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  The mass converted to energy was millions of tons per second. The Sun’s bulk was so huge that this was a tiny fraction; in all its five-billion-year history so far the Sun had burned only five percent of its hydrogen fuel…

  But, relentlessly, the fuel in the core would be exhausted. Gradually an ash of helium would accumulate in the core, and the central temperature would drop. The delicate balance between gravity and radiation pressure would be lost, and the core would implode under the weight of the surrounding, cooler layers.

  Paradoxically, the implosion would cause the core temperature to rise once more — so much so that new fusion processes would become possible — and the star’s overall energy output would rise.

  The outer layers would expand enormously, driven out by the new-burning core. The Sun would engulf Mercury, and perhaps more of the inner planets, before reaching a new gravity-pressure equilibrium — as a red giant. This hundred million-year phase would be spectacular, with the Sun’s luminosity increasing by a factor of a thousand.

  But this profligate expansion was not sustainable. Complex elements would be burned with increasing desperation in the expanding, clinker-ridden core, until at last all the available fuel was exhausted.

  As the core’s temperature suddenly fell, equilibrium would be lost with sudden abandon. The Sun would implode once more, seeking a new stability. Finally, as a white dwarf, the Sun would consist of little more than its own dead core, its density a million times higher than before, with further contraction opposed by the pressure of high-speed electrons in its interior.

  Slowly, the remnant would cool, at last becoming a black dwarf, surrounded — as if by betrayed children — by the charred husks of its planets.

  …At least, Lieserl thought, that was the theory.

  If the laws of physics were allowed to unravel, following their own logic unimpeded, the Sun’s red giant stage was still billions of years away… not mere millions of years, as Superet’s evidence suggested was the case.

  Lieserl’s brief was to find out what was damaging the Sun.

  Lieserl. Try to pick up the p-modes; we want to see if that sensory mechanism works…

  “Absolutely. Helioseismology, here I come,” she said flippantly.

  She opened her eyes once more.

  A new pattern was built up by her processors, a fresh overlay on top of the images of convective cells and tangled flux tubes: gradually, she made out a structure of ghostly-blue walls and spinning planes that propagated through the convective cavern. These were p-modes: sound waves, pressure pulses fleeing through the Solar gas from explosive events like the destruction of granules on the surface. The waves were trapped in the convective layer, reflected from the vacuum beyond the photosphere and bent away from the core by the increasing sound speed in the interior. The waves canceled and reinforced each other until only standing waves survived, modes of vibration which matched the geometry of the convective cavern.

  The modes filled the space around her with ghostly, spinning patterns; their character varied as she surveyed the depth of the cavern, with length scales increasing as she looked into the interior. Looking up with her enhanced vision Lieserl could see how patches — thousands of miles wide — of the Sun’s surface oscillated as the waves struck, with displacements of fifty miles and speeds of half a mile a second.

  The Sun rang, like a bell.

  Good… good. This is terrific data, Lieserl.

  “I’m glad to oblige,” she said drily.

  All right. Now let’s try putting it together. Use the neutrino flux, such as it is, and the helioseismology data, and everything else you’ve got… Let’s find out how much we can see.

  Lieserl felt a thrill of excitement — subtle, but real — as she began to comply. Now she was moving to the core of her mission, even of her life: to look into the heart of the Sun, as no human had done before.

  As the processors worked to integrate the data she called up from her long-term memory a template: the Standard Model of the Sun. The processors overlaid the cavern around her with yet another level of complexity, as they populated it with icons, graphics, grid lines and alphanumeric labels, showing her the basic properties of the Standard Model. The Model — refined and revised over millennia — represented humanity’s best understanding of how the Sun worked. She looked in toward the core and saw how, according to the Model, the pressure and temperature rose smoothly toward the core; the temperature graph showed as a complex three-dimensional sphere in pink and red, reaching an intensely scarlet fifteen million degrees at the very heart.

  Slowly, her processors plotted the reality — as she perceived it now — against the theory; graphs and schematics blossomed over each other like clusters of multicolored flowers.

  After a few minutes, her vision stabilized. She stared around at the complex imagery filling the cavern, zooming in on particular aspects, highlighting differences.

  Oh, no, Scholes said. No. Something’s wrong.

  “What?”

  The discrepancies, Lieserl. Particularly toward the core. This simply can’t be right.

  She felt amused. “You’ve gone to all the trouble of constructing me, of sending me in here like this, and now that I’m here you’re going to disbelieve what I tell you?”

  But look at the divergences from the Model, Lieserl. Under a command from Scholes, the actual and predicted temperature gradients were picked out in glowing, radiant pinks. Look at this.

  “Hmm…”

  According to the Standard Model, the temperature should have fallen quite rapidly away from the fusion region — down by a full twenty percent from the central value after a tenth of the Sun’s radius. But in fact, the temperature drop was much more shallow… falling only a few percent, Lieserl saw, over more than a quarter of the radius.

  “That’s not so surprising. Is it?” In riposte she superimposed a graphic of her own, a variant of the Standard Model. “Look at this. Here’s a model with a dark matter component — photinos, orbiting the core.” The dark matter — fast-moving, almost intangible particles kept clustered around the heart of the Sun by its gravity field — transferred energy out of the core and to the surrounding layers. “See? The photinos just leak kinetic energy — heat energy — out of the core. The central temperature is suppressed, and the core is made isothermal uniform temperature — out to about ten percent of the radius.”

  Scholes sounded testy, impatient. Yes, he said, but what we’re looking at here is an isothermal region covering three times that radius — twenty-five times the volume predicted even by the widest of the Standard Model’s variants. It’s impossible, Lieserl. Something must be going wrong with —

  “With what? With the eyes you’ve built for me? Or with your own expectations?”

  Irritated, she canceled all the schematics. The spheres and contour lines imploded in sparkles of pixels, exposing the native panorama of the convective cavern, a complex, ghostly overlay of flux tubes, p-modes and convection cells.

  Frustrated, with some analogue of nervous energy building in her, she sent her Virtual self soaring around the cavern. She chased the rotating p-wave modes, sliced through flux tubes. “Kevan. What if the effect we’re seeing is real? Maybe this divergence in the core is what you’ve sent me in here to find.”

  Maybe… Lieserl, what will you do next?

  “It’s early days, but I think I’ll soon have learned all I can out here.”

  Out here?

  “In the cavern — the convective zone. All the evidence we have is indirect, Kevan. The real action is deeper in, at the core.”

  But you can’t go any deeper, Lieserl. Your design… the wormhole will implode if you try to penetrate the radiative zone…

  “Maybe. Well, it’s up to you to sort that out, Kevan.”

  She swooped up to the glowing roof of the cavern, and plunged down, at hundreds of miles a second, toward the plasma sea, past the slow-pulsing flanks of giant p-modes.

  6

  L
ike an insect circling an elephant the pod skimmed around the hull of the Great Northern.

  Mark Wu, Louise Armonk, Garry Uvarov and Serena Milpitas sat and watched as their tiny pod skirted the starship. Their silence, Mark thought, was suitably deep and awe-struck, even for four who had been as close to the final stages of the project as these. And maybe that was Louise’s intention today, he thought, the subtext under what was ostensibly a simple inspection tour of the ship by her top management team.

  Well, if so, she was certainly succeeding.

  The lifedome of the Northern was a squat, transparent cylinder a mile wide. It was extraordinary to think that the whole of Michael Poole’s GUTship — drive section and all — would have fitted inside that sparkling box; Mark tried to imagine the Hermit Crab suspended in that great cylinder like some immense model under glass.

  Mark could see clearly the multiple decks of the dome, and throughout the dome there was movement and light, and the deep, refreshing green of growing things. He was aware that the adaptation of much of the dome, and the rest of the ship, was still unfinished; most of what he saw was little more than a Virtual projection. But still he was impressed by the scale and vigor of it all. This lifedome would be a self-contained city — no, more than that: a world in itself, a biosphere suspended between the stars.

  Home to five thousand people for a thousand years.

  Now they wheeled to the underside of the lifedome. The pod approached the immense, tangled structure of the Northern’s main spine, and flew parallel to the spine for some three hundred yards toward the base of the dome.

  The spine was a three-mile highway of metal littered with supply modules and antennae and other sensors, turned up to the distant stars like mouths. Behind them the spine led to the mysterious darkness of the drive section, where the lights of workers — human and robotic — crawled like flies. And, attached to the spine by bands of gold just before the drive section, was the huge Interface, the wormhole terminus which they would tow to the future. The tetrahedral frame looked like a gaudy, glittering toy of shining blue ribbon.

  Uvarov spread his long, intelligent fingers and rested his hands against the gleaming hull of the pod. “Lethe,” he said. The pod’s lights struck highlights from his bony profile as he peered out at the spine. “It might not be real, but it’s beautiful.”

  Louise laughed; beside the thin, gaunt eugenicist she looked short, compact, Mark thought. “Real enough,” she said. “The spine’s framework is a hundred percent realized. It’s just the superstructure that remains nebulous.” She thought for a moment, then called, “Configure 3-B.”

  The flower-like antennae clustered along the spine melted away, dissolving into showers of pixel cubes which tumbled like snowflakes. For a few surreal seconds Virtual configurations of equipment modules blossomed over the spine; through the snowstorm of modules Mark could see the basic — and elegant structure of triangular vertebrae at the core of the spine.

  At last the storm of images stilled; the spine settled into a new scattering of lenses and antennae. To Mark’s untutored eye this looked much the same as the original — perhaps rather sparser — but he became aware that Serena Milpitas was nodding, almost wistfully.

  “This is the original configuration,” she said. “It’s what was planned when the ship was being designed for its oneway hop to Tau Ceti, just a century away.”

  Mark studied Milpitas curiously. The project’s new chief engineer affected physical-forty, but Mark knew she was at least twice as old as that. He also knew there had been quite a bit of friction between Milpitas and Louise; so he was surprised to find, now, Milpitas praising Louise’s design. “You sound a little — nostalgic. Do you really think this is a better design?”

  “Oh, yes.” Milpitas’ broad face split in a smile; she seemed surprised by the question. “Don’t you? Can’t you see it?”

  Uvarov grunted. “Not particularly.”

  “Inelegance was forced on us. Look — for a thousand-year flight the problems of reliability are enormous.” Her accent was broad, confident Martian. “This ship has around a thousand million distinguishable components. And all of them have to work perfectly, all of the time. Right? Now, we estimate that the chance of a significant failure of any one of those components — of a failure serious enough to knock out a ship’s system, say — is a tenth of one percent per year. Pretty good odds, you might think. But as the years go by the chances of a failure mount up, and they work cumulatively.” She fixed Mark with a direct stare. “What would you guess the chances of such a failure would be after a hundred years?”

  Uvarov growled, “Oh, please, spare us games.”

  Mark shrugged. “A few percent?”

  “Not bad. Ten percent. Not wonderful, but liveable with.”

  Uvarov clicked his tongue. “I hate your Mons Olympus grammar, engineer.”

  Milpitas ignored him. “But after a thousand years, you’re looking at a failure probability of over sixty percent. You reach fifty-fifty after just seven centuries — ”

  “What she’s trying to tell you,” Uvarov said heavily, his flat Lunar tones conveying his boredom, “is the obvious fact that they’ve had to perform extensive redesign to enable the ship to survive a thousand-year flight.”

  “How? Louise doesn’t tell me a damn thing.”

  Uvarov grinned. “Ex-wives never do. I should know. I — ”

  Milpitas cut in, “With current technology, we couldn’t get the reliability rates high enough for the mechanical, electrical or semisentient components.” She waved a hand at the half-Virtual panorama beyond the hull. “Amazing, isn’t it? We think we’ve come so far. We thought that with nanobotic technology continual repair and replacement at the sub-visible level — reliability problems were a thing of the past. I mean, look at that spine out there. There’s sentience in it everywhere, right down to the nuts and bolts.”

  “There are no nuts and bolts, Serena,” Louise said drily.

  Milpitas ignored her. “And yet it doesn’t take much of a challenge to move us beyond the envelope of our capabilities. Strictly speaking, a thousand-year flight is still beyond our means.”

  “That sounds ominous,” Mark said uneasily.

  “So,” Louise said, “we had to look to the past — simple methods used to improve reliability on projects like the first off-Earth flights.” She called out, “Central configuration,” and the blizzard of virtual components swirled once more around the spine, settling at last into the pattern Mark remembered from before Louise’s change.

  Milpitas pointed. “And this is what we’re going to the stars with. Look at it. Even at this gross macroscopic level you can see there are many more components.”

  And, indeed, Mark realized now that there were more antennae, more sensor snouts, more maintenance pods; the spine structure looked busier, far more cluttered.

  “Triple redundancy,” Milpitas said with a grimace. “Words — and a technique from the twenty-fifth century. Or further back, even, for all I know; probably from the time of those disgusting old fission reactors. Carrying three of everything — or more, for the key components — to reduce the chance of a catastrophe to the invisibly small.”

  “Gripping,” Uvarov said. “But shall we move on, some time today? We do have the whole of the ship to inspect, as I recall.”

  The base of the lifedome expanded in Mark’s vision until it covered the sky, becoming an immense, complex, semi-transparent roof; guide lights and the outlines of ports — large and small — encrusted the surface with color, and everywhere there was movement, a constant flow of cargo, pods and spacesuited figures through the multiple locks. Again Mark had the impression that this was not so much a ship as a city: immense, busy, occupied with the endless business of maintaining its own fabric.

  Suspended beneath the lifedome, cradled in cables, was the dark, wildly incongruous form of the Great Britain. It looked like an immense lifeboat, suspended there, Mark thought; he grinned, relishing this evidence of
Louise’s sentimentality.

  The pod, working autonomously, made a flawless entry into one of the huge airlocks. After a couple of minutes the lock had completed its cycle.

  The four of them emerged, drifting, into the air at the base of the Northern’s lifedome. It seemed to Mark that the base itself — constructed with the universal semisentient transparent plastic — was a wall dividing the Universe into two halves. Before him was the elaborate, sparkling-clean interior of the lifedome; behind him was the tough, angular spine of the GUTship, and the static darkness of transPlutonian space.

  Louise led them to a row of zero-gee scooters; the scooters nuzzled against the transparent base, neat and efficient. Mark took a scooter. It was a simple platform, its pneumatic jets controlled by twists of its raised handles.

  They formed into pairs — Louise and Uvarov in the lead, with Mark and Milpitas following. With near-silent sighs of scooter air the four moved off in formation, up toward the heart of the lifedome.

  The lower fifth of a mile of the lifedome was known as the loading bay: a single, echoing hall, brilliantly lit and free of partitions. The roof of the loading bay — the underside of the first habitable section, called the maintenance bulkhead — was a mist-shrouded tangle of infrastructure, far above. Today, the loading bay was filled with bulky machinery and crates of supplies; huge masses, towed by people on scooters or by ’bots, crossed the air in all directions, emerging from a dozen locks.

  Serena Milpitas performed a slow, easy spiral as she rose up through the air beside Mark. “I love these scooter things, don’t you?”

  Mark smiled. “Sure. But they’re a lazy way to travel in zero gee. And they won’t be a lot of use when we’re underway.”

  “No. A constant one-gee drive for a thousand years. What a drag.”

  Mark studied the engineer as she went through her rolls; her expression was calm, almost vacuous, with every sign that she was lost in the simple physical pleasure of the scooter-ride. Mark said, “How did you feel about having to dig up those old techniques — the reliability procedures?”