Xeelee Redemption

by Baxter, Stephen

  Nicola grinned. ‘Imagine. Yeah. This is a wet dream for you, isn’t it? Did Jules Verne write about a ring around a black hole?’

  ‘Not Verne, though he could have handled the maths. Later thinkers did, though. Storytellers. Visionaries, edge-of-the-possible engineers. The structures they imagined had names like Bishop rings and Banks orbitals and Niven ringworlds.’

  Asher said, ‘We ourselves built stations like Larunda – a wheel a couple of kilometres across, turning at a revolution a minute to give you that one-gravity spin weight. But you could imagine a much bigger artefact. A wheel a few kilometres wide would turn in an hour, say. And a one-gravity wheel a few million kilometres across would have to turn in a day. There are limits to the tensile strength of any material, though; enough spin would pull it apart. The tension in the structure scales as the square of the spin velocity. You can’t build a one-gravity wheel out of steel much bigger than a few kilometres wide. With graphene you might reach a thousand kilometres.’

  Nicola waved a hand. ‘And to build a thing two light years across? With a velocity of lightspeed, nearly?’

  Jophiel shrugged. ‘This is Xeelee engineering. Remember we guessed that the Xeelee used cosmic string to hold their Great Attractor artefact together? Maybe . . .’

  Asher nodded. ‘Good thought. I’ll look for that.’

  Jophiel sighed. ‘Maybe Michael will get the chance to figure it out. There are other technical issues. To put it mildly. Such as, isn’t this thing dynamically unstable? If it is fixed on a central mass, like the black hole. Eventually it slips off centre, until one edge comes into contact with the middle. If it hasn’t shaken itself to pieces by then.’

  Asher said, ‘Somehow I doubt very much that that is a problem for the Xeelee. There are probably layers of subtle engineering beyond our capabilities.’

  ‘But it still makes no sense,’ Nicola broke in. ‘The whole design. Yes, this thing is clearly a Xeelee artefact. Maybe Asher is right that there are habitable zones, in those splashes of mud in the decks. But why one Earth gravity?’

  ‘Not exactly,’ Asher murmured. ‘But near enough.’

  ‘Why would the Xeelee build some monstrous habitat to suit us?’

  That stumped Jophiel. ‘Good point,’ he said grudgingly.

  But Asher said diffidently, ‘Actually it does make a certain sense, Nicola. There is a peculiarity about the distribution of the gravity fields of planets . . .’

  Planets came in all sizes, Asher said, from barely spherical balls of rock and ice smaller than Earth’s Moon, all the way up to monstrous gas giants larger than Jupiter, blending into brown dwarfs and dwarf stars at the upper end of the mass scale.

  ‘And you might think,’ she said, ‘that the bigger a world is, the higher its surface gravity. Because a bigger planet has more mass. But it doesn’t work out that way. If you keep adding mass, there are complications. Rocky worlds get denser, with liquid cores. Giants of ice and gas heat up in their interiors, so they expand, and become less dense. So, it turns out, the gravity field graph has a kink in the curve . . .’

  Jophiel, intrigued, pulled up some numbers on a softscreen. ‘It shows up in the Solar System. This never struck me before, but she’s right. Venus has – had, before the Xeelee destroyed it – eighty per cent Earth’s mass, ninety per cent the gravity. Neptune had seventeen times Earth’s mass, but only fourteen per cent higher gravity, in the high clouds anyhow. Saturn ninety-five times Earth’s mass, but only six per cent higher gravity. Quite a plateau.’

  ‘Not only that,’ Asher said, ‘the exobiologists came to believe that life was more likely to evolve on these transitional worlds – the plateau worlds. Neither too small so they lack a gravity field strong enough to retain an atmosphere, nor too large so the pressures are crushing . . . And therefore, if you had to build a single habitat to host a wide variety of planetary life forms, your best bet for the artificial gravity would be—’

  ‘About Earth’s,’ Nicola said. ‘OK. I’m impressed by the logic. But why build a habitat? And why on this scale?’

  ‘I suspect that when we know that,’ Jophiel said, ‘we’ll know most of it.’

  Asher said, ‘The flyby is over. We probably ought to shut down until the next mission milestone. This little missile we’re living in has plenty of juice, but we don’t want to burn out before we get to Chandra. There’s one more thing you ought to see before we go to sleep, though. Another clue – not about what this thing is, but how the Xeelee built it. Maybe, anyhow . . .’

  She assembled a series of images, to show them what she meant. Objects, swarms of them, crossing space in immense streams.

  Both Nicola and Jophiel recognised them immediately.

  Boxes of Xeelee hull plate, or so it appeared. Each the size of the object that had once been retrieved by the Xeelee from its five-billion-year storage deep in the crust of Mercury, then grown in the light of the Sun, and sent on a destructive course through the Solar System: a box a thousand kilometres on a side, a box big enough to contain a small moon. Humanity had called it the Cache.

  Now they saw thousands of caches. Millions. Uncountable.

  The individual caches were actually following orbits around the black hole, Asher said, elongated elliptical orbits that seemed to reach down to the black hole itself, and then came back out all this way, a light year out – orbits that must take millennia to complete.

  Asher was able to pick out streams of the structures, running more or less parallel: one heading into the black-hole system, towards Chandra, the other coming back out the other way, and heading for the Wheel.

  ‘I’m tweaking our course,’ Asher said. ‘To parallel the cache flow. You see, I figure this flow might have something to do with the Wheel’s constructon. Well, the Cauchy crew can figure out what happens to these caches when they reach the Wheel. Our job is to see what happens at the black hole.

  ‘We’re following the stream in, towards Chandra.’

  And Jophiel could only stare at a flow of material organised on a scale that matched the distances between the stars in the vicinity of Sol.

  Another sleep. Another period of unconsciousness.

  Another waking, to fresh wonders.

  38

  Ship elapsed time since launch: 21 years 240 days

  Jophiel found himself stretching, reflexively, as if waking. But he hadn’t been asleep. Rather, non-existent.

  He checked the screens. Over seven months this time, since the last revival. More than a year and a half, by the mother ship’s clocks, since the probe had left the Cauchy. And, outside—

  ‘Quite a view,’ Nicola said.

  The cache river was close now.

  And it snaked through a sky full of stars. Crimson globes, many of them near enough to show as discs.

  Jophiel ought to feel exhilarated by such a sight. He felt anything but. ‘Another little death,’ he said gloomily. ‘Another unwelcome birth. And only, what, a half light year out from the black hole?’

  Asher laughed. ‘Just look, Poole. Look . . .’

  Jophiel looked, and saw it.

  An artefact. Not the wheel. Something else. Something closer.

  That was obvious from his first glance. A plane sliding over the top of his field of view – sharp-edged, but the detail of its face elusive, even in the bright light of the swarming stars. And the colours were sombre, deep rust-reds and browns. The colours of a Martian autumn, he thought.

  Still that great plane unfolded over them, and still more. Ever wider.

  ‘Asher, can you give us an overall view?’

  ‘We did swoop in from far out . . .’

  The viewpoint zoomed out smartly. The window-filling plane dwindled, all its edges gathering – a triangular face, then – and then the complete object slid into view.

  ‘Lethe,’ Nicola said. ‘Another tetrahedron. Ma
ybe tetrahedra are some kind of universal. How big, Asher?’

  ‘Over fifteen million kilometres to an edge.’

  When Jophiel tried to make out the fine detail on that upper face, he lost his way again. It was like looking for patterns in frost congealing on a window. Indeed it looked fractal, a nesting, structure within structure. The artefact combined the finest of scales with the largest, it seemed.

  It hung in the complex sky, motionless, elusive.

  ‘What’s it for?’ Nicola snapped. ‘Any signs of life?’

  ‘No.’

  ‘Motive power?’

  ‘It seems to be following its own long, unpowered orbit around Chandra. Safely away from the accretion disc and other hazards. Just sitting there. The interior is very complex – well, you can see that. But it is . . . inert. Passive. My guess is it’s some kind of observing instrument. That complex interior might reflect a rich memory store. The thing could be very ancient,’ Asher murmured. ‘I’ve no way of proving that, but—’

  ‘It feels that way,’ Jophiel said.

  ‘Yeah. And this isn’t the only, umm, artefact I’ve encountered here. I didn’t wake you for them all.’

  She gave them a quick show and tell. A blizzard of three-dimensional dioramas. Much of this had been glimpsed from afar, and much of it was bewildering.

  A little family of cylinders, tumbling over each other like baby mice.

  A crumpled sphere that looked no bigger than the Cauchy’s lifedome, orbiting a treelike structure of branches and sparkling leaves.

  Bundles of spheroids and tetrahedrons, pencils and rods and wands.

  Jophiel found it hard to retain the detail. There was no sign of life, no purposeful movement.

  ‘Wrecks,’ Nicola said. ‘The sky is full of wrecks.’

  ‘Well, it’s a sparse scattering – but yes. Wrecks of what look like ships, what look like habitats – and stuff we can’t classify, like this big fractal tetrahedron.’

  Jophiel thought it over. ‘We came into this space, around the black hole, from a more or less random direction. And if we’re encountering this much clutter here—’

  ‘It must be everywhere,’ Asher said. ‘Throughout the volume around the black hole.

  ‘But it makes sense, if you think about it. These inner few light years are the most significant region in this entire hundred-thousand-light-year-wide Galaxy. The Xeelee came here; we followed it. The same idea must have occurred to other starfaring species. Everybody is going to send a probe here, looking for – what? Treasure? Scientific understanding? Their gods?’

  Nicola grunted. ‘Or vengeance, as we have.’

  Jophiel nodded. ‘And a fair proportion didn’t make it out alive. Billions of years of detritus, trapped in this giant filter. Who knows what wonders we are hurrying past here? Archaeological, cultural, scientific.’

  ‘And who knows,’ Nicola said drily, ‘what we might have found here in that other timeline, Jophiel? Human fortresses. Raging battles. Billions of corpses, maybe. Oh, and a statue.’

  Asher said, ‘I’m recording everything I see. Sending it all back to the Cauchy.’

  Nicola said, ‘These relics are depressing. Wake me when there’s something life-threatening.’ And she lay back in her couch, mummy style, eyes closed, hands crossed over her chest.

  39

  Ship elapsed time since launch: 22 years 84 days

  Nova dreams.

  Dreams of burning forests, boiling seas. Of spacecraft fleeing a wall of light.

  ‘. . . We are still following the cache stream, as it follows its own three-thousand-year orbit in from the Wheel, from a light year out . . .’

  Jophiel woke with a start, sat up in his couch.

  Nova dreams.

  He tried to concentrate on where he was. He stared out of the window.

  Dead ahead, a black sun, rising from a sea of fire. It wore a halo of white gold, smoothly drawn on one side.

  The black hole.

  Asher murmured, ‘Jophiel? Are you OK?’

  Jophiel felt as if he was having trouble waking, in this latest revival. Asher’s words came into focus slowly, and his vision more slowly still. ‘I’m fine.’

  Nicola snapped, ‘No, you’re not. This gets worse every time you’re revived.’

  ‘I’m . . . sorry.’

  ‘I mean, look at that.’ She pointed at the view. ‘We made it. There’s Chandra. A supermassive black hole. Another first for human eyes, or quasi-human, and you made it here. Even big brother Michael never got this far, and never will. So why are you depressed?’

  ‘Who says I’m depressed?’

  ‘Come on, Poole. I know you. I met your mother.’

  ‘Maybe the support software is faulty. Or my memory store got corrupted.’

  ‘It’s not that, Jophiel,’ Asher said gently – more gently than Nicola anyhow, Jophiel thought wryly. ‘I think she might be right. Virtuals are as human as their originals – or at least they start out that way. With humans, depression is a complex thing, a negative feedback loop between body and mind. Similarly there could be a deleterious feedback between your projected consciousness and the systems supporting you. You feel bad, the software develops bugs, the memory corrupts, you feel even worse . . . And I think I know the cause. After one of the downloads from the Cauchy, your consciousness has been synced with a message that came in from Goober c. From the partial you left there, the witness.’

  ‘I’ve been dreaming of novas,’ Jophiel blurted.

  Nicola laughed again. ‘Are Virtuals supposed to dream?’

  ‘Shut up, Nicola. Jophiel – nova dreams?’

  Jophiel sighed. He closed his eyes. ‘I’m fleeing in a ship which can’t outrun the light.’

  ‘This is clearly deriving from your upload from the Goober c witness. It will have sent its upload at lightspeed – a message that only just recently caught up with the Cauchy, and has been passed on to you.’

  Jophiel smiled faintly. ‘So I’m not going crazy.’

  Nicola said, ‘Maybe you should embrace the discomfort you’re feeling, even the confusion, as a tribute to it. The witness. It was another you, and it died, after all. And it’s all an entirely rational process.’

  But Jophiel wasn’t so sure about that. Because, floating in his mist of a memory, was an image of that other version of himself, older, in that battered coverall. His wry smile. How to figure out what his presence meant?

  ‘I guess I earned this pain. But, Asher, if you think some problem I have is compromising the mission, you have my permission to close me down.’

  Nicola just looked at him. ‘You know, I always wondered why it was you who came on this trip. I mean, a Jophiel copy, as opposed to a new spin-off from Michael, the template. A fresh copy instead of a faded second-generation clone.’ You’re still him. You still bear his monstrous freight of guilt, don’t you? Anyhow, you won’t need to “survive” much longer, will you?’

  ‘What do you . . . oh.’

  And Jophiel started to take in where he was.

  Chandra was close. And the mission was coming to an end.

  The black hole itself was the heart of a complex system. It sat in a sea of fire, centred on that great hemispherical dome of darkness. And that dome itself was rimmed by brilliant white-gold light, a pure arc. Like a distorted dawn, Jophiel thought.

  ‘That lake of fire. The accretion disc?’

  Asher said, ‘We’ll make an astrophysicist of you yet, Jophiel. We are about five astronomical units from the event horizon. Meaning, about the orbit of Jupiter.’

  ‘The orbit of Jupiter,’ Nicola said. ‘Sounds an awfully long way. But in this strange system we’re falling awfully fast.’

  ‘Right. How long before—’

  ‘Forty-four minutes,’ Asher said. ‘Before the closest approach to the eve
nt horizon. Our velocity is over ninety per cent of lightspeed; our mission was to get here as fast as possible, so we didn’t decelerate. I unfolded the light aberration . . . I thought we deserved to see the view. We aren’t going to fall into the black hole itself. We won’t get that far. But—’

  ‘We know the plan,’ Jophiel said. ‘This probe was designed as a one-way mission. I designed it that way. At closest approach, or anyhow at closest safe approach, we’ll send a message back to the Cauchy. They’ll receive it in a year and a half – as they are one and a half light years out – and all the data we’ve retrieved, and the essences of us, will be returned. Reintegrated into the memories of our templates. We will be remembered. Everything we saw, felt – even this conversation, I guess. But we—’

  ‘We all knew the deal,’ Nicola said brutally. ‘You said it yourself, Poole, when we woke up. Your first words.’

  ‘I did?’

  ‘“Lucked out.”’

  He forced a smile. ‘Should be on my gravestone.’

  ‘You’re not going to get a gravestone. Still less a statue.’

  ‘But what we have got,’ Asher said, ‘is work to do.’

  They all shut up and focused.

  And the fragile little missile, still tracking a stream of Xeelee caches like a cautious fish following a school of whales, fell steadily in towards the centre of everything.

  The accretion disc itself was extraordinary. As the Virtual flitter, nestling in a ragged stream of caches, flew low over the swirling pool of debris, Jophiel stared out, stunned, baffled. Delighted.

  Cautiously Asher identified features. Such as a long, streaky wound, evidently the relic of a recent explosion. ‘Most of the accretion disc is star stuff. Every ten thousand years or so a star wanders in too close to the black hole. The tides get hold of it, and it is stretched out, turned into a tube of hydrogen – still fusing; the magnetic and gravitational fields close to the hole keep hold of it that tightly. But when it passes closest approach, those fields weaken, and the star explodes, with the savagery of a hundred supernovas. But even that is just a detail, this close to Chandra.’