The Science of Discworld II

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 descriptionsof 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 6 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’. In 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 aclosed ‘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.
    There are practical problems in making such a gadget, the main one(!) being that the wormhole will