Xeelee: An Omnibus: Raft, Timelike Infinity, Flux, Ring by Stephen Baxter Page B

lab - and many other ideas. Hollerbach listened gravely, occasionally nodding.
    None of it convinced Rees. He stared at the simple device, willing it to offer up its secrets.
    At last Baert said hesitantly, ‘What about gravity?’
    Hollerbach raised his eyebrows. ‘What about it?’
    Baert was a slender, tall boy; now he rubbed his thin nose uncertainly. ‘We’re a little further from the Raft’s centre of gravity here, aren’t we? So the pull of gravity on the pendulum bob will be a bit less . . .’
    Hollerbach eyed him fiercely, saying nothing. Baert flushed and went on, ‘It’s gravity that makes the bob swing, by pulling at it. So if gravity’s less, the period will be longer . . . Does that make sense?’
    Hollerbach rocked his head from side to side. ‘At least that’s a little less dubious than some of the other proposals I’ve heard. But if so, what precisely is the relationship between the strength of gravity and the period?’
    ‘We can’t say,’ Rees blurted. ‘Not without more data.’
    ‘Now that,’ Hollerbach said, ‘is the first intelligent thing any of you have said this shift. Well, ladies and gentlemen, I suggest you proceed to gather your facts. Let me know what you find out.’ He stood, stiffly, and walked away.
    The students dispersed to their task with varying degrees of enthusiasm. Rees went at it with a will, and for the next few shifts scoured the deck, armed with his pendulum, notepad and supply of candles. He recorded the period of the pendulum, made careful notes and drew logarithmic scale graphs - and more; carefully he observed how the plane of the pendulum’s swing formed various angles with the surface, showing how the local vertical was changing as he moved across the face of the Raft. And he watched the slow, uncertain oscillations of the pendulum at the Rim itself.
    At last he took his findings to Hollerbach. ‘I think I have it,’ he said hesitantly. ‘The period of the pendulum is proportional to the square root of its length . . . and also inversely proportional to the square root of the acceleration due to gravity.’
    Hollerbach said nothing; he steepled liver-spotted fingers before his face and regarded Rees gravely.
    At length Rees blurted, ‘Am I correct?’
    Hollerbach looked disappointed. ‘You must learn, boy, that in this business there are no right answers. There are only good guesses. You have made an empirical prediction; well, fine. Now you must check it against the body of theory you have learned.’
    Inwardly Rees groaned. But he went away and did so.
    Later he showed his findings on the strength and direction of the Raft’s gravitational field to Hollerbach. ‘The way the field varies is quite complex,’ he said. ‘At first I thought it might fall off as the inverse square of the distance from the centre of the Raft; but you can see that’s not true . . .’
    ‘The inverse square law holds only for point masses, or for perfectly spherical objects. Not for something shaped like a dinner plate, like the Raft.’
    ‘Then what is . . .?’
    Hollerbach merely eyed him.
    ‘I know,’ Rees sighed. ‘I should go and work it out. Right?’
    It took him longer than the pendulum problem. He had to learn to integrate in three dimensions . . . and how to use vector forces and equipotential surfaces . . . and how to make sensible approximating assumptions.
    But he did it. And when he’d done that, there was another problem. And another, and still another . . .
    It wasn’t all work.
    One shift Baert, with whom Rees struck up a diffident friendship, offered Rees a spare ticket to something called the Theatre of Light. ‘I won’t pretend you’re my first choice companion,’ Baert grinned. ‘She was a bit better looking than you . . . But I don’t want to miss the show, or waste a ticket.’
    Rees thanked him, turning the strip of cardboard over in his hands. ‘The Theatre of Light? What is it? What goes on there?’
    ‘There aren’t too many