of its own accord. If a spinning body contracts, but the rate of spin doesn't change, angular momentum will be lost: therefore the rate of spin must increase to compensate. This is how ice skaters do rapid spins: they start with a slow spin, arms extended, and then bring their arms in close to their body. Moreover, spinning matter experiences a force, centrifugal force, which seems to pull it outwards, away from its centre.

  Laplace wondered whether centrifugal force acting on a spinning gascloud might throw off a belt of gas round the equator. He calculated that this ought to happen whenever the gravitational force attracting that belt towards the centre was equal to the centrifugal force trying to fling it away. This process would happen not on
ce, but several times, as the gas continued to contract — so the shrinking Sun would surround itself with a series of rings of material, all lying in the same plane as the Sun's equator. Now suppose that each belt coalesced into a single body ... Planets!

  What Laplace's theory got right, but Kant's did not, was that the planets lie roughly in a plane and they all rotate round the Sun in the same direction that the Sun spins. As a bonus, something rather similar might have occurred while those belts were coalescing into planets, in which case the motion of satellites is explained as well. It's not hard to combine the best features of Kant's and Laplace's theories, and this combination satisfied scientists for about a century. However, it slowly became clear that our solar system is far more unruly than either Kant or Laplace had recognized. Asteroids have wild orbits, and some satellites revolve the wrong way. The Sun contains 99% of the solar system's mass, but the planets possess 99% of its angular momentum: either the Sun is rotating too slowly or the planets are revolving too quickly.

  As the twentieth century opened, these deficiencies of the Laplacian theory became too great for astronomers to bear, and several people independently came up with the idea that a star developed a solar system when it made a close encounter with another star. As the two stars whizzed past each other, the gravitational attraction from one of them was supposed to draw out a long cigar-shaped blob of matter from the other, which then condensed into planets. The advantage of the cigar shape was that it was thin at the ends and thick at the middle, just as the planets are small close to the Sun or out by Pluto, but big in the middle where Jupiter and Saturn live. Mind you, it was never entirely clear why the blob had to be cigar-shaped ...

  One important feature of this theory was the implication that solar systems are rather uncommon, because stars are quite thinly scattered and seldom get close enough together to share a mutual cigar. If you were the sort of person who'd be comforted by the idea that human beings are unique in the universe, then this was a rather appealing suggestion: if planets were rare, then inhabited planets would be rarer still If you were the sort of person who preferred to think that the Earth isn't especially unusual, and neither are its life-forms, then the cigar theory definitely put a crimp on the imagination.