The Science of Discworld II - The Globe tsod-2

by Terry Pratchett

  This point is well understood on Discworld. In Lords and Ladies we find the following passage: There are indeed such things as parallel universes, although parallel is hardly the right word universes swoop and spiral around one another like some mad weaving machine or a squadron of Yossarians[72] with middle-ear trouble.

  And they branch. But, and this is important, not all the time. The universe doesn't much care if you tread on a butterfly. There are plenty more butterflies. Gods might note the fall of a sparrow but they don't make any effort to catch them.

  Shoot the dictator and prevent the war? But the dictator is merely the tip of the whole festering boil of social pus from which dictators emerge; shoot one and there'll be another one along in a minute. Shoot him too? Why not shoot everyone and invade Poland? In fifty years', thirty years', ten years' time the world will be very nearly back on its old course. History always has a great weight of inertia.

  Almost always ...

  At circle time, when the walls between this and that are thinner, when there are all sorts of strange leakages ... Ah, then choices are made, then the universe can be sent careening down a different leg of the well-known Trousers of Time.

  This kind of question can be asked of any dynamical system, emergent or not; but it takes on a special aspect when the dynamic 'makes itself up as it goes along'. In a rerun, would it make up the same thing? Would it tell the same story? If so, that story is robust; it has a degree of inevitability, not just in some particular run of history, but in all of them.

  Science fiction writers explore historical phase space in 'alternate[73] universe' stories, where one historical event is changed and the author develops possible consequences. Philip K. Dick's The Man in the High Castle explores a history in which Germany won World War II. Harry Harrison's West of Eden trilogy explores a world in which the K/T meteorite missed and the dinosaurs survived. Science writers also ask about historical phase space, especially in the context of evolution. The most celebrated example is Stephen Jay Gould's Wonderful Life, which asks whether humans would arise again on Earth if evolution were to be run again. His answer,

  'no', rests on a very literal interpretation of 'human'. Harrison's answer in West of Eden is that intelligent mosasaurs -contemporaries of the dinosaurs that had returned to the sea -would evolve, and play the same role on the evolutionary stage that humans have played in this world.

  (For plot reasons he also has genuine humans in his alternate universe, but the Yilane, the smart mosasaur descendants, were there first.)

  Where Gould sees divergence and massive changes brought about by chance events, Harrison sees convergence: same play, different actors. To Gould, a change of actor is significant; to Harrison, what matters is the play. Both have good arguments to present, but the main point is that they are tackling different questions.

  A second way in which science fiction writers explore alternative historical tracks is through the time travel story, and this brings us back to the wizards of Unseen University and their battle against the elves. There are two kinds of time travel story. In the first kind, the protagonists mainly use their ability to travel in time as a way of observing the past or future; a good example is the first significant time travel novel, H.G Wells's The Time Machine of 1895. The time machine is a vehicle for Wells to discuss the future of humanity, but his Time Traveller makes no real effort to change history. In contrast, the narrative theme of Robert Silverberg's 1969 novel Up the Line is the paradoxes that arise if it is possible to travel into the past and change it.

  In this story, the Time Service does not set out to change the past; on the contrary, its prime objective is to preserve the past and avoid paradoxes, despite the activities of observers from the future, who are cataloguing the past by visiting it and seeing what actually happened.

  The classic time travel paradox is 'what if I went back and killed my grandfather?' The logic of the situation, of course, is that with granddad dead, you wouldn't have been born, so you wouldn't be able to go back and kill him, so he'd have lived, so you would have been born ... All attempts to resolve this self-contradictory causal loop are cheats: perhaps granddad dies, but you get born anyway with different grandparents, but then it wasn't really granddad that you killed. In the 'many worlds' interpretation of quantum mechanics, the causal logic of the universe holds together provided the grandfather that gets killed was in a different parallel universe from that of the killer. But then he wasn't your real granddad, either, just a parallel version in some other universe.

  A slightly more subtle time paradox is the Cumulative Audience Paradox. If people in the future have access to time machines, then they are bound to want to go back and witness all of the great historical events, like the crucifixion. But we know, from existing descriptions of these events, that they did not happen in front of crowds of thousands of visitors from the future. So where were they? This is a temporal analogue of the Fermi Paradox[74] about intelligent aliens: if they're all over the galaxy, then why aren't they here? Why haven't they visited us? Other time paradoxes are used as essential plot elements in Robert A. Heinlein's short stories 'By his bootstraps' and 'All you zombies', fn the latter, a time-traveller manages to be his own father, son, and - via a sex change - mother. When asked where he comes from, he replies that he knows exactly where he comes from. The big puzzle is: where does everybody else come from? This idea is taken to serious extremes by David Gerrold in The Man Who Folded Himself.

  Over the last few decades, serious physicists have started thinking about the possibility of time travel and the resolution of any associated paradoxes. Their work is a tribute to narrative imperative on Roundworld. The reason they are asking such questions is no doubt that as children they read stories like those of Wells, Silverberg, Heinlein and Gerrold. When they became professional physicists, the stories bubbled up from their subconscious, and they began to take the idea seriously - not as a practical engineering issue, but as a theoretical challenge.

  Do the laws of physics permit time travel, or not? You'd expect the answer to be 'no', but the remarkable consequence of the theorists' research is that it is 'yes'. A working time machine is still a long way off, and it may be that we're missing some basic physical principle that would change the answer to 'no', but the fact is that today's accepted frontier physics does not forbid time travel. It even offers a few scenarios in which it could occur.

  The context for such research is general relativity, in which the continuum of space and time can be distorted by gravity. Or, more accurately, in which gravity is caused by such distortions,

  'curved spacetime'. In place of a time machine, the physicists look for a 'closed timelike curve'.

  Such a curve corresponds to an object that travels into the future and ends up in its own past, and so becomes trapped in a closed 'time loop'.

  The best known way to generate a closed timelike curve is to use a wormhole. A wormhole is a short-cut through space, obtained by fusing a Black Hole to its time-reversal, a White Hole. Just as Black Holes suck in anything that comes near them, White Holes spit things out. A wormhole sucks things in at its black end and spits them out at its white end. Of itself, a wormhole is more a matter-transmitter than a time machine, but it becomes a time machine when allied to the famous Twin Paradox. In relativity, time slows down for objects moving at very high speeds. So if one member of a pair of twins heads out to a distant star at very high speed, and then returns, she will have aged less than the other twin who stayed at home. Suppose that the travelling twin takes with her the white end of a wormhole, while her sister keeps the black end. Then when the travelling twin returns, the white end is younger than the black end: the exit from the wormhole lies in the past of the entrance. So anything that is sucked into the black end is spat out in its own past. Because the white end is now right next to the black one -the twin has come back home the object can hop across to the Black Hole and go round and round this closed loop in spacetime, tracing a closed timelike curve.
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br />   There are practical problems in making such a gadget, the main one(!) being that the wormhole will collapse too quickly for an object to pass through it, unless it is held open by threading

  'exotic matter' with negative energy through it. Nonetheless, none of this is forbidden by the current laws of physics. So what of the paradoxes? It turns out that the laws of physics forbid genuine paradoxes, although they permit many apparent paradoxes. A useful technique for understanding the difference is known as a Feynman diagram, which is a picture of the motion of an object (usually a particle) in space and time.

  For example, here is an apparent time travel paradox. A man is imprisoned in a concrete cell, locked from the outside, with no food, no water and no possibility of escape. As he sits in a corner in despair, waiting for death, the door opens. The person who has opened it is ... himself.

  He has returned in a time machine from the future. But how (the paradox) did he get to the future in the first place? Well, a kind person opened the door and set him free ...

  There seems to be something very odd about the causality in the story, but the corresponding Feynman diagram shows that it violates none of the laws of physics. First, the man follows a space-time path that puts him inside the cell and then removes him from it through opened door.

  This time-line continues into his future until he encounters a time machine. Then the time-line reverses direction, heading into the past, until he encounters a locked cell. He opens it, and his time-line reverses again, propelling him into his own future. So the man follows a single zig-zag path through time, and at every step the laws of physics hold good. Provided his time machine violates no physical law, of course.

  If you try to 'explain' the grandfather paradox by this method, it doesn't work. The time-line leading from grandfather to killer is severed when the killer returns; there is no consistent scenario, even in a Feynman diagram. So some stories of time travel are consistent with the laws of physics, and have their own kind of causal logic, albeit twisted; but other equally plausible stories are inconsistent with the laws of physics. You can rescue the Grandfather Paradox by assuming that changing the past in a logically inconsistent way switches you into a different alternate universe -say a quantum-mechanical parallel world. But then it wasn't your grandfather that you killed, but the grandfather of an alternate you. So this 'resolution' of the Grandfather Paradox is a cheat.

  Faced with all this, the way that the wizards handle the complications of time travel seems quite reasonable!

  29. ALL THE GLOBE'S A THEATRE

  The elves did not spend a lot of time in serious thought. They could control people who could do the thinking for them. They didn't play music, they did not paint, they never carved stone or wood. Control was the talent, and it was the only one they had ever needed.

  Nevertheless, there were ones who had survived for many thousands of years, and while they had no great intelligence they had accumulated that mass of observations, experience, cynicism and memory that can pass for wisdom among people who don't know any better. One of the wisest things they did was not read.

  They had found some clerks to read the play.

  They listened.

  Then, when it was over, the Queen said: 'And the wizards have been showing great interest in this man?'

  'Yes, your majesty,' said one of the old ones.

  The Queen frowned. 'This ... play is ... good. It treats us ... kindly. We are firm but fair with mortals. We offer rewards to those who deal well with us. Our beauty is satisfactorily referred to.

  Our ... issues with our husband are treated more romantically than I would like, but, nev ertheless ... it is positive, it enhances us, it places us yet more firmly in the human world. One of the wizards was actually carrying this.'

  One of the senior elves cleared its throat. 'Our grip is loosening, your majesty. Humanity is becoming more, shall we say, questioning?'

  The Queen shot it a glance. But it was older than many Queens, and did not step back.

  'You think it will do us harm? Is it a plot against us?'

  The senior elves looked at one another. The main reason that they thought it was a plot was that they were predisposed to see plots. In the court of Faerie, an inability to see it coming meant that it took you by the throat.

  'We think it may be,' one said at last.

  'How? In what way?'

  'We know the wizards have been seen in the company of the author,' said the elf.

  'Then perhaps they are endeavouring to stop him writing the play, have you thought of that?'

  snapped the Queen. 'Can you see any way in which those words harm us?'

  'We are agreed that we cannot ... nevertheless, we have a sense that in some way—'

  'It is so simple! At last we are done some real honour and the wizards will try to stop it! Are you so stupid that you cannot see that?'

  Her long dress swirled as she turned on her heel. 'It will happen,' she said. 'I will see to it!'

  The senior elves filed out, not looking at her face. They knew those moods.

  On the stairs one said to the others: 'Purely out of interest ... can any of us put a girdle around the Earth in three minutes?'

  "That would be a very big girdle,' said an elf.

  'And would you wish to be called Peaseblossom?'

  The eyes of the old elf were grey, flecked with silver. They had seen horrible things under many suns, and in most cases had enjoyed them. Humans were a valuable crop, the elf conceded. There had never been a species like it for depth of awe, terror and superstition. No other species could create such monsters in its heads. But sometimes, it considered, they were not worth the effort.

  'I think not,' it said.

  'Well, now, Will - do you mind if I call you Will? Oh, Dean, fetch Will another pint of this really unpleasant ale, will you? Now ... where was I ... oh yes, I really enjoyed that play of yours.

  Magnificent, I thought!' Ridcully beamed. Around him, the inn hummed with life.

  Will tried to focus. 'Which one was that, good sir?' he said.

  Ridcully's smile remained fixed, but began to unravel around the edges. He was never one for unnecessary reading.

  'The one with the king in it,' he said, aiming for safety.

  On the other side of the table Rincewind did some desperate pantomime.

  'The rabbit,' said Ridcully. 'The rat. The ferret. Sounds like ... hat. Rat. Rodent. Thing with teeth.'

  Rincewind gave up, leaned across and whispered.

  'Something about the shrew,' said Ridcully. Rincewind whispered a little harder.

  'The one about the tame shrew. The man married a shrew. A shrewish woman. Not a real shrew, obviously, haha. No one would marry a real shrew. It would be a completely foolish idea.'

  Will blinked. He was not, as an actor and a writer, averse to alcohol bought by other people, and these people were being very good hosts. It was just that they seemed to be completely deranged.

  'Er ... I thank you,' he said. He was aware of being stared at, and also of a strange but not unpleasant animal smell. He turned on the bench and was rewarded with a grin. It occupied all the space between a deep hood and a jerkin. There were a couple of brown eyes, too, but it was the grin his gaze kept coming back to.

  The Librarian raised his tankard and gave Will a friendly nod. This caused the grin to get bigger.

  'Now I'm sure you hear this all the time,' said Ridcully, slapping Will so hard on the back that his drink slopped, 'but we've got an idea for you. Dean, more ale all round, eh? It really is very weak stuff. Yes, an idea.' He poked Will in the chest. 'Too many kings, that's the trouble. What the public wants now, what puts bums on seats—'

  'Feet,' said Rincewind.

  'What?'

  'Bums on feet, Archchancellor. It's mostly standing room in the theatre.'

  'Feet, then. Bums, anyway. Thank you, Dean. Cheers.' Ridcully wiped his mouth delicately and turned his attention again to Will, who tried to avoid the prodding finger.


  'Bums on, haha, feets,' he said, and blinked. 'Funny thing, funny thing, something similar happened to us, 'smatterofact, few years ago, Midsummer's Eve, these chaps were going to put on a play thingy for the king, next thing, elves all over the place, haha. Why, yes, Runes, I'll have another if you're paying, it's far too sweet to be a serious drink. Where was I? Ah. Elves. What you've got to do, what you've gotta do ... is ... why aren't you writing this down?'

  In the morning Rincewind opened his eyes at the fourth attempt and with the assistance of both hands. There was a moment of brain lag, where the little wheels spun happily with no work to do, and then big horrible machinery cut in.

  'Whg d'hl der ...' he said, and then got control of his mouth as well.

  Bits of last night crept out of hiding to do their treacherous dance before his eyes. He groaned.

  'We couldn't have done that, could we?' he muttered.

  And memory said: that was only the start ...

  Rincewind sat up and waited until the world stopped moving.

  He'd been on the floor in the library. The other wizards lay scattered around the room or sprawled across piles of books. The air smelled of beer.

  A veil will be drawn over the following half an hour, and lifted to find the wizards sitting around the table.

  'It must've been the pork scratchings,' said the Dean.

  T don't remember any pork scratchings,' muttered Ponder.

  'Something crunchy, anyway. They may have been moving about.'

  'There's no doubt in my mind that it was caused by all this travelling we've been doing,' said Ridcully. 'That sort of thing must take a terrible toll on the system. We've been concentrating so hard, d'yer see, that the moment we relaxed the strain we just unwound, like a big spring.'

  The wizards brightened up. Rascally drunkenness was too much of an embarrassment to men who could sit through an entire meal at the UU high table, but time sickness ... yes, that had a certain cachet. They could live with time sickness although, at the moment, they were wishing they didn't have to.