The Science of Discworld Revised Edition

by Terry Pratchett

  For a helpful analogy, contemplate our own Earth. Like a pulsar, it spins on an axis. Like a pulsar, it has a magnetic field. The magnetic field has an axis too, but it’s different from the axis of rotation – that’s why magnetic north is not the same as true north. There’s no good reason for magnetic north to be the same as true north on a pulsar, either. And if it isn’t, that magnetic axis whips round thirty times every second. A rapidly spinning magnetic field emits radiation, known as synchrotron radiation – and it emits it in two narrow beams which point along the magnetic axis. In short, a neutron star projects twin radio beams like the spinning gadgetry on top of a terrestrial lighthouse. So if you look at a neutron star in radio light, you see a bright flash as the beam points towards you, and then virtually nothing until the beam comes round again. Every second, you see thirty flashes. That’s what Bell had noticed.

  If you’re a living creature of remotely orthodox construction, you definitely do not want your star to be a pulsar. Synchrotron radiation is spread over a wide range of wavelengths, from visible light to x-rays, and x-rays can seriously damage the health of any creature of remotely orthodox construction. But no astronomer ever seriously suspected that pulsars might have planets, anyway. If a big star collapses down to an incredibly dense neutron star, surely it will gobble up all the odd bits of matter hanging around nearby. Won’t it?

  Perhaps not. In 1991 Matthew Bailes announced that he had detected a planet circling the pulsar PSR 1829-10, with the same mass as Uranus, and lying at a distance similar to that of Venus from the Sun. The known pulsars are much too far away for us to see planets directly – indeed all stars, even the nearest ones, are too far away for us to see planets directly. However, you can spot a star that has planets by watching it wiggle as it walks. Stars don’t sit motionless in space – they generally seem to be heading somewhere, presumably as the result of the gravitational attraction of the rest of the universe, which is lumpy enough to pull different stars in different directions. Most stars move, near enough, in straight lines. A star with planets, though, is like someone with a dancing partner. As the planets whirl round the star, the star wobbles from side to side. That makes its path across the sky slightly wiggly. Now, if a big fat dancer whirls a tiny feather of a partner around, the fat one hardly moves at all, but if the two partners have equal weight, they both revolve round a common centre. By observing the shape of the wiggles, you can estimate how massive any encircling planets are, and how close to the star their orbits are.

  This technique first earned its keep with the discovery of double stars, where the dancing partner is a second star, and the wobbles are fairly pronounced because stars are far more massive than planets. As instrumentation has become more accurate, ever tinier wobbles can be detected, hence ever tinier dancing partners. Bailes announced that pulsar PSR 1829-10 had a dancing partner whose mass was that of a planet. He couldn’t observe the wiggles directly, but he could observe the slight changes they produced in the timing of the pulses in the signal. The only puzzling feature was the rotational period of the planet: exactly six Earth months. Bit of a coincidence. It quickly turned out that the supposed wiggles were not caused by a planet going round the pulsar, but by a planet much closer to home – Earth. The instruments were doing the wiggling at this end, not the pulsar at the far end.

  Scarcely had this startling claim of a pulsar planet been withdrawn, however, when Aleksander Wolszczan and Dale Frail announced the discovery of two more planets, both circling pulsar PSR 1257+12. A pulsar solar system with at least two worlds! The way you wiggle when you have two dancing partners is more complex than the way you do it with one, and it’s difficult to mistake such a signal for something generated at the receiving end by the motion of the Earth. So this second discovery seems to be fairly solid, unless there is a way for pulsars to vary their output signals in just such a complex manner without having planets – maybe the radio beam could be a bit wobbly? We can’t go there to find out, so we have to do the best we can from here; and from here it looks good.

  So there do exist planets outside our solar system. But it’s the possibility of life that really makes distant planets interesting, and a pulsar planet with all those x-rays is definitely not a place for anything that wants to be alive for very long. But now conventional stars are turning out to have planets, too. In October 1995 Michel Mayor and Didier Queloz found wobbles in the motion of the star 51 Pegasi that were consistent with a planet of about half Jupiter’s mass. Their observations were confirmed by Geoffrey Marcy and Paul Butler, who found evidence for two more planets – one seven times the mass of Jupiter orbiting 70 Virginis, and one two or three times Jupiter’s mass orbiting 47 Ursae Majoris.

  By 1996 seven such planets had been found. As we write, about 70 extrasolar planets have been detected, either by the wobble method, or by observing the light output from a star and seeing whether it changes as an orbiting planet reflects different amounts of its light. Theoretical calculations show that with improved telescopes, this method might even be able to detect how fast the planet is rotating. Even now, new extrasolar planets are being found virtually every week. The exact number fluctuates because every so often astronomers discover problems with previous measurements that cast doubt on somebody else’s favourite new planet, but the general trend is up. And our nearest sunlike neighbour, epsilon Eridani, is now known to possess an encircling dustcloud, perhaps like our Sun’s Oort cloud, thanks to observations made in 1998 by James Greaves and colleagues. We can’t see any wobbles, though, so if it has planets, their mass must be less than three times that of Jupiter. A year earlier, David Trilling and Robert Brown used observations of a similar dustcloud round 55 Cancri, which does wobble, to show that it has a planet whose mass is at most 1.9 Jupiters. This definitely rules out alternative explanations of the unseen companion, for example that it might be a ‘brown dwarf’ – a failed star.

  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). A successor, Star Light, will follow in 2005. 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 all 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 finding, 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 already have 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.1