Phase Space

by Stephen Baxter

  Research and development, carried on by a community of goddamn seals, with no hands or tools. Greenberg didn’t know how they did it. It was one of the many things about the Weissmans he didn’t understand.

  He did know they were trying to extend their consciousness. Mind, it seemed, was a quantum process, intimately bound to the structure of space and time. And in space, after ten thousand years free of the distortions of the muddy pond of atmosphere at the bottom of Earth’s gravity well, consciousness – the Weissmans claimed – was taking a huge evolutionary leap forward, to new realms of power and control and depth.

  Maybe.

  To Greenberg, it was all very well to dream of super-minds of the future, but right now, he suspected there was nobody left, for instance, who was giving thought to pushing a troublesome asteroid out of its orbit, where once the children of man had rearranged worlds almost at will.

  And, Greenberg was coming to realize, that might make a big difference in the future.

  He still had some of his old monitoring systems, or patiently reconstructed copies anyhow. He studied Ra’s evolving trajectory around the sun.

  And, gradually, he’d learned something that had disturbed him to his core.

  Near-Earth asteroids wandered in steadily from the main belt, their orbits tweaked by the gravity of Jupiter, Venus, Mars and Earth itself. They hung around for thirty megayears or so, their orbits slowly evolving. Then they would encounter one of three fates, with equal probability: they would hit Earth, or hit Venus, or be slingshot out of the system altogether.

  The cratering record on Earth showed this had been going on for billions of years. The smaller the object type, the more frequent the collision. Every few thousand years, for instance, Earth would be hit by an object a hundred metres or so across, big enough to dig out a new Meteor Crater, as in Arizona, where Apollo Moonwalkers had once trained. Earth had actually suffered a few fresh strikes like that while Greenberg had been observing.

  And every few tens of millions of years, a much larger body would strike.

  Such an object had struck the Earth sixty-five million years ago, at Chicxulub in Mexico. It had caused the extinction of most of the species extant at that time.

  It was known as the dinosaur killer.

  Earth was overdue for another impact like that.

  Near-Earth asteroid orbits were pretty much chaotic. It was like the weather used to be, back when he lived in a place that had weather. But as computers had gotten smarter, the path of Ra-Shalom had been pushed out, in the computer’s digital imagination, further and further. Finally it had become clear to Greenberg what Ra’s ultimate fate would be.

  Ra wasn’t going to hit Venus, or be thrown out of the system to the stars. Ra was going to hit Earth.

  Ra was the next dinosaur killer.

  It was a long time ahead: all of a million years from now. But it worried him that right now, nobody seemed to know how to deflect this damn rock.

  Whenever he got the chance, he sounded off about the dinosaur killer problem. The Weissmans told him they had plans to deal with it, when the time came. Greenberg wasn’t sure whether he believed that.

  And he wasn’t sure he wanted to be around to see this chewed-up rock auger in on the surface of the planet where he was born. But he couldn’t turn his back.

  Within the confines of the tiny world, civilizations fell and rose; by turns, the refugee race fell to barbarism, or dreamed of the stars. The guardians had planned for this.

  But the little world was not stable. This they had not anticipated.

  Its orbit was close to a resonance with that of Jupiter: it circled the sun three times in each of Jupiter’s stately years. The powerful tug of Jupiter worked on the asteroid’s trajectory, millennium after millennium.

  Quite suddenly, the orbit’s ellipticity increased. The asteroid started to swing deep into the warm heart of the solar system.

  There was nothing the inhabitants could do to steer their rock. Some adapted. Many died. Superstitions raged.

  For the first time, the asteroid dipped within the orbit of Earth.

  Scale: Exp 5

  … Crossing time in unimaginable jumps, drifting between sleeping and waking, eroding towards maximum entropy like some piece of lunar rock …

  He never knew, he didn’t understand, he couldn’t believe how much time had passed. A hundred thousand years? It was a joke.

  But even the sky was changing.

  The nearby stars, for instance: Alpha Centauri and Barnard’s Star and Sirius and Procyon and Tau Ceti, names from the science fiction of his youth. You could see the changes in the light, the stain of oxygen and carbon, chlorophyll green. Even from here you could see how humans, or post-humans anyhow, had changed the stars themselves.

  And to think he used to be awed by the Vehicle Assembly Building at Canaveral.

  And the expansion must be continuing, further out, inexorably. On it would go, he thought dimly, a growing mass of humanity filling up the sphere centred on Sol, chewing up stars and planets and asteroids, until the outer edge of the inhabited sphere had to move at the speed of light to keep up, and then what would happen, he wanted to know?

  But none of that made a difference here, in the ancient system of Sol, the dead heart of human expansion. It was hard for him to trace the passing of the years because so little changed any more, even on the heroic timescales of his intervals of consciousness.

  Conditions in a lot of the inhabited rocks had converged, in fact, so that the worlds came to resemble each other. Most of them finished up with the kind of simple, robust ecosystem that sustained Ra, even though their starting points might have been very different. It was like the way a lot of diverse habitats on Earth – forests and jungles and marshes – would, with the passage of time, converge into a peat bog, the same the world over, as if they were drawn to an attractor in some ecological phase-space.

  And most of the rocks, drifting between uninhabited gravity wells, were about as interesting as peat bogs, as far as Greenberg was concerned.

  Meanwhile, slowly but inexorably, life was dying back, here in the solar system. which had once hosted billions of jewel-like miniature worlds.

  There were a lot of ways for a transformed asteroid to be destroyed: for instance, a chance collision with another object. Even a small impact on a fragile bubble-world like Ra could puncture it fatally. But nobody around seemed capable of pushing rocks aside any more.

  But the main cause of the die-back was simple ecological failure.

  An asteroid wasn’t a planet; it didn’t have the huge buffers of mass and energy that Earth had. A relatively small amount of matter circulated in each mass loop, and so the whole thing was only marginally stable, and not always self-recovering.

  It had even happened here, on Ra-Shalom. Greenberg had woken once to find concentrations of the amino acid called lysine had crashed. The Weissmans were too busy on dreaming their cetacean dreams to think too much about the systems that were keeping them alive. Many died, before a new stability was reached. It drove Greenberg crazy.

  But the Weissmans didn’t seem too upset. You have to think of it as apoptosis, they said to him. The cells in the hands of an archaic-form human embryo will die back in order to sculpt out tool-making fingers. Death is necessary, sometimes, so that life can progress. It is apoptosis, not necrosis …

  Greenberg just couldn’t see that argument at all.

  And in the meantime, Ra was still on its course to become the next dinosaur killer. The predictions just got tighter and tighter. And still, nobody seemed to be concerned about doing anything about it.

  When what the Weissmans said to him made no sense at all – when they deigned to speak to him – Greenberg felt utterly isolated.

  But then, all humans were alone.

  Nobody had found non-terrestrial life anywhere, in the solar system or beyond, above prokaryotes: single-celled creatures without internal structures such as nuclei, mitochondria and chloroplasts. Mars
was typical, it had turned out: just a handful of crude prokaryote-type bugs shivering deep in volcanic vents, waiting out an Ice Age that would never end. Only on Earth, it seemed, had life made the big, unlikely jump to eukaryotic structure, and then multi-celled organisms, and the future.

  It seemed that back when he was born Earth had been one little world holding all the life there was, to all intents and purposes. And it would have stayed that way if his generation and a couple before, Americans and Russians, hadn’t risked their lives to enter space in converted ICBMs and ridiculous little capsules.

  Makes you think, he reflected. The destiny of all life, forever, was in our hands. And we never knew it. Probably would have scared us to death if we had.

  For if we’d failed, if we’d turned ourselves to piles of radioactive ash, there would now be no life, no mind, anywhere.

  Gravitational tweaks by Earth and Venus gradually wore away the asteroid’s energy, and its orbit diminished. The process took a hundred million years.

  At last, the asteroid with its fragile cargo settled into a circle, a close shadow of Earth’s orbit. Its random walk across the solar system was complete.

  The inhabitants adapted. They even flourished, here in the warmer heart of the solar system.

  For a time, it seemed that a long and golden afternoon lay ahead of the refugees within the rock. Once more, they forgot what lay beyond the walls of their world …

  But there seemed to be something in the way.

  Scale: Exp 6

  We have an assignment for you.

  He came swimming up from a sleep as deep as death. He wondered, in fact, if he was truly in any sense alive, between these vivid flashes of consciousness.

  … And Earth, ocean-blue, swam before Ra, a fat crescent cupping a darkened ocean hemisphere, huge and beautiful, just as he’d seen it from a Shuttle cargo bay.

  In his vision there was water everywhere: the skin of Earth, the droplet body of Ra-Shalom, and in his own eyes.

  We have an assignment for you. A mission.

  ‘What are you talking about? Are you going to push this damn rock out of the way? I can’t believe you’ve let it go this far.’

  This has happened before. There has been much apoptosis.

  ‘Hell, I know that …’

  He looked up at a transformed sky.

  Everywhere now, the stars were green.

  There was old Rigel, for instance, one of the few stars he could name when he was a kid, down there in Orion, at the hunter’s left boot. Of course all the constellations had swum around now. But Rigel was still a blue supergiant, sixty thousand times more luminous than the sun.

  But now even old Rigel had been turned emerald green, by a titanic Dyson cloud twice the diameter of Pluto’s orbit.

  Not only that, the people up there were starting to adjust the evolution of their giant star. Rigel only had a few million years of stable life – compared to Sol’s billions – before it would slide off the Main Sequence and rip itself apart as a supernova.

  But the people up there were managing Rigel, managing a goddamn supergiant, deflecting its evolution into realms of light and energy never before seen in the history of the universe. And that emerald colour, visible even to a naked archaic human eye, was the symbol of that achievement.

  It was a hell of a thing, a Promethean triumph, monkey paws digging into the collapsing heart of a supergiant.

  Nobody knew how far humans had got from Earth, or what technical and other advances they had achieved, out there on the rim. But if we don’t have to fear supernovas, he thought, we need fear nothing. We’ve come a long way since the last time I climbed into the belly of a VentureStar, down there at Canaveral, and breathed in my last lungful of sea air …

  … an assignment, the Weissmans were saying to him.

  Earth swam close, and was growing closer.

  We want to right the ancient necrosis as far as we can. We want you to help us.

  ‘Me? Why me?’

  It is appropriate. You are an ambassador from exponent zero. This is a way of closing the loop, in a sense. The causal loop. Do you accept?

  ‘I don’t know what you’re talking about. I accept. I don’t know what you mean …’

  … The walls of the hab module dissolved around him. Suddenly he didn’t have hold of anything, and he was falling.

  Oh, shit, he thought.

  But there were shadows around him, struts and blocks. And a heavy, liquid mass at his lower body he hadn’t felt for a long time.

  Legs. He had legs.

  His breathing was loud in his ears. Oxygen hissed over his face.

  He was back in his Shuttle-era pressure suit, and he was encased in his PMU once more, the original model, its spidery frame occluding the dusting of stars around him.

  He grasped his right-hand controller. It worked. There was a soft tone in his helmet; he saw a faint sparkle of exhaust crystals, to his left.

  Still, Earth swam before him.

  It is time.

  ‘Wait – what –’

  Earth was gone.

  Ra-Shalom sailed through the space where the Earth had been, its meniscus shimmering with slow, complex waves as it rolled, the life at its heart a dim green knot against the blue.

  My God, he thought. They pushed Earth aside. I didn’t know they got so powerful –

  ‘What did you do? Is it destroyed?’

  No. Earth is in a stable orbit around Jupiter. The ice will return, for now. But later, when the sun starts to die, Earth will be preserved, as it would not have been –

  ‘Later?’

  We must plan for exponent seven, eight, nine. Even beyond. The future is in our hands. It always has been.

  ‘But how –’

  Goodbye, the Weissmans said, a tinny voice in the headphones in his Snoopy hat. Goodbye.

  And now there was another hulking mass swimming into view, just visible at the edge of his faceplate.

  He worked his attitude thrusters, and began a slow yaw. Strange, he didn’t seem to have forgotten any of the old skills he had practised in the sims at Houston, and in LEO, all those years ago.

  He faced the new object.

  It was an asteroid. It looked like Ra-Shalom – at any rate, how that rock had looked when he first approached it – but it was a lot bigger, a neat sphere. The sun’s light slanted across craters and ravines, littered with coal-dust regolith. And there was a structure there, he saw: tracings of wire and panelling, bust up and abandoned, and a big affair that stuck out from the rock, a spider-web of wires and threads. Maybe it was an antenna. Or a solar sail.

  Artefacts.

  It looked like the remains of a ship, in fact. But not human.

  Not human. My God, he thought.

  And now the light changed: to the stark planes of the sun’s eternal glow was added a new, softer glow.

  Water blue.

  He turned, clumsily, blipping his attitude thrusters.

  Earth was back, a fat crescent, directly ahead of him. This is a hell of a light show, he thought.

  But Earth looked different. It had spun around on his axis. Before he’d been over the Pacific; now he could make out, in a faint dawn glow, the familiar shapes of the continents – North and South America, painted over the ocean under bubbling wisps of cloud.

  There were no lights, anywhere. And the arrangement of continents didn’t look right. Earth didn’t match his memories of schoolroom globes, under the Stars and Stripes, back in Iowa.

  The Atlantic looked too skinny, for instance.

  This new rock was heading for Earth, just like Ra-Shalom had been. It couldn’t be more than a few minutes from reaching the atmosphere. And it looked to him as if it was going to hit somewhere in Mexico …

  Oh, he thought. I get it.

  This was the dinosaur killer, the original, destined to gouge out a two-hundred-kilometre crater at Chicxulub, and to have its substance rained around the planet.

  He shielded his eyes with a gloved
hand, and studied the stars.

  They were different. The stars were bone white: no green, anywhere.

  He was displaced in time, a long way. But this was not the far future, but the deep past.

  He turned again to face the plummeting rock, with its fragile cargo of artefacts.

  One last time the kerosene thrusters fired, fat and full. The asteroid started to approach him, filling his sky. The suit was quiet, warm, safe.

  He just let himself drift in, at a metre or so a second. The close horizon receded, and the cliff face turned into a wall that cut off half the universe.

  He collided softly with the rock. Dust sprays were thrown up from around the PMU’s penetrator legs. Greenberg was stuck there, clinging to the surface like a mountaineer to a rock face.

  He turned on his helmet lamp. Impact glass glimmered a few centimetres from his face. He reached out and pushed his gloved hand into the compacted-snow surface, a monkey paw probing.

  … There was something here. Something alive, something sentient, inside the rock. He could feel it, though he couldn’t tell how.

  Maybe the Weissmans were using him as some kind of conduit, he thought. Maybe they wanted to save some of whatever was here from the destruction of the rock, take it with them to whatever future awaited mankind.

  Or maybe it was just him.

  He smiled. He was a million years old after all; maybe a little of the Weissman had rubbed off on him.

  He took a handful of dust and pulled out his hand. A cloud of dust came with it that gushed into his face like a hail of meteorites, glittering particles following dead-straight lines.