The Science of Discworld

by Terry Pratchett

  Although today's telescopes cannot detect an alien planet directly, future telescopes might. Conventional astronomical telescopes use a big, slightly dish-shaped mirror to focus incoming light, plus lenses and prisms to pick up the image and send it to what used to be an eyepiece for an astronomer to look down, but then became a photographic plate, and is now likely to be a 'charge-coupled device' — a sensitive electronic light-detector — hooked up to a computer. A single telescope of conventional design would need a very big mirror indeed to spot a planet round another star — a mirror some 100 yards (100 m) across. The biggest mirror in existence today is one-tenth that size, and to see any detail on the alien world you'd need an even bigger mirror, so none of this is really practicable.

  But you don't have to use just one telescope.

  A technique known as 'interferometry' makes it possible, in principle, to replace a single mirror 100 yards wide by two much smaller mirrors 100 yards apart. Both produce images of the same star or planet, and the incoming light waves that form those images are aligned very accurately and combined. The two-mirror system gathers less light than a complete 100-yard mirror would, but it can resolve the same amount of tiny detail. And with modern electronics, very small quantities of incoming light can be amplified. In any case, what you actually do is use dozens of smaller mirrors, together with a lot of clever trickery that keeps them aligned with each other and combines the images that they receive in an effective manner.

  Radio astronomers use this technique all the time. The biggest technical problem is keeping the length of the path from the star to its image the same for all of the smaller telescopes, to within an accuracy of one wavelength. The technique is relatively new in optical astronomy, because the wavelength of visible light is far shorter than that of radio waves, but for visible light the real killer is that it's not worth bothering if your telescopes are on the ground. The Earth's atmosphere is in continual turbulent motion, bending incoming light in unpredictable ways. Even a very powerful ground-based telescope will produce a fuzzy image, which is why the Hubble Space Telescope is in orbit round the Earth. Its planned successor, the Next Generation Space Telescope, will be a million miles away, orbiting the Sun, delicately poised at a place called Lagrange point L2. This is a point on the line from the Sun to the Earth, but further out, where the Sun's gravity, the Earth's gravity, and the centrifugal force acting on the orbiting telescope all cancel out. Hubble's structure includes a heavy tube which keeps out unwanted light, especially light reflected from our own planet. It's a lot darker out near L2, and that cumbersome tube can be dispensed with, saving launch fuel. In addition, L2 is a lot colder than low Earth orbit, and that makes infra-red telescopy much more effective.

  Interferometry uses a widely separated array of small telescopes instead of one big one, and for optical astronomy the array has to be set up in space. This produces an added advantage, because space is big, or, in more Discworldly terms, a place to be big in. The biggest distance between telescopes in the array is called the baseline. Out in space you can create interferometers with gigantic baselines — radio astronomers have already made one that is bigger than the Earth by using one ground-based telescope antenna and one in orbit. Both NASA and the European Space Agency ESA have missions on the drawing-board for putting prototype optical interferometer arrays — 'flocks' is a more evocative term — into space.

  Some time around 2003 NASA will launch Space Technology 3 (previously named Deep Space 3), involving two spacecraft flying 0.6 miles (1 km) apart and maintaining station relative to each other to a precision of less than half an inch (1 cm). Another NASA venture, the Space Interferometry Mission, will employ three interferometers with a 10-metre baseline and is tentatively due to launch in 2009. And NASA is thinking about a Terrestrial Planet Finder in 2012, which will look not just for planets, but for carbon dioxide, water vapour, ozone, and methane, which could be signs of life — or, at least, of a planet that might be able to support life similar to ours. Life Finder, with no specific date, would look more closely.

  The European Space Agency (ESA) has similar missions on the drawing-board. SMART-2, consisting of two satellites orbiting in formation, is planned for 2006. A more ambitious ESA project is Darwin, a flotilla of 6 telescopes that could be in space by 2014.

  The biggest dream of all, though, is NASA's Planet Imager, pencilled in for 2020. A squadron of five spacecraft, each equipped with four optical telescopes, will deploy itself into an interferometer with a baseline of several thousand miles, and start mapping alien planets. The nearest star is just over four light years away; computer simulations show that 50 telescopes with a baseline of just 95 miles (150 km) can produce images of a planet 10 light years away that are good enough to spot continents and even moons the size of ours. With 150 telescopes and the same baseline, you could look at the Earth from 10 light years away and see hurricanes in its atmosphere. Think what could be done with a thousand-mile baseline.

  Planets outside our solar system do exist, then — and they probably exist in abundance. That's good news if you're hoping that somewhere out there are alien lifeforms. The evidence for those, though, is controversial.

  Mars, of course, is the traditional place where we expect to find life in the solar system — partly because of myths about Martian 'canals' which astronomers thought they'd seen in their telescopes but which turned out to be illusions when we sent spacecraft out there to take a close look, partly because conditions on Mars are in some ways similar to those on Earth, though generally nastier, and partly because dozens of science-fiction books have subliminally prepared us for the existence of Martians. Life does show up in nasty places here, finding a foothold in volcanic vents, in deserts, and deep in the Earth's rocks. Nevertheless, we've found no signs of life on Mars.

  Yet.

  For a while, some scientists thought we had. In 1996 NASA announced signs of life on Mars. A meteorite dug up in the Antarctic with the code number ALH84001 had been knocked off Mars 15 million years ago by a collision with an asteroid, and plunged to Earth 13,000 years ago. When it was sliced open and the interior examined at high magnification we found three possible signs of life. These were markings like tiny fossil bacteria, crystals containing iron like those made by certain bacteria, and organic molecules resembling some found in fossil bacteria on Earth. It all pointed to: Martian bacteria! Not surprisingly, this claim led to a big argument, and the upshot is that all three discoveries are almost certainly not evidence for life at all. The fossil 'bacteria' are much too small and most of them are steps on crystal surfaces that have caused funny shapes to form in the metal coatings used in electron microscopy; the iron-bearing crystals can be explained without invoking bacteria at all; and the organic molecules could have got there without the aid of Martian life.

  However, in 1998 the Mars Global Surveyor did find signs of an ancient ocean on Mars. At some point in the planet's history, huge amounts of water gushed out of the highlands and flowed into the northern lowlands. It was thought that this water just seeped away or evaporated, but it now turns out that the edges of the northern lowlands are ail at much the same height, like shorelines eroded by an ocean. The ocean, if it existed, covered a quarter of Mars's surface. If it contained life, there ought to be Martian fossils for us to find, dating from that period.

  The current favourite for life in the solar system is a surprise, at least to people who don't read science fiction: Jupiter's satellite Europa. It's a surprise because Europa is exceedingly cold, and covered in thick layers of ice. However, that's not where the life is suspected to live. Europa is held in Jupiter's massive gravitational grasp, and tidal forces warm its interior. This could mean that the deeper layers of the ice have melted to form a vast underground ocean. Until recently this was pure conjecture, but the evidence for liquid water beneath Europa's surface has now become very strong indeed. It includes the surface geology, gravitational measurements, and the discovery that Europa's interior conducts electricity. This f
inding, made in 1998 by K. K. Khurana and others, came from observations of the worldlet's magnetic field made by the space probe Galileo, The shape of the magnetic field is unusual, and the only reasonable explanation so far is the existence of an underground ocean whose dissolved salts make it a weak conductor of electricity. Callisto, another of Jupiter's moons, has a similar magnetic field, and is now also thought to have an underground ocean. In the same year, T. B. McCord and others observed huge patches of hydrated salts (salts whose molecules contain water) on Europa's surface. This might perhaps be a salty crust deposited by upwelling water from a salty ocean.

  There are tentative plans to send out a probe to Europa, land it, and drill down to see what's there. The technical problems are formidable — the ice layer is at least ten miles (16 km) thick, and the operation would have to be carried out very carefully so as not to disturb or destroy the very thing we're hoping to find: Europan organisms. Less invasively, it would be possible to look for tell-tale molecules of life in Europa's thin atmosphere, and plans are afoot to do this too. Nobody expects to find Europan antelopes, or even fishes, but it would be surprising if Europa's water-based chemistry, apparently an ocean a hundred miles (160 km) deep, has not produced life. Almost certainly there are sub-oceanic 'volcanoes' where very hot sulphurous water is vented through the ocean floor. These provide a marvellous opportunity for complicated chemistry, much like the chemistry that started life on Earth.

  The least controversial possibility would be an array of simple bacteria-like chemical systems forming towers around the hot vents — much as Earthly bacteria do in the Baltic sea. More complicated creatures like amoebas and parameciums would be a pleasant surprise; anything beyond that, such as multicellular organisms, would be a bonus. Don't expect plants — there's not enough light that far from the sun, even if it could filter down through the layers of ice. Europan life would have to be powered by chemical energy, as it is around Earth's underwater volcanic vents. Don't expect Europan lifeforms to look like the ones round our vents, though: they will have evolved in a different chemical environment.

  In 2001 Brad Dalton, an astrogeophysicist ('geologist' for other worlds) wondered whether we might have already seen alien life. Europa’s surface is covered with red-brown marks, especially along what appear to be cracks in the ice. Dalton discovered that the infrared spectrum of these marks is very similar to that of Earthly bacteria that can survive in very cold environments. In fact, for three species of bacteria the spectrum is closer than that for the most likely alternative explanation of the marks: mineral salts seeping up through the cracks. Europa’s surface is too cold for even these bacteria, but they might thrive in the ocean and somehow be transported to the surface.

  FIFTEEN

  THE DAWN OF DAWN

  PONDER OPENED HIS EYES and looked up into a face out of time. A mug of tea was thrust towards him.

  It had a banana stuck in it.

  'Ah ... Librarian,' said Ponder weakly, taking the cup. He drank, stabbing himself harmlessly in the left eye. The Librarian thought that practically everything could be improved by the addition of soft fruit, but apart from that he was a kindly soul, always ready with a helping hand and a banana.*

  The wizards had put Ponder to sleep on a bench in the storeroom. Dusty items of magical gear were stacked from floor to ceiling. Most of it was broken, and all of it was covered in dust.

  Ponder sat up and yawned.

  'What time is it?'

  'Ook.'

  'Gosh, that late?'

  As the warm clouds of sleep ebbed, it dawned on Ponder that he had left the Project entirely in the hands of the senior faculty. The Librarian was impressed at how long the door kept swinging.

  Most of the main laboratory was empty, except for the pool of light around the Project.

  The Dean's voice said, 'Mappin Winterley ... that's a nice name?'

  'Shutup.'

  'Owen Houseworthy?'

  'Shutup.'

  'William.'

  'Shut up, Dean. That's not funny. It never was funny.' This was the voice of the Archchancellor.

  'Just as you say, Gertrude.'

  Ponder advanced towards the glowing Project.

  'Ah, Ponder,' said the Senior Wrangler, stepping in front of it hurriedly. 'Good to see you looking so —'

  'You've been ... doing things, haven't you,' said Ponder, trying to see around him.

  'I'm sure everything can be mended,' said the Lecturer in Recent Runes.

  'And it's still nearly circular,' said the Dean, 'Just ask Charlie Grinder here. His name's definitely not Mustrum Ridcully, I know that.'

  'I'm warning you, Dean...’

  'What have you done?'

  Ponder looked at his globe. It was certainly warmer now, and also rather less globular. There were livid red wounds across one side, and the other hemisphere was mainly one big fiery crater. It was spinning gently, wobbling as it did so.

  'We've saved most of the bits,' said the Senior Wrangler, watching him hopefully

  'What did you do?'

  'We were only trying to be helpful,' said the Dean. 'Gertrude here suggested we make a sun, and —’

  'Dean?' said Ridcully

  'Yes, Archchancellor?'

  'I would just like to point out, Dean, that it was not a very funny joke to begin with. It was a pathetic attempt, Dean, at dragging a sad laugh out of a simple figure of speech. Only four-year-olds and people with a serious humour deficiency keep on and on about it. I just wanted to bring this out into the open, Dean, calmly and in a spirit of reconciliation, for your own good, in the hope that you may be made well. We are all here for you, although I can't imagine what you are here for.' Ridcully turned to the horrified Ponder. 'We made a sun —’

  ‘— some suns —’ muttered the Dean.

  ‘— some suns, yes, but ... well, this "falling in circles" business is very difficult, isn't it? Very hard to get the hang of.'

  'You crashed a sun into my world?' said Ponder.

  'Some suns,' said Ridcully.

  'Mine bounced off,' said the Dean.

  'And created this rather embarrassingly large hole here,' said the Archchancellor. 'And incidentally knocked a huge lump out of the place.'

  'But at least bits of my sun burned for a long time,' said the Dean.

  'Yes, but inside the world. That doesn't count.' Ridcully sighed. 'Yet your machine, Mister Stibbons, says a sun sixty miles across won't work. And that's ridiculous.'

  Ponder stared hollow-eyed at his world, wobbling around like a crippled duck.

  'There's no narrativium,' he said dully. 'It doesn't know what size a sun should be.'

  'Ook,' said the Librarian.

  'Oh dear,' said Ridcully. 'Who let him in here?'

  The Librarian was informally banned from the High Energy Magic building, owing to his inherent tendency to check on what things were by tasting them. This worked very well in the Library, where taste had become a precision reference system, but was less useful in a room occasionally containing bus bars throbbing with several thousand thaums. The ban was informal, of course, because anyone capable of pulling the dooknob right through an oak door can obviously go where he likes.

  The orangutan knuckled over to the dome and tasted it. The wizards tensed as delicate black fingers twiddled the knobs of the omniscope, bringing into focus the furnace that had exploded yesterday. It was a tiny point of light now, surrounded by coruscating streamers of glowing gas.

  The focus moved in to the glowing ember.

  'Still too big,' said Ridcully. 'Nice try, old chap.'

  The Librarian turned towards him, the light of the explosion moving across his face, and Ponder held his breath.

  It came out in a rush. 'Someone give me a light!'

  The globes on his desk rolled off and bounced on the floor as he tried to grab one. He held it as the Senior Wrangler obligingly lit a match, and waggled it this way and that. 'It'll work!' 'Jolly good!' said Ridcully. 'What will?'

&n
bsp; 'Days and nights!' said Ponder. 'Seasons, too, if we do it right! Well done, sir! I'm not sure about the wobble, but you might have got it just right!'

  'That's the kind of thing we do,' said Ridcully, beaming. 'We're the chaps for getting things right, sure enough. What things did we get right this time?'

  'The spin!'

  'That was my sun that did that,' the Dean pointed out, smugly.

  Ponder was almost dancing. And then, suddenly, he looked grave.

  'But it all depends on fooling people down there,' he said. 'And there isn't anyone down there ,.. HEX?'

  There was a mechanical rattle as HEX paid attention.

  +++Yes? +++

  'Is there any way we can get onto the world?'

  +++ Nothing Physical May Enter The Project +++

  'I want someone down there to observe things from the surface.'

  +++ That Is Possible. Virtually Possible +++

  'Virtually?'

  +++ But You Will Need A Volunteer. Someone To Fool +++

  'This is Unseen University,' said the Archchancellor 'That should present no problem.'

  A magical accident had once turned the University’s Librarian into an orangutan, a state which he enjoyed sufficiently to threaten, with simple and graphic gestures, anyone who suggested turning him back. The wizards noticed no difference now. An orangutan seemed such a natural shape for a librarian.