Ring xs-4

by Stephen Baxter

  “No. But it’s approaching its helium flash point.”

  “Won’t the action of the birds suppress this helium runaway — the helium flash just as they’ve suppressed hydrogen fusion, all this time?”

  “No, Uvarov. They’re not taking out enough energy to stop the flash… Maybe they don’t intend to. And, of course, the fact that the core of Sol is so unusually hydrogen-rich is going to make a difference to the outcome. Perhaps there will be some hydrogen fusion in there as well, a complex multiple reaction.”

  “Mark. You said a new equilibrium will be reached, after the helium flash.” Uvarov didn’t like the sound of that. He wondered if it would be healthy to be around, while an artificially induced red giant struggled to find a new stability after the explosion of its core… “What will happen, after the helium flash?”

  “Well, the pulse of heat energy released by the flash will take time — some centuries — to work its way through the envelope. The envelope will expand further, seeking a new balance between gravity and radiation pressure. And the energy released in the flash will be immense, Uvarov.”

  “Immense?”

  “Uvarov, there will be a superwind.”

  Superwind…

  The helium flash would blow away half the mass of the Sun, into an expanding shell ballooning outwards at hundreds of miles a second.

  The core — exposed, a shrunken thing of carbon-choked helium — would become a white dwarf star: cooling rapidly, with half the mass of Sol but just a few thousand miles across, no larger than old Earth. The flocks of photino birds, insubstantial star-killers, would continue to swoop around the heart of Sol’s diminished gravity well.

  At present — before the flash — Sol was a red giant around two astronomical units across. After the superwind the envelope would be blown into a globe twenty thousand times that size, a billowing, cooling cloud three hundred light days across.

  The furthest planet from the heart of old Sol was only forty astronomical units out — six light-hours. So the swelling envelope would, at last, smother all of Sol’s children.

  Then, when the superwind was done, the dwarf remnant would emit a new wind of its own: a fizz of hot, fast particles which would blow at the expanding globe, pushing out the inner layers. The globe would become a planetary nebula — a huge, cooling, hollow shell of gas, fluorescing in the light of the dying dwarf at its heart.

  Mark said, “At last, of course, the fusing helium in the core will be exhausted. Then the core will shrink once more, until the temperature of the regions around the core becomes high enough for helium fusion to start — in a shell outside the core, but within the hydrogen-burning shell. And the helium fusion will deposit carbon ash onto the core, growing in mass and heating it up — until the fusion of carbon begins…

  “The cycle repeats, Uvarov. There will be carbon flashes — and, later, flashes of oxygen and silicon… At last, the giant might have a core of almost pure iron, with an onion-shell structure of fusing silicon, oxygen, carbon, helium and hydrogen around it. But iron is a dead end; it can only fuse by absorbing energy, not liberating it.”

  “And all this will happen to the Sun?”

  Mark hesitated. “Our standard models say that the reactions go all the way to iron only in stars a lot more massive than the Sun — say, twelve Solar masses or more.” He sighed, theatrically. “Will we get onion-shell fusion in the heart of the Sun? I don’t know, Uvarov. We may as well throw out our theoretical models, I guess. If the photino birds are as widespread as they seem to be, there may not be a single star in the Universe which has followed through a ‘standard’ lifecycle.”

  “Superwind,” Uvarov breathed. “How soon is Sol’s helium flash?”

  “Lieserl’s observations are sketchy on this. But, Uvarov, the conditions are right. The flash may even have happened by now. The superwind could already be working its way out…”

  “How soon, damn you?”

  “We have a few centuries. No more.”

  Uvarov swept his blind face around the saloon. He pictured the ruined Jovian system beyond these walls, the bloated star dominating the sky outside.

  “Then we can’t stay here,” he said.

  21

  By the time she’d climbed to the top of the giant kapok tree her hand-grips were slick with sweat, and her lungs were pumping rapidly. Spinner-of-Rope took off her spectacles and wiped the lenses on a corner of her loincloth. Zero-gee or not, it still took an effort to haul her bulk around this forest… an effort that seemed to be increasing with age, despite all the AS treatment in the world.

  She was at the crown of the kapok. The great tree was a dense, tangled mass of branches beneath her. Seeds drifted everywhere, filling the rippling canopy with points of light — like roaming stars, she thought. Somewhere a group of howler monkeys shrieked out their presence. Their eerie ululations, rising and falling, reminded her of the klaxon which had once called the Undermen to their dreary work…

  She put that thought out of her mind with determination. She pulled some dried meat from her belt and chewed on it, relishing the familiar, salty taste. She felt tired, damn it; she’d come here, alone, because she wanted — just for a few hours — to put all of the strangeness below the forest Deck, and beyond the skydome, out of her mind, to immerse herself once more in the simple world in which she’d grown up.

  In the distance a bird flapped, shrieking, its colors gaudy against the bland afternoon blue of the skydome.

  The bird was flying upside down.

  “Spinner-of-Rope.”

  The voice was close to her ear. Still chewing her meat, Spinner turned, slowly.

  Louise Ye Armonk hovered a few feet away, standing on the squat, neat platform of a zero-gee scooter. Louise grinned. “Did I make you jump? I’m sorry for cheating with this scooter; I’m not sure I would have managed the climb.”

  Spinner-of-Rope glared at her. “Louise. Never — never — sneak up on someone at the top of a tree.”

  Louise didn’t look too concerned. “Why not? Because you might lose your grip, and drift off the branch a couple of feet? What a disaster.”

  Spinner tried to maintain her anger, but she started to feel foolish. “Come on, Louise. I’m trying to make a point.”

  Louise, skillfully, brought her scooter in closer to Spinner; without much grace she clambered off the scooter and onto the branch beside Spinner. “Actually,” she said gently, “so am I.” She breathed deeply of the moist forest air, and looked around the sky. “I saw you watching that bird.”

  Spinner pushed her spectacles up the bridge of her nose. “So what?”

  Louise picked at the tree bark. “Well, the bird seems to be doing its best to get by, in zero-gee.”

  “Maybe. Not everyone here is doing so well,” Spinner said heavily. The loss of gravity was, slowly but surely, devastating the forest biota. “The higher birds and animals seem to be adapting okay… The monkeys quickly learned to adjust the way they climb and jump. But otherwise, things are falling apart, in a hundred tiny ways.” She thought of spiders which could no longer spin webs, of tree-dwelling frogs which found their tiny leaf-bound ponds floating away into the air. “We’re doing our best to keep things working — to save whatever we can,” she said. “But, damn it, even the rain doesn’t fall right any more.”

  Louise reached out and took her hand; the old engineer’s skin was cold, leathery. “Spinner, we have to reestablish all of this. Permanently.” Louise lifted her face; the diffuse light of the dome softened the etched-in age lines. “I designed this forest Deck, remember. And this is the only fragment of Earth that’s survived, anywhere in the Universe — as far as we know.”

  Spinner-of-Rope pulled her hand away. “I know what your little parable about the bird was about, Louise. I should adapt, just like the plucky little bird. Right? You want me to come back to the nightfighter.”

  Louise nodded, studying her.

  “Well, it was a dumb parable. The bird is the exception, n
ot the rule. And — ”

  “Spinner, I know you needed a break. But you’ve been climbing around these trees for a long time, now. I need you to come back — we all do. I know it’s difficult for you, but you’re the only person I have who can do the job.”

  Spinner watched her face, skeptically. “But we’re not talking about mere discontinuity-drive jaunts around the Solar System now. Are we, Louise?”

  “No.” Louise wouldn’t meet her eyes.

  Spinner felt a hollowness in her chest — as if it had expanded, leaving her heart fluttering like a bird in some huge cavity. Hyperdrive…

  “Spinner, we need the hyperdrive. You understand that, don’t you? The Sun is dying. Perhaps we could attempt to establish some sort of colony here, in the Solar System. But we need to find out what’s happening beyond the System. Are there any people left, anywhere? Maybe we can join them — find a better place than the Solar System has become.

  “But, without the hyperdrive, journeys like that would take millennia, more even with the discontinuity drive. And I don’t think we have millennia…”

  Spinner took a deep breath. “Yes, but… Louise, what will happen when I throw the switch? How will it feel?”

  Louise hesitated. “Spinner, I don’t know. That’s the truth; that’s what we want to find out from the first flight. We aren’t going to know for sure until we try it in anger. Mark and I have only just begun to put together theories on how the damn hyperdrive works… Spinner, all we know is it’s something to do with dimensionality.”

  A conventional craft (Louise said) worked in a “three-plus-one” dimensional spacetime — three spatial dimensions, plus one of time. And within those dimensions nature was described by a series of fundamental constants — the charge on the electron, the speed of light, the gravitational constant, Planck’s constant, and others.

  But — humans believed — physics was governed by the Spin (10) theory, which described symmetries among the forces of nature. And the symmetries needed to be expressed in higher dimensions than four.

  “So, Spinner-of-Rope, there are more than three spatial dimensions,” Louise said. “But the ‘extra dimensions’ are compactified—”

  “They’re what?”

  “Collapsed down to the smallest possible scale — to the Planck scale, below which quantum physics and gravitation merge.”

  Once — just after the initial singularity — the forces of physics were one, and the Universe was fully multi-dimensional. Then the great expansion started.

  “Three of the spatial dimensions expanded, rapidly, to the scales we see today. The other dimensions remained compactified.”

  “Why did three dimensions expand? Why not four, or two, or one — or none at all?”

  Louise laughed. “That’s a good question, Spinner. I wish I had a good answer.

  “Geometrically, three-dimensional spaces have some unique attributes. For instance, only in three dimensions is it possible for planets to have stable orbits governed by the central forces exerted by stars. Did you know that? Planets in a four-dimensional cosmos would drift into space, or spiral into their suns. So if life needs billions of years of a stable planetary environment, three dimensions are the only possibility. Matter isn’t stable in higher dimensions, even: the Schrödinger wave equation would have no bound solutions… And waves can propagate without distortion, only in three dimensions. So if we need high-fidelity acoustic or electromagnetic signals to be able to make sense of the world, then again, three dimensions is the only possibility.

  “Spinner, maybe there are alternate universes, out there somewhere, where more than three dimensions ballooned up after the initial singularity. But as far as we can see, life — our kind of life — couldn’t have evolved there; the fundamental geometry of spacetime wouldn’t have allowed it…

  “Remember, though, the extra dimensions are here, still, but they’re rolled up very tightly, into high-curvature tubes a Planck length across.”

  “So we can’t see them.”

  “No. But — and here’s the trick we think the Xeelee have exploited, Spinner — the extra dimensions do have an impact on our Universe. The curvature of these Planck tubes determines the value of the fundamental constants of physics. So the way the tubes are folded up determines things like the charge of an electron, or the strength of gravity.”

  Spinner nodded slowly. “All right. But what has this to do with the hyperdrive?”

  “Spinner-of-Rope, we think the Xeelee found a way to adjust some of those universal numbers. By changing the constants of physics — in a small region of space around itself — the hyperdrive can make spacetime unfurl, just a little.” Louise lifted her face. “Then the nightfighter can move, a short distance, through one of the higher dimensions.

  “Think of a sheet of paper, Spinner. If you’re confined to two dimensions — to crawling over the paper — then it will take you a long time to get from one side to the other. But if you could move through the third dimension — through the paper — then you could move with huge apparent speed from one place to another…”

  Spinner frowned. “I think I see that. Is this something like wormhole travel?”

  Louise hesitated. “Not really. Wormholes are defects in our three-plus-one dimensional spacetime, Spinner; they don’t involve the higher collapsed dimensions. And worm-holes are fixed. With a wormhole you can travel only from one place to another, unless you drag the termini around with you. With the Xeelee drive — we think — you can travel anywhere, almost at will. It’s like the difference between a fixed rail route and a flitter.”

  Spinner thought it over. “It sounds simple.”

  Louise laughed. “Believe me, it’s not.” She turned, distracted. “Hey. Look,” she said, pointing to the skydome.

  Spinner looked up, squinting through her spectacles against the glare of the dome. “What?”

  Louise leaned closer so that Spinner could sight along her outstretched arm. “See? Those shadows against the dome, over there…”

  The shadows, ten or a dozen forms, clambered across a small corner of the skydome, busy, active.

  Spinner smiled. “Howler monkeys. They’ve colonized the skydome. I wonder how they got up there.”

  “The point is,” Louise said gently, “they’ve adapted, too. Just like that parrot.”

  “Another parable, Louise?”

  Louise shrugged, looking smug.

  Spinner felt, she decided, like one of Morrow’s Under-men. She was no longer free; she was bowed down by the need to serve Louise’s vast, amorphous project.

  “All right, Louise, you’ve made your point. Let’s go back to the nightfighter.”

  For the first time, Lieserl understood the photino birds.

  She thought of novae, and supernovae.

  As the newly shining stars had settled into their multi-billion-year Main Sequence lifetimes, the Universe must have seemed a fine place to the photino birds. The stars had appeared stable: eternal, neat little nests of compact gravity wells and fusion energy.

  Then had come the first instabilities.

  Red giant expansions and novae must have been bad enough. But even a nova was a limited explosion, which could leave a star still intact: survivable, by the infesting birds. A supernova explosion, however, could destroy a star in seconds, leaving behind nothing more than a shriveled, fast-spinning neutron star.

  Lieserl tried to see these events from the point of view of the photino birds. The instabilities, the great explosions, must have devastated whole core-flocks. Perhaps, she speculated now, the birds had even evolved a civilization in the past; she imagined huge, spinning cities of dark matter at the heart of stars cities ripped apart by the first star-deaths.

  If she were a photino bird, she wouldn’t tolerate this.

  The birds didn’t need spectacular, blazing stars. They certainly didn’t need instability, novae and supernovae, the disruption of dying stars. All they demanded from a star was a stable gravity well, and a tri
ckle-source of proton photino interaction energy.

  She thought of Sol.

  When the birds were finished with the Sun — after the superwind had blown through the wrecked System — a white dwarf would remain: a small, cooling lump of degenerate matter smaller than the Earth. The Sun’s story would be over. It could expect no change, except a slow decline; there would certainly be no cataclysmic events in Sol’s future…

  But the dwarf would retain over half the Sun’s original mass. And there would be plenty of dense matter to interact with, and energy from the slow contraction of the star.

  The Sun would have become an ideal habitat for photino birds.

  Lieserl saw it all now, with terrifying clarity.

  The photino birds were not prepared to accept a Universe full of young, hot, dangerous stars, likely to explode at any moment. So they had decided to get it over with — to manage the aging of the stars as rapidly as possible.

  And when the birds’ great task was done, the Universe would be filled with dull, unchanging white dwarfs. The only motion would come from the shadowy streams of photino birds sailing between their neutered star-nests.

  It was a majestic vision: an engineering project on the grandest possible of scales — a project which could never be equalled.

  But it was making the Universe — the whole of the Universe — into a place inimical to humans.

  She studied the swelling core of the Sun. Its temperature climbed higher almost daily; the helium flash was close — or might, indeed, already have occurred.

  The humans seemed to have assimilated the data she had sent them. A reply came to her, via her tenuous maser-light pathways.

  She translated it slowly. A smiling face, crudely encoded in a binary chain of Doppler-distorted maser bursts. Words of thanks for her data. And — an invitation.

  Join us, the human said.

  Once again, Spinner-of-Rope sat in the cage of the Xeelee nightfighter. Arcs of construction material wrapped around her; beyond them the bloated bulk of the Sun loomed, immense and pale, like some vast ghost.