Science of Discworld III

encounter with a giant hippopotamus.
    These are purely relativistic time machines, but the universe has quantum features too, and these should be taken into account. The search for a unification of relativity and quantum theory – respectably known as ‘quantum gravity’ and often derided as a Theory of Everything – has turned up a beautiful mathematical proposal, string theory. In this theory, instead of fundamental particles being points, they are vibrating multidimensional loops. The best-known version uses six-dimensional loops, so its model of spacetime is really ten-dimensional. Why has no one noticed? Perhaps because the extra six dimensions are curled up so tightly that no one has observed them – very possibly, can observe them. Or perhaps – the Irishman again – we can’t go that way from here.
    Many physicists hope that string theory, as well as unifying relativity and quantum mechanics, will also supply a proof of Hawking’s chronology protection conjecture – that the universe conspires to keep events happening in the same temporal order. In this connection, there is a five-dimensional string-theoretic rotating black hole called a BMPV 3 black hole. If this rotates fast enough, it has CTCs outside the black hole region. Theoretically, you can build one from gravitational waves and esoteric string-theoretic gadgets called ‘D-branes’.
    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. 4
    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