Science of Discworld III

End of Eternity .
    An entire class of paradoxes arises from time loops, closed loops of causality in which events only get started because someone comes from the future to initiate them. For example, the easiest way for today’s humanity to get hold of a time machine is if someone is presented with one by a time traveller from the far future, when suchmachines have already been invented. He or she then reverse engineers the machine to find out how it works, and these principles later form the basis for the future invention of the machine. Two classic stories of this type are Robert Heinlein’s ‘By His Bootstraps’ and ‘All you Zombies’, the second being noteworthy for a protagonist who becomes his own father and his own mother (via a sex change). David Gerrold took this idea to extremes in The Man Who Folded Himself .
    Science-fiction authors are divided on whether time paradoxes always neatly unwrap themselves to produce consistent results, or whether it is genuinely possible, in their fictional setting, to change the past or the present. (No one worries much about changing the future, mind you, presumably because ‘free will’ amounts to precisely that. We all change the future, from what it might have been to what it actually becomes, thousands of times every day. Or so we fondly imagine.) So some authors write of attempts to kill your grandfather that, by some neat twist, bring you into existence anyway. For example, your true father was not his son at all, but a man he killed. By mistakenly eliminating the wrong grandfather, you ensure that your true father survives to sire you . Others, like Asimov and Silverberg, set up entire organisations dedicated to making sure that the past, hence the present, remains intact. Which may or may not work.
    The paradoxes associated with time travel are part of the subject’s fascination, but they do rather point towards the conclusion that time travel is a logical impossibility, let alone a physical one. So we are happy to allow the wizards of Unseen University, whose world runs on magic, the facility to wander at will up and down the Roundworld timeline, switching history from one parallel universe to another, trying to get Charles Darwin – or somebody – to write That Book. The wizards live in Discworld, they operate outside Roundworld constraints. But we don’t really imagine that Roundworld people could do the same, without external assistance, using only Roundworld science.
    Strangely, many scientists at the frontiers of today’s physics don’t agree. To them, time travel has become an entirely respectable 2 research topic, paradoxes notwithstanding. It seems that there is nothing in the ‘laws’ of physics, as we currently understand them, that forbids time travel. The paradoxes are apparent rather than real; they can be ‘resolved’ without violating physical law, as we will see in Chapter 8 . That may be a flaw in today’s physics, as Stephen Hawking maintains; his ‘chronology protection conjecture’ states that as yet unknown physical laws conspire to shut down any time machine just before it gets assembled – a built-in cosmological time cop.
    On the other hand, the possibility of time travel may be a profound statement about the universe. We probably won’t know for sure until we get to tackle the issue using tomorrow’s physics. And it’s worth remarking that we don’t really understand time , let alone how to travel through it.
    Although (apparently) the laws of physics do not forbid time travel, it turns out that they do make it very difficult. One theoretical scheme for achieving that goal, which involves towing black holes around very fast, requires rather more energy than is contained in the entire universe. This is a bit of a bummer, and it does seem to rule out the typical science fiction time machine, about the size of a car. 3
    The most extensive descriptions of Discworld time are found in Thief of Time . The ingredients for this novel include a member of the Guild of Clockmakers, Jeremy Clockson, who is determined to make a completely accurate clock. However, he is up against a theoretical barrier, the paradoxes of the Ephebian philosopher Xeno, which are firstmentioned in Pyramids . A Roundworld philosopher with an oddly similar name, Zeno of Elea, born around 490 BC, stated four paradoxes about the relation between space, time and motion. He is Xeno’s Roundworld counterpart, and his paradoxes bear a curious resemblance