Science of Discworld III

‘Time’, states The New Discworld Companion , ‘is one of the Discworld’s most secretive anthropomorphic personifications. It is hazarded that time is female (she waits for no man) but she has never been seen in the mundane worlds, having always gone somewhere else just a moment before. In her chronophonic castle, made up of endless glass rooms, she does at, er, times, materialise into a tall woman with dark hair, wearing a long red-and-black dress.’
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    Even Discworld has trouble with time. In Roundworld it’s worse. There was a time (there we go) when space and time were considered to be totally different things. Space had, or was, extension – it sort of spread itself around, and you could move through it at will. Within reason, maybe 20 miles (30km) a day on a good horse if the tracks weren’t too muddy and the highwaymen weren’t too obtrusive.
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    Time, in contrast, moved of its own volition and took you along with it. Time just passed , at a fixed speed of one hour per hour, always in the direction of the future. The past had already happened, the present was happening right now – oops, gone already – and the future had yet to happen, but by jingo, it would, you mark my words, when it was good and ready.
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    You could choose where you went in space, but you couldn’t choose when you went in time. You couldn’t visit the past to find out what had really happened, or visit the future to find out what fate had in store for you; you just had to wait and find out. So time was completely different from space. Space was three-dimensional, with three independent directions: left/right, back/forward, up/down. Time just was .
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    Then along came Einstein, and time started to get mixed up with space. Time-like directions were still different from space-like ones, in some ways, but you could mix them up a bit. You could borrow time here and pay it back somewhere else. Even so, you couldn’t head off into the future and find yourself back in your own past. That would be time travel, which played no part in physics.
    Ti —
    What science abhors, the arts crave. Time travel may be a physical impossibility, but it is a wonderful narrative device for writers, because it allows the story to move to past, present, or future, at will. Of course you don’t need a time machine to do that – the flashback is a standard literary device. But it’s fun (and respectful to narrativium) to have some kind of rationale that fits into the story itself. Victorian writers liked to use dreams; a good example is Charles Dickens’s A Christmas Carol of 1843, with its ghosts of Christmas past, present, and yet-to-come. There is even a literary subgenre of ‘timeslip romances’, some of them really quite steamy. The French ones.
    Time travel causes problems if you treat it as more than just a literary device. When allied to free will, it leads to paradoxes. The ultimate cliché here is the ‘grandfather paradox’, which goes back to René Barjavel’s story Le Voyageur Imprudent . You go back in time and kill your grandfather, but because your father is then not born, neither are you, so you can’t go back to kill him … Quite why it’s always your grandfather isn’t clear (except as a sign that it’s a cliché, a low-bred form of narrativium). Killing your father or mother would have the same paradoxical consequences. And so might the slaughter of a Cretaceous butterfly, as in Ray Bradbury’s 1952 short story ‘A Sound of Thunder’, in which a butterfly’s accidental demise at the hands 1 of an unwitting time traveller changes present-day politics for the worse.
    Another celebrated time paradox is the cumulative audience paradox. Certain events, the standard one being the Crucifixion, are so endowed with narrativium that any self-respecting time tourist will insist on seeing them. The inevitable consequence is that anyone who visits the Crucifixion will find Christ surrounded by thousands, if not millions, of time travellers. A third is the perpetual investment paradox. Put your money in a bank account in 1955, take it out in 2005, with accumulated interest, then take it back to 1955 and put it in again … Be careful to use something like gold, not notes – notes from 2005 won’t be valid in 1955. Robert Silverberg’s Up the Line is about the Time Service, a force of time police whose job is to prevent such paradoxes from getting out of hand. A similar theme occurs in Isaac Asimov’s The