`Oh, shame. I was looking forward to that,' said Ridcully.

`No, sir,' said Ponder, as patiently as possible. `We have to deal with people. Remember? We agreed last time it's not ethical to leave that to Hex. Remember the rain of fat women?[1]

`That never actually happened,' said the Lecturer in Recent Runes, wistfully.

`Quite so,' said Ridcully, firmly. `And just as well. Lead on, Mr Stibbons.'

`So much to do, so much to do,' muttered Ponder, leafing through the paperwork. `I suppose we'd better do things in order ... so first, we must see that Mr Habbakuk Souser's cook throws away the fish.'

It was a scullery boy who opened the back door, in a street of quite prosperous-looking houses. Ponder Stibbons raised his very tall hat.

`We wish to see - ' he consulted the clipboard ` - Mrs Boddy,' he said. `She is the cook here, I believe? Tell her we are the Committee for Public Sanitation, and the matter is urgent, so look sharp about it!'

`I hope you know what you're doing, Stibbons,' hissed Ridcully as the boy scurried away.

`Totally, Archchancellor. Hex says the line of causality is - ah, Mrs Boddy?'

This was to a skinny, worried woman who was advancing on them from the dim interior, wiping her hands on her apron.

'I am, sir,' said the cook. `The boy said you gentlemen was Hygienic?'

`Mrs Boddy, you had some fish delivered this morning?' said Ponder, sternly.

[1] A rare meteorological phenomenon discussed briefly in The Science of Discworld II

`Yes sir. Nice piece o' hake.' Sudden uncertainty seized her features. `Er ... that was all right, wasn't it?

'Alas it was not, Mrs Boddy!' said Ponder. `We have just come from the fishmonger. All his hake is completely off. We have had many complaints. Some of them were from next of kin, Mrs Boddy!'

`Oh, what shall we do to be saved!' the cook burst out. `I've got it cookin'! It smelled all right, sir!'

`Thankfully, then, there is no harm done,' said Ponder. `Shall I give it to the cat?

'Do you like the cat?' said Ponder. `No, wrap it in some paper and bring it out to us right now! I'm sure Mr Souser will understand when you give him some of the cold ham from yesterday.'

`Yessir!' The cook scurried away, and returned shortly with a parcel of very hot, very damp fish. Ponder grabbed it from her and thrust it into Rincewind's arms.

`Scour the pan thoroughly, Mrs Boddy!' said Ponder, as Rincewind tried to juggle hake. `Gentlemen, we must hurry!'

He started to walk very fast towards the end of the street, the wizards jogging along behind him, and turned sharply into an alleyway just ahead of a shout of `Sir? Sir? How did you know about the cold ham?'

`Location 9, Hex,' said Ponder. `And remove the fish, please!'

`Was all that about?' said Ridcully. `Why did we take that poor woman's fish?'

Rincewind said `ow!' as the fish disappeared.

`Mr Souser will travel, er, tomorrow to meet some businessmen,' said Ponder, as a circle formed on the ground around the wizards. `One of them will be a man called Josiah Wedgwood, a famous industrialist. Mr Souser will tell him about his son James, who is currently working with the Navy. It has made a man of him, Mr Souser will say. Mr Wedgwood will listen with interest, and form the opinion that the adventure of a long sea voyage in respectable company may well be of benefit to a young man on the verge of adult life. At least, he will 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.'

FORWARD TO THE PAST

WELL, 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 Godel, better known for his fundamental work in mathematical logic. In 1949 he worked out the relativistic mathematics of a rotating universe, and discovered that the past and future of every point intersect. Start wherever and whenever you like, travel into your future, and you'll end up in your own past. However, observations indicate that the universe is not rotating, and spinning up a stationary universe (especially from inside) doesn't look like a plausible way to make a time machine. Though, if the wizards were to give Roundworld a twirl ...