The Science of Discworld

by Terry Pratchett

  Ponder disappeared for a moment, and then came back.

  'HEX feels that this is a dumb-waiter,' he said. 'Er ... you know ... for lifting things to another level.'

  'Going where?' said Ridcully.

  'Er ... up, sir. Into that ... necklace around the world. HEX has been speaking to the intelligence here. It's a sort of HEX, sir. And it's nearly dead.'

  'That's a shame,' said Ridcully He sniffed. 'Where's everyone gone, then?'

  'Er ... they made huge ... sort of... big metal balls to live in. I know it sounds stupid, sir. But they've gone. Because of the ice. And there was a comet, too. Not very big. But it scared everyone. They built the ... the beanstalk things, and then they ... er ... mined metal out of floating rocks, and ... they left.'

  'Where've they gone?'

  'The ... intelligence isn't sure. It's forgotten. It says it's forgotten a lot.'

  'Oh, I understand,' said the Dean, who'd been trying to follow this, 'Everyone's climbed up a great big beanstalk?'

  'Er ... sort of, Dean,' said Ponder, in his diplomatic voice. 'In a manner of speaking.'

  'Certainly messed the place up before they went,' said Ridcully.

  Rincewind had been watching a rat scuttle away into the debris, but the words sunk in and exploded in his head.

  'Messed up?' he growled. 'How?'

  'Say again?' said Ridcully.

  'Did you see the weather report for this world?' said Rincewind, waving his hands in the air. 'Two miles of ice, followed by a light shower of rocks, with outbreaks of choking fog for the next thousand years? There will be widespread vulcanism as half a continent's worth of magma lets go, followed by a period of mountain building? And that's normal.'

  'Yes, well —’

  'Oh, yes, there are some nice quiet periods, everything settles down, and then — whammo!'

  'There's no need to get so excited —’

  'I've been here!' said Rincewind. 'This is how this place works! And now, please, you tell me how, I mean how, can anything living on this world possibly mess it up? I mean, compared to what happens anyway?' He paused, and gulped air. 'I mean, don't get me wrong, if you pick the right time, yes, sure, it's a great world for a holiday, ten thousand years, even a few million if you're lucky with the weather but, good grief, it's just not a serious proposition for anything long term. It's a great place to grow up on, but you wouldn't want to live here. If any thing's got off, the best of luck to them.'

  He waved a finger at the rat, who was watching them suspiciously. Underneath them, the ground trembled again.

  'See him?' he said. 'We know what's going to happen. In a million years or so his kids are going to be saying, wow, what a great world the Big Rat made for us. Or it'll be the turn of the jellyfish, or something that's still bobbing around under the sea that we don't even know about yet! There's no future here! No, that's wrong ... I mean there's always a future, but it belongs to someone else. You know what chalk's made of here? Dead animals! The actual rock is made of dead animals! There were some ...'

  Even in his overheated state, he paused. It probably wasn't a good idea to remind people about the apes. A vague, suspicious guilt was nudging him.

  'There were these creatures,' he said, 'and they were using limestone caves. Limestone's made from ancient blobs, I saw it being made, like snow in the water ... and these creatures are living in the bones of their ancestors! Really! This place ... this place is a kaleidoscope. You smash it up, wait a moment, and there's another pretty pattern. And another one. And another o ...'He stopped. And sagged. 'Could I have a glass of water, please?'

  'That was a very ... interesting speech,' said Ponder.

  'A point of view, certainly,' said Ridcully.

  The other wizards had, however, lost interest. They usually did, if the speeches were not given by them.

  'Shall I tell you something else?' said Rincewind, a little more calmly. 'This world is an anvil. Everything here is between a rock and a hard place. Every single thing on it is the descendant of creatures that have survived everything the world could throw at them. I just hope they never get angry ...'

  The Senior Wrangler and the Dean had ambled towards a huge cylinder. The word 'MAETNANS' was painted in large black letters on the side.

  'Hey, you chaps!' the Dean shouted. 'There's something talking in here ...'

  The inside of the cylinder reminded the wizards of a lighthouse. There was a spiral staircase; shaped cupboards lined the walls. Lights glowed dimly, whole constellations of them. Certainly the builders of this thing had discovered magic.

  The 'A-L-A-A-M' word still blinked on and off in the air.

  'I wish that wretched thing would stop,' said the Senior Wrangler.

  The light vanished. The sound stopped.

  'They've probably invented demons,' said the Dean airily. 'Listen ... hello.'

  A pleasant female voice said, 'Elevator Unstable.'

  'Oh, magic,' said Ridcully flatly. 'Well, we know how to deal with magic. We want to go up in the magic box, voice.'

  'Do we?' said Ponder

  'Anything better than staying in this gloomy place,' said Ridcully. 'It'd be quite an interestin' experience, too. We'll take one last look the world and then, well ... frankly, that's it.'

  'Instability Rising', said the voice. It did not sound worried by the news.

  'What did it say?' said the Dean. 'Sounded like name of a place,'

  'Very good, very good,' said Ridcully 'Now let's be going shall we?'

  The pattern of lights moved. Then the voice said, as if it'd been thinking it over, 'Emerjansi Override.'

  The door slid shut. The cylinder jerked. Shortly afterwards, some pleasant music started, and didn't really get on anyone's nerves for several minutes.

  The rat watched the thing rise up the cables in the centre of the pyramid.

  The ground shook again.

  Slowly, the web around the world came apart.

  Ice walls had attacked some of the cable moorings on the ground, but instability was already there, working inexorably as it had done for the past few weeks, turning little movements into big movements.

  Slowly, one cable broke free from its pyramid, glowing red-hot as it was jerked through the atmosphere, flailing across the sky.

  Around the curve of the world, the others danced and groaned...

  When the end finally came, it took only a day. The lines folded around the centre of the world, writhing incandescently across hundreds of miles of snow. The necklace tore apart far above. Some bits drifted away. Others spun gently towards the surface, to impact hours later.

  A ring of fire burned for a while around the equator.

  And then the cold returned.

  As the wizards said, it would all be the same in a hundred million years' time. But it would be different tomorrow.

  In the deserted High Energy Building, HEX turned the omniscope outwards, homing in on signs of the strange new life.

  It found comet cores, strung on cables thousands of miles long. There were dozens of these trains, many millions of miles from the frozen world, accelerating into the abyss between the stars.

  Lights twinkled on their surfaces. The extelligence inside appeared to be travelling hopefully.

  A yellow cylinder tumbled gently across the darkness.

  It was empty.

  FORTY-SIX

  WAYS TO LEAVE YOUR PLANET

  RINCEWIND'S IMPASSIONED SPEECH HAS A POINT. If you think he's overstating his case, and that the Earth is really an idyllic place to live, bear in mind that he's been on our planet a lot longer than we have, and he's seen a lot that we've missed, because we experience the world on a much shorter timescale than the wizards have done. We think the planet's a great place. We grew up here. We were made for it, and it's just right for us ... at the moment.

  Tell that to the dinosaurs.

  You can't, can you. That's the point.

  We're not suggesting that you sell up everything and start building a lif
eboat. But even the United States congress is beginning to wonder just how safe our planet really is, and politicians are not usually known for taking long-term views. The sight of Shoemaker-Levy 9 smashing into Jupiter raised a few political eyebrows. Tentative schemes are afoot to set up a defence system against incoming comets and asteroids. Spotting them early enough is the trick. Find them quickly, and a modest little rocket motor can save our planetary bacon.

  It is in many ways amazing that life on Earth has survived everything that the universe has so far thrown at it. Evolution runs on Deep Time — less than a hundred million years hardly counts. Life is extremely resilient, but individual species are not. They last a few million years and then they become obsolete. Life persists by changing — by being a series of opening chapters. But, being human, we'd like to see our own story turn into at least a blockbuster dekalogy.

  We can take small comfort in one thing. Although right now we don't worry enough about incoming disaster from Up There, we do worry a lot about home-grown disaster Down Here: nuclear warfare, biological warfare, global warming, pollution, overpopulation, destruction of habitat, burning of the rainforests, and so on. However, there's no danger that human actions will wipe out the planet. Compared to what nature has already done, and will do again, our activities barely show up. One large meteorite packs more explosive power than all human wars put together, a hypothetical World War III included. One Ice Age changes the climate more than a civilization's worth of carbon dioxide from car exhausts. As for something like the Deccan Traps ... you wouldn't want to know how nasty the atmosphere could become.

  No, we can't destroy the Earth. We can destroy ourselves.

  No one would care. The cockroaches and the rats will come back, or if the worst comes to the worst the bacteria miles below ground will start to write a new opening chapter in the Book of Life. Someone else will read it.

  If we really deserve the name Homo sapiens, then we can do at least two things to improve our chances. First, we can learn to manage our impact on the environment. The fact that nature deals the occasional death blow doesn't hand us an excuse to imitate it. We invented ethics. Our environment is sufficiently buffeted by various forces that the last thing it needs is humanity throwing extra spanners in the works. At the most selfish level, we might be buying ourselves some time.

  We could use that time to put some of our eggs in another basket.

  One of the great dreams of humanity has been to visit other worlds. It's starting to look as though this might be a very good idea — not just for fun and profit, but for survival.

  We'd better say right now that none of this is science fiction. Or, rather, yes, it is science fiction, it's the very stuff of science fiction, because some of the best science-fiction writers (you don't see their stuff on TV) have been dealing with it for many decades. But that does not mean it's not real. Ices Ages happen. Big, big rocks come screaming out of the sky, and you need rather more than Bruce Willis flying the Space Shuttle as if it was the Millennium Falcon to stop them.

  Our urge to explore the universe may be just another case of monkey curiosity, but there seems to be a deep impulse that urges us to find new lands to map and new worlds to conquer. Maybe there's an inbuilt urge to spread out — one leopard can't eat all of you if you spread out.

  It is an urge that has driven us into every corner and crevice of our own planet, from the ice-floes of the Arctic to the deserts of Namibia, from the depths of the Mariana Trench to the peak of Everest. Most of us incline to Rincewind's view of a comfortable lifestyle and much prefer to stay at home, but a few are too restless to be happy anywhere for very long. The combination is a powerful one, and it has shaped our species into something very unusual, with collective capabilities beyond the understanding of any individual. We may not always use that combination wisely, but without it we would be greatly diminished. And it's offering a real opportunity.

  Even a dream can work miracles. When Columbus (re-)discovered America, and Europe found out that it existed, he was looking for a new route to the Indies. He had convinced himself — on grounds that most scholars at the time found totally spurious — that the Earth was considerably smaller than was generally thought. He calculated that a relatively short voyage westward, from Africa, would lead to Japan and India. The scholars were right, Columbus was wrong — but it is Columbus that we remember, because he made the world smaller. He had the courage to set sail into an empty sea, sustained only by the belief that there was something important on the other side.

  At least we can see where we ought to go. Columbus had to back a hunch.

  A dirty great Saturn-V rocket with a tiny Apollo capsule on top was the first practical method for getting out of the Earth's gravity well altogether. By this we don't mean that the Earth's gravitational pull becomes zero if you go far enough away, which is a common misconception: we mean that if you go fast enough, then the Earth's gravity can never pull you back down. Celestial mechanics operates in the phase space of distance and velocity, its 'landscape' involves speeds as well as lengths. Only when we understood enough about gravity and dynamics to appreciate this point did we stand any chance of making technology like Apollo work.

  You can see this clearly from earlier suggestions — which were imaginative, in an earthbound sort of way — but fantastic and impractical, at least on Roundworld. In 1648 Bishop John Wilkins listed four possible ways to leave the ground: enlist the aid of spirits or angels, get a lift from birds, fasten wings to your body, or build a flying chariot. If we wanted to be charitable, we could interpret the last two as aircraft and rockets, but Wilkins was clearly unaware that the Earth's atmosphere doesn't extend all the way to the Moon. A sixteenth-century engraving by Hans Schauffelein depicts Alexander the Great carried into space by two griffins — no noticeable improvement. Bernard Zamagna conceived of an aerial boat, and others suggested the use of balloons.

  Every age fantasized about technology that already existed. In Jules Verne's From the Earth to the Moon of 1865 the journey was accomplished by firing a space capsule from a huge gun in Florida; its 1870 sequel Around the Moon involved a series of such capsules, forming a space train. Verne got Florida right — he knew that the Earth's spin produces centrifugal force, which helps the capsule to leave the planet more easily, and he knew that this force was greatest at the equator. Since the protagonists in his book were American, Florida was the best bet. When NASA started launching rockets, it came to the same conclusion, and the space facility at Cape Canaveral was born.

  Big guns have deficiencies, such as a tendency to laminate passengers to the floor because of rapid acceleration, but modern technology does make it possible to avoid this by applying the acceleration gradually. Rockets are more practical from the engineering point of view. In 1926 Robert Goddard invented the liquid fuel rocket. The first one rose to the dizzy height of 40 feet (12.5 m). Rockets have come a long way since then, taking men to the Moon and instruments to the edge of the solar system. And they are much better rockets. Even so, there's something ... inelegant about heading off the planet on a giant disposable firework.

  Until recently, there has been a general assumption that the energy to get into space has to be carried with the craft. However, we already have the beginnings of one way to get off the Earth that keeps the power source firmly on the ground. This is laser propulsion, in which a powerful beam of coherent light is aimed at a solid object and literally pushes it along. It takes a lot of power, but prototypes invented by Leik Myrabo have already been tested at the High Energy Laser System Test Facility at White Sands. In November 1997 a small projectile reached a height of 50 feet (15m) in 5.5 seconds; by December this had been improved to 60 feet (20 m) in 4.9 seconds. This may not sound impressive, but compare with Goddard's first rocket. The method involves spinning the projectile at 6000 revolutions per minute to achieve gyroscopic stability. Then 20 laser pulses per second are directed towards a specially shaped cavity, heating the air beneath the craft and creating a
pressure wave of thousands of atmospheres with temperatures up to 30,000° Kelvin — and that's what propels the projectile. At higher altitudes the air becomes very thin, and a similar craft would need an onboard fuel source. Fuel would be pumped into the cavity to be vapourized by the laser A megawatt laser could lift a 2-pound (1 kg) craft into orbit.

  It is also a very powerful weapon …

  Another possibility is power beaming. It is possible to 'beam' electromagnetic power from the ground in the form of microwaves. This isn't just fantasy: in 1975 Dick Dickinson and William Brown beamed 30 kilowatts of power — enough for thirty electric fires — over a distance of one mile. James Benford and Myrabo have suggested launching a spacecraft using millimetre range microwaves which are not attenuated by the atmosphere. This is a variation on the laser method and would use the same kind of projectile.

  Both of these methods rely on a lot of raw power, betraying traces of the basic engineering assumption that getting into space needs a lot of energy to overcome the Earth's gravity. They do have the advantage that the raw power is just sitting on the planet; the 1,000 megawatt power station your laser launcher would require could generate for the National Grid when a launch wasn't going on.

  A method of greater subtlety is the bolas, first proposed in the 1950s. Traditionally, a bolas is a hunting device made by tying three weights to strings and then tying the ends of the strings together. When thrown, it spins, pulling the weights apart, until the strings hit the target, at which point the weights spiral rapidly inwards and deal a killing blow. The same sort of device could be set up in a vertical plane above the equator, a bit like a giant ferris wheel with only three spokes. On the ends of the spokes would be pressurized cabins. The lowest part of the bolas's swing would be somewhere in the lower atmosphere, the top part way out in space. You would fly up in an aircraft, transfer to the first passing cabin, and be whisked skywards. The biggest obstacle to making such a machine is the cable, which has to be stronger than any known material — but carbon fibre is well on the way to combining enough strength with enough lightness. Friction with the atmosphere would gradually slow the bolas's rotation down, but that could be compensated for using solar power arrays up in space.