Science of Discworld III

now. If Mr Souser had eaten that fish, he would have been too ill to travel tomorrow.’
    ‘Well, that’s good news for Mr Souser, but what’s it got to do with us?’ said the Dean.
    ‘Mr Wedgwood is Charles Darwin’s uncle,’ said Ponder, as the air wavered. ‘He will have an influence on his nephew’s career. And now for our next call …’
    ‘Good morning! Mrs Nightingale?’
    ‘Yes?’ said the woman, as if she was now doubting it. She took in the group of people in front of her, her eye resting on the very bearded one whose knuckles touched the ground. Beside her, the housemaid who’d opened the door looked on nervously.
    ‘My name is Mr Stibbons, Mrs Nightingale. I am the secretary of The Mission to Deep Sea Voyagers, a charitable organisation. I believe Mr Nightingale is shortly to embark on a perilous mission to the storm-tossed, current-mazed, ship-eating giant-squid infested waters of the South Americas?’
    The woman’s gaze tore itself away from the Librarian and her eyes narrowed.
    ‘He never said anything to me about giant squid,’ she said.
    ‘Indeed? I’m very sorry to hear that, Mrs Nightingale. Brother Bookmeister here,’ Ponder patted the Librarian on the shoulder, ‘would tell you about them himself were it not that the dire experience quite robbed him of the power of speech.’
    ‘Ook!’ said Brother Bookmeister plaintively.
    ‘Really?’ said the woman, setting her jaw firmly. ‘Would you gentlemen care to step into the parlour?’
    ‘Well, the biscuits were nice,’ said the Dean, as the wizards strolled out into the street half an hour later. ‘And now, Stibbons, would you care to tell us what all that was about?’
    ‘Gladly, Dean, and may I say your story about the sea snake was very useful?’ said Ponder. ‘But Rincewind, that tale about the killer flying fish was rather over the top, I thought.’
    ‘I didn’t make it up!’ Rincewind said. ‘They had teeth on them like—’
    ‘Well, anyway … Darwin was the second choice for the post on the Beagle ,’ said Ponder. ‘Mr Nightingale was the captain’s initial choice. History will record that after his wife’s pleading he declined the offer. This he will do within about five minutes of when he gets home tonight.’
    ‘Another fine ruse?’ said Ridcully.
    ‘I’m rather pleased with it, as a matter of fact,’ said Ponder.
    ‘Hmm,’ said Ridcully. Cunning in younger wizards is not automatically applauded in their elders. ‘Very clever, Stibbons. You are a wizard to watch.’
    ‘Thank you, sir. My next question is: does anyone here know anything about shipbuilding? Well, perhaps that won’t be necessary. Hex, take us to Portsmouth, please. The Beagle is being refitted. You will need to be naval inspectors which, ahaha, I’m sure you’ll be good at. In fact you will be the most observant inspectors there have ever been. Location 3, please, Hex.’
    1 Yes, they did – in The Science of Discworld II .
    2 A rare meteorological phenomenon discussed briefly in The Science of Discworld II .

EIGHT
FORWARD TO THE PAST
    W ELL, THE WIZARDS HAVE MADE a good start. And with the might of Hex behind them, the wizards can travel at will along the Roundworld timeline. We’re happy for them to do that, in a fictional context – but could we do the same thing, in a factual one?
    To answer that, we must decide what a time machine looks like within the framework of general relativity. Then we can talk about building one.
    Travel into the future is easy: wait. It’s getting back that’s hard. A time machine lets a particle or object return to its own past, so its world-line, a timelike curve, must close into a loop. So a time machine is just a closed timelike curve , abbreviated to CTC. Instead of asking, ‘Is time travel possible?’ we ask, ‘Can CTCs exist?’
    In flat Minkowski spacetime, they can’t. Forward and backward light cones – the future and past of an event – never intersect (except at the point itself, which we discount). If you head off across a flat plane, never deviating more than 45° from due north, you can never sneak up on yourself from the south.
    But forward and backward light cones can intersect in other types of spacetime. The first person to notice this was Kurt Gödel, better known for his fundamental work in mathematical logic. In 1949 he worked out the relativistic mathematics of a rotating universe, anddiscovered that the past and future of every point intersect. Start