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

by Stephen Baxter

  She looked up. There was something before the ship, occluding the blue-shifted galaxy fragments, hiding the Ring.

  She saw night-dark wings, spread to their fullest extent, looming over the Northern.

  Nightfighters.

  She twisted in her seat. There were hundreds of them - impossibly many, dark lanterns hanging in the sky.

  They were Xeelee. The Northern was surrounded.

  Spinner screamed, and slammed her fists against the hyperdrive waldo.

  The ‘fighters moved through electric-blue cosmic string like birds through the branches of a forest. There were so many of them in this era. They were cool and magnificent, their night-dark forms arrayed deep into space all around her. Lieserl stared at the swooping, gliding forms, willing herself to see them more clearly. Had any humans ever been closer to Xeelee than this?

  The Xeelee moved in tight formation, like bird-flocks, or schools of fish; they executed sudden changes of direction, their domain wall wings beating, in squads spanning millions of miles - absolutely in unison. Now Lieserl saw how ‘fighters should be handled, in contrast to Spinner’s earnest, clumsy work. The nightfighters were sculptures of spacetime, with a sleek beauty that made her shiver: this was baryonic technology raised to perfection, to a supreme art, she thought.

  She was struck by the contrast between this era and the age of devastation - of victory for the photino birds - to which the Northern had first brought them. Here, the Ring was complete and magnificent, and the Xeelee, in their pomp, filled space. Already, she knew, the final defeat was inevitable, and the Xeelee were, in truth, huddling inside their final retreat. But still, her heart beat harder inside her as she looked out over this, the supremacy of baryonic life.

  The overlapping lengths of string slid down, smoothly, past the lifedome, as the Northern climbed. The nightfighters swooped like starlings through the string, and around the Northern - no, Spinner realized suddenly; the nightfighters were flickering across space.

  ‘They’re using their hyperdrive,’ she breathed.

  Yes. Poole stared up at the nightfighters, his lined face translucent. And we’re hyperdriving too. You’re pushing it, Spinner; we’ve never tried jumps of this scale, even in test. Do you know how fast you’re travelling? Ten thousand light-years with every jump . . . But even so, the Xeelee are easily keeping pace with us.

  Of course they are, Spinner thought. They are Xeelee.

  These ‘fighters could have stopped the Northern at any time - even destroyed it. But they hadn’t.

  Why not?

  The ship was rising high above the plane of the Ring. The tangle of string fell away from the foreground, and she could see easily now the million-light-year curve of the structure’s limb. And at the heart of the Ring, the singularity seemed to be unfolding towards her, almost welcoming.

  The Xeelee ‘fighters rose all around her, like leaves in a storm. They can’t believe we’re a threat. I guess humans never were a threat, in truth. Now, it’s almost as if the Xeelee are escorting us, she thought.

  ‘Lieserl,’ she said.

  ‘I hear you, Spinner-of-Rope.’

  ‘Tell me what in Lethe’s name we’re doing.’

  ‘You’re taking us out of the plane of the Ring . . .’

  ‘And then?’

  ‘Down . . . ’ Lieserl hesitated. ‘Look, Spinner, we’ve got to get away from the Xeelee, before they change their mind about us. And we’ve nowhere else to run, not in all of the Universe.’

  ‘And this is your plan?’ Spinner was aware of the hysteria in her own voice; she felt fear spread through her stomach and chest, like a cold fluid. ‘To fly into a singularity?’

  Mark punched his thigh. ‘I was right, damn it,’ he said. ‘I was right all along.’

  The tension was a painful presence, clamped around Louise’s throat. ‘Damn it, Mark, be specific.’

  He turned to her. ‘About the significance of the radio energy flux. Don’t you see? The photino birds have manufactured this immense cavity, of stars and smashed-up galaxies, to imprison the Ring.’ He glanced around the skydome. ‘Lethe. It must have taken them a billion years, but they’ve done it. They’ve built a huge mirror of star-stuff, all around the Ring. It’s a feat of cosmic engineering almost on a par with the construction of the Ring itself.’

  ‘A mirror?’

  ‘The interstellar medium is opaque to the radio energy. So each radio photon gets reflected back into the cavity. The photon orbits the Ring - and on each pass it’s superradiant-amplified, as Lieserl described, and so sucks out a little more energy from the inertial drag of the Ring’s rotation. And then the photon heads out again . . . but it’s still trapped by the galaxy mirror. Back it goes again, to receive a little more amplification . . . Do you see? It’s a classic example of positive feedback. The trapped radio modes will grow endlessly, leaching energy from the Ring itself . . .’

  ‘But the modes can’t grow indefinitely,’ Morrow said.

  ‘No,’ Mark said. ‘The process is an inertial bomb, Morrow. All that electromagnetic pressure will build up in the cavity, until it can no longer be contained. And in the end - probably only a few tens of millennia from now - it will blow the cavity apart.’

  Louise glanced around the sky, seeing again the smooth distribution of galaxies she’d noted earlier. ‘Right. And, in a hundred thousand years, the Northern will fly right into the middle of the debris from that huge explosion.’

  Now the ship had sailed high above the plane of the Ring; Louise could see the whole structure, laid out before her like the rim of a glimmering mirror, with the sparkle of the singularity at its heart.

  Lieserl said, ‘Louise, the hostile photino bird activity we’ve noted before - the direct assault on the Ring itself with lumps of matter - is spectacular, but Mark’s right: this radio bomb trick is what will truly bring down the Ring.’ A subtle smile played on her lips. ‘It’s damn clever. The birds are draining the Ring itself, drawing energy out of the gravitational field using inertial drag. They’re going to use the Ring’s own mass-energy to wreck it.’

  Subvocally, Louise checked her chronometer. Less than twenty minutes had elapsed since Mark and Lieserl had ordered Spinner to start moving the ship, but already they must have crossed eight million light-years - already they must be poised directly above the singularity.

  ‘Mark. Where are we going?’

  Poole, evidently trying to calm Spinner, told her what would happen to the nightfighter as it approached the disc singularity.

  A timelike trajectory could reach the upper surface of the disc, Poole told her. A ship could reach the plane of the singularity. But - so said the equations of the Kerr metric - no timelike trajectory could pass through the singularity loop and emerge from the other side.

  ‘So what happens? Will the ship be destroyed?’

  No.

  ‘But if the ship can’t travel through the loop - where does it go?’

  There can be no discontinuity in the metric, you see, Spinner-of-Rope. Poole hesitated. Spinner-of-Rope, the singularity plane is a place where universes kiss.

  ‘Lethe,’ Louise said. ‘You’re planning to take us out of the Universe?’

  Mark swivelled his head towards her, unnaturally stiffly; the degradation of the image of his face - the crawling pixel-defects, the garish colour of his eyes - made him look utterly inhuman. ‘We’ve nowhere else to run, Louise. Unless you have a better idea . . .’

  She stared up at the singularity. The AIs, working together at inhuman speed, had come up with a response to this scenario. But are they right? She felt the situation slipping away from her; she tried to plan, to come to terms with this.

  Lieserl said dryly, ‘Of course, timing is going to be critical. Or we might end up in the wrong universe . . .’

  Morrow clung to his scooter, his eyes wide, his knuckles bloodless. ‘What in Lethe’s name are you talking about now?’

  Mark hesitated. ‘The configuration of the string is changin
g constantly. It’s a dynamic system. And that’s changing the topology of the Kerr metric - it’s changing the basis of the analytical continuation of space through the singularity plane . . .’

  ‘Damn you,’ Morrow said. ‘I wish you’d stick to English.’

  ‘The singularity plane is a point at which this Universe touches another smoothly. Okay? But because of the oscillations of the Ring, the contact point with the other universe isn’t a constant. It’s changing. Every few minutes - sometimes more frequently - the interface changes to another continuation region - to another universe.’

  Morrow frowned. ‘Is that significant for us?’

  Mark ran a hand through his hair. ‘Only because the changes aren’t predictable, either in timing or scope. Maybe the changes cycle round, for all I know, so if we wait long enough we’ll get a second chance.’

  ‘But we don’t have time to wait.’

  ‘No. Well, we’re not exactly planning this . . . We won’t be able to choose which universe we end up in. And not every universe is habitable, of course . . .’

  Louise pressed her knuckles to her temples. Good point, Mark. We’ve decided to commit ourselves to crashing out of our Universe, and we have half the Xeelee nightfighters in creation on our tails already and now you bring me this. What am I supposed to do about it?

  ‘Tell me what you see through there right now,’ she said. ‘Tell me about the universe on the other side of the Kerr interface.’

  ‘Now?’ Mark looked doubtful. ‘Louise, you’re asking me to come up with an analysis of a whole cosmos - based on a few muddled glimpses - in a few seconds. It’s taken all of human history even to begin a partial—’

  ‘Just do it,’ she snapped.

  He studied her briefly, his expression even. ‘Some of the twin universes feature a degree of variation to our physical laws. That’s no great surprise; the constants of physics are just an arbitrary expression of the way the symmetries at the beginning of time were broken . . . But even those universes with identical laws to ours can be very different, because of changed boundary conditions at the beginning of time - or even, simply, from being at a different stage of their evolutionary cycles to ours.’

  ‘And in this particular case?’ she asked heavily.

  He closed his eyes. Louise could see that stray pixels, yellow and purple, were again migrating across the Virtual images of his cheeks. His eyes snapped open, startling her. ‘High gravity,’ he said.

  ‘What?’

  ‘Variation of the laws. In the neighbouring universe, the constant of gravity is high - enormously high - compared to, uh, here.’

  Morrow looked nervous. ‘What would that mean? Would we be crushed?’

  More pixels, glitches in the image, trekked across Mark’s cheeks. ‘No. But human bodies would have discernible gravity fields. You could feel Louise’s mass, Morrow, with a pull of about half a gee.’

  Morrow looked even more alarmed.

  ‘Stars could be no more than a mile wide, and they would burn for only a year,’ Mark said. ‘Planets the size of Earth would collapse under their own weight immediately . . .’

  Lieserl frowned. ‘Could we survive there?’

  Mark shrugged. ‘I don’t know. The lifedome would implode immediately under its own weight. We’d need to find a source of breathable air, and fast. And we’d have to live in free-fall; any sizeable mass would exert unbearably high gravitational forces. But maybe we could make some kind of raft of the wreckage of the Northern . . .’

  Lieserl looked up into the singularity plane, and her expression softened. ‘We know there have been human assaults in the Ring - like the neutron star missile. So perhaps we are not the first human pilgrims to fall through the Ring. Mark, you said the bridge to the other universe goes through cycles. I wonder if there are humans on the other side of that interface even now, clinging to rafts made from wrecked warships, struggling to survive in their high-gravity world . . .’

  Mark smiled; he seemed to be relaxing. ‘Well, if there are, we won’t meet them. That continuation has closed off; a new one is opening . . . Wherever we’re going, it won’t be there.’

  Louise glanced up at the false-colour sky. ‘. . . I think it’s time to find out,’ she said.

  The Northern reached the zenith of its arc, high over the plane of the Ring.

  Spinner felt as if she were suspended at the top of some huge cosmic tree, a million light-years high. The ship was poised above the singularity’s central, glittering pool of muddled starlight, and beyond that, at the edge of her field of view, was the titanic form of the Ring itself.

  The flock of nightfighters hovered in a rough cap around her and above her, their wings spread. The ‘fighters were sharp, elegant forms, filling space.

  Spinner-of-Rope closed her hands over the hyperdrive waldo.

  Now, it was like tumbling out of the tree.

  The nightfighter fell through space, covering ten thousand light-years every second.

  The singularity is a gateway to other universes, Michael Poole said. Who knows? - perhaps to better ones than this.

  In fact, Poole told her, there had to exist further gateways, in the universe beyond, to still more cosmoses . . . He painted a picture of a mosaic of universes, connected by the glowing doorways of positive and negative Kerr singularities. It’s wonderful, Spinner-of-Rope.

  Spinner stared down at the singularity. ‘Is this what they intended? Did the Xeelee mean to construct the singularity as a gateway?’

  Of course they did. Why do you think they made the singularity so damned big? . . . So that ships could pass through it, without being destroyed by tidal forces from the singularity thread.

  Spinner-of-Rope, this is the Xeelee’s most magnificent achievement. I would have liked to tell you some day how this Ring was built . . . how the Xeelee returned through time and even re-engineered their own evolution, to give themselves the capabilities to achieve this.

  ‘You would have liked to tell me . . . ?’

  Yes. Poole sounded sad. Spinner, I’m not going to get the chance . . . I can’t follow you.

  ‘What?’

  It was as if she descended through an immense tunnel, walled by the distant, irrelevant forms of blue-shifted galaxies. The singularity was the starlit open base of that tunnel, out of which she would fall into—

  Into what?

  Still, the starling flocks of nightfighters swirled around the ship.

  ‘You know,’ she said, ‘the Xeelee could have stopped us at almost any point. I’m sure they could destroy us even now.’

  I’m sure they could.

  ‘But they haven’t.’

  Perhaps they are helping us, Spinner-of-Rope. Maybe there is some residual loyalty among the baryonic species, after all.

  ‘ . . . Spinner-of-Rope.’

  ‘Yes, Lieserl.’

  ‘Listen to me. The trip through the singularity is going to be - complicated.’

  ‘Oh, good,’ Spinner said dryly.

  ‘Spinner, the spacetime manifold around here isn’t simple. Far enough out the singularity will attract us - draw us in. But close to the plane of the singularity, there is a barrier of potential in the gravitational field.’

  She sighed. ‘What does that mean?’

  ‘ . . . Antigravity, Spinner-of-Rope. The plane will actually repel us. If we don’t have enough kinetic energy as we approach the plane, we’ll be pushed away: either back to the asymptotically flat regions - I mean, to infinity, far from the plane - or else back into the zone of attraction. We could oscillate, Spinner, alternately falling and being repelled.’

  ‘What happens on the other side? Will we be drawn back into the plane?’

  ‘No.’ Lieserl hesitated. ‘When we pass through the plane, there is a co-ordinate sign change in the metric . . . The singularity will push us away. It will hurl us on, deep into the new universe.’

  ‘So what do I have to do?’

  ‘To get over the potential barrier, we need t
o build up our kinetic energy before we hit the plane of the singularity. Spinner, you’re going to have to operate your discontinuity drive in parallel with the hyperdrive. The fractions of a second between jumps, when we’re in normal space, will be enough to let us begin our normal-space acceleration.’

  Spinner felt sweat trickle over her face, pooling under her eyes behind her spectacles. She was afraid, suddenly, she realized: but not of the singularity, or what might lie beyond, but of failing. ‘That’s ridiculous, Lieserl. How am I supposed to pull that off? What am I, a spider-monkey?’

  Lieserl laughed. ‘Well, I’m sorry, Spinner-of-Rope. We’re making this up as we go along, you know . . .’

  ‘I can’t do it.’

  ‘I know you can,’ Lieserl said calmly.

  ‘How do you know?’

  Lieserl was silent for a pregnant moment. Then she said, ‘Because you have help. Don’t you, Spinner-of-Rope?’

  And Spinner felt the warm hands of Michael Poole close over hers once more, strong, reassuring.

  The discontinuity-drive wings unfurled behind the hulk of the lifedome, powerful and graceful.

  ‘If it’s any consolation, Spinner, we’ll be a spectacular sight as we hit the plane,’ Lieserl said. ‘We’ll shed our Kerr plunge radiation in a single burst of gravity waves . . .’

  The singularity plane was widening; it was a disc, filled with jumbled starlight, opening like a mouth.

  ‘Michael, will there be photino birds, in the new universe?’

  I don’t know, Spinner.

  ‘Will there be Xeelee?’

  I don’t know.

  ‘I want you to come with me.’

  I can’t. I’m sorry. The quantum functions which sustain me don’t traverse the plane of the singularity.

  The Xeelee ‘fighters swirled around her cage, graceful, their night-dark wings beating. They filled space to infinity, magnificent here at the heart of their final defeat. The plane of the singularity was a sea of silver light below her.

  The construction material of her cage, of the wings, began to glow, as if white-hot.