Xeelee: An Omnibus: Raft, Timelike Infinity, Flux, Ring

by Stephen Baxter

  ‘I know, Lieserl. I’m sorry.’ Louise had never found it easy to express affection. With a struggle, she said: ‘In fact, you’re one of the most human people I’ve ever met.’

  Lieserl looked around at the makeshift hospital, following the soft cries of the wounded. ‘Louise,’ she said slowly, ‘I have a long perspective. Think of the story of the race. Our timelines emerged from the oceans, and for millions of years circled the Sun with Earth. Then, in a brief, spectacular explosion of causality, the timelines erupted in wild scribbles, across the Universe. Humanity was everywhere.

  ‘But now, our possibilities have reduced.

  ‘Louise, all the potential paths of the race - all the timelines, running from those ancient oceans of the past, through millions of years to an unknown future - all of them have narrowed to a single event in spacetime: here, on this ship, now. And that event is under your control.’

  Lieserl’s face loomed before Louise now, filling her vision; Louise looked into her soft, vulnerable eyes, and - for the first time, really - she had a sudden, deep insight into Lieserl’s personality. This woman really is ancient - ancient, and wise.

  ‘Louise, you are not a woman - or rather, you are more than a woman. You are a survival mechanism: the best to be found, for this crucial instant, by our genes, and our culture, and our minds. If you didn’t have the strength within you now, to deliver us through this causal gateway to the future, you would not have been chosen. But you do have the strength to continue,’ Lieserl said. ‘To find a way through. Look within yourself, Louise. Tap into that strength . . .’

  There was a deep, almost subsonic groan, all around Louise. It sounded like thunder, she thought.

  It was the sound of metal, under immense stress.

  She pulled away from Lieserl and twisted in the air. She looked across at the section of hull breached by the arc of string. The patch that had been applied across the string damage gleamed brightly, fresh and polished, at the centre of the grass-coated hull surface. A stress failure - another breach of the lifedome - would kill them all. But the patch looked as if it was holding up okay . . . not that a visual inspection from this distance meant anything.

  As if on cue, a projection of Mark’s head materialized before her. ‘Louise, I’m sorry.’

  ‘What is it?’

  ‘Come with me. We need to talk.’

  ‘No,’ she said. Suddenly, she felt enormously weary. ‘No more talk, Mark. I’ve done enough damage already.’

  Behind her, Lieserl said warningly: ‘Louise . . .’

  ‘I heard what you said, Lieserl.’ Louise smiled. ‘But it’s all a little too mystical for a tired old engineer like me. I’m going to stay here. Help out in the hospital.’

  Lieserl frowned at her. ‘Louise, you’re an engineer, not a doctor. Frankly, I wouldn’t want you treating me.’

  Mark smiled. ‘Besides, we don’t have time for all this self-pity, Louise. This is important.’

  She sighed. ‘What is?’

  He whispered, in a surprisingly unrealistic hiss, ‘Didn’t you hear the hull stress noise? Spinner is moving the ship again.’

  Think of spacetime as a matrix, Michael Poole whispered. A four-dimensional grid, labelled by distance and duration. There are events: points in time and space, at nodes of the grid. These are the incidents that mark out our lives. And, connecting the events, there are trajectories.

  The starbow across the sky broadened, now. That meant her speed had reduced, since the relativistic distortion was lessened. Spinner called up a faceplate display subvocally. Yes: the ship’s velocity had fallen to a fraction over half lightspeed.

  Trajectories are paths through spacetime, Poole said. There are timelike trajectories, and there are spacelike trajectories. A ship going slower than light follows a timelike path. And, Spinner, we - all humans, since the beginning of history - work our snail-like way along timelike trajectories into the future. At last, our world-lines will terminate at a place called timelike infinity - at the infinitely remote, true end of time.

  But ‘spacelike’ means moving faster than light. A tachyon - a faster-than-light particle - follows a spacelike path, as does this nightfighter under hyperdrive.

  She twisted in her seat. Already the neutron star system had vanished, into the red-shift distance. And directly ahead of her there was a cloud of cosmic string; space looked as if it were criss-crossed by fractures, around which blue-shifted star images slid like oil drops.

  Poole’s hands, invisible, tightened around hers as the ship threw itself into the cloud of string.

  We know at least three ways to follow spacelike paths, Spinner-of-Rope: three ways to travel faster than light. We can use the Xeelee hyperdrive, of course. Or we can use spacetime wormholes. Or, Poole said slowly, we can use the conical spacetime around a length of cosmic string . . .

  Think of the gravitational lensing effect that produces double images of stars around strings. A photon coming around one side of the string can take tens of thousands of years longer to reach our telescopes than a photon following a path on the other side of the string.

  So, by passing through the string’s conical deficit, we could actually outrun a beam of light . . . There was string all around the ship, now, tangled, complex, an array of it receding to infinity. A pair of string lengths, so twisted around each other they were almost braided, swept over her head. She looked up. The strings trailed dazzling highways of refracting star images.

  Behind her the huge wings spread wide, exultant.

  This damn nightfighter was made for this, she thought.

  Under Poole’s guidance, Spinner brought the craft to a dead halt; the discontinuity wings cupped as they tore at space. Then Spinner turned the craft around rapidly - impossibly rapidly - and sent it hurtling at the string pair once more. The nightfighter soared upwards, and this time the two strings passed underneath the ship’s bow.

  . . . And if you can move along spacelike paths, Spinner-of-Rope, you can construct closed timelike curves.

  The neutron star system was old.

  Once the system had been a spectacular binary pair, adorning some galaxy lost in the sky. Then one of the stars had suffered a supernova explosion, briefly and gloriously outshining its parent galaxy. The explosion had destroyed any planets, and damaged the companion star. After that, the remnant neutron star slowly cooled, glitching as it spun like some giant stirring in its sleep, while its companion star shed its life-blood hydrogen fuel over the neutron star’s wizened flesh. Slowly, too, the ring of lost gas formed, and the system’s strange, spectral second system of planets coalesced.

  Then human beings had come here.

  The humans soared about the system, surveying. They settled on the largest planet in the smoke ring. They threw microscopic wormhole mouths into the cooling corpse of the neutron star, and down through the wormholes they poured devices and - perhaps - human-analogues, made robust enough to survive in the neutron star’s impossibly rigorous environment.

  The devices and human-analogues had been tiny, like finely jewelled toys.

  The human-analogues and their devices swarmed to a magnetic pole of the neutron star, and great machines were erected there: discontinuity-drives, perhaps powered by the immense energy reserves of the neutron star itself.

  Slowly at first, then with increasing acceleration, the neutron star - dragging its attendant companion, ring and planets with it - was forced out of its parent galaxy and thrown across space, a bullet of stellar mass fired at almost lightspeed.

  ‘A bullet. Yes.’ In the pod, Uvarov mused. ‘An apt term.’

  Lieserl stared at the swirling, unresolved pixels inside the Virtual image’s clear tetrahedral frame. ‘I wonder if there are still people in there,’ she said.

  Mark frowned. ‘Where?’

  ‘People-analogues. Inside the neutron star. I wonder if they’ve survived.’

  He shrugged, evidently indifferent. ‘I doubt it. Unless they were needed for maintenance, th
ey would surely have been shut down after their function was concluded.’

  Shut down . . . But these were people. What if they hadn’t been ‘shut down’? Lieserl closed her eyes and tried to imagine. How would it be, to live her life as a tiny, fish-like creature less than a hair’s-breadth tall, living inside the flux-ridden mantle of a neutron star? What would her world be like?

  ‘A bullet,’ Uvarov said again. ‘And a bullet, fired by our forebears - directly at the heart of this Xeelee construct.’

  She opened her eyes.

  Mark was frowning. ‘What are you talking about, Uvarov?’

  ‘Can’t you see it yet? Mark, what do you imagine the purpose of this great engineering spectacle was? We already know - from the Paradoxa data, and the fragments provided to us by Lieserl - that the rivalry between humanity and Xeelee persisted for millions of years. More than persisted - it grew in that time, becoming an obsession which - in the end - consumed mankind.’

  Lieserl said, ‘Are you saying that all of this - the discontinuity engines, the hurling of the neutron star across space - all of this was intended as an assault on the Xeelee?’

  ‘But that’s insane,’ Mark said.

  ‘Of course it is,’ Uvarov said lightly. ‘My dear friends, we’ve plenty of evidence that humanity isn’t a particularly intelligent species - not compared to its great rivals the Xeelee, at any rate. And I have never believed that humanity, collectively, is entirely sane either.’

  ‘You should know, Doctor,’ Mark growled.

  ‘I don’t understand,’ Lieserl said. ‘Humans must have known about the photino birds - damn it, I told them! They must have seen what danger the birds represented to the future of all baryonic species. And they must have seen that the Xeelee - if remote and incomprehensible - were at least baryonic too. So the goals of the Xeelee, if directed against the birds, had to be in the long-term interests of mankind.’

  Uvarov laughed at her. ‘I’m afraid you’re still looking for rational explanations for irrational behaviour, my dear. Lieserl, I believe that the Xeelee grew into the position in human souls once occupied by images of gods and demons. But here, at last, was a god who was finite - who occupied the same mortal realm as humans. A god who could be attacked. And attack we did: down through the long ages, while the stars went out around us, all but ignored.’

  ‘And so,’ Mark said grimly, ‘we fired off a neutron star at the Ring.’

  ‘A spectacular gesture,’ Uvarov said. ‘Perhaps humanity’s greatest engineering feat . . . But, ultimately, futile. For how could a mere neutron star disrupt a loop of cosmic string? And besides, the Xeelee starbreaker technology was surely sufficient to destroy the star before—’

  ‘But it didn’t work,’ Lieserl said slowly.

  Mark had been staring at the sensor ‘bot; the squat machine had come to a halt before the chair, its sensor arms suspended in the air. ‘What do you mean?’

  ‘Think about it,’ she said. ‘The neutron star is heading away from the site of the Ring. And it’s clearly not been disrupted by starbreakers.’

  ‘Yes. So something went wrong,’ Uvarov said. ‘Well, the precise sequence hardly matters, Lieserl. And—’

  It happened in a heartbeat.

  The light died. The ancient structure was flooded with darkness.

  Louise and Mark left the improvised hospital and found an abandoned house. The house was bereft of furniture, its owners gone to live in the zero-gee sky (but, of course, the zero-gee dwellings were gone now, Louise noted morosely, swept out of the sky by the cosmic string incursion).

  Mark quickly created a Virtual diagram in the air: a geometrical sketch of lines and angles, lettered and arrowed.

  Louise couldn’t help but smile. ‘Lethe, Mark. At a time like this, you give me a diagram Euclid would have recognized.’

  He looked at her seriously. ‘Louise, working out the spacetime geometry of a cosmic string is a hard problem in general relativity. But, given that geometry, all the rest of it is no more than Pythagoras’ theorem . . .

  ‘As near as I can figure out, this is what Spinner is up to.’ There was a pair of tubes in the air, glowing electric blue, like neon. ‘We are flying around a pair of cosmic strings. Now, here are the angle deficits of the strings’ conical spacetimes.’ Wedges of air, like long cheese slices, were illuminated pale blue; one wedge trailed each string length.

  ‘Okay. Here comes the Northern.’ The ship was represented by a cartoon sketch of a sycamore seed in black. ‘You can see we’re travelling on a curving path around the string pair, going against the strings’ own rotation.’

  Now the seed arced into the wedge-shaped angle deficit glow of one of the strings. As soon as it had entered the boundary it vanished, to reappear instantly at the far side of the deficit.

  Mark snapped his fingers. ‘See that? Faster-than-light travel: a spacelike trajectory right across the deficit.’

  Now the little ship-model came arcing back and flickered through the second string’s angle deficit. ‘Louise, the strings are travelling just under the speed of light - within three decimal places of it, actually. Spinner has the Northern travelling at a little over half lightspeed. The turning curves, and the accelerations, are incredible . . . The domain wall inertial shielding seems to be working pretty well, although there’s a little leakage.’

  Louise nodded. ‘Right. Which is why the Northern is complaining.’

  ‘Yeah. Louise, the Northern wasn’t designed for this - and neither was our bastardized lash-up of Northern and nightfighter. But there’s nothing we can do. We’ll just have to pray the whole mess holds together until Spinner-of-Rope finishes her joy-riding . . .

  ‘Anyway, the trajectory she’s following is quite precisely machined . . . We’re passing from side to side of the string pair in light-minutes, but we’re crossing light-years thanks to the spacelike savings. Louise, I think Spinner-of-Rope is assembling closed timelike curves, from these spacelike trajectories.’

  Louise stared at the seed-craft; she felt an impulse to reach out and pluck it from the air. ‘But why, Mark? And how?’

  ‘I know what a closed timelike curve is,’ Spinner said. Again she dragged the ship to a halt and whirled its nose around towards the string; although she was still shielded from the impossible accelerations she felt herself gasp as the Universe lurched around her. ‘The original mission of the Great Northern, with its wormhole, was to follow a segment of a closed timelike curve . . .’

  Yes. A closed timelike curve is a circle in time. By following a closed timelike curve all the way to its starting point, you would at last meet yourself, Spinner-of-Rope . . . Closed timelike curves allow you to travel through time, and into the past.

  Again the nightfighter hurled itself at the cosmic string pair; again Spinner hauled at the waldoes, dragging the ship around. The huge wings beat at spacetime.

  She screamed, ‘How much longer, damn it?’

  Spinner, each traverse around the string pair is taking us a thousand years into the past. But we need to travel back through a hundred millennia, or more . . .

  ‘A hundred traverses,’ she whispered.

  Can you do it, Spinner? Do you have the strength?

  ‘No,’ she said. ‘But I don’t think I have much choice, do I?’

  Lieserl looked around the darkened chamber, confused. The ‘bot’s brilliant lantern had been extinguished. Suddenly the walls were dim grey sheets, closing over her head, claustrophobic.

  ‘Lieserl.’ Mark’s face loomed before her, erupting out of the darkness; his blue eyes, white teeth where vivid. He moved with nanosecond speed, the slowness of humanity finally abandoned.

  Dimly, she was aware of poor Uvarov sitting in the pod. He was frozen in human-time, and unable to follow their high-speed insect-buzz. ‘What is it? What’s happened?’

  ‘The ‘bot has failed. Lieserl, it was controlled by the ship’s processors. The download link from the ship must have gone down . . .’

  I
mmediately, she felt that loss of processor support. She felt as if her mind had been plunged into a twilight cavern, echoing; she felt herself drift away.

  ‘They’ve abandoned us.’

  ‘Probably they had no choice, Lieserl.’

  I am to experience death, then. But - so suddenly?

  Lieserl would survive, of course - as would Mark, as projections on board the Northern. But this projection - she, this unique branch of her ancient consciousness - couldn’t be sustained solely by the limited processors on the pod.

  She felt a spasm of regret that she would never be able to tell Louise and Spinner-of-Rope about the wonderful little people embedded inside the neutron star flux.

  She reached for Mark. Their environment suits melted away; desperately they pressed their bodies against each other. With deep, savage longing, she sought Mark’s warm mouth with her lips, and—

  ‘Lethe. And we can’t even talk to her.’ Louise looked out of the house and across the lifedome, in the vague direction of the nightfighter cage. ‘Mark, Spinner is a smart woman, but she’s no expert on string dynamics. And she’s out there without significant processor support. I don’t see how she’s even calculating the trajectories we’re following.’

  Mark frowned. ‘I - wait.’ He held up a hand, and his expression turned inward, becoming blank.

  ‘What is it?’

  ‘We’ve stopped. I mean, the traverses around the string pair have been halted.’ He thought for a moment. ‘Louise, I counted a hundred and seven complete circuits . . .’

  ‘Louise? Mark?’

  The voice sounded out of the air close to Louise’s ear. ‘Yes, Trapper-of-Frogs. I hear you. Where are you?’

  ‘I’m in the forest. I—’

  ‘Yes?’

  ‘I think you’d better get up here.’

  Louise looked at Mark; he was frowning, and no doubt some sub-projection of him was already with Trapper.

  ‘Why?’ Louise asked. ‘What’s wrong, Trapper?’

  ‘Nothing’s wrong. Not exactly. It’s just - different . . .’