Darwin's Watch Read online

  [1] Jason Breckenridge, Rob Myers, Amanda Peet, and Cumrun Vafa.

  And here we see a hint of Hawking's cosmological time cops. Lisa Dyson took a careful look at just what happens when you put the gravitational waves and D-branes together. Just as the black hole is within a gnat's whisker of turning into a time machine, the components stop collecting together in the same place. Instead, they form a shell of gravitons (hypothetical particles of gravity, analogous to photons for light). The D-branes are trapped inside the shell. The gravitons can't be persuaded to come any closer, and the BMPV can't be made to spin rapidly enough to create an accessible CTC.

  The laws of physics won't let you put this kind of time machine together, unless some clever kind of scaffolding can be invented.

  Quantum mechanics adds a new spin to the whole time-travel game. For a start, it may open up a way to create a wormhole. On the very tiny length scale of the quantum world, known as the Planck length (around 10-35 metres), spacetime is thought to be a quantum foam - a perpetually changing mass of tiny wormholes. Quantum foam is a kind of time machine. Time is slopping around inside the foam like spindrift bobbing on the ocean waves. You just have to harness it. An advanced civilisation might be able to use gravitational manipulators to grab a quantum wormhole and enlarge it to macroscopic size.

  Quantum mechanics also sheds light, or possibly dark, on the paradoxes of time travel. Quantum mechanics is indeterminate - many events, such as the decay of a radioactive atom, are random. One way to make this indeterminacy mathematically respectable is the `many worlds' interpretation of Hugh Everett III. This view of the universe is very familiar to readers of SF: our world is just one of an infinite family of `parallel worlds' in which every combination of possibilities occurs. This is a dramatic way to describe quantum superposition of states, in which an electron spin can be both up and down at the same time, and (allegedly) a cat can be both alive and dead.'

  It's OK for electrons and probably nonsense for cats. See Greebo's cameo appearance in The Science of Discworld.

  In 1991 David Deutsch argued that, thanks to the many worlds interpretation, quantum mechanical time travel poses no obstacles to free will. The grandfather paradox ceases to be paradoxical, because grandad will be (or will have been) killed in a parallel world, not in the original one.

  We find this a bit of a cheat. Yes, it resolves the paradox, but only by showing that it wasn't really time travel at all. It was travel to a parallel world. Fun, but not the same. We also agree with a number of physicists, among them Roger Penrose, who accept that the `many worlds' interpretation of quantum theory is an effective mathematical description, but deny that the parallel worlds involved are in any sense real. Here's an analogy. Using a mathematical technique called Fourier analysis you can resolve any periodic sound, such as the note played by a clarinet, into a superposition of `pure' sounds that involve only one vibrational frequency. In a sense, the pure sounds form a serious of `parallel notes', which together create the real note. But you don't find anyone asserting that there must therefore exist a corresponding set of parallel clarinets, each producing one of the pure notes. The mathematical decomposition need not have a literal physical analogue.

  What about paradoxes of genuine time travel, no faffing about with parallel worlds? In the relativistic setting, which is where such questions most naturally arise, there is an interesting resolution. If you set up a situation with paradoxical possibilities, it automatically leads to consistent behaviour.

  A typical thought-experiment here is to send a billiard ball through a wormhole, so that it emerges in its own past. With care, you can send it in so that when it comes (came) out it bashes into its earlier incarnation, deflecting it so that it never enters the wormhole in the first place. This is the grandfather paradox in less violent form. The question for a physicist is: can you actually set such paradoxical states up? You have to do so before the time machine is built, then build it, and see what physical behaviour actually occurs.

  It turns out that, at least in the simplest mathematical formulation of this question, the usual physical laws select a unique, logically consistent behaviour. You can't suddenly plonk a billiard ball down inside a pre-existing system - that act involves human intervention, `free will', and its relation to the laws of physics is moot. If you leave it up to the billiard ball, it follows a path that does not introduce logical inconsistencies. It is not yet known whether similar results hold in more general circumstances, but they may well do.

  This is all very well, but it does beg the `free will' question. It's a deterministic explanation, valid for idealised physical systems like billiard balls. Now, it is possible that the human mind is actually a deterministic system (ignoring quantum effects to keep the discussion within bounds). What we like to think of as making a free choice may actually be what it feels like when a deterministic brain works its way towards the only decision that it can actually reach. Free will may be the 'quale' of decision-making - the vivid feeling we get, like the vivid sense of colour we get when we look at a red flower.' Physics does not yet explain. how these feelings arise. So it is usual to dismiss effects of free will when discussing possible temporal paradoxes.

  This sounds reasonable, but there's a catch. The whole discussion of time machines, in physics terms, is about the possibility of people constructing the various warped spacetimes that are involved. `Get a black hole, join it to a white one ...' Specifically, it is about people choosing or deciding to construct such a device. In a deterministic world, either they are bound to construct it from the beginning, in which case `construct' isn't a very appropriate word, or the thing just puts itself together, and you find out what sort of

  [1] See Ian Stewart, Jack Cohen, Figments of Reality: the origins of the curious mind (Cambridge University Press,

  universe you are in. It's just like Godel's rotating universe: either you're in it, or you're not, and you don't get to change anything. You can't bring a time machine into being unless it was already implicit in the unfolding of that universe anyway.

  The standard physics viewpoint really only makes sense in a world where people have free will and can choose to build, or not to build, as they see fit. So physics, not for the first time, has adopted inconsistent viewpoints for different aspects of the same question, and has got its philosophical knickers in a twist as a result.

  For all the clever theorising, the dreadful truth is that we do not yet have the foggiest idea how to make a practical time machine. The clumsy and energy-wasteful devices of real physics are a pale shadow of the elegant machine of Wells's Time Traveller, whose prototype was described as `a glittering metallic framework, scarcely larger than a small clock, very delicately made. There was ivory in it, and some transparent crystalline substance.'

  There's still some R&D needed.

  Probably this is a Good Thing.

  NINE

  AVOIDING MADEIRA

  THE JOINER WAS AMAZED, As he told his mates in the pub after work

  - so I was just finishing, and this feller comes down the ladder and says beggin' your pardon, sir, but I'd just like to check that bulkhead, please. Nothing wrong with it, says I, it's as sound as a bell. Then he says, right, right, of course, but I've just got to check something. He pulls this piece of paper out of his pocket and reads it careful, and says he's got to check that the new timber hasn't got a rare tropical worm that'll leave it looking like good wood but weaken it so much that the ship will take in too much water and will have to put in to Madeira for repairs, or something, possibly. I'll soon see about that, says I and whacks it with my hammer and, blow me, it cracks in half. I'd have sworn it was prime timber, too. Little worms everywhere!'

  `Funny you should say that,' said the man opposite. `One of 'em came up when I was working and asked if he could look at the copper nails I was usin'. Well, he takes out a knife, scrapes away at one, it's a bit of rubbish iron under a skin 'o copper! Had to do half a day's work again! Beats me how he knew. Tom said the
chandler swore they were all copper when he supplied 'em.'

  `Hah,' said a third man, `one came up to me and said what would I do if a giant squid pulled the ship under. I told him I'd do nothing, being as I live in Portsmouth.' He drained his mug. `Damned thorough, these inspectors.'

  `Yeah,' said the first man, reflectively. `They think of everything. ..'

  `A goose is an inconvenient bird, I've always thought,' said Mustrum Ridcully, carving it. Just a bit too much for one but not quite enough for two.' He extended a fork. `Anyone else want some? Rincewind, just get the man to send up some more oysters, will you? What do you say, gentlemen? Another six dozen? Let's push the boat out, eh? Hahah ...'

  The wizards had taken rooms at an inn, and the owner, watching the bustling staff down in his kitchen, was already thinking happily of an early retirement.

  Money had not been a problem. Hex had merely teleported some from a distant bank. The wizards had debated the moral implications of this for some time, with their mouths full, but had come down in favour of the idea. They were, after all, Doing The Right Thing.

  Only Ponder wasn't eating much. He nibbled a biscuit and updated his notes, before announcing: `We have covered everything, Archchancellor. The nails, the leaking water barrels, the defective compass, the bad meat ... there were nine reasons why the Beagle would have called in at the island of Madeira. Hex believes the giant squid may be a red herring. As for the nine ... yes, I think we have assured that they will no longer occur.'

  `Remind me why that's important, will you?' said the Dean. `And pass the wine, Mustrum.'

  `Without this intervention it's more than likely that Darwin will leave the ship at Madeira, should the Beagle call there,' said Ponder. `He will be terribly seasick on the voyage.'

  `Madeira being - ?' said the Dean.

  'One of a group of islands on the way, Dean. After that it's a long haul down to the South Atlantic, round the bottom of South America with a few stops, and straight up to the Galapagos Islands.'

  `Down, bottom, up,' muttered the Dean. `How can anyone get the hang of globular navigation?'

  `The phenomenon we call The Love Of Iron, sir,' said Ponder, smoothly. 'We only find it in rare metals that drop from the sky, but it's very common here. Iron here tries to point north.'

  Silence fell around the table.

  `North? Is that the bit at the top?' said Ridcully.

  `Conventionally, sir, yes,' said Ponder, and rather foolishly added,

  `but on a globe it doesn't really matter, of course.'

  `Ye gods,' muttered the Dean, putting his hand over his eyes. `How does the iron know which way to point?' Ridcully persisted.

  `Metal can't think.'

  `It's a bit like ... like peas turning to follow the Sun, sir,' hazarded Ponder, not sure if they actually did; perhaps it was pea farmers.

  `Yes, but peas are living things,' said Ridcully. `They ... know about the Sun, right?'

  `Peas aren't exactly renowned for their brains, Archchancellor,' said the Chair of Indefinite Studies, `hence the term pea-brained.'

  `But a pea must be a bloody genius compared to a lump of iron, yes?' said Ridcully.

  Ponder knew he had to put a stop to this. The wizards were still determined to apply common sense to Roundworld, and that would get them nowhere.

  `It's a force that can occur on globe-shaped worlds,' he said. `It's caused by the molten iron core spinning, and helps prevent life on the surface being fried by the Sun.'

  `Sounds like Deitium in disguise, doesn't it,' said Ridcully. `Planet gets this big magical umbrella so that life can survive? Shows forethought.' `It doesn't work quite like that, Archchancellor,' said Ponder. `Life evolved because conditions allowed it to do so.'

  `Ah, but if conditions hadn't been right, there wouldn't have been any life,' said Ridcully. `Therefore the whole exercise would have been pointless.'

  `Not really, sir. There wouldn't have been anyone to point out the pointlessness of it,' said Ponder. `I was about to add that some birds, like pigeons, use The Love Of iron to help them navigate long distances. They have tiny things called "magnets" in their head, says Hex. They're ... little bits of iron that know where the North Pole is

  `Ah, I know that bit,' said the Lecturer in Recent Runes. `The North and South Poles are those bits on a globe where the spindle comes out. But they're invisible, of course,' he added.

  `Um,' said Ponder.

  `Just a minute, can we get back to these birds?' said Ridcully. `Birds

  with magnet heads?'

  `Yes?' said Ponder, knowing that this was going to be loaded. `How?' said Ridcully, flourishing a goose leg. `On this globe, birds grew out of great big monstrous lizard beasts, isn't that so?'

  `Er ... small great monstrous beasts, sir,' said Ponder, wishing not for the first time that his Archchancellor did not have a knack for remembering inconvenient details.

  `Did they have to fly long distances through fog and bad weather?' said the Archchancellor.

  `I doubt it, sir,' said Ponder.

  'So did they already have these magnets in their heads from day

  one, or did they turn up by some godly hand? What does Mr Darwin

  of The Origin say about that?

  'Not very much, sir,' said Ponder. It had been a long day.

  `But it suggests, does it not, that The Ology haha, is right and The

  Origin is wrong. Perhaps the magnets were added when needed?' `Could be, sir,' said Ponder. Just don't let him start on the eyeball, he thought.

  `I've got a question,' said Rincewind, from the end of the table. `Yes?' said Ponder, quickly.

  `There's going to be monster creatures on these islands we're heading for, yes?'

  `How did you know that?' said Ponder.

  `It just came to me,' said Rincewind gloomily. `So there are monsters?' `Oh, yes. Giants of their kind.'

  `With big teeth?'

  `No, not really. They're tortoises.'

  `How big?'

  `About the size of an easy chair, I think.'

  Rincewind looked suspicious.

  `How fast?'

  `I don't know. Not very fast.'

  `And that's it?

  'From a Darwinian perspective, the islands are famous for their many species of finches.'

  `Any of them carnivorous?'

  `They eat seeds.'

  `So ... there's nothing dangerous where we're going?'

  `No. Anyway, we don't have to go there. All we have to do now is find the point where he decides to write The Ology instead of The Origin.'

  Rincewind pulled the dish of potatoes towards him. 'Sez you,' he said.

  +++ I need to communicate grave news +++

  The words came out of the air. In Roundworld, Hex had a voice. `We're having a bit of a celebration here,' said Ridcully. `I'm sure

  your news can wait, Mr Hex!'

  +++ Yes. It can +++

  `Good. In that case, Dean would you pass me-'

  +++ I would not wish to spoil your appetite +++ Hex went on. `Glad to hear it.'

  +++ The destruction of the human race can wait until after the pudding +++.

  Ridcully's fork hovered between his plate and his mouth. Then he said: `Would you care to explain this, please, Mr Stibbons?'

  `I can't, sir. What is happening, Hex? We completed all those tasks properly, didn't we?'

  +++ Yes. But, pause for significance, have you heard of a mythical creature called a, pause again, hydra? +++

  `The monster with many heads?' said Ponder. `You don't need to tell us when you pause, by the way.'

  +++ Thank you. Yes. Cut off one head and a dozen grow in its place. This history is a hydra +++

  Rincewind nodded at Ponder. `Told you,' he said, with his mouth full.

  +++ I am unable to explain why this is the case, but there are now 1457 reasons why Darwin did not write The Origin of Species. The book has never been written in this history. The voyage has never taken place +++

  `Don't be s
illy! We know it did!' said the Dean.

  +++ Yes. It did. But now, it hasn't. Charles Darwin the scientist has been removed from this history while you ate. He was, and now he was not. He became a little-remembered priest who caught butterflies. He wrote no book. The human race dies in five hundred years +++

  `But yesterday-' Ridcully began.

  +++ Consider time not as a continuous process but as a succession of discrete events. Darwin's scientific career has been excised. You remember him, but that is because you are not part of this universe. To deny this is simply to scream at the monkeys in the next tree +++

  `Who did it?' said Rincewind.

  `What sort of question is that?' said Ponder. `No one did it. There isn't anyone to do things. This is some kind of strange phenomenon.'

  +++ No. The act shows intelligence +++ said Hex. +++ Remember, I detected malignity. I surmise that your interference in this history has led to some counter-measure +++

  `Elves again?' said Ridcully.

  +++ No. They are not clever enough. I can detect nothing except natural forces +++

  `Natural forces aren't animate,' said Ponder. `They can't think!' +++ pause for dramatic effect ... Perhaps the ones here have learned to +++ said Hex.

  TEN

  WATCH-22

  IN THE STANDARD VERSION OF Roundworld history,

  Charles Darwin's presence on the Beagle came about only because of a highly improbable series of coincidences - so improbable that it is tempting to view them as wizardly intervention. What Darwin expected to become was not a globetrotting naturalist who revolutionised humanity's view of living creatures, but a country vicar.

  And it was all Paley's fault.

  Natural Theologys seductive and beautifully argued line of reasoning found considerable favour with the devout people of Georgian (III and IV) England, and after them, the equally devout subjects of William IV and Victoria. By the time Victoria ascended to the throne, in 1837, it was indeed almost compulsory for country vicars to become experts in some local moth, or bird, or flower, and the Church actively encouraged such activities because they were continuing revelations of the glory of God. The Suffolk rector William Kirby was co-author, with the businessman William Spence, of a lavish four-volume treatise An Introduction to Entomology, for example. It was fine for a clergyman to interest himself in beetles. Or geology, a relatively new branch of science that had grabbed the young Charles Darwin's attention.