Raft

The students nervously followed, clutching their retorts; Doav rolled his eyes at the tedium of it all.

Hollerbach led them on a respectable hike, out along an avenue beneath the canopy of turning trees. The sky was clear of cloud today and starlight dappled the plates of the deck. Despite Ms age Hollerbach kept up a good pace, and by the time he paused, under open sky a few yards beyond the edge of the flying forest, Rees suspected that his weren't the only young legs that ached a little. He looked around curiously, blinking in the direct starlight; since beginning classes he had scarcely had a chance to come out this way, and the apparent tilt of the riveted deck under his feet felt strange.

Solemnly Hollerbach lowered himself to the deck plates and sat cross-legged, then bade his students do the same. He fixed a series of candles to the plates. 'Now, ladies and gentlemen,' he boomed, 'I would like you to repeat your experiments of our last class. Set up your pendulum.'

There were stifled groans around the class, presumably inaudible to Hollerbach. The students began work, and Hollerbach, restless, got up and paced among them. 'You are Scientists, remember,' he told them. 'You are here to observe, not judge; you are here to measure and understand…'

Rees's results were… odd. As Hollerbach's supply of candles burned through he went over his results carefully, repeating and testing.

At last Hollerbach called them to order. 'Conclusions, please? Doav?'

Rees heard the cadet's breathy groan. 'No difference,' he said languidly. 'Same result curve as last time.»

Rees frowned. That was wrong; the periods he had measured had been greater than yesterday's — by a small amount, granted, but greater consistently.

The silence gathered. Doav shifted uneasily.

Then Hollerbach let him have it. Rees tried not to grin as the old Scientist tore into the cadet's sloppy methods, his closed mind, his laziness, his lack of fitness to wear the golden braids. By the end of it Doav's cheeks burned crimson.

'Let's have the truth,' Hollerbach muttered, breathing hard. 'Baert…?'

The next apprentice supplied an answer consistent with Rees's. Hollerbach said, 'Then what has happened ? How have the conditions of this experiment changed?'

The students speculated, listing the effect of the starlight on the pendulum bobs, the greater inaccuracy of the timing method — Hollerbach's candles flickered far more out here than in the 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 the student 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 center 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 Hol-lerbach. '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 center 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 theatres in the Belt, eh? Well, if you haven't heard, wait and see…'

The Theatre was situated beyond the tethered forest, about three-quarters of the way to the Rim. There was a bus service from the Raft's central regions but Baert and Rees chose to walk. By the time they had reached the head-high fence which surrounded the Theatre the deck appeared to be sloping quite steeply, and the walk had become a respectable climb. Out here on the exposed deck, far from the cover of the forest canopy, the heat of the star above the Raft was a tangible thing, and both of them arrived with faces slick with sweat.

Baert turned awkwardly, slippered feet gripping at the riveted slope, and grinned down at Rees. 'Kind of a hike,' he said. 'But it'll be worth it. Do you have your ticket?'

Rees fumbled in his pockets until he found the precious piece of cardboard. Bemused, he watched as Baert presented the tickets to a doorkeeper and then followed Baert through a narrow gate.

The Theatre of Light was an oval some fifty yards along its long axis, which ran down the apparent slope of the deck. Benches were fixed across the upper part of the Theatre. Rees and Baert took their places and Rees found himself looking down the slope at a small stage which was fixed on stilts so that it rested at the local horizontal — so at an angle to the 'tilted' deck — and beyond the stage, serving as a mighty backdrop to the show, he could see the center of the Raft tip away, a vast metal slope of boxy buildings and whirling, rustling trees.

The Theatre filled up rapidly. Rees estimated there was room for about a hundred people here, and he shivered a little, uncomfortable at the thought of so many people in one place.

'Drinks?'

He turned with a start. A girl, luminously pretty, stood beside his seat with a tray of glasses. He tried to smile back and form an answer, but there was something odd about the way she was standing…

Without effort or discomfort she was standing perpendicularly to the deck; she ignored the apparent tilt of the deck and stood as naturally as if it were level. Rees felt his jaw drop, and all his carefully constructed reasoning about the illusory tilt of the deck evaporated. For if she was vertical then he was sitting at an angle with