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

by Stephen Baxter

  ‘I see it,’ Spinner breathed. Subvocally, she called for magnification.

  ‘Do you know what you’re looking at, Spinner?’ Louise’s flat voice contained awe, humility. ‘It’s what we suspected must have gouged out this valley. It’s a fragment of cosmic string . . .’

  At the centre of an immense cavity, walled by crowded galaxies, Lieserl and Mark rotated slowly around each other, warm human planets.

  The sky was peppered with the dusty spirals of galaxies, more densely than the stars in the skies of ancient Earth. But the cavity walls were ragged and ill-defined, so that it was as if Lieserl was at the centre of some immense explosion. And every one of the galaxies was tinged by blue shift: the light from each of these huge, fragile star-freights was compressed, visibly, by its billion-year fall into this place.

  Mark took her hand. His palm was warm against hers, and when he pulled gently at her arm, her body slowly rotated in space until she faced him.

  ‘I don’t understand,’ Lieserl said. ‘This - cavity - is empty. Where’s the Ring?’

  The light of a hundred thousand galaxies, blue-shifted, washed over his face. Mark smiled. ‘Have patience, Lieserl. Get your bearings first.

  ‘Look around. We’ve arrived at a cavity, almost free of galaxies, ten million light-years across: a cavity right at the site of the Great Attractor. The whole cavity is awash with gravitational radiation. Nothing’s visible, but we know there’s something here, in the cavity . . . It just isn’t what we expected.’

  Lieserl raised her face to stare around the crowded sky, at the galaxies embedded in the walls of this immense cave of sky. One galaxy with an active nucleus - perhaps a Seyfert - emitted a long plume of gas from its core; the gas, glowing in the searchlight beam of ionizing radiation from the core, trailed behind the infalling galaxy like the tail of some immense comet. And there was a giant elliptical which looked as if it was close to disintegration, rendered unstable by the fall into the Attractor’s monstrous gravity well; she could clearly see the elliptical’s multiple nuclei, orbiting each other within a haze of at least a thousand billion stars.

  Some of the galaxies were close enough for her to make out individual stars - great lacy streams of them, in disrupted spiral arms - and, in some places, supernovae glared like diamonds against the paler tapestry of lesser stars. She picked out one barred-spiral with a fat, gleaming nucleus, which trailed its loosening arms like unravelling bandages. And there was a spiral - heart-breakingly like her own Galaxy - undergoing a slow, stately collision with a shallow elliptical; the galaxies’ discs had cut across each other, and along the line where they merged exploding stars flared yellow-white, like a wound.

  It was, she thought, as if the Universe had been wadded up, compressed into this deep, intense gravity pocket.

  Everywhere she caught a sense of motion, of activity: but it was motion on an immense scale, and frozen in time. The galaxies were like huge ships of stars, Lieserl thought, voyaging in towards here, to the centre of everything - but they were ships caught suspended by the flashbulb awareness of her own humanity. She longed for the atemporal perspective of a god, so that she could run this immense, trapped diorama forward in time.

  ‘It’s all very beautiful,’ she said. ‘But it almost looks artificial - like a planetarium display.’

  Mark grunted. ‘More like a display of trapped insects. Moths, maybe, drawn in to an invisible gravitational flame. We’re still sifting through the data we’re gathering,’ he said softly. ‘I wonder if any astronomers in human history have ever had such a rich sky to study . . . even if it does mark the end of time.

  ‘But we’ve found one anomaly, Lieserl.’

  ‘An anomaly? Where?’

  He raised his arm and pointed, towards an anonymous-looking patch of sky across the cavity. ‘Over there. A source in the hydrogen radio band. As far as we can tell it’s coming from a neutron star system - but the neutron star is moving with an immense velocity, not far below lightspeed. Anomalies all round, right? The source is difficult to pick out against all this galactic mush in the foreground. But it’s undoubtedly there . . .’

  ‘What’s so special about it?’

  He hesitated. ‘Lieserl, it seems to be a signal.’

  ‘A signal? From who?’

  ‘How should I know?’

  ‘Maybe it’s a freak; an artifact of our instruments.’

  ‘Quite possibly. But we’re thinking of checking it out anyway. It’s only a million light-years away.’ He smiled ruefully. That’s all of eight hours’ travel, if you hitch a ride on a nightfighter . . .’

  A signal, here at the end of space and time . . . Was it possible the motley crew of the Northern wasn’t alone after all?

  The hair at the base of her skull prickled. At the end of this long, long life, she’d thought there was nothing left to surprise her.

  Evidently, she was wrong.

  Mark said, ‘Lieserl, what you’re looking at here is visible light: the Virtual display we’re drifting around inside is based on images from right at the centre of the human visible spectrum. You’re seeing just what any of the others would see, with their unaided vision. But the image has been enhanced by blue shift: red, dim stars have been made to look blue and bright.’

  ‘I understand.’

  Now the blue stain faded from the galaxy images, seeping out like some poor dye.

  A new colour flooded the galaxy remnants, but it was the colour of decay - dominated by flaring reds and crimsons, though punctuated in places by the glaring blue-white of supernovae. And without the enhancement offered by the blue shift, some of the galaxies faded from her view altogether.

  The galaxies had turned into ships of fire, she thought.

  Mark’s profile was picked out, now, in colours of blood. ‘Take a good look around, Lieserl,’ he said grimly. ‘I’ve adjusted out the blue shift; this is how things really are.’

  She looked at him curiously; his tone had become hostile, suddenly. Though he still held her hand, his fingers felt stiff around hers, like a cage. ‘What are you saying?’

  ‘Here’s the result of the handiwork of your photino bird pets,’ he said. ‘In the week since we arrived, we’ve been able to catalogue over a million galaxies, surrounding this cavity. In every one of those million we see stars being pushed off the Main Sequence, either explosively as a nova or supernova or via expansion into the red-giant cycle. Everywhere the stars are close to the end of their lifecycles - and, what’s worse, there’s no sign of new star formation, anywhere.’

  Suddenly she understood. ‘Ah. This is why you’ve set up this display for me. You’re testing me, aren’t you?’ She felt anger build, deep in her belly. ‘You want to know how all this makes me feel. Even now - even after we’ve been so close - you’re still not sure if I’m fully human.’

  He grinned, his red-lit teeth like drops of blood in his mouth. ‘You have to admit you’ve had a pretty unusual life history, Lieserl. I’m not sure if any of us can empathize with you.’

  ‘Then,’ she snapped, ‘maybe you should damn well try. Maybe that’s been the trouble with most of human history. Look at all this: we’re witnessing, here, the death of galaxies. And you’re wondering how it makes me feel? Do you think all this has somehow been set up as a test of my loyalty to the human race?’

  ‘Lieserl—’

  ‘I’ll tell you how I feel. I feel we need a sense of perspective here, Mark. So what if this - this cosmic discontinuity - is inconvenient for the likes of you and me?’ She withdrew from him and straightened her back. ‘Mark, this is the greatest feat of cosmic engineering our poor Universe will ever see - the most significant event since the Big Bang. Maybe it’s time we humans abandoned our species-specific chauvinism - our petty outrage that the Universe has unfolded in a way that doesn’t suit us.’

  He was smiling at her. ‘Quite a speech.’

  She punched him, reasonably gently, beneath the ribs, relishing the way her fist sank into his flesh.
‘Well, you deserve it, damn it.’

  ‘I didn’t mean to imply—’

  ‘Yes, you did,’ she said sharply. ‘Well, I’m sorry if I’ve failed your test, Mark. Look, you and I - by hook or by crook - have survived the decline and destruction of our species. I know we’re going to have to fight for survival, and I’ll be fighting right alongside you, as best I can. But that doesn’t remove the magnificence of this cosmic engineering - any more than an ant-hill’s destruction to make way for the building of a cathedral would despoil the grandeur of the result.’

  Still holding her hand within his stiff fingers, he turned his face to the galaxy-stained sky. His offence at her words was tangible; he must be devoting a great deal of processing power to this sullen rebuke. ‘Sometimes you’re damn cold, Lieserl.’

  Lethe, she thought. People. ‘No,’ she said. ‘I just have a longer perspective than you.’ She sighed. ‘Oh, come on, Mark. Show me the Ring,’ she said.

  The sculpture of string, driving itself into the heart of the scarred galaxy, was not symmetrical. It was in the form of a rough figure-of-eight; but each lobe of the figure was overlaid with more complex waveforms - a series of ripples, culminating in sharp, pointed cusps.

  ‘Do you see it, Spinner?’ Mark asked. ‘That is a loop of string nearly a thousand light-years wide.’

  Spinner smiled. ‘That’s not a loop. That’s a knot.’

  ‘It’s moving towards the galactic core at over half the speed of light. It’s got the mass of a hundred billion stars . . . Can you believe that? It’s as massive as a medium-size galaxy itself. No wonder it’s cutting this swathe through the stars; the damn thing’s like a scythe, driving across the face of this galaxy.’

  Louise laughed. ‘A knot. Knot-making is a skill, up there in the forest, isn’t it, Spinner? I’ll bet you’d have been proud to come up with a structure like that.’

  ‘Actually,’ Mark said, ‘and I hate to be pedantic, but that isn’t a knot, topologically speaking. If you could somehow stretch it out - straighten up the cusps and curves - you’d find it would deform into a simple loop. A circle.’

  Spinner heard Garry Uvarov’s rasp. ‘And I hate to be a pedant, in my turn, but in fact a simple closed loop is a knot - called the trivial knot by topologists.’

  ‘Thank you, Doctor,’ Louise said dryly.

  Spinner frowned, peering at the detailed image of the string loop; in the false colours of her faceplate it was a tracery of blue, frozen against the remote background of the galaxy core. She realized now that she was looking at one projection of a complex three-dimensional object. Subvocally she called for a depth enhancement and change in perspective.

  The loop seemed to loom towards her, lifting away from the starry background, and the string was thickened into a three-dimensional tubing, so that she could see shadows where one strand overlaid another.

  The image rotated. It was like a sculpture of hosepipe, rolling over on itself. Mark commented, ‘But the string isn’t stationary, of course. I mean, the whole loop is cutting through this galaxy at more than half lightspeed - but in addition the structure is in constant, complex motion. Cosmic string is under enormous tension - a tension that increases with curvature - and so those loops and cusps you see are struggling to straighten themselves out, all the time. Most of the length of the string is moving at close to lightspeed - indeed, the cusps are moving at lightspeed.’

  ‘Absurd,’ Spinner heard Uvarov growl. ‘Nothing material can reach lightspeed. ’

  ‘True,’ Mark said patiently, but cosmic string isn’t truly material, in that sense, Uvarov. Remember, it’s a defect in spacetime . . . a flaw.’

  Spinner watched the beautiful, sparkling construct turn over and over. It was like some intricate piece of jewellery, a filigree of glass, perhaps. How could something as complex, as real as this, be made of nothing but spacetime?

  ‘I can’t see it move,’ she said slowly.

  ‘What was that, Spinner?’

  ‘Mark, if the string is moving at close to lightspeed - how come I can’t see it? The thing should be writhing like some immense snake . . .’

  ‘You’re forgetting the scale, Spinner-of-Rope,’ Mark said gently. ‘That loop is over a thousand light-years across. It would take a millennium for a strand of string to move across the diameter of the loop. Spinner, it is writhing through space, just as you say, but on timescales far beyond yours or mine . . .

  ‘But watch this.’

  Suddenly the three-dimensional image of the string came to life. It twisted, its curves straightening or bunching into cusps, lengths of the string twisting over and around each other.

  Mark said, ‘This is the true motion of the string, projected from the velocity distribution along its length. The motion is actually periodic . . . It resumes the same form every twenty thousand years or so. This graphic is running at billions of times true speed, of course - the twenty-millennia period is being covered in around five minutes.

  ‘But the graphic is enough to show you an important feature of this motion. It’s non-intersecting . . . The string is not cutting itself at any point in the periodic trajectory. If it did, it would bud off smaller sub-loops, which would oscillate and cut themselves up further, and so on . . . the string would rapidly decay, shrivelling through a thousand cuts, and leaking away its energy through gravitational radiation.’

  Spinner wished, suddenly, that she wasn’t human: that she could watch the motion of this loop unfold, without having to rely on Mark’s gaudy projections. How wonderful it would be to be able to step out of time!

  . . . Close your eyes, Spinner.

  ‘What?’

  You can step out of time, just as you desire. Close your eyes, and imagine you are a god.

  . . . And here, in her mind’s eye - so much more dramatic than any Virtual! - came the knot of string, sailing out of space. The knot wriggled like some huge worm, closed on itself as if swallowing its own tail.

  The knot struck the rim of this defenceless galaxy and scythed towards the core, battering stars aside like blades of grass.

  It was a disturbing, astonishing image. She snapped open her eyes, dispelling the vision; fear flooded her, prickling over her flesh.

  She wasn’t normally quite so imaginative, she thought dryly. Maybe her companion had had something to do with that brief, vivid vision . . .

  She returned her attention to the harmless-looking Virtual display. Now Mark showed Spinner the loop’s induced magnetic field, a yellow glow of energy which sleeved the fake blue of the string itself.

  ‘As it hauls through the galaxy’s magnetic field, that string is radiating a lot of electromagnetic energy,’ Mark said. ‘I see a flood of high-energy photons . . .’

  Cosmic string wasn’t actually one-dimensional; it was a Planck length across, a fine tube containing charged particles: quarks, electrons and their antiparticles, gathered into super-heavy clusters. As a result, string acted as a superconducting wire.

  The string knot was cutting through this galaxy’s magnetic field. As it did so immense electrical currents - of a hundred billion billion amps or more - were induced in the string. These currents generated strong magnetic fields around the string.

  The string’s induced field was stronger than a neutron star’s, and dominated space for tens of light-years around the knot.

  Mark said, ‘The string has a maximum current capacity. If it’s overloaded, the string starts to shed energy. It glows with gamma radiation: And the lost energy crystallizes into matter: ions and electrons, whispering into existence all along the length of the string.’ Spinner saw representations of particles - out of scale, of course - popping into existence around the string image. ‘So the string is glowing as brightly as a star.’

  ‘Yes,’ Louise put in. ‘But the distribution of the radiation is odd, Mark. Look at this. The radiation is beamed forward of the loop’s motion - parallel to that forward spike of gravitational radiation.’

  ‘Like a searchl
ight,’ Morrow said.

  Or a spear . . ..

  She heard Morrow saying, ‘Mark, what is driving the string? What is impelling it through space, and into this galaxy?’

  ‘Gravitational radiation,’ Mark said simply.

  Louise said, ‘Morrow, gravity waves are emitted whenever large masses are moved through space. Because the loop is asymmetrical it’s pushing out its gravitational radiation in particular directions - in spikes, ahead of and behind it. It is pushing out momentum . . . It is a gravitational rocket, using its radiation to drive through space.’

  Mark said, ‘Of course the gravitational radiation is carrying away energy - the string is shrinking, slowly. In the end it will collapse to nothing.’

  ‘But not fast enough to save this galaxy,’ Uvarov growled.

  ‘No,’ Louise said. ‘Before it has time to decay away, the string is going to reach the core - and devastate the galaxy.’

  Close your eyes.

  Spinner-of-Rope shivered. Once again the voice had come from her left - from somewhere outside her suit. She stared at the Virtual image in her faceplate, not daring to look around.

  Close your eyes. Think about your vision again - of the string loop, cutting through the stars. It frightened you, didn’t it? What did that image mean, Spinner-of-Rope? What was it telling you?

  Suddenly she saw it.

  ‘Mark,’ she said. ‘This is not just a gravitational rocket.’

  ‘What?’

  ‘Think about it. The string knot must be a missile.’

  The galaxy images dimmed, leaving Mark and Lieserl suspended in a crimson-tinged darkness. Then, against that background, new forms began to appear: speckles of light, indistinct, making up the ghostly outline of a torus, its face tipped open towards her.

  ‘Of course this is a false colour representation,’ Mark said. ‘The images have been reconstructed from gravity wave and gamma ray emissions . . .’