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

by Stephen Baxter

  Governed by the Northern’s processors, ‘bots had drawn in the straps comprising the cage, slowly and steadily tugging the lifedome against the nightfighter.

  ‘So,’ the Planner said, ‘the strap arrangement hugs the nightfighter tightly against us, without fixing us to it. But that’s obviously enough to persuade the ‘fighter to carry the lifedome safely through hyperspace.’

  ‘And - creep?’

  ‘Because the cage is not fixed to the ‘fighter - and because we are subject to various stresses - the cage’s bands slip over the construction-material surface. They creep. But we have nanobots out there working continually, readjusting the straps and compensating for stress.’

  Lieserl nodded. ‘It’s a smart solution, Milpitas.’

  He bowed, sardonically. ‘Perhaps. But I can’t take the credit for it. I merely implemented the design which—’

  Suddenly she felt a stab of pity for this scarred, stunted man. ‘Don’t underestimate yourself,’ she said on impulse. ‘Believe me, you’ve achieved so much . . .’

  ‘For a madman?’ he asked disarmingly. He smiled at her. ‘I know you think I’m a rather foolish, rigid person, Lieserl.’

  Startled, she opened her mouth to deny this, but he held up his hand.

  ‘Well, perhaps I am. But I was responsible, in large part, for the teams of ‘bots which constructed this frame for the nightfighter. I know that our sensors could tell us much more about the state of the infrastructure which fixes us to this nightfighter than my naked gaze ever could. And yet—’

  ‘And yet, you feel you want to see it for yourself?’ She smiled. ‘You’re wrong, Planner. You’re not the easiest person I’ve ever had to get along with, but I don’t think you’re a fool to follow your instincts.’

  He studied her, coolly appraising. ‘You believe so?’

  ‘I know so,’ she said firmly. ‘After all, that was the whole point of my stay in the Sun - in fact, the point of my very existence. Plenty of probes were dropped into the Sun ahead of me, and after me. I was sent in so that - at least through a surrogate - human eyes could see what was happening in there.’

  He grunted. ‘Although, it seems, we made precious little use of the insights you gained.’

  ‘That’s as may be.’ She laughed. ‘But I couldn’t control that.’

  He studied her. ‘You may be a surrogate,’ he said. ‘But, Lieserl, despite that, your humanity is powerful and obvious.’

  That left her confused. She kept her face straight, determinedly. She issued subvocal commands, overriding the autonomic simulation of her face; she was adamant that her cheeks shouldn’t show a hint of colouring. ‘Thank you,’ she said lightly. ‘Although I’m not sure you need thanks. You’re not proffering compliments, are you? I suspect you don’t praise, Planner; you appraise,’ she said.

  ‘Perhaps.’ He turned away, closing the subject.

  She studied his battered profile. Milpitas gave the impression of a man in control, but maybe he gave away more than he bargained for. With Milpitas, the communication of information was only one function - and a subsidiary one at that - of speech. The real purpose of conversation, for Milpitas, was control. She felt he was constantly fencing with her - testing her sharpness, and strength of will.

  This was a man who was used to power, and used to exerting it, even in the most trivial conversation. But what type of person was this who - after centuries of subjective existence - would bother to fence with a tired old Virtual like her?

  Milpitas continued his inspection, slowly, methodically.

  Perhaps he was a little less than human - less, even, than her, she thought. Still - she conceded warily - there was a core of strength in Milpitas she had to admire.

  Milpitas had been forced to watch his world - a world he’d controlled - fall apart, before his eyes. And he’d fought hard to preserve it. But then he’d stopped fighting, when he realized his old world was gone - that his beliefs were actually indefensible.

  And that was the hard part. That, she reflected, was the point from which the endless strings of martyrs strewn across mankind’s bloody history had failed to return. And since then he’d kept functioning - contributing to the mission.

  She grinned. ‘I think you’re tougher than you look, Planner Milpitas. I mean, you have managed to break out of the prison of your past . . .’

  He turned. ‘But the past is not a prison,’ he said softly. ‘The past is altered, constantly, by our actions in the present. Every new act revalues the meaning of the past . . .’

  She was surprised. ‘That sounds like the surface of a deep philosophy.’

  ‘Deep, and old,’ he said. He eyed her, the tracery of scars over his scalp vivid in the flat light of the loading bay. ‘We in Paradoxa were never one-dimensional oppressors, Lieserl. We saw ourselves as preserving the best of humanity’s wisdom, and we sought constantly to interpret our present and future in history’s light . . .’

  She grunted. ‘Hmm. Interesting. Perhaps the notion of a fluid past, recast in the light of our changing assumptions, is the only philosophy which will allow a race of immortals to stay sane. Maybe I’m still underestimating you, Milpitas.’

  He touched his control bar and, gently, rose into the air. His face was impassive. ‘Perish the thought,’ he said dryly.

  The Universe-image expanded, focusing on a comparatively small volume; Spinner studied a nondescript chunk of cosmic foam, a collection of threads, voids and sheets of shining matter.

  ‘Okay, Spinner-of-Rope: here’s a three-dimensional map of our neighbourhood. The voids are around a hundred million light-years across, on average.

  ‘Now here’s a local landmark - a famous void called the Hole in Boötes, two hundred million light-years across - and, look, here’s the Great Wall: the largest coherent structure in the Universe, a sheet of galaxies five hundred million light-years long.’ Louise paused, and when she spoke again her voice was darker, tinged with the resentment and half-suppressed anger Spinner had come to recognize. ‘Of course the Wall isn’t quite the tourist site it was when I was a girl,’ she said sourly. ‘The damn photino birds have been active there as well . . . All across the Wall, as far as we can observe, there’s evidence of bird degradation.’

  Spinner allowed herself to smile. She could imagine what Louise was thinking. Damn it, it’s our Wall!

  Louise was saying, ‘This cloud’ - a mist fragment the size of Spinner’s hand, labelled by a small red arrow - ‘is the Virgo Cluster. Our local supercluster.’ A small region within the Virgo cloud began to flash yellow, and a straight blue line snaked out of the yellow clump, piercing the heart of the Virgo. ‘The little yellow volume is the Local Group, where Sol is,’ Louise said, ‘and the line represents our journey so far with the nightfighter: right through the middle of the Virgo super-cluster.’

  Spinner grunted. ‘Not very far.’

  ‘Oh, come on, Spinner; think about the scale of this picture!

  ‘Now look at this,’ Louise said. Small, lime-green vector arrows appeared, bristling over the dusty surface of the Virgo Cluster. ‘See that? The whole of our supercluster is moving through space - and it’s at a significant speed, a million miles an hour or more. So fast that the motion was even observable from Earth - it imposed a Doppler shift on the whole Universe, Spinner: on the microwave background radiation itself.’

  Now more velocity arrows appeared on another massive cluster close to the Virgo Cluster. ‘There’s another supercluster, called Hydra-Centaurus,’ Louise said. ‘And guess what: that’s streaming in the same direction as the Virgo.’

  Velocity arrows bristled now all over the foamy region of space . . . and all the arrows, Spinner saw, pointed inwards, to an anonymous region at the heart of the three-dimensional diagram.

  And the projected blue line of the nightfighter’s voyage reached towards the centre of the immense implosion.

  ‘I know what that is,’ Spinner breathed. ‘At the centre of the implosion. That’s the Great Attra
ctor.’ The place all the galaxies are falling to . . .

  ‘Yes. There seems to be a mass concentration there, attracting galaxies across hundreds of millions of light-years. The Attractor is a hundred and fifty million light-years from Sol, and with the mass of ten thousand galaxies . . .’

  Staring into the toy Universe, Spinner-of-Rope felt her heart flutter. ‘And if it really is an artifact—’

  ‘If it is, then it’s an artifact so massive it’s drawing in superclusters like moths, Spinner; so massive it’s actually counteracting the expansion of the Universe, in this part of space . . . It’s an artifact beyond our imagination.’

  Yes, thought Spinner. Beyond imagination. And that’s where we’re heading . . .

  25

  ‘I don’t know why you had to drag me up here, into the forest,’ Louise grumbled. ‘Not now. Couldn’t you wait until you were sure of your data?’

  Mark said, ‘But the data—’

  ‘Is partial, and incomplete, and hardly conclusive. What have you got - just two double images?’

  ‘But the spectral match of the double galaxy images is almost perfect, in each case. I tell you it must be string,’ Mark insisted.

  ‘And I’m telling you that’s impossible,’ Louise growled. She felt her irritation rise. ‘How could there be cosmic string in the middle of a void like this?’

  Uvarov raised his skull-like face and cackled, relishing the conflict.

  The three of them were suspended just below the forest skydome. Louise was on a zero-gee scooter, and Uvarov had been strapped into a stripped-down life support chair attached to three of the flexible little scooters.

  Mark, irritatingly, was choosing to manifest himself as a disembodied head, twice life-size, hovering in the air. ‘How’s Spinner-of-Rope?’ he asked Louise.

  She grunted. ‘Bearing up. We’re thirty-three days into the mission, now - thirty-three days for Spinner in that couch. And the last ten of them inside this damn hole in the sky.’

  ‘Well, this is really a pretty exciting part of the journey,’ Mark said. ‘We’re crossing the edge of the greatest cosmological void ever detected: more than two hundred million light-years across. As far as we can tell, we’re the only scrap of baryonic matter in all that immensity. That’s an exciting thought even without my evidence of cosmic string . . .’

  ‘Not exciting for Spinner,’ Louise said dryly. ‘For her this void is nothing but sensory deprivation.’

  ‘Hmm,’ said Uvarov. ‘The Universe as an immense sensory deprivation tank . . . maybe that’s a good image to sum up the photino birds’ cosmic handiwork.’

  Now schematic graphics of remote galaxies - sheets of them, at the boundary of the huge void - peppered the dome with splashes of false colour; here and there fragments of text and supplementary images were interspersed amid the insect-like galactic swarms.

  Mark’s head swivelled around towards Louise. ‘Look, I’m sorry you don’t think it’s appropriate for me to have dragged you up here. Maybe I should have waited for proof of the string’s existence. Well, I didn’t realize we were out here to do science. I thought we were trying to find ways to stay alive - to anticipate what we’re up against. And that means reacting - and thinking, Louise - as quickly and as flexibly as possible. All right, maybe I’m guessing. But - what if it is cosmic string out there? Have you thought about that?’

  Louise turned her face, uncertainly, up to the dome. ‘If it is string - here - then, perhaps, we’re heading into something even more extraordinary than we’ve anticipated.’

  Uvarov chuckled. ‘Perhaps we should stick to the facts, my dear Mark.’

  ‘There are no facts,’ Louise said. ‘Only a handful of observations. And - across distances measured in hundreds of millions of light-years, and taken from a platform moving through a hyperdrive journey - they’re damned imprecise observations at that.’

  Uvarov turned his head to the Virtual. ‘Tell me about your observations, then. Why are these double images so all-fired important?’

  ‘I’ve been taking observations of the far side of the void’ the Virtual said. ‘I’ve been looking for evidence of gravitational lensing . . . The distortion of light from distant objects by the gravitational field of some huge, interposed mass. I wasn’t looking for strings specifically. I was trying to see if I could detect any structure within the void - any concentrations of density.’

  ‘Are the strings so massive, then, that they can distort light so far?’

  Louise said, ‘It isn’t really as simple as that, Uvarov. Yes, strings are massive: their width is only the Planck length, but their density is enormous - a one-inch length would have a mass of around ten million billion tons . . . a string stretching from Sol to Saturn, say, would have around one Solar mass. We expect strings to be found either in loops thousands of light-years across, or else they will be endless - stretched right across the Universe by the expansion from the singularity.’

  Uvarov nodded. ‘Therefore, if they are so massive, their gravitational fields are correspondingly huge.’

  ‘Not quite,’ Louise said. ‘Strings are very exotic objects. They aren’t like stars, or planets, or even galaxies. They simply aren’t Newtonian objects, Uvarov. The relativistic gravitational fields around them are different.’

  Uvarov turned to her. ‘Are you telling me the strings are antigravitational, like the domain walls of the nightfighter’s discontinuity-drive wings?’

  ‘No . . .’

  Far enough from a loop - a finite length of string - the mass of the string would attract other bodies, just as would any other massive object. But an observer close to a string, either a loop or part of an infinite string, would not experience the gravitational effects to be expected from such massive concentrations of matter.

  Louise said, ‘Uvarov, gravitational attraction works by distorting spacetime. Spacetime is flat if no heavy objects are present; an object will sail across it in a straight line, like a marble across a tabletop. But the spacetime close to a Newtonian object, like a star, is distorted into a well, into which other objects fall. But close to a string, spacetime is locally flat - it’s what’s called a Minkowski spacetime. Objects close by aren’t attracted to the string, despite the huge mass . . .’

  ‘But,’ Mark said, ‘the spacetime around a string is distorted. It is conical.’

  Uvarov frowned. ‘Conical?’

  ‘Imagine spacetime as a flat sheet. The presence of the string removes a slice from that sheet - like a slice of a pie, cut out of spacetime. What’s left of the spacetime is joined up - the hole left by the missing slice is closed up - so that the spacetime is like a cone. Still flat, but with a missing piece.

  ‘If you were to draw a circle around a string, you would find its circumference shorter than you would expect from its radius - it’s just like drawing a circle around the apex of a cone.’

  ‘And this small spacetime defect is sufficient to cause the double images you speak of?’

  ‘Yes,’ Mark said.

  A cosmic string wasn’t visible directly. But its path could be made visible, by a track of double images of remote objects, separated by about six arc seconds, along the length of the string.

  Louise said, ‘Uvarov, imagine two photons setting off towards us from a remote galaxy, beyond a string. One of them comes to us directly. The second, passing on the far side of the string, travels through the conical defect. The second photon actually has less distance to travel to reach us, thanks to the defect; its journey time is less than the first’s by around ten thousand years. Hence, the double images.’

  Uvarov grunted. ‘Louise, you have explained to me how the network of strings was the web around which the galaxies coalesced. I do not understand how this can be, if the gravitational effects of these strings are so slight.’

  Louise sighed. ‘The strings are primeval objects: they were formed within an invisible fraction of a second after the Big Bang itself, during the symmetry loss caused by the decomposition of
the unified superforce. Since then, the expansion of the Universe has stretched the strings. So the strings are under great tension - a tension caused by the expansion of the Universe itself . . . The strings whip through space, at close to the speed of light.

  ‘Where the strings pass, their conical defects cause them to leave a wake. Matter falls in towards the two-dimensional, sheet-like path swept out by the string. And it’s this infalling that caused the formation of the baryonic matter structures we observe now: clusters of galaxies, in threads and sheets.’

  ‘In fact,’ Mark said, ‘the wake is itself observable. Or should be. It imposes a slight Doppler shift on the microwave background radiation. I should be able to see a slightly brighter sky on one side of the invisible string than on the other . . .’

  ‘And have you seen this?’ Uvarov snapped.

  ‘No,’ Mark admitted. ‘Damn it. The Northern couldn’t be a much worse platform for this kind of measurement; the microwave Doppler is below my level of resolution.’

  ‘But you do think you’ve found some image pairs,’ Uvarov persisted.

  ‘Yes,’ Mark said, sounding excited again. ‘Two pairs so far, and a few other candidates. The two pairs are aligned, just as you’d expect them to be if a string is the cause . . .’

  ‘Enough,’ Uvarov snapped. He raised his chair into the air above them and prowled across the underside of the skydome, his ravaged profile silhouetted against the false colours of the galaxies. ‘Now tell me what this means. Let us accept, Louise, that your Virtual lover has found a fragment of this - string. So what? Why should we care?’

  ‘We’re in a void, Uvarov,’ Louise said patiently. ‘We’d expect to find string at the heart of huge baryonic structures - like the Great Wall, for instance, a sheet of clusters half a billion light-years long, which—’

  ‘But we are not at the heart of such a huge baryonic structure. Is that your point, Louise?’

  ‘Yes. That’s the point. There’s no reason why we should find string here, in this void, away from any concentrations of matter.’