/ Language: English / Genre:sf

Schild’s Ladder

Грег Иган

In Schild’s Ladder, humanity has transcended both death and Earth, and discovered its home world is nearly unique as a cradle of life. As it spreads throughout the galaxy, humanity enjoys an almost utopian existence — until a scientist accidentally creates an impenetrable, steadily expanding vacuum that devours star systems and threatens the entire universe with destruction. Tchicaya is a Yielder, member of the faction that believes this "novo-vacuum" deserves study. The opposing Preservationists — among them Mariama, his first love — seek to save worlds and destroy the novo-vacuum. Discord heats to terrorist violence; then enmities and alliances are turned upside-down by a discovery that may mean the novo-vacuum is, instead, a new and very different universe — and one which may contain life.

Thanks to John Baez, Jennifer Brehl, Caroline Oakley, Anthony Cheetham, John Douglas, Simon Spanton, Oisín Murphy-Lawless, Devi Pillai, Peter Robinson, Russell Galen, Carol Jackson, Emma Bailey, Diana Mackay, Philip Patterson, Christodoulos Litharis, Nicola Fantini, Giancarlo Carlotti, Albert Solé, Petr Kotrle, Makoto Yamagishi, Florin Pîtea, and Mihai-Dan Pavelescu.

Part One

Chapter 1

In the beginning was a graph, more like diamond than graphite. Every node in this graph was tetravalent: connected by four edges to four other nodes. By a count of edges, the shortest path from any node back to itself was a loop six edges long. Every node belonged to twenty-four such loops, as well as forty-eight loops eight edges long, and four hundred and eighty that were ten edges long. The edges had no length or shape, the nodes no position; the graph consisted only of the fact that some nodes were connected to others. This pattern of connections, repeated endlessly, was all there was.

In the beginning? Waking more fully, Cass corrected herself: that was the version she remembered from childhood, but these days she preferred to be more cautious. The Sarumpaet rules let you trace the history of the universe back to the vicnity of the Diamond Graph, and everything you could ask for in a Big Bang was there: low entropy, particle creation, rapidly expanding space. Whether it made sense to follow these signposts all the way back, though, was another question.

Cass let the graph’s honeycomb pattern linger in the darkness of her skull. Having relinquished her child’s-eye view of the world, she was unable to decide which epoch of her life she actually inhabited. It was one of the minor perils of longevity: waking could be like to trying to find your way home on a street with ten thousand houses, all of which had once been your own. That the clues on the other side of her eyelids might be more enlightening was beside the point; she had to follow the internal logic of her memories back into the present before she could jolt herself awake.

The Sarumpaet rules assigned a quantum amplitude to the possibility of any one graph being followed by another. Among other things, the rules predicted that if a graph contained a loop consisting of three trivalent nodes alternating with three pentavalent ones, its most likely successors would share the same pattern, but it would be shifted to an adjoining set of nodes. A loop like this was known as a photon. The rules predicted that the photon would move. (Which way? All directions were equally likely. To aim the photon took more work, superimposing a swarm of different versions that would interfere and cancel each other out when they traveled in all but one favored direction.)

Other patterns could propagate in a similar fashion, and their symmetries and interactions matched up perfectly with the known fundamental particles. Every graph was still just a graph, a collection of nodes and their mutual connections, but the flaws in the diamond took on a life of their own.

The current state of the universe was a long way from the Diamond Graph. Even a patch of near-vacuum in the middle of interstellar space owed its near-Euclidean geometry to the fact that it was an elaborate superposition of a multitude of graphs, each one riddled with virtual particles. And while an ideal vacuum, in all its complexity, was a known quantity, most real space departed from that ideal in an uncontrollable manner: shot through with cosmic radiation, molecular contaminants, neutrinos, and the endless faint ripple of gravitational waves.

So Cass had traveled to Mimosa Station, half a light-year from the blue subgiant for which it was named, three hundred and seventy light-years from Earth. Here, Rainzi and his colleagues had built a shield against the noise.

Cass opened her eyes. Lifting her head to peer through a portal, still strapped to the bed at the waist, she could just make out the Quietener: a blue glint reflecting off the hull a million kilometers away. Mimosa Station had so little room to spare that she’d had to settle for a body two millimeters high, which rendered her vision less acute than usual. The combination of weightlessness, vacuum, and insectile dimensions did make her feel pleasantly robust, though: her mass had shrunk a thousand times more than the cross sections of her muscles and tendons, so the pressures and strains involved in any collision were feather-light. Even if she charged straight into a ceramic wall, it felt like being stopped by a barricade of petals.

It was a pity the same magical resilience couldn’t apply to her encounters with less tangible obstacles. She’d left Earth with no guarantee that the Mimosans would see any merit in her proposal, but it was only in the last few days that she’d begun to face up to the possibility of a bruising rejection. She could have presented her entire case from home, stoically accepting a sevenhundred-and-forty-year delay between each stage of the argument. Or she could have sent a Surrogate, well briefed but nonsentient, to plead on her behalf. But she’d succumbed to a mixture of impatience and a sense of proprietorship, and transmitted herself blind.

Now the verdict was less than two hours away.

She unstrapped herself and drifted away from the bed. She didn’t need to wash, or purge herself of wastes. From the moment she’d arrived, as a stream of ultraviolet pulses with a header requesting embodiment on almost any terms, the Mimosans had been polite and accommodating; Cass had been careful not to abuse their hospitality by pleading for frivolous luxuries. A self-contained body and a safe place to sleep were the only things she really needed in order to feel like herself. Being hermetically sealed against the vacuum and feeding on nothing but light took some getting used to, but so did the customs and climate of any unfamiliar region back on Earth. Demanding the right to eat and excrete, here, would have been as crass as insisting on slavish recreations of her favorite childhood meals, while a guest at some terrestrial facility.

A circular tunnel, slightly wider than her height, connected her spartan quarters to a chamber where she could interact with the software she’d brought from Earth, and through it the Mimosans themselves. She bounced down the borehole, slapping the wall with her hands and feet, bumping her head and elbows deliberately.

As she entered the chamber, she seemed to emerge from the mouth of a burrow to float above a lush, wide meadow beneath a cloud-dappled sky. The illusion was purely audiovisual — the sounds encoded in radio waves — but with no weight to hold her against the ceramic hidden beneath the meadow, the force of detail was eerily compelling. It only took a few blades of grass and some chirping insects to make her half-believe that she could smell the late-summer air.

Would it really have been an act of self-betrayal, if this landscape had stretched all the way inside her — right down to the sensations of inhabiting her old, two-meter body, gorging on a breakfast of fruit and oats after swimming across Chalmers Lake? If she could drift in and out of this soothing work of art without losing her grip on reality, why couldn’t she take the process a few steps further?

She pushed the argument aside, though she was glad that it never stopped nagging at her. When the means existed to transform yourself, instantly and effortlessly, into anything at all, the only way to maintain an identity was to draw your own boundaries. But once you lost the urge to keep on asking whether or not you’d drawn them in the right place, you might as well have been born Homo sapiens, with no real choices at all.

A short distance from the burrow stood a marble statue of Rainzi, arms folded, smiling slightly. Cass gestured at the messenger and it came to life, the white stone taking on the hue and texture of skin. Rainzi himself was several generations removed from anyone who’d bothered to simulate a living dermis, let alone possess one, but Cass was not equipped to make sense of the Mimosans' own communications protocols, so she’d chosen to have everything translated into the visual dialect used back on Earth.

"We’ll give you our decision at nine o’clock, as promised," the messenger assured her. "But we hope you won’t mind if we precede this with a final review session. Some of us feel that there are matters that have yet to be entirely resolved. We’ll begin at half past seven." The messenger bowed, then froze again, expecting no reply.

Cass tried not to read too much into the sudden change of plan. It was unnerving to discover that her hosts still hadn’t been able to reach a verdict, but at least they weren’t going to keep her waiting any longer than she’d expected. The fact that she’d alread briefed them in detail on every aspect of the experiment that had crossed her mind during three decades of preparation, and they now hoped to hear something new and decisive from her in twenty minutes' time, was no reason to panic. Whatever loose ends they’d found in her analysis, they were giving her the chance to put things right.

Her confidence was shaken, though, and she couldn’t stop thinking about the prospect of failure. After a month here, she still wasn’t lonely, or homesick; that was the price she’d pay upon returning. Even at the leisurely pace of the embodied, seven hundred and forty years cut a deep rift. It would be millennia before the changes that her friends on Earth had lived through together would cease to set her apart from them. Millennia, if ever.

She still believed that she could come to terms with that loss, so long as she had something to weigh against it. Being a singleton meant accepting that every decision had its cost, but once you understood that this state of affairs was a hard-won prize, not a plight to rail against, it gave some dignity to all but the most foolish choices.

If the Mimosans turned her down, though? Maybe there was something daring and romantic in the mere act of traveling hundreds of light-years, inhabiting the body of a vacuum-dwelling insect, and alienating herself from the world where she belonged, all in the hope of seeing her ideas tested as rapidly as possible. But for how long would she be able to take comfort from the sheer audacity of what she’d done, once that hope had come to nothing?

She curled into a ball and tried to weep. She could not shed tears, and the sobs rebounding against her membrance-sealed mouth were like the drone of a mosquito. But the shuddering as she worked her vestigial lungs still provided some sense of release. She had not entirely erased the map of her Earthly body from her mind; too much of the way she experienced emotions was bound up in its specific form. So everything she’d amputated lingered as a kind of phantom — nowhere near as convincing as a true simulation, but still compelling enough to make a difference.

When she was spent, Cass stretched out her limbs and drifted over the meadow like a dandelion seed, as calm and lucid as she’d been at any time since her arrival.

She knew what she knew about Quantum Graph Theory, backward. Whatever insights she was capable of extracting from that body of knowledge, she’d extracted long ago. But if the Mimosans had found a question she couldn’t answer, a doubt she couldn’t assuage, that in itself would be a chance to learn something more.

Even if they sent her home with nothing else, she would not be leaving empty-handed.

It was Livia who asked the first question, and it was far simpler than anything Cass had anticipated.

"Do you believe that the Sarumpaet rules are correct?"

Cass hesitated longer than she needed to, a calculated attempt to imbue her response with appropriate gravity.

"I’m not certain that they are, but the likelihood seems overwhelming to me."

"Your experiment would test them more rigorously than anything that’s been tried before," Livia observed.

Cass nodded. "I do see that as a benefit, but only a minor one. I don’t believe that merely testing the rules one more time would justify the experiment. I’m more interested in what the rules imply, given that they’re almost certainly correct."

Where was this heading? She glanced around at the others, seated in a ring in the meadow: Yann, Bakim, Darsono, Ilene, Zulkifli, and Rainzi. Her Mediator had chosen appearances for all of them, since they offered none themselves, but at least their facial expressions and body language were modulated by their own intentional signals. By choice, they all looked politely interested, but were giving nothing away.

"You have a lot of confidence in QGT?" Clearly, Livia did realize just how strange her questions sounded; her tone was that of someone begging to be indulged until her purpose became apparent.

Cass said, "Yes, I do. It’s simple, it’s elegant, and it’s consistent with all observations to date." That handful of words sounded glib, but other people had quantified all of these criteria long ago. QGT as a description of the dynamics of the universe with the minimum possible algorithmic complexity. QGT as a topological redescription of some basic results in category theory — a mathematical setting in which the Sarumpaet rules appeared as natural and inevitable as the rules of arithmetic. QGT as the most probable underlying system of physical laws, given any substantial database of experimental results that spanned both nuclear physics and cosmology.

Darsono leaned toward her and interjected, "But why, in your heart" — he thumped his chest with an imaginary fist — "are you convinced that it’s true?" Cass smiled. That was not a gesture in the staid vocabulary her Mediator used by default; Darsono must have requested it explicitly.

"In part, it’s the history," she admitted, relaxing slightly. "The lineage of the ideas. If some alien civilization had handed us Quantum Graph Theory on a stone tablet — out of the blue, in the eighteenth or nineteenth century — I might not feel the same way about it. But general relativity and quantum mechanics were among the most beautiful things the ancients created, and they’re still the best practical approximations we have for most of the universe. QGT is their union. If general relativity is so close to the truth that only the tiniest fragment can be missing, and quantum mechanics is the same…how much freedom can there be to encompass all of the successes of both, and still be wrong?"

Kusnanto Sarumpaet had lived on Earth at the turn of the third millennium, when a group of physicists and mathematicians scattered across the planet — now known universally as the Sultans of Spin — had produced the first viable offspring of general relativity and quantum mechanics. To merge the two descriptions of nature, you needed to replace the precise, unequivocal geometry of classical space-time with a quantum state that assigned amplitudes to a whole range of possible geometries. One way to do this was to imagine carrying a particle such as an electron around a loop, and computing the amplitude for its direction of spin being the same at the end of the journey as when it first set out. In flat space, the spins would always agree, but in curved space the result would depend on the detailed geometry of the region through which the particle had traveled. Generalizing this idea, crisscrossing space with a whole network of paths taken by particles of various spins, and comparing them all at the junctions where they met, led to the notion of a spin network. Like the harmonics of a wave, these networks comprised a set of building blocks from which all quantum states of geometry could be constructed.

Sarumpaet’s quantum graphs were the children of spin networks, moving one step further away from general relativity by taking their own parents' best qualities at face value. They abandoned the idea of any preexisting space in which the network could be embedded, and defined everything — space, time, geometry, and matter — entirely on their own terms. Particles were loops of altered valence woven into the graph. The area of any surface was due to the number of edges of the graph that pierced it, the volume of any region to the number of nodes it contained. And every measure of time, from planetary orbits to the vibrations of nuclei, could ultimately be rephrased as a count of the changes between the graphs describing space at two different moments.

Sarumpaet had struggled for decades to breathe life into this vision, by finding the correct laws that governed the probability of any one graph evolving into another. In the end, he’d been blessed by a lack of choices; there had only been one set of rules that could make everything work. The two grandparents of his theory, imperfect as they were, could not be very far wrong: both had yielded predictions in their respective domains that had been verified to hair’s-breadth accuracy. Doing justice to both had left no room for errors.

Livia said, "Conceptually, that argument is very appealing. But there could still be deviations from the rules — far too small to have been detected so far — that would change the outcome of your experiment completely."

"So it’s a sensitive test," Cass agreed. "But that’s not why I’ve proposed it." They were talking in circles. "If the rules hold, the graph I’ve designed should be stable for almost six-trillionths of a second. That’s long enough to give us a wealth of observations of a space-time utterly different from our own. If it doesn’t last that long, I’ll be disappointed. I’m not doing this in the hope of proving Sarumpaet wrong!"

Cass turned to Darsono, seeking some hint that he might share her exasperation, but before she could gauge his mood, Livia spoke again.

"What if it lasts much longer?"

Finally, Cass understood. "This is about safety? I’ve addressed the potential risks, very thoroughly — "

"On the basis that the Sarumpaet rules are correct."

"Yes. What other basis should I have used?" Phoenician astrology? Californian lithomancy? Cass resisted the urge to lapse into sarcasm; there was too much at stake. "I’ve admitted that there’s no certainty that the rules hold in every last untested circumstance. But I have nothing better to put in their place."

"Nor do I," Livia said gently. "My point is, we mustn’t over-interpret the success of the Sarumpaet rules. General relativity and quantum field theory confessed from the start that they were just approximations: pushed to extremes, they both yielded obvious nonsense. But the fact that QGT doesn’t — the fact that there is no fundamental reason why it can’t be universally applicable — is no guarantee that it really does stretch that far."

Cass gritted her teeth. "I concede that. But where does it leave us? Refusing to perform any experiment that hasn’t been tried before?"

Rainzi said, "Of course not. Livia is proposing a staged approach. Before attempting to construct your graph, we’d move toward it in a series of experiments, gradually bridging the gap."

Cass fell silent. Compared to outright rejection this was a trivial obstacle, but it still stung: she’d worked for thirty years to refine her own proposal, and she resented the implication that she’d been reckless.

"How many stages?"

"Fifteen," Livia replied. She swept a hand through the vacuum in front of her, and a sequence of target graphs appeared. Cass studied them, taking her time.

They’d been well chosen. At first one by one, then in pairs, then triples, the features that conspired to render her own target stable were introduced. If there was some undiscovered flaw in the rules that would make the final graph dangerous, there could be no more systematic way to detect it in advance.

"It’s your choice," Rainzi said. "We’ll vote on whichever proposal you endorse."

Cass met his eyes. The openness of his face was an act of puppetry, but that didn’t mean he was insincere. This wasn’t a threat, an attempt to bully her into agreeing. It was a mark of respect that they were letting her decide, letting her weigh up her own costs, her own fears, before they voted.

She said, "Fifteen experiments. How long would that take?"

Ilene answered, "Perhaps three years. Perhaps five." Conditions varied, and the Quietener wasn’t perfect. Planning an experiment in QGT was like waiting for a stretch of ocean to grow sufficiently calm that a few flimsy barriers could block the waves and keep out the wildlife long enough to let you test some subtle principle of fluid dynamics. There was no equivalent of a laboratory water tank; space-time was all ocean, indivisible.

In terms of separation from her friends, five years was nothing compared to the centuries she’d already lost. Still, Cass found the prospect daunting. It must have shown on her face, because Bakim responded, "You could always return to Earth immediately, and wait for the results there." Some of the Mimosans had trouble understanding why anyone who found life in the station arduous would feel obliged to be here in person at all.

Darsono, empathetic as ever, added quickly, "Or we could give you new quarters. There’s a suitable cavity on the other side of the station, almost twice as large; it’s just a matter of rerouting some cables."

Cass laughed. "Thank you." Maybe they could build her a new body, too, four whole millimeters long. Or she could abandon her scruples, melt into software, and wallow in whatever luxuries she desired. That was the hazard she’d face every day, here: not just the risk that she’d give in to temptation, but the risk that all the principles she’d chosen to define herself would come to seem like nothing but masochistic nonsense.

She lowered her gaze toward the illusory meadow, laserpainted on her retinas like everything around her, but her mind’s eye conjured up another image just as strongly from within: the Diamond Graph, as she saw it in her dreams. She could never reach it, never touch it, but she could learn to see it from a new direction, understand it in a new way. She’d come here in the hope of being changed, by that knowledge if by nothing else. To flee back to Earth out of fear that she might test her own boundaries more rigorously here, in a mere five years of consciousness, than if she’d spent the same three-quarters of a millennium at home, would be the greatest act of cowardice in her life.

"I’ll accept the staged experiments," she declared. "I endorse Livia’s proposal."

Rainzi said, "All in favor?"

There was silence. Cass could hear crickets chirping. No one? Not Livia herself? Not even Darsono?

She looked up.

All seven Mimosans had raised their hands.

Chapter 2

Riding her ion scooter the million kilometers to the Quietener, Cass found herself reveling in the view for the first time in years. The scooter was doing one-and-a-quarter gees, but the couch pressed against her back so gently that she might have been floating. Floating in dark water, beneath an alien sky. Even at half a light-year, Mimosa punched a dazzling violet hole in the blackness, a pinprick ten times as bright as a full moon. Away from its glare, the stars were far too plentiful to suggest constellations; any stick-figure object that she began to sketch between them was soon undermined by an equally compelling alternative, then a third, then a fourth — like a superposition of graphs, each with a different choice of edges between the same nodes. When she’d first arrived, she’d homed in on her own star, watching with a mixture of fear and exaltation as it hovered at the edge of visibility to her thousandth-scale eyes. Now, she’d forgotten all the cues she’d need to find it, and she felt no urge to ask her navigation software to remind her. The sun was no beacon of reassurance, and she’d be seeing it close-up again soon enough.

Each time one of Livia’s staged targets had been achieved, Cass had dispatched a small army of digital couriers to pass on the news to seven generations of her ancestors and descendants, as well as all her friends in Chalmers. She’d received dozens of messengers herself, mostly from Lisa and Tomek, full of inconsequential gossip, but very welcome. It must have grown strange for her friends as the years had passed, and they no longer knew whether or not there was any point continuing to shout into the void. If she had traveled embodied, as a handful of ancients still did, she could have caught up with centuries of mail on the return voyage. Reduced to a timeless signal en route, though, she’d have no choice but to step unprepared into the future. Her homecoming was going to be the hardest thing she’d ever faced, but she was almost certain now that her time here would prove to have been worth it.

Half an hour before arrival, Cass rolled onto her stomach and poked her head over the edge of the couch. Her engine’s exhaust was a barely perceptible flicker, fainter than a methanol flame by daylight, but she knew that if she reached down and placed her hand in the stream of plasma, she’d rapidly lose any delusion that her Mimosan body was indestructible.

She watched the Quietener growing beneath her, the silvery sphere glinting Mimosa-blue. Surrounding it was a swarm of smaller, twinned spheres, unevenly colored and far less lustrous. Tethers, invisibly slender, allowed the twins to orbit each other, while ion jets balanced the slight tug of the Quietener’s gravity, keeping each pair’s center of mass fixed against the stars.

The Quietener made it possible to perform experiments that could never be carried out elsewhere. The right distribution of matter and energy could curve space-time in any manner that Einstein’s equations allowed, but creating a chosen state of quantum geometry was a very different proposition. Rather than simply bending space-time in bulk, like a slab of metal in a foundry, it had to be controlled with the same kind of precision as the particles in a two-slit interference experiment. But the "particles" of geometry were twenty-five orders of magnitude smaller than atoms, and they could never be vaporized, ionized, or otherwise coaxed apart to be handled one by one. So the same degree of delicacy had to be achieved with the equivalent of a ten-tonne lump of iron.

Refining the starting material helped, and the Quietener did its best to screen out every form of impurity. Ordinary matter and magnetic fields absorbed or deflected charged particles, while a shell of exotic nuclei, trapped by gamma-ray lasers in states from which they could not decay without absorbing neutrinos, mopped up a greater fraction of the billions wandering by than would have been stopped by a galaxy’s-worth of lead.

Gravitational waves passed through anything, so the only antidote was a second train of waves, tailored to cancel out the first. There was nothing to be done about sporadic cataclysms — supernovae, or black holes gorging on star clusters in the centers of distant galaxies — but the most persistent gravitational waves, coming from local binary stars, were cyclic, predictable, and faint. So the Quietener was ringed with countersources, their orbits timed to stretch space at the center of the device when the bodies they mimicked squeezed it, and vice versa.

As Cass passed within a few kilometers of one of the counter-sources, she could see the aggregate rocky surface that betrayed its origins in Mimosa’s rubble of asteroids. Every scrap of material here had been dragged out of that system’s gravity well over a period of almost a thousand years, a process initiated by a package of micron-sized spores sent from Viro, the nearest inhabited world, at ninety percent of lightspeed. The Mimosans themselves had come from all over, traveling here just as Cass had once the station was assembled.

The scooter’s smooth deceleration brought her to a halt beside a docking bay, and she was weightless again. Whenever she was close enough to either the station or the Quietener to judge her velocity, it seemed to be little more than that of a train, giving the impression that in the five-hour journey she might have traveled the width of a continent on Earth. Not to the moon and back, and more.

One wall of the bay had handholds. As Cass pulled herself along, Rainzi appeared beside her. The Mimosans had dusted projectors and cameras all over the walls of the places she visited in the Quietener, rendering guest and host mutually visible.

"This is it!" Rainzi said cheerfully. "Barring untimely supernovae, we’ll finally get to see your graph complete." The software portrayed him with a jet pack, to rationalize his ability to follow her uneven progress up the wall without touching anything.

Cass replied stoically, "I’ll believe it when it happens." In fact, from the moment Ilene had scheduled the run, twelve hours before, Cass had felt insanely confident that no more hurdles remained. Eight of the fourteen previous targets had been achieved at the first attempt, making the prospect of one more tantalizingly plausible. But she was reluctant to admit to taking anything for granted, and if something did go wrong it would be easier to swallow her disappointment if she’d been pretending from the start that her expectations had always been suitably modest.

Rainzi didn’t argue, but he ignored her feigned pessimism. He said, "I have a proposition for you. A new experience you might like to try, to celebrate the occasion. I suspect it will be against all your high-minded principles, but I honestly believe you’d enjoy it. Will you hear me out?"

He wore a look of such deadpan innocence that Cass felt sure he knew exactly how this sounded in translation. If that was his meaning, the idea wasn’t entirely absurd, or unwelcome. She’d grown fond of Rainzi, and if he’d never been quite as solicitous or as eager to understand her as Darsono, the truth was, that made him more intriguing. If they could find enough common ground to become lovers, it might be a fitting way to bid Mimosa farewell: sweeping away the mutually distorted views they had of each other. To remain loyal to the ideals of embodiment, here, she’d been forced to adopt a kind of asceticism, but that was definitely not a quality to which she’d ever aspired, let alone one for which she hoped to be remembered.

She said, "I’m listening."

"For special events like this, we sometimes go nuclear. So I thought I’d ask whether you’d like to join us."

Cass froze, and stared at him. "Nuclear? How? Has someone solved all the problems?" Femtomachines built from exotic nuclei had been employed as special-purpose computers ever since the basic design had been developed, six thousand years before. For sheer speed, they left every other substrate in the dust. But as far as Cass knew, no one could make a femtomachine stable for more than a few picoseconds; they could perform a great many calculations in that time, but then they blew themselves apart and left you hunting through the debris for the answer. Gamma-ray spectroscopy could only extract a few hundred kilobytes, which was orders of magnitude too small even for a differential memory — a compressed description of experience that could be absorbed by a frozen reference copy of the person who’d actually lived through it. Cass might have missed the news of a breakthrough while she’d been on her way here from Earth, but if word had reached Mimosa Station at all she should have heard by now.

"Nothing’s changed in the technology," Rainzi said. "We do it freestyle. One-way."

Freestyle meant implementing your mind on a substrate that underwent quantum divergence. One-way meant none of the end products of any version of the computation could be retrieved, and transferred back into your usual hardware. Rainzi was asking her to clone herself into a nuclear abacus-cum-time-bomb that would generate a multitude of different versions of her, while holding out no prospect of even one survivor.

Cass said haltingly, "No, I’m sorry. I can’t join you." So much for feeling smugly unshockable for daring to contemplate cross-modal sex. She joked, "I draw the line at any implementation where I experience detectable weight changes every time I learn something." Femtomachines shuffled binding energies equivalent to a significant portion of their own mass; it would be like gaining or losing half a kilogram several times a second, from the sheer gravity of your thoughts.

Rainzi smiled. "I thought you’d say no. But it would have been discourteous not to ask."

"Thank you. I appreciate that."

"But you’d see it as a kind of death?"

Cass scowled. "I’m embodied, not deranged! If a copy of my mind experiences a few minutes' consciousness, then is lost, that’s not the death of anyone. It’s just amnesia."

Rainzi looked puzzled. "Then I don’t understand. I know you prefer embodiment, for the sake of having honest perceptions of your surroundings, but we’re not talking about immersing you in some comforting simulation of being back on Earth. Your experiment should last almost six picoseconds. Running on a strong-force substrate, you’d have a chance to watch the data coming in, in real time. Of course, you’ll receive a useful subset of the same information eventually, but it won’t be as detailed, or as immediate. It won’t be as real."

He smiled provocatively. "Suppose the ghost of Sarumpaet came to you in your sleep, and said: I’ll grant you a dream in which you witness the decay of the Diamond Graph. You’ll travel back in time, shrink to the Planck scale, and see everything with your own eyes, exactly as it happened. The only catch is, you won’t remember anything when you wake. You say you don’t believe that the dreamer would be dying. So wouldn’t you still want the dream?"

Cass let go of one handhold and swiveled away from the wall. There wasn’t much point objecting that he was offering her a view billions of times coarser than that, of a much less significant event. It wasn’t a ringside seat at the birth of the universe, but it was still the closest she could hope to get to an event for which she’d already sacrificed seven hundred and forty-five years of her life.

She said, "It’s not the fact that I wouldn’t remember the experience. If you’ve lived through something, you’ve lived through it. What worries me is all the other things I’d have to live through. All the other people I’d have to become."

Cass dated the advent of civilization to the invention of the quantum singleton processor. The Qusp. She accepted the fact that she couldn’t entirely avoid splitting into multiple versions; interacting with any ordinary object around her gave rise to an entangled system — Cass plus cloud, Cass plus flower — and she could never hope to prevent the parts that lay outside her from entering superpositions of different classical outcomes, generating versions of her who witnessed different external events.

Unlike her hapless ancestors, though, she did not contribute to the process herself. While the Qusp inside her skull performed its computations, it was isolated from the wider world — a condition lasting just microseconds at a time, but rigidly enforced for the duration — only breaking quarantine when its state vector described one outcome, with certainty. With each operating cycle, the Qusp rotated a vector describing a single alternative into another with the same property, and though the path between the two necessarily included superpositions of many alternatives, only the final, definite state determined her actions.

Being a singleton meant that her decisions counted. She was not forced to give birth to a multitude of selves, each responding in a different way, every time she found her conscience or her judgment balanced on a knife edge. She was not at all what Homo sapiens had actually been, but she was close to what they’d believed themselves to be, for most of their history: a creature of choice, capable of doing one thing and not another.

Rainzi didn’t pursue the argument; he followed her in silence as she clambered into the display chamber. This was a small cavity in the Quietener’s outer structure, not much larger than her room at the station, equipped with a single chair. There was no question of Cass being allowed any closer to the action; even the processor on which the Mimosans were running, scrupulously designed to spill as little noise into the environment as possible, was banished to the rim of the Quietener. Lacking the same antinoise features herself, she had to agree to be snap-frozen to a few Kelvin, three minutes before each run. Apart from being immobilized, this had no unpleasant side effects, but it served as an uncomfortable reminder of the fact that the closed-cycle "breathing" of her Mimosan body was pure placebo. Still, she’d been willing to put up with it twenty times so far, merely for the sake of sparing herself the three-second time lag for data to make its way back to the station.

As she took her place in the cryogenic chair, the other Mimosans began to appear around her. Teasing her, congratulating her on her stamina. Livia joked, "We should have had a wager as to whether or not the incremental targets would turn out to be a waste of time. You could have relieved me of all my worldly goods by now." Livia’s sole material possession was a replica of an ancient bronze coin, carved from leftover asteroid metal.

Cass shook her head. "What would I have put up? My left arm?" They’d been right to do things Livia’s way, and Cass had long ago ceased resenting it. Not only was it safer, it was better science, testing each novel structure one by one.

It turned out that Livia was alluding to a real wager: Bakim admitted that he’d made a bet with Darsono that Cass would not remain at Mimosa to the end. But he was unable to explain the stakes to her; her Mediator couldn’t find a suitable analogy, and nothing she suggested herself was even close. No precious object or information would change hands, nor was there any token act of servitude or humiliation in store for the loser. Cass was amused by the bet itself, but it bothered her that she could only grasp half of what was going on. When her friends asked her about the Mimosans, would all her stories end with apologies for her own incomprehension? She might as well have visited one of the great cities back on Earth and spent her time living in a storm-water drain, having shouted conversations through a narrow grill with the people at street level, full of misunderstandings about objects and events she couldn’t even glimpse.

Rainzi had clearly been delegated to put the Nuclear Question to her, because no one else broached the subject. Cass found it slightly galling that they wouldn’t even suffer a moment’s embarrassment when they took up their superior vantage point. They wouldn’t depart, they wouldn’t abandon her; they’d simply clone their minds into the nuclear substrate. With no expectation of recovering the clones, the originals would have no reason to pause, even for a picosecond, while their faster versions ran.

The target graph appeared on the wall in front of her. The four distinctive node patterns they’d tried in every other combination were all present now. Just as virtual particles stabilized the ordinary vacuum — creating a state of matter and geometry whose most likely successor was itself — Cass’s four patterns steered the novo-vacuum closer to the possibility of persistence. The balance was only approximate: according to the Sarumpaet rules, even an infinite network built from this motif would decay into ordinary vacuum in a matter of seconds. At the Planck scale, that was no small achievement; a tightrope walker who managed to circum-navigate the Earth a few billion times before toppling to the ground might be described as having similarly imperfect balance. In reality, any fragment of novo-vacuum they managed to create would be surrounded from the start by its older, vastly more stable relative, and would face the inevitable about a trillion times faster.

Ilene reeled off a list of measurements from the instrument probes that were monitoring their environment, out to a radius of more than a light-hour. There was nothing on its way that could wreck the experiment — or at least, nothing traveling slower than ninety-five percent of lightspeed. Zulkifli followed with a status report from the machinery deep inside the Quietener. Systems that had been preparing themselves for the last twelve hours were now minutes away from readiness.

The single graph on the wall was just a useful shorthand for the state they were hoping to create; the novo-vacuum itself was the sum of equal parts of forty-eight variations of the target graph, all generated by simple symmetry transformations of the original. All the individual variations favored one direction over another, but the sum combined every possible bias, canceling them all out and giving rise to a perfectly isotropic state. Since none of the graphs could be found in nature, this elegant description was useless as a recipe, but it wasn’t hard to show that the same state vector could also be described by a different sum: forty-eight regions of ordinary vacuum, each slightly curved, oriented in forty-eight different directions.

Inside the Quietener, an asteroid’s-mass worth of helium had been cooled into a Bose-Einstein condensate, and manipulated into a state where it was equally likely to be found in any of forty-eight different places. These alternative locations were distributed across the surface of a sphere six kilometers wide. Ordinary matter — or any kind of matter interacting with the outside world — would have behaved as if each distinct position had already become the sole reality; if a swarm of dust particles wandering by had made themselves part of the system, or if the helium’s behavior en masse had merely hinted at the detailed motion of its own atoms, then that behavior could only have told half the story — the classical half — and all the quantum subtleties would have been lost in the fine print. But the condensate was isolated as scrupulously as any cycling Qusp, and it had been cooled to the point where the states of all its individual atoms were dictated completely by its macroscopic properties. With no hidden complications, inside or out, the result was a quantum-mechanical system the size of a mountain.

The geometry of the vacuum in the Quietener inherited the helium’s multiplicity: its state vector was a sum of the vectors for forty-eight different gravitational fields. Once the condensate’s components had all been nudged into place, the quantum geometry at the center of the sphere would be equivalent to the novo-vacuum, and a new kind of space-time would blossom into existence.

That was the idealized version: a predictable event in a known location. In reality, the outcome remained hostage to countless imperfections and potential intrusions. If the experimenters were lucky, sometime over a period measured in minutes, somewhere over a region measured in meters, a few thousand cubic Planck lengths of novo-vacuum would be created, and survive for an unprecedented six-trillionths of a second.

Yann turned to Cass. "Are you ready to freeze?" The first time he’d asked her this, she’d been almost as nervous as the moment before she’d been transmitted from Earth, but the question had rapidly become a formality. Of course she was ready. That was how things were done. Just a few minutes of numb immobility, watching the data appear on the screen in front of her, and the odds were good that it would be the last time. A five-hour trip back to the station, a day or two of analysis, a brief celebration, and she would depart. Her Earth body, frozen more deeply than this one had ever been, was waiting for her. She’d step across the light-years in a subjective instant, a new set of memories to sweep away the icy cobwebs of her old self.

She said, "No. I’m not ready."

Yann looked alarmed, but only for a moment. Cass suspected that he’d just conferred privately with someone better able to guess what she had in mind. Though the Mimosans didn’t think any more rapidly than she did — running on Qusps themselves, they faced the same computing bottlenecks — they could communicate with each other about five times faster than her own form of speech allowed. That only annoyed her when they used it to talk about her behind her back.

She added dryly, "Tell Rainzi I’ve changed my mind."

Yann smiled, clearly delighted, and then his icon was instantly replaced by Rainzi’s. Fair enough: with the countdown proceeding, the Mimosans had better things to do than fake inertia for its own sake.

Rainzi’s response was more cautious than Yann’s. "Are you certain you want to do this? After everything you told me?"

"I’m the quintessential singleton," Cass replied. "I weigh up all my choices very carefully."

There was no time to spell out in glacial words everything she was feeling, everything that had swayed her. Part of it was the same sense of ownership that had brought her all this distance in the first place: justifiably or not, she didn’t want the Mimosans to have a better view than she did of the thing they were about to create together. There was the same longing for immediacy, too: she would never see, or touch, any graph as it really was, but to remain locked in a body that could only perceive a fraction of the data, milliseconds after the fact, would leave her feeling almost as detached from the event, now, as if she’d stayed on Earth, waiting for the centuries-old news of an experiment conducted light-years away. Every viewpoint was a compromise, but she had to be as close as she could get.

Beyond the experiment itself, though, it was clear to her now that she couldn’t leave Mimosa without doing at least one thing that went against the grain. After five years of monastic restraint, five years of denying herself the dishonest comforts of virtual reality, she was sick of placing that principle above everything else. Beyond the fact that this disembodiment would be entirely in the service of honesty, she needed, very badly, to drag herself out of the absolutist rut she’d been digging from the moment she’d arrived. If she’d compromised a little from the start, maybe she wouldn’t have felt the same sense of desperation. But it was too late now for half-measures. If she returned to Earth unchanged, it wouldn’t be a triumph of integrity. It would be a kind of death. She’d implode into something as hermetic and immutable as a black hole.

All this, weighed against the thing she hated most: lack of control. Every choice she made rendered meaningless. What choices, though? Her clones would run for a few subjective minutes, most of them in rapt attention as the data poured in. What was the worst that one of these transient selves might do? Utter a few unkind words to Livia or Darsono? Disclose some small guilty secret from her past to people who either wouldn’t understand, wouldn’t care, or at the very least, wouldn’t have the chance to reproach her for long? She wasn’t opening up the gates to the old human nightmare: endless varieties of suffering, endless varieties of stupidity, endless varieties of banality. She would diffuse a very small distance into the space of possibilities, and whatever unhappiness she might experience, whatever misdemeanors she might commit, would be erased beyond recovery.

Rainzi looked skeptical, and she couldn’t blame him. But there was no time left for him to play devil’s advocate, to test her resolve. Cass stood her ground, silently, and after a moment he nodded assent.

She felt a stream of low-level requests for data, and she willed her Mediator to respond. She’d been through the same process before her transmission from Earth: sending the preliminaries first, things that needed to be known about the structure of her mind before it could be implemented in a new environment.

Rainzi said, "Take my hand. We’ll step through together." He placed his ghost-fingers over hers, and asked her for everything.

Cass examined his face. It was pure chance that her Mediator had given him an appearance that inspired trust in her, but the faces of the embodied were no better guides to character, whether they’d been sculpted by genes or by their wearer’s wishes. If Rainzi’s eyes still seemed kind to her, after five years, wasn’t that because he’d shown her genuine kindness? This was not the time for paranoid delusions about the unknowable mind behind the mask.

She said, "Are you ever afraid of this, yourself?"

"A little," he admitted.

"What frightens you the most? What is it that you think might happen?"

He shook his head. "There’s no terrible fate that I fear is lying in store for me. But however many times I do this, I come no closer to knowing what it’s actually like. Don’t you think there’s something frightening about that?"

She smiled. "Absolutely." They weren’t so different that she’d be insane to follow him, the way it would be insane to follow an armored robot into a volcano. This would not be strange or painful beyond her power to bear. If she truly wanted it, she had nothing to fear.

Cass opened the floodgates.

Rainzi’s hand passed through her own, intangible as ever. Cass shuddered. She was who she always was, and the part of her who valued that above all else could not disguise its relief.

"Don’t worry," he assured her, "you won’t be hanging around waiting. And you won’t be disappointed. The femtomachine will only start up on a definite signal from the Quietener; if there’s nothing, it won’t ever be run."

Cass protested, "Aren’t you telling the wrong person?" He might have mentioned this before she’d been split.

Rainzi shrugged. "To the clone, it will be self-evident. If it gets the chance to think anything at all."

If the vacuum at the heart of the Quietener changed, her other self would wake, watch the whole event unfold in slow motion, bifurcate a million times, then vanish, before Cass had even noticed the good news. Neither the price nor the payoff were part of her own future, now.

Yet they would all be one person: awake, asleep. The dream she would not remember would be her own.

Here and now, though?

She would have to make do with whatever glimpses she could steal.

She turned to Yann. "Freeze me. One last time."

Chapter 3

Cass looked around the simulated chamber. The display on the wall was densely inscribed with new data, but nothing else appeared to have changed. The Mimosans were the usual icons drawn by her Mediator; she still had no hope of perceiving them as they perceived themselves. The structures in her mind where sensory data was represented hadn’t changed; they simply weren’t coupled to genuine sense organs anymore. It was only the touch of Rainzi’s nonexistent skin against her own — a translation interacting with a simulation — that proved she’d stepped from her world into his.

Or rather, they’d both stepped together into a new world, from which neither of them could hope to emerge.

Cass felt no anxiety, just a bittersweet sense of everything her newfound freedom did and didn’t mean. If she’d abandoned embodiment a year or two earlier, she might have had some prospect of going further: finding a path of gradual change that led to new abilities, such as the power to interpret the Mimosans' language firsthand. As it was, she didn’t even have time for the smallest act of self-indulgence: a simulated swim, a solid meal, a glass of cool water. After five years, all the pleasures she’d been pining for had become attainable at the very moment when they would be nothing but unwelcome distractions.

She slipped her hand free of Rainzi’s and turned to examine the display. A faint spray of particles was radiating out from the center of the Quietener, the sign of an unstable boundary between old vacuum and new.

The data had only been coming in for a few hundredths of a picosecond, so the statistics were still ambiguous. As she watched, rows of figures were updated, the sprinkling of points on half a dozen charts grew denser, curves shifted slightly. Cass knew where every number and every curve was heading; it was like watching the face of a long-awaited friend materialize out of the darkness, having pictured the reunion a thousand times. And if the face might yet turn out to be a stranger’s, that had nothing to do with the way she felt. There was pleasure enough in anticipation; she didn’t need to conjure up traces of doubt just to savor the added suspense.

"What we’re doing isn’t all that unusual," Darsono mused. "I think everyone lives in at least two time scales: one of them fast and immediate, and too detailed to retain in anything but outline; the other slow enough to be absorbed completely. We think our memory has no gaps, we think we carry our entire past inside us, because we’re accustomed to looking back and seeing only sketches and highlights. But we all experience more than we remember."

"That’s not true of everyone," Bakim countered. "There are people who record every thought they have."

"Yes, but unless every part of that record has the potential to be triggered automatically by subsequent thoughts and perceptions — which no one ever allows, because the barrage of associations would drive them mad — it’s not true memory. It’s just a list of all the things they’ve forgotten."

Bakim chortled. "True memory? And I suppose if I perceive something with so much spatial resolution that I can’t give immediate, conscious attention to every last detail simultaneously, it’s not a true perception — it’s just a cruel taunt to drive home all the things I’ve failed to perceive?"

Cass smiled, but stayed out of the argument. With certainty? Probably not. But it was pointless dwelling on every potential branching; if and when she experienced something unpleasant, firsthand, or did something foolish herself, she could regret it. Anything else was both futile and a kind of masochistic doublecounting. (And she would not start wondering if that resolution was universal — a constant across histories, an act of inevitable good sense — or just the luck of one branch.)

Livia said, "I don’t understand what’s happening with the energy spectrum." In the feigned weightlessness of the chamber, she appeared upside down, her face at the upper edge of Cass’s vision. "Does that make sense to anyone?"

Cass examined the histogram showing the number of particles that had been detected in different energy ranges; it did not appear to be converging on the theoretically predicted curve. She’d noticed this earlier, but she’d assumed it was just an artifact of the small sample they’d collected. The histogram’s rim was quite smooth, though, and its overall shape wasn’t fluctuating much, so its failure to match the curve really didn’t look like an accident of noise. Worse, all the high-powered statistics beneath the chart suggested that there was now enough data to give a reliable picture of the underlying spectrum.

"Could we have miscalculated the border geometry?" Rainzi wondered. The particles they were seeing reflected the way the novo-vacuum was collapsing. Cass had first modeled the process back on Earth, and her calculations had shown that, although the border’s initial shape would be a product of both pure chance and some uncontrollable details of conditions in the Quietener, as it collapsed it would rapidly become spherical, all quirks and wrinkles smoothed out.

At least, that was true if some plausible assumptions held. She said, "If the converted region had a sufficiently pathological shape to start with, it might have retained that as it shrank. But I don’t know what could have caused that in the first place."

"Some minor contaminant that wasn’t quite enough to wreck coherence?" Ilene suggested.

Cass made a noncommittal sound. It would be nice to have a view from several different angles, allowing them to pick up any asymmetry in the radiation. But they’d been woken by the arrival of data from the cluster of detectors closest to the femtomachine; information from the second-closest would take almost another microsecond to reach the same spot, by which time they’d be long gone. Her old embodied self would get to see the big picture, albeit more coarsely grained. Her own task — her own entire raison d'être — was to make what sense she could of the clues at hand.

The energy spectrum wasn’t jagged and complicated, or even particularly broad. It didn’t look wrong enough to be the product of a sausage- or pancake- or doughnut-shaped region of novo-vacuum, let alone some more exotic structure with a convoluted fractal border. The peak had about the same width, and the same kind of smooth symmetry as the predicted curve; it was merely displaced upward along the energy scale, and the shoulders on either side were reversed. It wasn’t literally a mirror image of the expected result, but Cass felt sure it was the product of some fairly simple transformation. If you changed a single plus sign to a minus, somewhere deep in the underlying equations, this would be the outcome.

Zulkifli was one step ahead of her. "If you modify the operator that acts on the border, swapping the roles of the inside and outside of the region, you get a perfect match."

Cass experienced a shiver of fear, all the more disturbing for evoking the phantom viscera of her Earth body. If Zulkifli’s claim was true, then the region was expanding, not collapsing.

She said, "Are you sure that works?"

Zulkifli made his private calculations visible, and superimposed the results on the histogram. His curve ran straight through the tops of all the bars. He’d found the plus sign that had turned into a minus. Except —

"That can’t be right," she declared. The simple role reversal he’d suggested was elegant, but nonsensical: it was like claiming that they were seeing the light from a fire in which ashes were burning into wood. Conservation of energy was a subtle concept, even in classical general relativity, but in QGT it came down to the fact that the flat vacuum state remained completely unchanged from moment to moment. An awful lot of physics flowed from that simple requirement, and though it was remote from everyday notions of work, heat, and energy, a billion commonplace events that Cass had witnessed throughout her life would have been impossible, if the truth were so different that Zulkifli’s border operator was the right choice.

There was silence. No one could contradict her, nor could they deny that Zulkifli’s curve matched the data.

Then Livia spoke. "The Sarumpaet rules make our own vacuum perfectly stable; that’s the touchstone Sarumpaet used from the start. But the novo-vacuum is not decaying in the way those rules predict. So what’s the simplest way to reconcile the contradictions?" She paused for a moment, then offered her own solution. "Suppose both kinds of vacuum are perfectly stable, on their own. If there’s a wider law that makes that true — with the Sarumpaet rules as a special case — we would never have stumbled on it in the staged experiments, because we never had the full set of virtual particles that constituted a viable alternative vacuum."

Yann grinned appreciatively. "All states with the potential to be a vacuum must be treated equally? However exotic we might think they are, they’re all eternal? Very democratic! But wouldn’t that imply a stalemate? Wouldn’t that freeze the novo-vacuum, leaving the border fixed?"

Ilene said, "No. The dynamics needn’t be that evenhanded. One side could still convert the other at a boundary. The one with the fewest species of particles, I expect."

By any count, the novo-vacuum was the more streamlined of the two. Cass was more angry than afraid, though. Talk of a runaway vacuum conversion was intolerable; they’d spent five years ruling that out, validating the Sarumpaet rules for every related graph. They could not have been more cautious.

Rainzi said calmly, "Suppose the novo-vacuum is growing. What happens when it encounters some contamination? It’s a coherent state that could only be created in perfect isolation, in the middle of the purest vacuum in the universe. It’s fragility incarnate. Once it hits a few stray neutrinos and decoheres, it will be forty-eight flavors of ordinary vacuum — all of them in separate histories, all of them harmless."

Livia glanced warily at Cass. It was as if she wanted Cass to be the bearer of bad news for a change, rather than always hearing it from her.

Cass obliged her. "I wish you were right, Rainzi, but that argument’s biased. It’s just as correct to say that our own vacuum is a superposition of different curved versions of the novo-vacuum. If there really is a new dynamic law at work here, and if it preserves the novo-vacuum precisely, then according to that law, it’s our vacuum that’s the delicate quantum object waiting to decohere."

Rainzi pondered this. "You’re right," he conceded. "Though even that doesn’t tell us much about the border. Neither of the specialized laws that apply on either side can hold there. We’ll only understand the fate of the border if we can understand the general law."

Cass laughed bitterly. "What difference does it make, what we understand? We won’t be able to tell anyone! We won’t be able to warn them!" The border wasn’t traveling at lightspeed — or they wouldn’t have been woken at all before it swept over the femtomachine — but it was unlikely to be spreading so slowly that their originals would see it coming, let alone have a chance to evacuate. In any case, what she and her fellow clones knew was worthless; they had no way to share their knowledge with the outside world. The femtomachine was designed to do no more than compute its inhabitants, for their own benefit. All it would leave behind was debris. Even if they could encode a message in the decay products, no one would be looking for it.

A lifetime’s worth of defensive slogans about the perils of VR started clamoring in her head. She wanted to scrape this whole illusion off her face, like a poisonous, blinding cobweb; she wanted to see and touch reality again. To have real skin, to breathe real air, would change everything. If she could only see the world through her own eyes, and react with the instincts of her own body, she knew she could flee from any danger.

It was so perverse it was almost funny. She was perceiving the danger a billion times more clearly than she could ever have hoped to if she’d been embodied. She had all her reflexes at her disposal, and all her powers of reasoning, operating a billion times faster than usual.

It was just a shame that all of these advantages counted for nothing.

Zulkifli said, "The brightness is increasing."

Cass examined the evidence as dispassionately as she could. A slow, steady rise in the rate of particle production was apparent now, clearly distinguishable from the background fluctuations that had initially masked it. That could only mean that the border was growing. Short of some freakishly benign explanation for this — a fractal crinkling that allowed the border to increase in area while the volume of novo-vacuum itself was shrinking — this left little room for doubt about which vacuum was being whittled away to produce the particles they were seeing. The thing she had always thought of as an elegant piece of whimsy — as charming and impractical as a mythical beast that might be bioengineered into existence, and kept alive briefly if it was pampered and protected, but which could never have lasted five minutes outside its glass cage — was now visibly devouring its ancient, wild cousin. She had summoned up, not a lone, defenseless exile from a world that could never have been, but the world itself — and it was proving to be every bit as autonomous and viable as her own.

Rainzi addressed her, gently but directly. "If the station is destroyed, we all have recent backups en route to Viro. What about you?"

She said, "I have my memories back on Earth. But nothing since I arrived here." The five years she’d spent among the Mimosans would be lost. It had still happened. She had still lived through it all. It would be amnesia, not death. But if that argument had been enough to let her step willingly into the cul-de-sac she inhabited now, she wasn’t sure she could push it far enough to reconcile herself to the greater loss. She had finally become someone new, at the station — someone different enough from her old self to be here now, beside the Mimosans. But the Cass who had steeled herself to leave the solar system for the very first time would wake from her frozen sleep unchanged, to learn that the emboldened traveler she’d hoped to become was dead.

"I don’t know how to help you make peace with that," Rainzi said. "But I can only think of one way to make my own peace with the people we’ve endangered." Mimosa was remote from the rest of civilization, but the process they’d begun would not burn itself out, would not fade or weaken with distance. With vacuum as its fuel, the wildfire would spread inexorably: to Viro, to Maeder, to a thousand other worlds. To Earth.

Cass asked numbly, "How?"

"If we can see a way to stop this," Rainzi replied, "then it doesn’t matter that we can’t enact it ourselves, or even get the word out to anyone else. We can still take comfort in uncovering the right strategy. I know we have certain advantages — in the time resolution with which we’re seeing the data, and in being the only witnesses to this early stage — but on balance, I think the combined population of the rest of the galaxy constitutes more than an even match. If we can find a solution, someone out there will find it, too."

Cass looked around at the others. She felt lost, rootless. Not guilty, yet. Not monstrous. The Mimosans would all wake on Viro, missing a few hours' memories but otherwise unscathed, and though she’d robbed them of their home, they’d understood the risks as well as she did when they’d chosen to conduct the experiment. But if the loss of the Quietener and the station was something she could come to terms with, it was still surreal to extrapolate from her own few picoseconds of helplessness to the exile of whole civilizations. She had to face the truth, but she was far from certain that the right way to do that was to hunt for a solution that would at best be a plausible daydream.

Darsono caught her eye. "I agree with Rainzi," he said solemnly. "We have to do this. We have to find the cure."

"Livia?"

"Absolutely." Livia smiled. "Actually, I’m far more ambitious than Rainzi. I’m not willing to concede yet that we can’t stop this ourselves."

Zulkifli said dryly, "I doubt that. But I want to know if my family will be safe."

Ilene nodded. "It’s not much, but it’s better than giving up. I’m not bailing out just to spare myself the sense of being powerless — not while data’s pouring in, and we can still look for an answer."

"The danger doesn’t seem real to me," Yann admitted. "Viro is seventeen light-years away, and we can’t be sure that this thing won’t snuff itself out before it even grazes the shell of the Quietener. But I would like to know the general law that replaces the Sarumpaet rules. It’s been twenty thousand years! It’s about time we had some new physics."

Cass turned to Bakim.

He shrugged. "What else are we going to do? Play charades?"

Cass was outnumbered, and she wanted to be swayed. She ached to get her hands on even the smallest piece of evidence that the disaster could be contained, and if they failed, it would still be the least morbid way to go out: struggling to the end to find a genuine cause for optimism.

But they were fooling themselves. In the few subjective minutes left to them, what hope did they have of achieving that?

She said simply, "We’ll never make it. We’ll test one hunch against the data, find it’s wrong, and that will be it."

Rainzi smiled as if she’d said something comically naive. Before he spoke, Cass recalled what it was she had forgotten.

What it was she had become.

He said, "That’s how it will seem for most of us. But that shouldn’t be disheartening. Because every time we fail, we’ll know that another version of ourselves will have tested another idea. There will always be a chance that one of them was right."

Part Two

Inhabited Space

Only a small proportion of all systems are shown. Shaded systems have been lost behind the border as Tchicaya arrives on the Rindler, 605 years after Mimosa.

Chapter 4

By choice, Tchicaya’s mind started running long before his new body was fully customized. As his vision came into focus, he turned his gaze from the softly lit lid of the crib to the waxen, pudgy template that he now inhabited. Waves of organizers swarmed up and down his limbs and torso like mobile bruises beneath the translucent skin, killing off unwanted cells and cannibalizing them, stimulating others to migrate or divide. The process wasn’t painful — at worst it tickled, and it was even sporadically sexy — but Tchicaya felt an odd compulsion to start pummeling the things with his fists, and he had no doubt that squashing them flat would be enormously satisfying. The urge was probably an innate response to Earthly parasites, a misplaced instinct that his ancestors hadn’t got around to editing out. Or perhaps they’d retained it deliberately, in the hope that it might yet turn out to be useful elsewhere.

As he raised his head to get a better view, he caught sight of an undigested stretch of calf, still bearing traces of the last inhabitant’s body hair and musculature. "Urrggh". The noise sounded alien, and left a knot in his throat. The crib said, "Please don’t try to talk yet." The organizers swept over the offending remnant and dissolved it.

Morphogenesis from scratch, from a single cell, couldn’t be achieved in less than three months. This borrowed body wouldn’t even have the DNA he’d been born with, but it had been designed to be easy to regress and sculpt into a fair approximation of anyone who’d remained reasonably close to their human ancestors, and the process could be completed in about three hours. When traveling this way, Tchicaya usually elected to become conscious only for the final fitting: the tweaking of his mental body maps to accommodate all the minor differences that were too much of a nuisance to eliminate physically. But he’d decided that for once he’d wake early, and experience as much as he could.

He watched his arms and fingers lengthen slightly, the flesh growing too far in places, then dying back. Organizers flowed into his mouth, re-forming his gums, nudging his teeth into new locations, thickening his tongue, then sloughing off whole layers of excess tissue. He tried not to gag.

"Dith ith horrible," he complained.

"Just imagine what it would be like if your brain was flesh, too," the crib responded. "All those neural pathways being grown and hacked away — like a topiary full of tableaux from someone else’s life being shaped into a portrait of your own past. You’d be having nightmares, hallucinations, flashbacks from the last user’s memories."

The crib wasn’t sentient, but pondering its reply made a useful distraction from the squirming sensation Tchicaya was beginning to feel in his gut. It was a much more productive rejoinder than: "You’re the idiot who asked to be awake for this, so why don’t you shut up and make the best of it?"

When his tongue felt serviceably de-slimed, he said, "Some people think the same kind of thing happens digitally. Every time you reconfigure a Qusp to run someone new, the mere act of loading the program generates experiences, long before you formally start it running."

"Oh, I’m sure it does," the crib conceded cheerfully. "But the nature of the process guarantees that you never remember any of it."

When Tchicaya was able to stand, the crib opened its lid and had him pace the recovery room. He stretched his arms, swiveled his head, bent and arched his spine, while the crib advised his Qusp on the changes it would have to make in order to bring his expectations for kinesthetic feedback and response times into line with reality. In a week or two he would have accommodated to the differences anyway, but the sooner they were dealt with, the sooner he’d lose the distracting sense that his own flesh was like poorly fitted clothing.

The clothes that were waiting for him had already been informed of his measurements, and the styles, colors, and textures he preferred. They’d come up with a design in magenta and yellow that looked sunny without being garish, and he felt no need to ask for changes, or to view a range of alternatives.

As he dressed, Tchicaya examined himself in the wall mirror. From the whorl of dark bristles on his scalp to the glistening scar running down his right leg, every visible feature had been reproduced faithfully from a micrometer-level description of his body on the day he’d left his home world. For all he could tell, this might as well have been the original. The internal sense of familiarity was convincing, too; he’d lost the slight tension in his shoulder muscles that had been building up over the last few weeks before his departure, but having just rid himself of all the far more uncomfortable kinks he’d acquired in the crib, that was hardly surprising. And if this scar was not the scar from his childhood, not the same collagen laid down by the healing skin in his twelve-year-old body, nor would it have been the same in his adult body by now, if he’d never left home. All an organism could do from day to day was shore itself up in some rough semblance of its previous condition. The same was true, from moment to moment, for the state of the whole universe. By one means or another, everyone was an imperfect imitation of whatever they’d been the day before.

Still, it was only when you traveled that you needed to dispose of your own past, or leave behind an ever-growing residue. Tchicaya told the crib, "Recycle number ten." He’d forgotten exactly where the tenth-last body he’d inhabited was stored, but when his authorization reached it, the memories sitting passively in its Qusp would be erased, and its flesh would be recycled into the same kind of waxen template as the one he’d just claimed as his own.

The crib said, "There is no number ten, by my count. Do you want to recycle number nine?"

Tchicaya opened his mouth to protest, then realized that he’d spoken out of habit. When he’d left Pachner, thirty years before — a few subjective hours ago — he’d known full well that his body trail would be growing shorter by one while he was still in transit, and he wouldn’t have to lift a finger or say a word to make it happen.

He said, "Keep number nine."

As he stepped out of the recovery room, Tchicaya was grateful for his freshly retuned sense of balance. The deck beneath his feet was opaque, but it sat inside a transparent bubble a hundred meters wide, swinging for the sake of gravity at the end of a kilometer-long tether. To his left, the ship’s spin was clearly visible against the backdrop of stars, all the more so because the axis of rotation coincided with the direction of travel. The stars turning slowly in the smallest circles were tinted icy blue, while away from the artificial celestial pole they took on more normal hues, ultimately reddening slightly. The right half of the sky was starless, filled instead with a uniform glow that was untouched by the Doppler shift, and so featureless that there was nothing to be seen moving within it: not one speck of greater or lesser brightness rising over the deck in time with the stars.

From the surface of Pachner, the border of the Mimosa vacuum had appeared very different, a shimmering sphere of light blazing a fierce steely blue at the center, but cooled toward the edges by its own varied Doppler shift. The graded color had made it look distinctly rounded and three-dimensional, and the fact that you could apparently see it curving away from you had added to an already deceptive impression of distance. Because it was expanding at half the speed of light, the amount of sky the border blotted out was not a reliable measure of its proximity. Looking away from its nearest point meant looking back to a time when it had been considerably smaller, and starlight that had grazed the sphere centuries before — skirting the danger, and appearing to delineate it — actually told you nothing about its present size. When Tchicaya had left, Pachner had been little more than two years away from being engulfed, but the border had barely changed its appearance in the decade he’d spent there, and it would still have occupied a mere one hundred and twenty degrees of the view at the instant the planet was swallowed.

Tchicaya had been on Pachner to talk to people on the verge of making their escape. He’d had to flee long before the hard cases, who’d boasted that they’d be leaving with just seconds to spare, but as far as he knew he’d been the only evacuee who was planning to end up closer to the border than when he left. Doomed planets were useless as observation posts; no sooner did the object of interest come near than you had to retreat from it at the speed of light. The Rindler was constantly retreating, but no faster than was absolutely necessary. Matching velocities with the border transformed its appearance; from the observation deck, the celestial image that had become an emblem of danger for ten thousand civilizations was nowhere to be seen. The border finally looked like the thing it was: a vast, structureless, immaterial wall between two incomparably different worlds.

"Tchicaya!"

He looked around. There were a dozen people nearby, but they were all intent on the view. Then he spotted a lanky figure approaching, an arm stretched up in greeting. Tchicaya didn’t recognize the face, but his Mediator picked up a familiar signature.

"Yann?" Tchicaya had known for centuries that Yann was also weaving his way toward the Rindler, but the last place he’d expected to run into him was the observation deck. In all the time they’d been in contact, exchanging messengers across decades and light-years, Yann had been strictly acorporeal.

The half-stranger stood before him. "How are you?"

Tchicaya smiled. "I’m fine. You seem to have put on weight."

Yann shrugged apologetically. "Conforming to local fashions. I still think it’s an absurdity: boosting millions of tonnes of furniture into a trajectory like this, when a few hundred kilograms of instrumentation and Qusps could have achieved as much. But given that they’ve gone ahead and done it anyway, and given that most of the people here are wearing flesh, I have to take account of that. I need to be in the thick of things, or there’s no point being here at all."

"That makes sense," Tchicaya conceded. He hated the idea of anyone being forced out of their preferred mode, but the political realities were undeniable.

If the optimists were right, and the border’s current velocity was the highest it would ever be, the simplest way to avoid the threat would be to flee from it. If your whole world already consisted of compact, robust hardware that was designed to function in interstellar space, the prospect of engineering in the necessary shielding against relativistic collisions with gas and dust, accelerating to a suitable velocity — half c plus a chosen safety margin — then simply coasting away from the danger, was not unthinkable at all. A dozen acorporeal communities, and countless scattered individuals, had already done that.

For people accustomed to dwelling on a planetary surface, though, the notion of entering a permanent state of flight was more likely to be horrifying. So far, the Mimosan vacuum had swallowed more than two thousand inhabited systems, and while most of the planet-hopping refugees were willing to transmit themselves at lightspeed from point to point, in less than two millennia all the old, established colony worlds that had taken them in would themselves be gone. In principle, the process could be prolonged indefinitely: new, habitable planets could be prepared in advance by high-velocity spore packages, with people following close behind. Each temporary home would last a little longer than the one before, as the border was outpaced. People might even grow accustomed to the fact that every world they set foot upon would be obliterated, not in billions of years, but in a few thousand. It would take six times as long as recorded history before the entire Milky Way was lost, and by then, the gulf between neighboring galaxies might seem less daunting.

Even assuming a watertight proof, though, that the border would not speed up without warning and turn that whole scenario into a rosy-hued fantasy, exile was not a fate to be accepted lightly. If it was physically possible to turn back the novovacuum — to seed its destruction, the way the Mimosans had seeded its creation — Tchicaya’s fellow embodied had by far the greatest stake in making that happen. It was not going to be easy to persuade them that they shouldn’t try.

Yann said, "You’ve just come from Pachner?"

Tchicaya nodded. He was pleased to have met up with Yann, but he was having trouble maintaining eye contact; the spinning sky kept drawing his gaze. "When did you get here?" He’d lost track of Yann’s recent movements; communication between interstellar travelers had always been difficult, with line-of-sight time lags and transit insentience, but having to route signals around a constantly growing obstacle had added a further level of delays and fragmentation.

"Almost nine years ago."

"Ha! And there I was thinking you were the one out of your element."

Yann took a moment to interpret this. "You’ve never been in space before?"

"No."

"Not even planetary orbit?" He sounded incredulous.

Tchicaya was annoyed; it was a bit rich for a former acorporeal to put such stock in where he had or hadn’t been, in the flesh. "Why would I have been in space? Vacuum never used to be much of an attraction."

Yann smiled. "Do you want to take the grand tour, while I fill you in?"

"Definitely." Everything Tchicaya had heard about the state of play on the Rindler was out of date — though not by the full sixty years that his thirty-year journey would normally have implied. He did a quick calculation before confirming the result with the ship: fifty-two years had elapsed here, since the last bulletin that he’d received on Pachner had been sent.

Stairs led down from the observation deck to a walkway. The ship was made up of sixteen separate modules arranged in a ring; the tethers joining them to the hub were not traversable, but there were umbilicals linking adjacent modules. Once they’d left the shelter of the deck behind, Tchicaya could see the engines sitting at the hub as dark outlines clustered at the zenith. They were unlikely to be used again for some time; if the border suddenly accelerated, it would probably move too fast for the Rindler to escape, and everyone onboard would evacuate the way they’d arrived: as data. Even if the ship was destroyed without warning, though, most people would only lose a few hours' memories. Tchicaya had instructed his Qusp to transmit daily backups, and no doubt Yann was doing something similar, having escaped from the Mimosan vacuum once already that way.

The view from the narrow walkway was disorienting; without an expanse of deck imposing a visual horizon, the rim of the border became the most compelling cue. Tchicaya began to feel as if he was walking inside a huge horizontal centrifuge, hovering an indeterminate distance above an ocean shrouded in white fog. Any attempt to replace this mildly strange hypothesis with the idea that he was actually keeping pace with a shock wave six hundred light-years wide did nothing to improve his steadiness.

"The factions have names now," Yann began.

Tchicaya groaned. "That’s a bad sign. There’s nothing worse than a label, to cement people’s loyalties."

"And nothing worse than loyalties cementing while we’re still in the minority. We’re Yielders, they’re Preservationists."

"Yielders? Whose idea was that?"

"I don’t know. These things just seem to crystallize out of the vacuum."

"With a little seeding from the spin doctors. I suppose it’s a step up from being Suicidal Deviants, or Defeatist Traitors."

"Oh, those terms are still widely used, informally."

Without warning, Tchicaya’s legs buckled. He knelt on the walkway and closed his eyes. He said, "It’s all right. Just give me a second."

Yann suggested mildly, "If the view’s that unsettling, why not paste something over it?"

Tchicaya scowled. His vestibular system wanted him to curl up on the ground, block out all the contradictory visual signals, and wait for normality to be restored. He spread his arms slightly, reassuring himself that he was prepared to take action to recover his balance at short notice. Then he opened his eyes and rose to his feet. He took a few deep breaths, then started walking again.

"Both stances remain purely theoretical," Yann continued. "The Preservationists are no more prepared to erase the Mimosan vacuum than we are to adapt to it. But the team working on the Planck worms has just attracted a fresh batch of recruits, and they’re running experiments all the time. If it ever does come down to a technological race, it’s sure to be a close one."

Tchicaya contemplated this prospect glumly. "Whoever first gains the power to impose their own view decides the issue? Isn’t that the definition of barbarism?" They’d reached the stairs that led up to the deck of the next module. He gripped the rails and ascended shakily, relieved to be surrounded by the clutter of ordinary objects.

They emerged at the edge of a garden, engineered in a style Tchicaya hadn’t seen before. Stems coiled in elaborate helices, sprouting leaves tiled with hexagonal structures that glinted like compound eyes. According to the ship, the plants had been designed to thrive in the constant borderlight, though it was hard to see how that could have required some of their more exotic features. Still, the embellishments did not seem overdone here. Purebred roses or orchids would have been cloyingly nostalgic in the middle of interstellar space.

There were more people in the garden than on the observation deck. When strangers caught his eye, Tchicaya smiled and offered whatever gestures his Mediator deemed appropriate to greet them in passing, but he wasn’t ready for formal introductions, sorting everyone into opposing camps.

"Isn’t there a level where both sides can still cooperate?" he asked. "If we can’t agree on the theory that’s going to underpin whatever action finally gets taken, we might as well all give up and join the wagon train to Andromeda."

Yann was apologetic. "Of course. Don’t let my moaning give you too bleak a picture. We haven’t reached the point of hostility for its own sake; we still pool resources for the basic science. It’s only the goal-directed experiments that make things a little frosty. When Tarek started scribing graphs at the border that he believed stood a good chance of being viable proto-worms, we cut him out of all the theoretical discussion groups and data sharing agreements — though none of us thought he was in any danger of succeeding. Since then, he’s backed off slightly, and agreed to limit himself to graphs that can test his hunches without running amok if they happen to confirm them."

Tchicaya began to protest, but Yann cut him off. "Yes, I know that’s a treaty full of holes: it wouldn’t take much disingenuousness to pretend that success was just a terrible mistake. But who am I to lecture anyone about the results they should or shouldn’t have expected?"

Tchicaya muttered, "Everyone’s wise about the accident, after the fact." He’d met people who’d claimed they’d happily obliterate every extant version of Cass and her accomplices, though that was the rare, extremist view. More commonly, it was conceded that the Mimosans had been cautious, and could not be judged by the magnitude of the force they’d unleashed. Few people could honestly claim that in the Mimosans' place, they would have treated the Sarumpaet rules — inviolate for twenty thousand years — as being subject to serious doubt, let alone erasure.

The last Tchicaya had heard, seventeen people out of the billions of evacuees had chosen to stand their ground and die. He knew that these suicides weighed on Yann’s conscience — as did the distress of all those who’d been driven from their homes — but that didn’t dictate his attitude to the phenomenon. It might have been tactful to withdraw from the debate entirely, as the other seven had, but Tchicaya understood his refusal to do so. The fate of the vacuum had to be argued on its merits, not treated as a surrogate through which its creators could be condemned or absolved, and Yann intended the fact that he’d dared to take sides to highlight that distinction.

"So there’s been no theoretical progress while I was in transit?" A definitive breakthrough would have been the first thing Yann mentioned, but there might still have been promising developments.

Yann shrugged. "Three steps left, four steps down. We scribe these elaborate probe graphs and drop them through the border, then hope that whatever we can see of their decay will tell us something useful. Even when we make an inspired choice of probe and get a clean set of data, as evidence for competing models it’s all hideously indirect."

In the immediate aftermath of the accident, it had been easy to devise candidates for meta-rules that stabilized both the old and new vacuum in bulk. In those days, the theorists' biggest problem had been an excess of possibilities. The borderlight’s spectrum had helped narrow the choices somewhat, and even the single, fortunate fact that the border was traveling slower than light had ultimately been shown to rule out a class of theories in which the accident had merely changed some particle masses and triggered a boring old Higgs field collapse. In that case, the Mimosan vacuum would have been nothing but a lower-energy version of the ordinary vacuum, and coming to terms with its physics would have been as simple as altering a few numbers in the old equations. A careful analysis, though, had eventually confirmed most people’s instinctive hunch: any single kind of vacuum — even one that was undergoing such a collapse — had to appear exactly the same to anyone who was coasting through it, an ancient principle known as Lorentz invariance, dating back to the abolition of the aether. The only velocity at which a change could spread while satisfying that criterion was lightspeed.

Since the Rindler had provided a stable platform from which to probe the border experimentally — while vividly driving home the point that it was not Lorentz-invariant — the embarrassment of riches had proved illusory. Once it had become possible to put the new theories to the test, the only ones that hadn’t been falsified were those that remained too ill-defined to offer clear predictions. That provisional vagueness wasn’t necessarily a flaw, though; it could easily be the case that the correct grand generalization of the Sarumpaet rules simply couldn’t be pinned down from one example of a stable vacuum and a murky glimpse of another, and it was better to be forced to confront that fact than to be lulled for a second time into a false sense of security.

Yann said thoughtfully, "I suppose we could always stop messing about trying to peek behind the border, and just resurrect the Quietener." He punched his hands together enthusiastically. "A few well-planned experiments in the old style might cut straight to the heart of things."

"Oh, that’s a great idea. We could do it right here." A second seeding of the novo-vacuum, from a starting point that was already moving rapidly in the same direction as everyone who was fleeing the first, would be twice as difficult to escape. Yann’s sardonic suggestion was sobering, though, since it was far from being the only way in which the disaster might be magnified. However careful they were, whatever their motives, there was always the chance of simply making things worse.

"We’re dropping the next probe in about twelve hours' time," Yann said. "If you’re interested, I could probably swing it."

"Swing what?"

"Bringing you along."

Tchicaya’s throat tightened. "You mean, you go down there? In person?"

"Absolutely."

"Why?"

Yann laughed. "Don’t ask me! You’re the one with the flesh fetish; I thought you’d understand. That’s how they do things here. I just play along."

Tchicaya looked past him, into the opaque pearly light, more featureless than any darkness he’d ever encountered. The eyes relished darkness, conjuring up hints of what it might contain, but the borderlight flooded his vision with incontrovertible blankness.

And he believed he could live in that light? He believed the embodied should end their flight, end their resistance, and march straight into that blinding whiteness?

The borderlight was a surface phenomenon, a distractingly perfect veil. Whatever lay behind it could easily be as richly structured and complex as the universe he knew.

He said, "Let me sleep on it."

Half the Rindler's sixteen modules were devoted to accommodation. The ship informed Tchicaya of the cabin he’d been allocated, but he declined detailed directions, since Yann seemed eager to continue as his guide.

"I’ll show you where I am, myself, first," Yann offered. "It’s on the way, and you’re always welcome to drop by." The accommodation modules were all split into multiple levels; away from the edges, where you could still glimpse the sky, it was like being in a high-rise building. When they left the stairwell, Yann paced briskly down a corridor, and pointed out the room.

Tchicaya’s heart sank. The cabin was divided into two banks of narrow slots, each about a meter wide and half as high. A number of the slots contained inert figures. Rows of handholds between the pigeonholes were apparently intended to assist the occupants in gaining access. Yann followed his gaze and said, "It’s not that hard, once you’re used to it." He demonstrated, clambering up and sliding into his coffin-sized bunk, the fifth in a stack of eight.

Tchicaya said forlornly, "My embodiment request had the standard clause: if there was no room for me here at full size, the ship was meant to bounce me to the nearest alternative destination. Maybe I’m going to have to start spelling out the meaning of some of those terms." In four millennia of traveling between planetary surfaces, he’d encountered a wide range of living conditions deemed acceptable by the local people, whether through custom or necessity. On rare occasions, he’d even been provided with deliberately inhospitable accommodation. He’d never seen people squeezed together as tightly as this.

"Mmm." Yann’s response was noncommittal, as if in retrospect he wasn’t surprised by the complaint, but it honestly hadn’t occurred to him that a newcomer would see the Rindler as cramped. He deftly reversed his insertion maneuver and joined Tchicaya on the deck.

"I’d suggest they ease things by scrapping the garden," Tchicaya mused, "but given how little difference that would make, they probably should keep it, for sanity’s sake."

Yann squeezed past him, back into the corridor. Tchicaya trudged after him dejectedly. He’d felt no sense of panic upon waking in the confinement of the crib, but he hadn’t realized he’d soon be moving into something smaller.

He crossed the final walkway with his eyes locked straight ahead, still faltering every ten or fifteen meters when the false horizon became impossible to ignore. He was angry that he was letting these petty tribulations weigh on him. He was lucky: he was used to travel, he was used to change, and he should have been inured to this kind of minor disappointment. Most of the evacuees on the verge of leaving Pachner had lived there all their lives, and change of the kind they were about to confront was something metaphysically foreign to them. Never mind what lay behind the borderlight; those people knew the shape of every rock within a thousand-kilometer radius of their homes, and even if they ended up on a world miraculously similar by any planetologist’s standards, they’d still feel alienated and dispossessed.

As they climbed the stairs, Tchicaya joked, "Let’s head back to the garden. I can sleep in the bushes." His shoulders were already aching at the thought of having to lie so still. He could modify himself to lose his usual urge to turn over repeatedly as he slept, but the prospect of needing to do that only made him feel claustrophobic in a deeper sense. You could whittle away a hundred little things like that, and not miss any of them individually, but then you woke one day to find that half your memories no longer rang true, every minor joy and hardship drained of its flavor and significance.

"D37, wasn’t it?" Yann asked cheerfully. "That’s left here, then fourth door on the right." He stopped and let Tchicaya walk past him. "I’ll talk to you again soon about the probe drop, but I’m sure the others won’t object."

"Yeah. Thanks." Tchicaya raised a hand in farewell.

The doors he passed were all closed, but the fourth recognized him and opened to his presence.

In front of him stood a desk, two chairs, and a set of shelves. He stepped into the room, and saw one, quite spacious, bed. Behind a partition, there was a shower, toilet and basin.

He sprinted after Yann, who started fleeing halfheartedly, then gave up and doubled over with laughter.

"Bastard!" Tchicaya caught up with him, and thumped him on the arm, hard enough to elicit a satisfying yelp.

"Show some cultural sensitivity!" Yann pleaded. "Pain isn’t part of my traditional gestalt." Which made it unlikely that he’d actually felt any; even among the embodied, it was a shade conservative to let anything short of structural damage register as genuine discomfort.

"Nor is space, apparently."

Yann shook his head, and tried to appear earnest. "On the contrary. I’ve always had a sophisticated self-and-environment map; us ex-acorporeals just aren’t hung up about its correlations with the physical world. Whatever it looks like to you, what we experience in that crowded cabin is ten orders of magnitude beyond any luxury you’ve ever known." He said this without a trace of gloating or pomposity. It wasn’t hyperbole, or wishful thinking; it was simply true.

"You know I almost turned around and left the ship?"

Yann snickered, completely unconvinced.

Tchicaya was at a loss for any suitable parting threat, so he just raised his arms in resignation and walked back to his cabin.

Sweeping his gaze around the modest few square meters made him beam like an idiot. It was one-thousandth the size of the house he’d lived in on Pachner, but it was everything he needed.

"Bastard." He lay down on the bed and thought about revenge.

Chapter 5

The shuttle separated from the Rindler, sending Tchicaya’s stomach into free fall. He watched the docking module retreat, knowing full well that he’d been flung off at a tangent, backward, but so viscerally convinced that he’d fallen straight down that the sight of the module — continuing along its arc of rotation, yet dropping from the zenith in front of him rather than disappearing behind his head — scrambled his sense of balance and direction completely. At first he felt as if he was tumbling backward, which would at least have explained what he was seeing, but when his inner ears failed to confirm the motion, the illusion vanished — only to return a moment later, to take him through the same cycle again. The lurching fits and starts that followed might have made him less queasy if they’d actually been happening; it was the inability to make sense of his perceptions that was disturbing, far more than any direct, physical effect of the lack of gravity.

He began to get his bearings once the whole ship was visible, edge-on. A minute later it had shrunk to a sparse necklace of glass beads, and the newly fixed stars finally crystallized in his mind as cues worth taking seriously. The infinite plane of whiteness on his right might have been a moonlit desert seen through half-closed eyes. He’d once flown a glider high over sand dunes at night, on Peldan, nearly free-falling at times in the thin air. There’d been no moonlight, of course, but the stars had been almost as bright as these.

Yann, sitting beside him, caught his eye. "You okay?"

Tchicaya nodded. "In the scapes you grew up in," he asked, "was there a vertical?"

"In what sense?"

"I know you said once that you didn’t feel gravity…but was everything laid out and connected like it is on land? Or was it all isotropically three-dimensional — like a zero-gee space habitat, where everything can connect in any direction?"

Yann replied affably, "My earliest memories are of CP4 — that’s a Kähler manifold that looks locally like a vector space with four complex dimensions, though the global topology’s quite different. But I didn’t really grow up there; I was moved around a lot when I was young, to keep my perceptions flexible. I only used to spend time in anything remotely like this" — he motioned at the surrounding, more-or-less-Euclidean space — "for certain special kinds of physics problems. And even most Newtonian mechanics is easier to grasp in a symplectic manifold; having a separate, visible coordinate for the position and momentum of every degree of freedom makes things much clearer than when you cram everything together in a single, three-dimensional space."

So much for being a seasoned traveler. Tchicaya didn’t envy Yann’s upbringing, but it probably rendered the world behind the border less exotic to him than the notion of a jungle had been to Tchicaya as a child. It shook his confidence to be reminded that there were measures by which his millennia of experience had been laughably narrow.

He couldn’t have it both ways, though: he couldn’t claim that the embodied needed the shock and the strangeness of this burgeoning universe, and then wish it could be no more daunting to confront than one more mundane planetary surface.

Kadir turned around and interjected testily, "I can analyze the flows in a symplectic manifold perfectly well without pretending to inhabit it. That’s what mathematics is for. Imagining that you need to float through every last abstract space that shows up in a physics problem is just being literal-minded."

Yann smiled, unoffended. "I’m not going to argue with you. I haven’t come here to proselytize for acorporeality."

Zyfete, seated in front of Tchicaya, muttered, "Why bother, if you can render embodiment just as barren?"

Tchicaya bit his tongue. He’d been forewarned about the level of acrimony, and at some point everyone on the Rindler was going to have to wade waist-deep through their opponents' venom on their way to a resolution, but spur-of-the-moment bickering in a confined space wasn’t his idea of productive disharmony.

The shuttle’s drive kicked in, delivering a mild push that Tchicaya succeeded in interpreting as a precipitous dive, rather than a complete inversion of land and sky. He scanned the eye-watering whiteness, hunting for their destination, but the glare was impenetrable. It seemed miraculous to be skimming kilometers above an object that dominated the sky for hundreds of light-years — without being burnt to a cinder, as he would have been this close to the surface of a star — but it was sheer size that made the border visible from afar. Each square kilometer didn’t have to blaze fiercely for the total luminosity to outshine any supernova. Without the usual Doppler shift to boost the light’s power, a pinhole view looking straight at the border would actually have been dimmer, here, by a factor of three, than the equivalent view from any planet he’d visited. What dazzled was the fact that it filled his vision, leaving room for nothing else. On Pachner, for much of the year the border had been partly hidden by daylight, but even when it reached its furthest angle from the sun there’d always been a narrow strip of washed-out darkness left over somewhere on the horizon, with a few pallid stars on which to rest your eyes.

As the drive reversed, he finally spotted the silhouette of the Scribe. He made a mask against the surrounding glare with his hands, and managed to discern some structure. At the top of the machine was a sphere, rainbow iridescent in the light that grazed it. He knew it was embossed with a fine pattern of microjets, trillions of tiny devices capable of firing as few as one or two atoms in any direction. While the Rindler could keep pace with the border well enough simply by cruising, the Scribe’s stylus hovered so close that collisions with interstellar gas, and even the pressure of the borderlight itself, would have ruined the alignment if left uncompensated. Presumably, the visitors' own influence would be well within the machine’s defensive capacities, but to Tchicaya it was both marvelous and comical that their presence could be accommodated — like a calligrapher inscribing Gravitation on the head of a pin, while four fat infants clambered onto the artisan’s shoulders and proceeded to wrestle.

As the shuttle drew nearer, the Scribe’s modest size became apparent; it was smaller than one of the Rindler's modules, forty or fifty meters across, with the sphere of microjets held out on a boom above a flat deck. The shuttle’s drive made one last perceptible correction before a series of maneuvers too gentle to feel brought them into contact with the deck.

Kadir unstrapped himself, and approached the hatch in the floor of the shuttle. Tchicaya followed him.

"You keep an atmosphere in there?"

Kadir nodded. "People come and go, it’s easiest just to maintain the pressure."

Tchicaya frowned. "I’m never going to get to use this, am I?" He pinched the back of his hand to tug on the near-invisible membrane that he’d sprayed all over his skin; he’d been told it would let his body survive for up to a week in vacuum, and since it took three months to grow a new one, that had seemed like a precaution worth taking. The one thing the suit lacked was reaction mass. If he found himself drifting toward the border, the best thing to do would be to broadcast a final backup and resign himself to an interesting local death.

Kadir said, "I’ll see if I can arrange an opportunity on the way back." The remark was delivered without obvious malice, but it was still hard to know how to take it. Since Tchicaya had allowed Yann to introduce him to the two Preservationists as a fellow partisan, the tension he’d felt had ebbed and flowed, and he was never sure when to expect a bit of good-natured teasing, and when to brace himself for a genuinely chilly rebuff as an enemy of the cause.

Zyfete and Yann joined them as the hatch irised open, revealing a softly lit tunnel lined with handholds. Tchicaya hung back until last, not wanting to block anyone’s progress if he froze. The others all went feetfirst, as if they were descending a ladder, but he felt more secure crawling along the tunnel, imagining himself more or less horizontal. He recalled a playground back on Turaev, a maze of interconnected pipes. When Zyfete glanced up at him and scowled, he poked his tongue out at her and recited a few lines of childish rhyme. In spite of herself, she smiled.

The Scribe’s control room was octagonal, with eight slanted windows facing down toward the border. Judging the distance by eye was difficult, with no texture to the light to set the scale, but Tchicaya guessed he was now floating just five or six meters from the novo-vacuum. He suddenly noticed the beating of his heart, though the rhythm didn’t feel abnormal; it was a shift in his attention, rather than a rush of adrenaline. He wasn’t afraid, but he was acutely aware of his body: the softness and fragility of it, compared to most other things in the world. It was the way he felt when he found himself stranded in the middle of a harsh landscape, insufficiently prepared for its rigors, but not so threatened that he’d simply write off his current incarnation as unsalvageable. It would take a cosmic disaster even larger than Mimosa to rob him of more than a few minutes' memories, but while he inhabited a body he identified with it wholly. He was in a place where a mishap could shred him into something smaller than atoms, and under the circumstances he was more than happy to let instincts predicated on absolute life and death come to the fore and do their best to protect him.

A bank of displays in the center of the room surrounded an octagonal dome, the housing for the stylus. Tchicaya watched as Kadir and Zyfete issued a long series of spoken commands. The lack of automation was almost ritualistic; he glanced inquiringly at Yann, who whispered, "It’s a kind of transparency. There are more sophisticated ways we could monitor each other, but having observers from both sides at every experiment, and controlling everything with words, keeps the proceedings out in the open on one level — while we check the equipment and audit the software with a thousand different kinds of high-powered tools, offstage."

"That’s so much like Earth-era diplomacy it’s depressing."

Yann smiled. "I knew your arcane knowledge would come in handy here."

Tchicaya snorted. "Don’t look at me to spout Machiavelli. If you want that shit, go and dig up an ancient."

"Oh, I’m expecting anachronauts to arrive at the Rindler any day now — preceded by a few megatonnes of fusion by-products — and announce that they’ve come to save the universe."

"Any day, or any millennium." It was an eerie prospect to contemplate. Scattered remnants of pre-Qusp civilization, twenty thousand or so years old, still chugged between the stars in spluttering contraptions, spewing spent fuel and taking thousands of years for every journey. Tchicaya had never met any of the ancients himself, but his father had encountered one group, which had visited Turaev long before he was born. None had traveled more than eighty light-years from Earth, so as yet they hadn’t been endangered by the novo-vacuum, but unless the Preservationists triumphed, within decades the anachronauts would face a decision between adopting some of the hated new technologies and annihilation.

Kadir shot them a disapproving look, as if their chattering meant they weren’t taking their monitoring role seriously. Tchicaya had full-sensory recall, regardless of conscious attention, and Yann would undoubtedly boast something even fancier, but he disciplined himself and fell silent.

Zyfete was describing a sequence of particles to be emitted by the stylus. The disaster at Mimosa had provided at least one compensatory boon: experiments in quantum gravity had become far easier to perform. The border was only a few Planck lengths deep, providing experimenters with a tool compared to which an atomic blade would look wider than a planetary system. While the highest-energy particles the Scribe could create were laughably blunt instruments, the border itself could be made to carve them into shrapnel vastly more effective than each innocuous whole. When the stylus fired a coherent beam of mesons at the border, the razor wire of disrupted graphs sliced fragments of their own surreal dimensions from the knot of virtual quarks and gluons making up each meson, and it was possible to exploit coherence effects to make some of these fragments act in unison to modify the border itself. Natural sources of noise had no prospect of accidentally triggering the same effect, so the kind of exorbitant shielding the Quietener had used was no longer required.

Kadir turned to look at them inquiringly. Yann nodded approval. "That’s all as we agreed. Go ahead."

Zyfete addressed the Scribe. "Execute that."

With no perceptible delay, the Scribe began to answer with the results. Tchicaya’s skin tingled; he’d had no time to remind himself between risk and reprieve, but they’d just tickled a tiger that might have responded by raking the four of them into geometric quanta, swallowing the Rindler a fraction of a millisecond later, and redoubling its efforts to devour all their distant backups and more prudent friends.

Kadir started cursing, his Mediator politely tagging the words with a cue that would shut off translation for anyone inclined to be offended. Zyfete watched him, anguished but silent.

When the tirade stopped, Tchicaya asked cautiously, "Not what you were hoping for, but did it tell you anything?"

Kadir kicked the stylus housing, the recoil driving him back to hit the window behind him with a thud. Tchicaya couldn’t help wincing; however robust the participants in these collisions, precision machinery, living flesh, and windows facing interstellar vacuum all seemed to merit gentler treatment.

Zyfete said, "This sequence was meant to confirm a previous experiment, but it didn’t yield the same results as the last time we ran it. Our model can’t explain the discrepancy, either as a statistical variation, or any predictable change in the novo-vacuum."

Kadir turned and blurted out, "Either you genocidal traitors have corrupted this machine, or — "

Yann pleaded, "Or what? Give us the more likely alternative!"

Kadir hesitated, then smiled grimly. "I think I’ll keep that hypothesis to myself."

Tchicaya was dismayed, though he was prepared to put the outburst down to frustration, rather than genuine contempt. Both sides were equally helpless. If this went on, no one was going to get their own way, and no one was going to forge a compromise. The novo-vacuum would simply roll on over them.

Halfway back to the Rindler, Kadir apologized. Tchicaya didn’t doubt his sincerity, though the words were more formal than friendly. Yann tried to joke with him, making light of the incident, but Kadir withdrew from the conversation.

When they reached the dock and disembarked, the group broke apart. Yann wanted to observe some tests on a new spectrometer package that were being conducted in a workshop higher up in the same module, but Tchicaya didn’t feel like tagging along, so he headed back toward his cabin.

He hadn’t expected to witness a breakthrough on the trip, let alone gain some kind of dramatic insight himself from mere proximity to the border; he might as well have hoped to learn the secrets of the ordinary vacuum by gazing into thin air. Nevertheless, he felt a pang of disappointment. Before he’d arrived, there’d been an undeniable thrill to the notion of cruising just beyond reach of the fatal shock wave, and then compounding the audacity by turning around and studying it. Dissecting the danger, laying it bare. It was like a legend his mother had told him: in the Age of Barbarism, when humans had rained bombs on each other from the sky, people called Sappers had dived from airplanes to fall beside them and defuse them in midair, embracing the devices like lovers as they reached into their mechanical hearts and seduced them into betraying their malign creators. But if aerodynamics rendered this romantic fable unlikely, at least no one had expected the Sappers to teach themselves nuclear physics from scratch as they fell, then reach inside each atom of fissile material and pluck out the destabilizing protons one by one.

Zyfete caught up with Tchicaya on the stairs leading down to the walkway. She said, "Kadir’s home is this far away from the border." She held up her hand, thumb and forefinger almost touching. "Nine thousand years of history. In less than a year, it will be gone."

"I’m sorry." Tchicaya knew better than to respond with platitudes about history living on in memory. He said, "Do you think I want to see Zapata destroyed?" She didn’t need to name the planet; everyone knew the awful schedule by heart. "If we can halt the border without wiping out the entire novo-vacuum, I’ll back that. I’ll fight for that as hard as anyone."

Zyfete’s eyes flashed angrily. "How very evenhanded of you! You’d let us keep our homes, so long as there was no danger of you losing your precious new toy!"

"It’s not a toy to me," Tchicaya protested. "Was Zapata a toy nine thousand years ago, when it lay on the frontier?"

"That frontier spread out from Earth, and it was made up of willing settlers. It didn’t incinerate anyone who dared to stay put." She scowled. "What do you think you’re going to find in there? Some great shining light of transcendence?"

"Hardly." Transcendence was a content-free word left over from religion, but in some moribund planetary cultures it had come to refer to a mythical process of mental restructuring that would result in vastly greater intelligence and a boundless cornucopia of hazy superpowers — if only the details could be perfected, preferably by someone else. It was probably an appealing notion if you were so lazy that you’d never actually learned anything about the universe you inhabited, and couldn’t quite conceive of putting in the effort to do so: this magical cargo of transmogrification was sure to come along eventually, and render the need superfluous.

Tchicaya said, "I already possess general intelligence, thanks. I don’t need anything more." It was a rigorous result in information theory that once you could learn in a sufficiently flexible manner — something humanity had achieved in the Bronze Age — the only limits you faced were speed and storage; any other structural changes were just a matter of style. "All I want to do is explore this thing properly, instead of taking it for granted that it has to be obliterated for our convenience."

"Convenience?" Zyfete’s face contorted with outrage. "You arrogant piece of shit!"

Tchicaya said wearily, "If you want to save people’s homes, you have greater obstacles than me to overcome. Go and comfort your friend, or go and work on your model. I’m not going to trade insults with you."

"Don’t you think it’s insult enough that you come here and announce your intention to interfere, if we ever look like we might be on the verge of succeeding?"

He shook his head. "The Rindler was built by a coalition with no agenda beyond studying the novo-vacuum. The individual members all had their personal goals, but this was meant to be a platform for neutral observation, not a launching pad for any kind of intervention."

They’d reached the walkway. Tchicaya kept his eyes cast down, though he knew it made him look ashamed.

Zyfete said, "The bodiless I can understand: what lies outside their Qusps is irrelevant to them, so long as they can keep the same algorithms ticking over. But you’ve felt the wind. You’ve smelled the soil. You know exactly what we have to lose. How can you despise everything that gave birth to you?"

Tchicaya turned to face her, angered by her bullying but determined to remain civil. He said, "I don’t despise anything, and as I’ve said, if it’s possible, I’ll fight to preserve all the same things as you. But if all we’re going to do with our precious embodiment is cling to a few warm, familiar places for the next ten billion years, we might as well lock ourselves into perfect scapes of those planets and throw away the key to the outside world."

Zyfete replied coldly, "If you think a marriage has grown too stale and cozy, I suppose you’d step in and stave one partner’s head in?"

Tchicaya stopped walking and held up his hands. "You’ve made yourself very clear. Will you leave me in peace now?"

Zyfete faced him in silence, as if she’d run out of venom and would have been happy to depart at precisely this moment, if only he hadn’t asked her. After a delay long enough to preclude the misconception that she might be doing his bidding, she turned around and strode back along the walkway. Tchicaya stood and watched her, surprised at how shaken he was. He’d never concealed his views from the people he’d lived among — apart from keeping his mouth politely shut in the presence of anyone in genuine distress — and over the decades he’d had to develop a thick hide. But the closer he’d come to the source of the upheaval, the harder he had found it to believe that he was witnessing an unmitigated tragedy, like the floods and famines of old. On Pachner, where the sorrow and the turmoil had been at their most intense, he’d also felt most vindicated. Because beneath all the grief and fear, the undercurrent of excitement had been undeniable.

If Zyfete’s attack had stung him, though, it was mostly through the things she hadn’t said. Just being here meant that she had already left her own home behind, already tasted that amalgam of liberation and loss. Like Tchicaya, she had paid once, and no one was going to tell her that the price had not been high enough.

Tchicaya took a shower to wash off his vacuum suit, then lay on his bed, listening to music, brooding. He didn’t want to spend every waking moment on the Rindler questioning his position, but nor did he wish to grow impervious to doubt. He didn’t want to lose sight of the possibility that he had chosen the wrong side.

If the Preservationists did achieve their goal, the possibilities offered by the novo-vacuum need not be lost forever. Whatever was learned in the process of destroying it might open up the prospect of re-creating it, in a safer, more controlled fashion. In a few tens of millennia, there could be a whole new universe on their doorstep again, but this time it would pose no threat to anyone. No one would be forced from their homes. No one would be made to choose between exile and adaptation.

And in a few tens of millennia, how much tighter would the deadening spiral of familiarity have wound itself? If the nine-thousand-year history of Zapata was too precious to lose, after ninety thousand years every tradition, every grain of sand on every inhabited planet, would be positively sanctified.

Still, those who believed they were being smothered could always flee, as he’d fled Turaev. Those who were happy sleepwalking into eternity could stay. He had no right to force this cusp on anyone.

He didn’t have the right, but he didn’t have the power either, nor did he aspire to it. He was only here to state an unpopular case, and see if anyone could be swayed. If he believed that the novo-vacuum offered the greatest wealth of opportunities the species had faced since leaving Earth, what else would it be but cowardice and dishonesty if he failed to argue against its destruction?

The cabin was beginning to feel less spacious by the minute. He left it and made his way around the ship, heading for the garden. He still felt jittery on the walkways, but his confidence was slowly improving.

The garden was almost deserted. He found a bench that faced away from the border, offering a view he could take in without vertigo. The reel of the blue polar stars was slow enough to be soothing, and with the foliage to break up their perfect arcs the whole sight seemed less mechanical.

The Doppler shift was a novelty to him, but the motion of the stars was familiar. The night sky on Turaev had looked just like this, during a mild Slowdown. The only thing missing was the sun, rising and setting with each turn.

He’d stood by the crib that would prepare his body for storage, and his mind for transmission. It had asked him to state his wishes on the eventual recycling of this, his birth flesh. His father had pleaded gently, "We could still wait for you. For a thousand years, if that’s what you need. Say the word, and it will happen. You don’t have to lose anything."

Someone passing glanced his way, curious at the sight of an unfamiliar passenger. Their Mediators interacted, and the stranger requested an introduction. Tchicaya hadn’t asked not to be interrupted, and he allowed the exchange of information to proceed. Protocols were established, translators verified, mutually acceptable behavior delineated. There were no local customs to defer to, here, so their Mediators virtually flipped a coin to decide the manner in which they should greet each other.

"I don’t believe we’ve met. My name’s Sophus."

Tchicaya stood and gave his own name, and they touched each other lightly on the left shoulder. "I’ve only been here a day," he explained. "It’s my first time off-planet; I’m still adjusting."

"Do you mind if I join you? I’m waiting for someone, and this is the nicest spot to do it."

"You’d be welcome."

They sat on the bench. Tchicaya asked, "Who are you waiting for?"

"Someone who’ll usurp your present role as most junior arrival. In fact, technically, I suppose she’s already done that, but she’s not yet in a state to show herself and claim the position."

Tchicaya smiled at the memory of his own appearance in the crib. "Two arrivals in as many days?" That wouldn’t have been so strange if someone had been following him from Pachner, but he hadn’t come across anyone there who’d shared his travel plans. "They’ll be running out of bodies if this keeps up. We’ll have to squeeze the ex-acorporeals right into the ship’s processors."

Sophus frowned, mock-reprovingly. "Hey, no discrimination, please! It’s up to them to volunteer, not us to suggest it."

"The way they offered to share those cabins, to make room for new arrivals?"

Sophus nodded, apparently amused by the gesture. Tchicaya felt a twinge of unease, unsure whether he had just endeared himself to Sophus with some remarks that had been taken as evidence of bigotry, or whether he was just being hypersensitive. He wondered how long it would take Sophus to quiz him about his allegiance; either the answer had spread through the grapevine already, or Sophus was polite enough to make small talk for a while, and see if he could extract the information indirectly.

"Actually, we’ll start some new bodies growing soon," Sophus explained. "We were expecting a rush about now — give or take a decade. People will want to be here, it’s what the models predicted."

Tchicaya was puzzled. "What, because of Zapata?"

Sophus shook his head. "It’s far too late to save Zapata. Maybe not literally, but most people are realistic enough not to think that they can turn back the tide at the very last moment. Look a bit further down the track. A century, a century and a half."

"Ah." In the right company, Tchicaya might have made a joke of the prospect Sophus was raising, but it wasn’t the kind of casual blasphemy he’d try out on a stranger. And the truth was, he did feel genuine sorrow, in some ways deeper than his feelings about Turaev’s eventual demise. Like the uprooting of some much-loved, long-sedentary ancestor through whom a scattered family remained in touch, the exodus of Earth’s people, and the destruction of its soil, would scar the hearts of even the most cosmopolitan travelers.

"There’s still talk of moving it," Sophus said casually. "Pushing a white dwarf into the solar system, to carry it away. Sirius B is the obvious candidate." Tchicaya blinked at him, incredulous. "It wouldn’t be impossible," Sophus insisted. "When you dump matter on a white dwarf, it undergoes tidal compression heating. If you do it in the right way, a significant amount squirts off in jets. If you arrange for asymmetric jets, and if you have enough mass to play with, you can achieve a modest net acceleration. Then you get the Earth into orbit around the star; the acceleration displaces the orbit, but it can still be bound."

"But to get Sirius B up to half the speed of light — "

Sophus raised a hand. "I know, I know! You’d have to gather so much reaction mass, and move all of it so swiftly into place, the damage would rival Mimosa. To wreak that kind of havoc just to put the whole ball of rock into exile as an unbroken whole would be like saving New York from the floods by blasting it all the way to Io. The only sane response is to work on designing an effective sandbag, while being prepared to give up gracefully and watch the place sink if that proves to be impossible."

"Yeah." If Tchicaya remembered the story correctly, though, while New York hadn’t quite ended up on Io, gracefully watching the place sink would be putting things charitably. Hadn’t some famous statue ended up in Paris, and various bridges and buildings gone to scattered theme parks?

Sophus attended briefly to an internal perception. "My colleague is on the brink of emerging. Would you like to meet her?"

"I’d be delighted." They rose together and headed for the stairs. On the walkway, Tchicaya forced himself to keep pace with Sophus, as if no one would make allowances for his lack of experience now that he’d ceased to be literally the rawest recruit.

"Where’s she come from?"

"You mean, directly?"

"Yeah. I was on Pachner, and no one else there was talking about traveling to the Rindler. Maybe I just didn’t bump into her — "

Sophus shook his head. "She’s been in transit almost a century, standard time."

That was a long journey. Though it cost you more lost years in total to travel by an indirect route, breaking up the trip with as many stops as possible eased the sense of alienation. Whatever faction she supported, she had to be serious about the cause.

Tchicaya pictured a map of the region. "She’s come from Chaitin?"

"Right."

"But she wasn’t born there?"

"No. You know, you’ll be able to ask her for her life’s history directly, in a couple of minutes."

"Sorry." Maybe it was absurd to be so curious about the newcomer when he still knew next to nothing about the Rindler's other passengers, but Yann’s gloomy summary, and his own limited experience, had already made him long for someone who’d shake up the status quo.

As they crossed the observation deck, the door to the recovery room opened. Tchicaya smiled in recognition at the newcomer’s posture: loose-limbed and confident after the kinesthetic retuning, seizing up for a moment at the sight of the border.

Then he recognized something more, and his own body turned to stone again.

He didn’t need to check her signature; she hadn’t changed her appearance since their paths had last crossed. In fact, she hadn’t changed in four thousand years, since the day they’d first parted.

Tchicaya broke into a run, blind to everything around him, calling out her name.

"Mariama!"

She turned at the sound. He could see that she was shocked, and then uncertain how to respond. He halted, not wanting to embarrass her. It had been twelve hundred years since they’d set eyes on each other, and he had no idea what she’d make of his presence.

Mariama held out her hands, and he ran forward to grip them in his own. They whirled around, laughing, surefooted on the polished floor, leaning back into their own centrifugal force, moving ever faster, until Tchicaya’s arms ached and his wrists burned and his vision blurred. But he would not be the one to stop moving, and he would not be the one to let go.

Chapter 6

Something unseen stung Tchicaya’s hand, a vibration like a tuning fork held against the bone. He turned and stared at the empty space beside him, and a dark blur shivered into solidity.

"Quickly! Give your Exoself this code."

No sooner had the data passed between their Mediators than Tchicaya wished he’d rejected it. He felt as if he’d been tricked into catching something incriminatory thrown his way, the reflex action triggered by the object in flight turning out to have been the wrong response entirely.

"I can’t!"

Mariama said, "No one will ever know. They’re like statues. You’ll be invisible."

Tchicaya’s heart pounded. He glanced at the door, and caught himself straining his ears for footsteps, though he knew there’d be nothing to hear. Could she have really walked through the house undetected, marching right past his parents in that scandalous state?

"Our Exoselves scan for danger," he protested. "If anything happens at ordinary speed — "

"Did your Exoself detect me?"

"I don’t know. It might have."

"Did it signal you? Did it bring you out of Slowdown?"

"No." He wasn’t an adult, though. Who knew how differently theirs were programmed?

"We’ll stay clear of them," Mariama explained. "I’m not doing this to pick their pockets. If we’re not a threat to anyone, we won’t trigger any alarms."

Tchicaya stared at her, torn. He had never feared his parents, but he basked in their approval. It only took the faintest shadow of disappointment on his father’s face to make him ache with unhappiness. His parents were good people; valuing their high opinion was not just childish narcissism. If he did well in their eyes, he would be respected by everyone. Mariama was only Mariama: a law unto herself.

She inclined her head. "Please, Tchicaya. It’s fun doing this, but I’m lonely without you."

"How long have you been out of Slowdown?"

Mariama averted her eyes. "A week."

That hurt. How lonely could she be, if it had taken her a week to miss him?

She put a hand over her mouth and mumbled, "Or two."

Tchicaya reached out to grab her arm, and she danced back and vanished from sight. He froze for a second, then rushed for the door, and stood with his back pressed against it.

He searched the room with his eyes, knowing that it was pointless looking for her if she did not want to be seen. Shadows slid across the walls and floor with hypnotic regularity. Lighting panels in the ceiling came on at night, and softened the changes at dusk and dawn, but even when he looked away from the window the diurnal cycle was obvious, everywhere.

Another week had passed, while he stood there. She could not still be in the room with him; even if she was able to go that long without food and water, she would have gone mad from boredom.

She reappeared in front of him like a trembling reflection in a pan of water, jolted into turbulence but quickly stilled.

"How did you get in?" he demanded.

She pointed a thumb at the window. "The same way I left."

"You’re wearing my clothes!"

Mariama grinned. "They fit me nicely. And I’m teaching them lots of new tricks." She ran a hand down one sleeve and erased the old pattern, supplanting it with golden starbursts on black.

Tchicaya knew she was goading him, hoping to prod him into giving chase. She’d handed him the key; he didn’t need anything more in order to pursue her. If he gave in and joined her now, at least he’d be spared an elaborate game of hide-and-seek.

He said, "Two weeks." That sounded more than generous, and the risk of his parents noticing his absence would be microscopic.

"We’ll see."

Tchicaya shook his head. "I want you to agree to it. Two weeks, then we both come back."

Mariama chewed her lower lip. "I’m not going to make a promise I might not be able to keep." Then she read his face, and relented slightly. "All right! Barring exceptional circumstances, we’ll come back in two weeks."

Tchicaya hesitated, but he knew that this was the closest thing to a guarantee he could hope to extract from her.

She held out a hand to him, smiling slightly. Then she silently mouthed the word Now.

Their Mediators were smart enough to synchronize the process without needing to be told. Tchicaya sent the code to his Exoself, and the two of them dropped out of Slowdown together. Switching the metabolic modes of cells throughout his body, and reconfiguring all the higher-level systems responsible for maintaining posture, breathing, circulation, and digestion took nearly fifteen minutes. The time passed imperceptibly, though, since his Qusp only resumed its normal rate once his body had completed the shift.

The light in his room had frozen into a late-winter’s afternoon. He could hear a breeze moving through the trees beside the house, a different sound entirely to the throb of barometric pressure changes to which he’d grown accustomed. They were only six civil days into the Slowdown, but the new rhythms had seeped into his mind more rapidly than they’d had any right to, as if abetted by some process that his Exoself had neglected to retard.

Mariama tugged on his hand, pulling him toward the door. "Come on!" Her expression made a joke of it, but she couldn’t disguise the note of genuine impatience. They were like lightning now, their least purposeful meanderings a dazzling feat in everyone else’s eyes, but that still wasn’t fast enough.

"Not that way." He gestured at the window.

Mariama said accusingly, "You’re afraid to walk past them."

"Of course." Tchicaya gazed back at her calmly. It was perfectly reasonable not to want to be discovered, and however skillful she was at manipulating him, he wasn’t going to be made ashamed of every last instinct of his own. "It’s safer to use the window. So we’ll use the window."

Mariama managed to look both amused and martyred, but she didn’t argue. Tchicaya climbed out, then she followed him, carefully pulling the hinged pane closed behind her. He was puzzled for a moment; no one was going to notice an open window in the short time they’d be gone. But in two weeks, the night frosts would have left an indelible mark on some of his more fragile possessions.

As they crossed the garden, he said, "Don’t you go home to sleep?"

"No. I’ve set up camp in the power station. All my food’s there." She turned to face him, and Tchicaya was sure she was on the verge of demanding that he go back to the house to pilfer some supplies of his own, but then she said, "You can share it. I’ve got plenty."

The bright afternoon was eerily quiet, though Tchicaya doubted that he would have been unsettled if he’d heard no other voices for a minute, or an hour, on an ordinary day. As they stepped onto the road, he spotted two other pedestrians in the distance. During Slowdown, his Exoself had not only reprogrammed his own gait, it had tweaked his expectations of other people’s appearance: moving with both feet constantly on the ground, positioning the arms to maximize stability, had looked as normal as it had felt. With his old notions of bodily dynamics restored, the pedestrians appeared, not merely frozen, but cowed and timid, as if they expected an earthquake at any moment.

He looked back at his house, quickly lowering his eyes from the windows to inspect the garden. Wind and rain could shift soil and pebbles into unwanted places on a time scale of decades, but the plants were engineered to herd those unruly elements; he’d watched the process with his own eyes. Out in the fields, the crops would be tending themselves, collectively arranging whatever changes they needed in irrigation and drainage, glorying in the strange seasons of unharvested bounty.

Tchicaya said, "How did you find the code?" It was the first Slowdown for both of them; she couldn’t have stored it on a previous occasion.

Mariama replied casually, "It’s not a big secret. It’s not buried deep, or encrypted. Don’t you ever examine your Exoself? Take apart the software?"

Tchicaya shrugged. He’d never even dream of tinkering with things on that level: his Exoself, his Mediator. Next thing you were probing the working of your own Qusp, dissecting your own mind. He said, "I only take things apart if I can survive not putting them back together."

"I’m not stupid. I make backups."

They’d reached the park. Four giant hexapods huddled motionless in a corner. The decorative robots consisted of nothing but six coiled legs, arranged as three pairs that met at right angles in the center. If they’d been endowed with even the mildest form of sentience, they would have gone insane from the lack of stimulation, but they were little more than pattern-recognizers on springs.

Mariama ran up to them and clapped her hands. The nearest one stirred sluggishly, shifting its center of mass and wobbling on the tripod of the three legs currently touching the ground. She started dancing back and forth, encouraging it, and it began to tumble for her.

Tchicaya watched, laughing, biting back an admonition: someone would notice that they’d moved, and know that the Slowdown had been violated. He doubted that the hexapods had memories, but there was machinery everywhere, monitoring the streets, guarding the town against unlikely dangers. The fact that they hadn’t woken anyone didn’t prove that they wouldn’t be found out in the end.

Mariama weaved between the robots. "Aren’t you going to help me?"

"Help you do what?" She’d managed to get all four of them moving simultaneously, without his aid. Tchicaya hadn’t played with them since he was an infant, but he’d never been able to hold the attention of more than one at a time.

"Make them collide."

"They won’t do that."

"I want to get their legs tangled together. I don’t think they understand that that can happen."

"You’re a real sadist," he protested. "Why do you want to confuse them?"

Mariama rolled her eyes. "It can’t hurt them. Nothing can."

"It’s not them I’m worried about. It’s the fact that you enjoy it."

She kept her eyes on him without breaking step. "It’s just an experiment. It’s not malicious. Why do you always have to be such a prig?"

Tchicaya felt a surge of anger, but he fought it down and replied pleasantly, "All right, I’ll help you. Tell me what to do." He caught the flicker of disappointment in her eyes before she smiled and started issuing detailed instructions.

The hexapods were primitive, but their self-and-environment model was more reliable than Mariama had imagined. After fifteen minutes trying to trick them into tying their legs into knots, she finally gave up. Tchicaya collapsed on the grass, breathless, and she joined him.

He stared up into the sky. It had grown pale already, almost colorless. It had been summer when the Slowdown began; he’d forgotten how short the winter days were.

Mariama said, "Has anyone you know even heard of Erdal?"

"No."

She snorted, her expectations confirmed. "He probably lives on the other side of the planet."

"So? Do you want half the planet to go into Slowdown, and the other half not?" Everyone on Turaev was connected somehow. While Erdal traveled, the whole world would wait for him, together. It was either that, or they broke into a thousand shards.

Mariama turned to face him. "You know why they do it, don’t you?"

It was a rhetorical question. People always had an ulterior motive, and Tchicaya had always been taken in by their explanations. He squirmed like an eager child and asked with mock excitement, "No, tell me!"

Mariama shot him a poisonous look, but refused to be sidetracked. "Guilt. Cosmic apron strings. Do you think poor Erdal would dare not come home, with nine million people holding their breath for him?"

Tchicaya knew better than to dispute this claim directly; instead, he countered, "What’s so bad about Slowdown? It doesn’t hurt anyone."

Mariama was venomous. "While every other civilized planet is flowering into something new, we do nothing and go nowhere, ten thousand times more ponderously than before."

"Lots of other planets do Slowdown."

"Not civilized ones."

Tchicaya fell silent. A faint star had appeared directly above him, even before the sun had fully set.

He said, "So you’ll leave one day? For good?" The question produced an odd, tight sensation in his windpipe. He’d never lost synch with anyone; he couldn’t imagine that kind of unbridgeable separation.

"No."

He turned to her, surprised. She said, "I plan to whip the whole planet into life, instead. Anything less would just be selfish, wouldn’t it?"

The machinery inside the power station was robust and intelligent enough to defend itself, and to safeguard any visitors, without the need for high fences or locked doors. Tchicaya remembered the place as being noisier the last time he’d explored it, but Slowdown had reduced the flow of waste from the town to an inaudible trickle. Energy was extracted from the waste by an enzyme-driven electrochemical process that he was yet to study in detail; fortunately, some of the energy ended up as heat, and even the diminished output was enough to make the building habitable at night. Mariama had made a nest of blankets right up against the coolant pipes that led to the radiator fins on the roof.

Tchicaya sniffed the air cautiously, but there was no trace of the usual offensive odor, maybe because there was not only less sewage passing through, but the undiminished runoff from the fields was diluting it. There was a strange, boiled-vegetable smell to the place, but it was nothing he couldn’t tolerate.

Mariama had stockpiled cans of food, self-heating rations of the kind people took into the untouched, frozen lands to the south. It must have taken her a while to build up the collection without attracting suspicion. She handed him a can, and he pressed the tab to start it heating.

"How long were you planning this?" he asked.

"A bit more than a year."

"That’s before I even knew Erdal would be traveling."

"Me too. I just wanted to be prepared, whenever it happened."

Tchicaya was impressed, and a little daunted. It was one thing to watch the sun and the stars racing around the sky, and think: what if I could be as fast as them? Plotting to break out of Slowdown before she’d even experienced it required an entirely different line of thought.

"What were you doing? Before you came to my house?"

She shrugged. "Just exploring. Messing about. Being careful not to wake the drones."

Tchicaya felt his face harden at this contemptuous phrase, but then he wondered how much allowance to make for the fact that she was always striving to provoke him. The calculations became so difficult at times, it drove him mad. He wanted the two of them to be straightforward with each other, but he doubted that would ever be her style. And he didn’t want her to be different, he didn’t want her to change.

He opened the can and hunched over his meal, unsure what his face was betraying.

After they’d eaten, they switched off the lamp and lay beneath the blankets, huddled together. Tchicaya was self-conscious at first, as if the contented glow he felt at the warmth of her body against his was at risk of turning into something more complicated, but he knew that it was still physically impossible for anything sexual to happen between them. The prospect of that guarantee eventually failing disturbed him, but it couldn’t vanish overnight.

Mariama said, "Two weeks isn’t long enough. You need to walk out of your room a centimeter taller: just enough to make your parents feel something is wrong, without being able to put their finger on it."

"Go to sleep."

"Or learn something you didn’t know. Amaze them with your erudition."

"Now you’re just mocking me." Tchicaya kissed the back of her head. He immediately wished he hadn’t done it, and he waited, tensed, for some kind of rebuke. Or worse, some attempt to move further along a path on which he’d never meant to set foot.

But Mariama lay motionless in the darkness, and after a while he began to wonder if she’d even noticed. Her hair was thick at the back, and his lips had barely brushed a few loose strands.

In Tchicaya’s view, the town’s effective desertion didn’t render it more interesting, and the freedom to wander the streets and fields at any hour was less appealing now, in winter, than in the ordinary summers when it was barely curtailed by parental authority anyway. Tchicaya thought of suggesting that they drop back into Slowdown and reemerge when the weather was warmer, but he was afraid of compromising their original deal. If he didn’t stick to the letter of it, he could forget about holding Mariama to her word.

Mariama wanted to catch a train to Hardy, further if possible, preferably circumnavigating the entire continent. In one weird concession to practicality, the trains moved at their ordinary speed, whisking commuters to their destinations in an eye blink. Understandably, though, departures were rare, and on examining the schedules it turned out that they could not have traveled anywhere and back in less than ten years.

Tchicaya did his best to keep Mariama distracted, terrified that she might harbor a yearning for sabotage that went beyond playground equipment. She’d know it was futile to hope to succeed in damaging any of the town’s infrastructure, but he could picture her delight at sirens wailing and people shuddering into motion around her. This image might have been unfair, but there was no point asking her for assurances; at best, that would only offend her, and at worst it might tempt her to act out his fears. So he tried to go along with any suggestions she made that weren’t completely outlandish, but only after putting up enough resistance to keep her from becoming too bored, or too suspicious of his compliance.

On their tenth night out of Slowdown, Tchicaya was woken by lukewarm fluid dripping onto his face. He opened his eyes in the pitch blackness, and rashly poked his tongue out to sample the fluid. It was water, but it had a complicated, slightly metallic taint. He pictured a crack in the ceiling, the heat from the radiator fins above them on the roof melting the surrounding frost.

He slid out from the blankets without waking Mariama, and groped for the lamp. When he held it up, a faint liquid sheen was visible snaking down one thick coolant pipe, collecting in drops at a right-angled bend above the cushion where his head had lain.

Mariama stirred, then shielded her eyes. "What is it?"

"Just some water from the roof. We might have to shift." He moved the lamp about, hunting for leaks along the other pipes. Then something different caught his eye, a flash of iridescent colors at the very top of the pipe that had proved to be the original culprit. "Is that oil?" Why would there be oil leaking from the roof? As far as Tchicaya knew, the plant’s few moving parts were all inside the building, and they’d all be molecularly smooth if they made physical contact with each other at all. Maybe flakes of ice could catch the light like that. But what could make them thin and flat enough?

There was sure to be a simple answer, but the puzzle gnawed at him. It was cold, and part of him wanted nothing more than to curl up beneath the blankets again — but what was the point of achieving a state in which no one could tell him to stop worrying and leave it till morning, if he didn’t take advantage of his freedom to act on his curiosity immediately?

He said, "I’m going up on the roof."

Mariama blinked at him in the lamplight, apparently at a loss for words.

Tchicaya put on his shoes and walked outside, taking the lamp with him.

He circled the building twice, before settling on a sturdylooking drainpipe. The lamp was attached to a chain; he hung it around his neck, like a pendant worn backward, and gripped the drainpipe between his forearms and knees. There were no handholds, and the frosted surface was slippery. The first time he found himself sliding back down, he panicked and almost let go, but the friction from the polymer surface was never enough to really hurt him. After ending up back on the ground twice, he found that if he tightened his grip the instant he began to slip, he could bring himself to a halt in a fraction of a second, and retain most of his hard-won altitude.

He reached the roof with his limbs numb and his chest soaked in icy perspiration. He crouched on the sloped tiles, flapping his arms vigorously to try to restore the circulation, until he realized that this was driving him slowly backward toward the sevenmeter drop behind him. If he did real damage to his birth flesh, there’d be no prospect of concealing it from his parents. And to take on a new body at the age of twelve would make him a laughingstock for centuries.

He rose up on his haunches and waddled across the roof, as wary of gravity now as if he’d been back in Slowdown. He had no idea whether he was heading in the right direction; the dark shapes looming ahead of him might have been anything. He stopped to work the lamp around from his back to a more useful position, and noticed a long gash along the inside of his right leg, wet with blood. Something had cut him as he’d slipped along the drainpipe, but the wound wasn’t painful, so it couldn’t be too deep.

Up close, the radiator fins were massive, each as wide as his outstretched arms. He ambled around the structure, shining the lamp into the angled gaps between the fins, hunting for the source of the leak.

Mariama called out to him, "What have you found?" She was outside, on the ground somewhere.

"Nothing, yet."

"Do you want me to come up?"

"Suit yourself." He felt a twinge of guilt at the way that would sound, but it was hardly an expression of lofty disdain by the standards she’d set. This was the first thing he’d done since he’d joined her that wasn’t part of some complicated strategy to please her, or confound her. He had to be indifferent to her, just this once, or he’d go mad.

When the lamplight finally returned the rainbow sheen he’d glimpsed from inside the building, it was unmistakable. An irregular, glistening patch of some filmy substance covered half the fin. Tchicaya approached, and touched it with a fingertip. The substance was slightly sticky, and the film clung to his finger for a fraction of a millimeter as he pulled away. When it parted from his skin he could feel it snap back elastically, rather than tearing like something viscous and treacly. He held his finger up for inspection; the skin was unstained, and when he rubbed it against his thumb there was no moisture or slickness at all. This wasn’t any kind of oil he’d seen before, and it definitely wasn’t ice.

He held the lamp closer to the surface, hunting for some sign of a damaged coolant channel. This had to be the residue left behind by a leak, though why the coolant would contain some sticky impurity was beyond him. Antifreeze? He was shivering with cold, but he was in a stubborn frame of mind.

A small hole appeared in the film at the center of the circle of lamplight, and grew before his eyes. He held the lamp as still as he could; once the boundary of the film had retreated into the penumbra cast by the lamp’s housing, the hole stopped growing.

Tchicaya moved the lamp to another spot. The same thing happened: the lamplight seemed to melt the film away. But the beam carried no heat whatsoever. Was it driving some kind of photochemical reaction?

He turned back to the original rent in the film. It had shrunk to half the size it had grown to when he moved the lamp away. He made a hole in the film in a third location, then took the lamp back to inspect the second hole. It was closing up, too.

Tchicaya stepped out from the gap between the fins and sat huddled on the roof tiles, his teeth chattering. Maybe the light broke up whatever molecules the film was made from, while the chemical process that had formed it in the first place rebuilt it when he took the light away. Some mixtures of simple chemicals could behave in a complicated fashion. He had no right to start summoning up phrases from his biology lessons, like negative phototropism.

His arms were shaking. Mariama had been silent since their last exchange; she had probably gone back to bed.

He rose to his feet, and scrupulously searched the other parts of the radiator, but it was only one side of one fin that bore any visible trace of the film.

He took a knife from his pocket, opened it, and scraped it over the film. The surface appeared unchanged, but when he lifted the knife there was a waxy residue visible along the edge of the blade.

He walked around the structure, counting the fins as he went, orienting himself with the stars. He closed his eyes and pictured the arc the sun would make as it crossed the sky; it was an easier task now than it would have been before he’d sat for a year in the front room of his house and watched the ribbon of fire shift with the seasons. He stepped between two of the fins and dislodged whatever had adhered to the knife onto the clean surface of the radiator.

He looked up at the sky again. A million stars, a million dead worlds. Only four planets had ever held anything different. His hunch was sure to be disproved, but the prospect only made him smile. There were some things so large and outlandish that you could only wish for them with your tongue in your cheek, and to be disappointed when they failed to appear would be like throwing a tantrum and cursing the world because the sun failed to rise at your beck and call.

He made his way to the edge of the roof, his breath frosting in front of him.

As he was climbing down the drainpipe, his leg began to throb. His body had managed to close the wound, and now it was warning him not to break the temporary seal of collagen it had woven across the gap in his skin. As he adjusted his legs to shift the pressure away from the cut, Tchicaya made a decision: he wanted to remember this night, he wanted it to leave a mark. He instructed his Exoself never to permit the cells of his skin to grow back in their normal pattern across the wound. For the first time, he would let the world scar him.

"Why do we need to borrow your parents' ladder?"

Tchicaya waved Mariama back from the toolshed. "I’m hoping it won’t trigger any alarms. If I tried to borrow someone else’s, that might look like I was stealing." He didn’t want her taking part in the act, though. That the house had permitted her to enter uninvited, and even borrow his clothes without his permission, proved that it was prepared to show some tolerance toward his friends. His parents had never been obsessed with safeguarding their possessions, so it was not surprising that they hadn’t programmed any paranoid, hair-trigger responses. He didn’t want to push his luck, though.

When he emerged from the shed, Mariama said, "Yes, but what do we need it for? What’s so interesting, up on the roof?"

Tchicaya swung the ladder toward her, making her jump back. "Probably nothing." He had planned to show her the film on the coolant pipes inside the building when she woke that morning, but by daylight the sight had been so drab and uninspiring that he’d changed his mind; she’d probably looked herself, and seen nothing but a mild discoloration. She’d laugh at his naiveté when he finally described his experiment, but he didn’t care. "We’ll find out tonight."

Mariama was puzzled. "What’s to stop me going up there before nightfall?"

Tchicaya tightened his grip on the ladder, but even if he could keep it from her, she wouldn’t need it.

He said, "Nothing. I’m asking you to wait, that’s all."

This answer seemed to please her. She smiled back at him sunnily.

"Then I’ll wait."

The ladder couldn’t stretch to the full height of the roof, and Tchicaya had to argue with it before it would extend itself at all.

"It’s not safe," the ladder wailed.

"I’ve already been up there once, without any help from you," he protested. He showed it his new pink scar. "I’ll climb up the drainpipe again if I have to. You can either make this as safe as possible, or you can stay on the ground and be completely useless."

The ladder gave in. Tchicaya gripped the bottom end firmly while a wave of deformation swept along the length of the device. As the side rails stretched, material was redistributed into new rungs. In its final shape, paper-thin, the ladder was still a meter too short to touch the edge of the roof, but it would bring it within reach.

Mariama said, "After you."

Tchicaya had planned to follow her up, so he’d have a chance to catch her if she slipped, but he’d been assuming that she’d demand to go first anyway, so he had no argument prepared. He mounted the ladder and began to ascend. He didn’t need to look down to know when she’d joined him; he could feel the structure vibrating with a second load.

If she did fall and injure herself, she could retreat at will into the painless world of her Qusp. An accident would mean discovery and shame, but no great suffering. Yet Tchicaya’s hands shook at the thought of it, and he could not imagine feeling differently. The structure of his mind had been passed down with only a few small modifications from the original human form, shaped by evolution in the Age of Death, leaving him with the choice between embracing its impulses in all their absurdity — like ancient figures of speech whose literal meaning bore no resemblance to anything people still did — or struggling to invent a whole new vocabulary to replace them. If you cared about someone, what could replace the sick feeling of the misery you’d feel if they came to harm? The bodiless, he knew, had found their own, varied answers, but the idea that he might one day do the same made him giddy.

He peered down.

Mariama said, "What?"

"Nothing."

The long climb was far easier than it had been the night before, but Tchicaya found the act of reaching back to grab hold of the gutter a lot more disconcerting while perched on the top rung of the ladder than when he’d gripped the drainpipe firmly with his legs. He hoisted himself up and clambered onto the roof, then moved away from the edge quickly so he wouldn’t be in Mariama’s way. Seconds later, she was beside him.

"We should have used ropes, and grappling hooks," she said. "Like they do on mountains."

"I never thought of that," Tchicaya admitted.

"I was joking."

"It might have been fun, though." It might have been safer.

"Are you going to let me in on the big secret now?"

Tchicaya feigned indifference. "I did warn you: there’s probably nothing to see." He aimed the lamp’s beam across the roof, but deliberately kept it low. "This way."

They crossed the tiles together in silence. When they reached the radiator, Tchicaya showed her the patch of iridescent film he’d discovered the night before.

Mariama examined it. Tchicaya had half-expected her to identify the substance immediately, puncturing his fantasy with a far simpler explanation, but she was as baffled as he was. When he showed her how the film responded to the lamplight, she said, "Is that why you thought there’d be nothing here? You expected the sunlight to destroy it?"

"No. This surface ought to be in the shade all day."

"It would still get some light from the sky, though."

"That’s true," he conceded. "But if it was there last night, it either had to be able to survive that much indirect sunlight, or it had to have formed after sunset, at least once. So why wouldn’t it be here again?"

Mariama nodded patiently. "All right. So what were you warning me not to expect?"

Tchicaya’s throat tightened. "I scraped some off, and put it on another fin. One that should have been about equally shaded. To see if it would…" He couldn’t say the word.

"To see if it would grow?"

He nodded stupidly.

Mariama whooped with delight. "Where!" She clutched at the lamp, but when he held on to it she didn’t fight him for it. Instead, she took hold of his arm and said, "Will you show me? Please?"

They stumbled around the radiator, helping each other stay balanced. Tchicaya told himself he didn’t care what they found; when there turned out to be nothing, they could laugh at his grandiose delusions together.

"This is the one." He aimed the lamp into the wedge-shaped space between the fins, but he couldn’t hold it still. "Do you see anything?"

Mariama put an arm around him, steadying his whole body to steady the lamp.

There was a patch of the film in front of them, an oval about the size of his hand, at exactly the height where he would have scraped the knife clean.

Mariama took the lamp, and knelt to inspect the patch more closely. It began to shrink immediately; she pulled the light away.

"This wasn’t here last night?"

"No."

"So it must be a new…" She struggled for the right word.

"Colony? Do you think that’s what it is?"

"I don’t know."

She turned to him. "But it is alive? It has to be!"

Tchicaya was silent for a moment. He’d thought the result would settle the issue, but now he was having second thoughts. The evidence was still too flimsy to support the extraordinary conclusion. "There are chemicals that do some strange things," he said. "I’m not sure what this proves."

Mariama rose to her feet. "We have to wake someone, and show them. Right now."

Tchicaya was horrified. "But then they’ll know what we did.

They’ll know we broke Slowdown."

"No one will care. Don’t you know how rare this is?"

He nodded. "But you promised me — "

Mariama laughed. "We’re not going to be in trouble! This is a thousand times more important!"

Apart from Earth itself, native life had only been found on three worlds. Simple and microbial, but in each case unique. Every biosystem used different chemistry, different methods of gathering energy, different structural units, different ways of storing and transmitting information. On the crassest, most pragmatic level, this knowledge might be of little value: technology had long ago surpassed nature’s ability to do all of these things efficiently. But each rare glimpse at a separate accident of biogenesis cast light on the nature and prospects of life. The roof of this building would become the most talked-about location for a hundred light-years.

Tchicaya said, "What if it’s something we brought ourselves? That wouldn’t be much of a discovery."

"Such as what? Nothing we brought can mutate freely: every cell in every crop, every cell in our bodies, has fifty different suicide enzymes that kill off the lineage at the first genetic error. This could no more be ours than if they found some strange machine out in the ice that nobody owned up to making."

Tchicaya was growing tired of trying to keep his balance on the sloping roof; he sat down, his back slumped against the fin. It was lukewarm, body temperature. Once Slowdown ended, it would be hotter than the boiling point of water. So which extreme did the native life favor? Had it grown here before the Slowdown, and then managed to cling on in the relative cool? Or had it blown out of the icy wastes and only colonized the radiator once the Slowdown had rendered this tiny niche benign?

Mariama sat beside him. "We’ll have to leave," she said.

"Can’t that wait until morning?"

"I don’t mean us, now. We’ll have to leave Turaev. They’ll evacuate the planet. We’ll all have to go somewhere else." She smiled, and added with a kind of mock jealousy, "I always wanted to be the one to shake this place out of its stupor. But it looks as if you’ve beaten me to it."

Tchicaya sat motionless, scowling slightly. The words refused to sink in. He knew that she was right: it was a universal principle, accepted by every space-faring culture. In each of the other three cases, the planet in question had been strictly quarantined and left to its own fate. Only one of those worlds had been settled, though. Native life was supposed to have been ruled out, long before the colonists' first spores were launched. However microscopic, and however sparsely distributed, it should have left some detectable chemical signature in the atmosphere.

Tears stung his eyes. In his euphoria, he’d never thought beyond the unlikely confirmation that his own world, his own town, held the fourth known example of extraterrestrial life. He could have lived down the shame of this childish escapade, half-excused by that serendipitous discovery. But he’d been more than disobedient, more than disrespectful of the customs that bound the people of Turaev together. He’d destroyed their whole world.

He didn’t want to weep in front of Mariama, so he stammered out an incoherent stream of words instead. Everything he’d planned, everything he’d pictured for the future, had just turned to ashes. He might have traveled one day, like Erdal, but he would never have left his friends and family behind, never lost synch. Fifty-nine generations had made this planet their home; he could never belong anywhere else. Now it would all be torn away from him. And nine million people would suffer the same fate.

When he stopped to catch his breath, Mariama said soothingly, "Everything here can be moved! Every building, every field. You could wake up on New Turaev, a thousand light-years away, and if you didn’t check the stars you’d never know."

Tchicaya replied fiercely, "You know it will never happen like that! Five minutes ago, you were crowing about it!" He wiped his eyes, struggling not to turn his anger against her. He’d always understood what she wanted; he had no right to blame her for that. But any reassurance she offered him was hollow.

Mariama fell silent. Tchicaya buried his head in his hands. There was no escape for him: only adults had the right to shut down their Qusp, to choose extinction. If he threw himself from the roof and broke his spine, if he doused himself in oil and set himself alight, it would only make him more contemptible.

Mariama put an arm around his shoulders. "On how many worlds," she said, "do you think they’ve found life?"

"You know the answer. Three, since Earth."

"I don’t know that. There might have been ten. There might have been hundreds."

Tchicaya’s skin crawled. He looked up and searched her eyes in the starlight, wondering if she was testing him. What she was proposing now was infinitely worse than anything they’d done so far.

She said, "If you believe it will hurt so many people, so badly, then I’ll listen to you." Tears were trickling down his cheeks again; she wiped them away with the back of her hand. "I’ll trust you."

Tchicaya looked away. She had the power to incinerate everything around her, the power to break through every stifling absurdity she’d railed against from the day they’d met. When they’d spoken of the future, it was all she had ever talked about: finding a way to force the world to change. Now she could gut the planet with its own stupid rules, and nothing would ever be the same.

Unless he asked her to stay her hand.

Tchicaya slept through the end of Erdal’s Slowdown, and woke from deep dreams, refreshed but disoriented. He lay in bed, listening to the wind, thinking over what had happened in the last two hundred and seventy-two years.

Erdal had traveled to Gupta, a hundred and thirty-six light-years away, and stayed for ten days. When he rose from the crib, back in his birth flesh, he would find that ten days had passed on Turaev, too. He would be the one bearing news, eagerly describing his travels to his family and friends. He would not be a stranger to them, greeted with an incomprehensible litany of change.

The whole planet had waited for him. What else should they have done? Turaev’s sun would burn for four billion years. How much greed and impatience would it take to begrudge the wait, to cast someone aside for the sake of a few centuries?

Tchicaya felt more pride than guilt. Despite his lapse, his heart was still in the right place, and he had resolved never to be so weak again.

As he was dressing, his gaze ran over the scar on his leg. His was sure that his parents had noticed it, but neither of them had asked him to explain its meaning. It was his right to decide who to tell, and when.

Above the scar, between his legs, the skin was newly red and swollen. Tchicaya sat on the edge of his bed and probed the swelling gingerly. Touching it was like tickling himslef; it made him smile faintly, but there was no disguising the fact that he’d much rather be tickled by someone else.

He finished dressing, moving about the room slowly. He hadn’t thought it would happen so soon. Some people were fourteen, fifteen, sixteen. He was tall, but he wasn’t strong for his age. He was nothing like his mother or father yet. He wasn’t ready. It was some kind of sickness, some kind of mistake.

He sat down on the bed again, trying not to panic. Nothing was irreversible yet. Whatever his body was constructing might take another year to be completed; the first time always took longer. And he could still change his mind, change his feelings. Everything was voluntary, his father had explained. Unless you loved someone deeply, and unless they felt the same way toward you, neither of you could grow what you both needed to make love together.

Tchicaya exposed the raw skin again, and stared down glumly at the formless nub. Every couple grew something different, just as every couple would have a different child. The molecules that had already passed between them in the air would determine the pair of shapes that formed. The two of them would be bound together then, literally remade for each other, even the chemical signals that gave them pleasure fitting together in a complementary pattern as unique as their interlocking flesh.

Tchicaya whispered, "I don’t love you. You’re nothing to me. I don’t love you." He would picture her face and recite the words every day, once when he rose and once before he slept. If he was strong enough, stubborn enough, his body would have to listen.

Chapter 7

Sophus was far too tactful to ask Tchicaya how he and Mariama knew each other; it must have been obvious that the answer was long, complicated, and largely none of his business. Tchicaya volunteered the bare minimum that the situation seemed to require. "We grew up together, in the same town on Turaev," he explained. "It’s been a while since we last ran into each other."

When Mariama asked to hear what was happening on the Rindler, Tchicaya deferred to Sophus, who took up the task of outlining some seventeen decades' worth of advances and disappointments. Tchicaya listened politely, hoping Mariama was taking in more than he was. His thoughts were still so scattered by the shock of her arrival that he gave up trying to pay attention; he could replay the whole conversation later.

As Sophus talked, the three of them strolled around the ship. Mariama was unfazed by the view from the walkways; she might not have been this close to the border before, but apparently she’d become accustomed to space. Then again, it would not have surprised him if she had decided to choose equanimity in the new environment by fiat, even if this was her first time off-planet.

When Tchicaya tuned in to the discussion again, Mariama was saying, "So there’s no prospect of using universality-class arguments to design a generally effective Planck worm, before we pin down the detailed physics?"

Sophus said, "Tarek has looked into that, and even tried some experiments, but I believe it’s a dead end. For a start, we still don’t know what the bulk symmetries of this system are. I’ve more or less given up talking about the novo-vacuum; it’s too misleading. What vacuum? We don’t know that there’s state that lies in the null space of all annihilation operators for the Mimosan seed particles. And if there is such a state, we don’t know that it will obey anything remotely analogous to Lorentz invariance. Whatever’s behind the border might not even posses any kind of time-translation symmetry."

"You’re joking!"

"No. In fact, it’s looking more likely every day." Sophus glanced at Tchicaya meaningfully, as if he was waiting for the Preservationists' laudable openness to be acknowledged.

Tchicaya said, "That’s right. I watched one experiment myself, just a few hours ago." Mariama smiled at him, envious at this slight head start.

He smiled back at her, hoping his face wasn’t betraying his confusion. At the instant he’d seen her standing on the observation deck, he hadn’t consciously assumed anything about the faction she’d be joining; such ephemeral concerns had been swept from his thoughts entirely. Now that she’d casually revealed in passing that she’d come here to support the side that he would have sworn she’d be committed to opposing, the one part of his mind that resonated with this fact was the oldest, crudest model he had of her: someone whose only role in life was to confound and unsettle him. The original Mariama, who he had imagined would go to any lengths, not so much to spite him as to prove that he had no hope of pinning her down.

Tchicaya dragged his thoughts back to Sophus’s comments.

Kadir and Zyfete had been nowhere near as explicit, but then they’d not been in the friendliest of moods. Kadir’s despair made more sense now, though; it went beyond his growing fears for his home world, and one more ordinarily frustrating encounter with the border.

Time-translation symmetry was the key to all their hopes of predicting how the novo-vacuum would behave. In ordinary physics, if two people performed the same experiment, one starting work at midnight while the other began at noon, their separate versions could be compared, very easily: you merely added or subtracted half a day, and all their data could be superimposed. That sounded too obvious to be worth stating, but the fact that it was possible, and the fact that any laws of physics had to be compatible with this process of sliding the two sequences of events together, was a powerful constraint on the forms such laws could take.

Everything that happened in the universe was unique, on some level. If that were not true, there’d be no such thing as memory, or history; there’d be no meaningful chronology at all. At the same time, it was always possible to unpick some features of an event from the complicated tapestry of its context, and demand that this tiny patch of reality look the same as countless others, once you knew how to orient them all for the purpose of comparison. Taking a step north on Turaev on your eighteenth birthday could never be the same as taking a step west on Pachner four thousand years later, but in analyzing these two admittedly singular activities, you could safely abstract the relevant joints and muscles from the surrounding thicket of biographical and planetological detail, and declare that the applicable laws of mechanics were precisely the same in both cases.

It had been obvious since the accident that whatever the Mimosans had created in the Quietener did not possess the same symmetries as ordinary space-time, which allowed the unique location, time, orientation, and velocity of any physical system to be stripped away, revealing its essential nature. Still less had anyone expected the Mimosan vacuum to obey the "internal" symmetries that rendered an electron’s phase or a quark’s color as arbitrary as the choice of a planet’s prime meridian.

But everyone studying the novo-vacuum had been relying on the assumption that these familiar regularities had merely been replaced by more exotic ones. Mathematicians had long had a catalog of possibilities on offer that dwarfed those realized in nature: more or fewer dimensions, different invariant geometric structures, novel Lie groups for the transformations between particles. All of these things would be strange to encounter, but ultimately tractable. And at the very least, it had been taken for granted that there was some prospect of using the results of sufficiently simple experiments to deduce what would happen when those experiments were repeated. Once you lost that, prediction in the conventional sense became impossible. You might as well try to guess who you’d meet in a crowded theater on Quine by consulting the guest list for an opening night of Aeschylus.

Tchicaya said, "If you’re right, we’re wasting our time here."

Sophus laughed. "I wish all Yielders were so easily discouraged."

Tchicaya caught the change in Mariama’s demeanor as he was finally labeled for her. She did not appear surprised, or cooler toward him, but a look of resignation crossed her face, as if she was letting other possibilities slip away.

He replied, "I didn’t say I believed you. Now I know you’re just spreading misinformation."

Sophus said, "The data’s all public; you should judge for yourself. But I’m giving a presentation later today that might interest you."

"On why we should all give up and go home? Yielders first, of course."

"No. On why we shouldn’t, even if I’m right."

Tchicaya was intrigued. "Dishing out despair with one hand, taking it away with the other. You’re never going to drive us away like that."

"I’m really not interested in driving anyone away," Sophus protested. "The more people there are working on this, the sooner we’ll understand it. I’m happy to share my ideas with everyone — and if some Yielder beats me to the punch line because of it, and fails to show reciprocal generosity, what have I lost?"

"You’re not afraid we’ll get through the border first? And shore up what you hope to annihilate?"

Sophus smiled amiably. "There might come a point when that’s a real threat. If I’m ever convinced that we’ve reached it, I suppose I might change my strategy. For now, though, it’s like a game of Quantum Pass-the-Parcel: all the players work simultaneously to tear off the wrapping, and all the players share the benefits. Why convert to the classical version? This is faster, and much more enjoyable."

Tchicaya let the argument rest. It would have been impolite to state the obvious: when Sophus finally decided that sharing his insights had become too risky, it would not be to his advantage to announce the fact. At that point, the most logical strategy would be to continue displaying the same generosity as he’d shown in the past, but to replace the genuine, hard-won conjectures he’d revealed to his opponents in the past with equally well-crafted red herrings.

When they reached Mariama’s cabin, Sophus left them. Tchicaya hung back in the corridor, unsure whether she wanted him to stay or go.

She said, "Would you come in, if you’re coming in?"

He sat cross-legged on the bed while she moved around the cabin. She’d included some physical ornaments in her transmission — a handful of carved rocks and blown-glass objects that the Rindler's reception unit had obligingly re-created for her from spare materials — and now she couldn’t decide where to put them.

"I traveled light, myself," Tchicaya said teasingly. "It didn’t seem fair to ask them to cannibalize the ship to provide me with knickknacks."

Mariama narrowed her eyes. "Aren’t you the puritan? Not to the point of amnesia, I hope."

He laughed. "Not these days." In the past, he’d left some rarely used memories behind in the Qusps of his body trail. With fullsensory recall, the amount of data mounted up rapidly, and there’d come a point when knowing precisely what it had been like to shake water out of his ears in a river on Gupta or roll over and fart while camping in a desert on Peldan didn’t really strike him as a crucial part of his identity.

Yet he’d gathered up all the trivia again, before any of the Qusps were erased. And now that there was nowhere he could store his memories in the expectation that they’d remain secure — even if he archived them with a fleeing acorporeal community, their safety would come at the price of accessibility — they all seemed worth dragging around with him indefinitely.

Mariama finally settled on the shelf by the bed as the place for an elaborately braided variant of Klein’s bottle. "Holding on to your memories is one thing," she said. "It doesn’t stop you going over the horizon."

Tchicaya snorted. "Over the horizon? I’m four thousand and nine years old! Take out Slowdowns and travel insentience, and I’ve barely experienced half of that." Information theory put bounds on the kind of correlations anyone could sustain between their mental states at different times; the details depended on the structure of your mind, the nature of its hardware, and, ultimately, on the recently rather plasticized laws of physics. If there were unavoidable limits, though, they were eons away. "I think I can still lay claim to doing a far better job of resembling myself — at any prior age — than a randomly chosen stranger."

Mariama folded her arms, smiling slightly. "In the strict sense, obviously. But don’t you think people can cross another kind of horizon? The strict definition counts everything: every aspect of temperament, every minor taste, every trivial opinion. There are so many markers, it’s no wonder it takes an eternity for all of them to drift far enough to change someone beyond recognition. But they’re not the things that define us. They’re not the things that would make our younger selves accept us as their rightful successors, or recoil in horror."

Tchicaya gave her a warning look that he hoped would steer her away from the subject. With a stranger, he might have asked his Mediator to handle the subtext, but he didn’t believe either of them had changed so much that they couldn’t read each other’s faces.

He said, "Any more children?"

She nodded. "One. Emine. She’s six hundred and twelve."

Tchicaya smiled. "That’s very restrained. I’ve had six."

"Six! Are any of them with you here?"

"No." He took a moment to realize why she was asking; he’d always sworn that he’d never leave a child before a century had passed. "They’re all on Gleason; large families are common there. The youngest is four hundred and ninety."

"No travelers among them?"

"No. What about Emine?"

Mariama nodded happily. "She was born on Har’El. She left with me. We traveled together for a while."

"Where is she now?"

"I’m not certain." She admitted this without a trace of reticence, but Tchicaya still thought there was a hint of sadness in her voice.

He said, "One thing about being planet-bound is, once you’ve committed to the place, that’s it. Even if you wander off to the other side of the world, everyone else who’s chosen to stay is just a few hours away."

"But two travelers? What does that guarantee?" Mariama shrugged. "Chance meetings, every few hundred years. Or more often, if you make the effort. I don’t feel like I’ve lost Emine."

"Of course not. Nor the others. What’s to stop you visiting the ones who’ve stayed put?"

She shook her head. "You know the answer to that. You’re like a cross between a fairy-tale character and some kind of…rare climatic disaster."

"Oh, come on! It’s not that bad." Tchicaya knew there was a grain of truth in what she said, but it seemed perverse to complain about it. When he was made to feel welcome, it was as a visitor, a temporary novelty. When your child had lived with three or four generations of their own descendants, for centuries, you were not a missing piece of the puzzle. But he never expected to slot in, anywhere. Once he’d told the crib on Turaev that his birth flesh could be recycled, he’d given up the notion that somewhere there’d always be a room waiting for him.

He said, "So what about Emine’s other parent?"

Mariama smiled. "What about your partner back on Gleason? The one you raised six children with."

"I asked first."

"What is there to say? She stayed on Har’El. Not even Emine could drag her away." Mariama lowered here eyes and traced a fingertip over the edges of one of the abstract carvings.

Tchicaya said, "If you could drag everyone with you, what would be the point of leaving? There were cultures back on Earth that traveled across continents, whole extended families together — and they were usually more conservative than the ones that stayed put, or the ones that spawned diasporas."

Mariama scowled. "If two travelers happened to have a child, would that constitute a tribe?"

"No. But traveling is not about a change of scenery. It’s about breaking connections." Tchicaya felt a sudden sense of déjà vu, then realized that he was quoting her own words back at her. He’d got into the habit long ago of using them on other people. "I’m not saying that there’d be anything wrong if six whole generations uprooted themselves together, if that’s not a contradiction in terms. But they wouldn’t stay together for long — or at least, they wouldn’t without imposing rules on themselves a thousand times more restrictive than any they’d needed when they were planetbound."

Mariama said irritably, "You’re such a fucking ideologue sometimes! And before you call me a hypocrite: it’s always the converts who are the worst."

"Yeah? That’s not such a convenient axiom for you, if you remember that it cuts both ways." Tchicaya raised his hands in apology; he wasn’t really angry or offended yet, but he could see where they were heading. "Just…forget I said that. Can we change the subject? Please?"

"You can tell me what happened on Gleason."

Tchicaya thought for a while before replying. "Her name was Lesya. I was there for a hundred and sixty years. We were in love, all that time. We were like bedrock to each other. I was as happy as I’ve ever been." He spread his arms. "That’s it. That’s what happened on Gleason."

Mariama eyed him skeptically. "Nothing soured?"

"No."

"And you don’t wish you were still there?"

"No."

"Then you weren’t in love. You might have been happy, but you weren’t in love."

Tchicaya shook his head, amused. "Now who’s the ideologue?"

"You just woke up one morning and decided to leave? And there was no pain, and no rancor?"

"No, we woke up one morning, and we both knew I’d be gone within a year. Just because she wasn’t a traveler doesn’t mean it was all down to me. What do you think? I lied to her at the start?" He was becoming so animated he was messing up the bed; he stroked the sheet, and it tightened. "You know how I think she’ll feel, if the border reaches Gleason?"

Mariama resisted answering, knowing that she was being set up. After several seconds, she succumbed anyway.

"Terrified?"

"No. I think she’ll be grateful." Tchicaya smiled at Mariama’s expression of disgust. It was strange, but she’d probably given him more confidence in his stance, now that she’d turned out to be his opponent, than if they’d been allies willing to reassure each other endlessly.

He continued. "You don’t take a traveler for a partner if you hope that the world will always stay the same. You do it because you can’t quite break away, yourself, but you can’t live without the promise of change hanging over you every day.

"That’s what the border means, for a lot of people. The promise of change they’d never be able to make any other way."

Sophus’s presentation took place in a theater that the ship had improvised in the middle of one of the accommodation modules, folding up all the cabins that happened to be unoccupied to create a single large space. When Mariama realized that this included her own, she was not pleased.

"I have glass in there!" She pointed across the theater. "Right where that person’s sitting."

"It’ll be protected," Tchicaya reassured her, as if he were a veteran of the concertina effect. "Anyway, what’s there to lose? If anything’s broken, it can be reconstructed."

"They’ve never been broken," she complained.

Tchicaya said, "I hate to be the one to point this out, but — " He held up his thumb and forefinger and adjusted the spacing to atomic size.

Mariama glared at him until he dropped his hand. "It’s not the same thing. But I wouldn’t expect you to understand."

Tchicaya winced. "So now I’m an all-round philistine?"

Mariama’s face softened. She reached over and ran a hand affectionately across his stubbled scalp. "No. Your failings are much more specific than that."

Tchicaya spotted Yann coming through the entrance with a small group of people. He raised a hand and tentatively beckoned to him. Yann responded by bringing the whole group along to sit beside them.

Rasmah, Hayashi, Birago, and Suljan had been involved in designing the new spectrometer. Catching the tail end of the conversation they’d been having made it clear that all but Birago were Yielders; the other three were joking about his plans to sneak in a filter to conceal the telltale signature of Planck worms devouring the scenery. Birago seemed to be taking their teasing with equanimity, though it struck Tchicaya that he had the quietness of someone outnumbered, who had decided that there was no point in speaking his mind.

Perhaps Mariama felt outnumbered, too, but she appeared genuinely amiable toward the Yielders as introductions were made; she was certainly more than diplomatically polite. Tchicaya had been wondering whether their friendship had caused her to conceal the full measure of her distaste for his position, but whatever effort she was making for his benefit, she was nowhere near the point that Kadir and Zyfete had reached.

Yann said, "The new spectrometer looks good. We’ll be able to resolve a whole new band of gamma rays, and with twice the precision of the old machine."

Tchicaya nodded, unsure how much difference that would make. "Do you know what this is all about?" He gestured at the podium that was now growing before their eyes. His Mediator had explained that the timing was meant to encourage people to stop talking among themselves — like a change of lighting, or the raising of curtains — but apparently this was an aspect of the Rindler's local culture that had been documented without ever being practiced.

"Not really," Yann admitted. "There’s usually something on the grapevine about these talks, weeks in advance, but this one has come out of the blue. Sophus is always interesting, though. I’m sure he’ll be worth listening to."

"He said something to me earlier about time asymmetry."

"What, time-reversal asymmetry? He’s talking about an arrow of time in the novo-vacuum?"

"No, time-translation asymmetry."

Yann’s eyes widened. "Interesting might have been an understatement."

Sophus appeared and made his way to the podium, but then he stood to one side. People were still entering the theater, and it looked as if they’d keep on streaming in until it was completely full.

Mariama surveyed the latecomers irritably. "Why can’t they watch this in their heads?"

"It’s a flesh thing," Yann confided. "I don’t understand it either."

Tchicaya glanced up. People were sitting in chairs suspended from the ceiling, accessed via corridors through higher levels that would otherwise have come to a sudden end. The ship had made use of every square meter of available surface, even though there was no prospect of cramming every last passenger in. Rasmah caught Tchicaya’s eye and joked, "I always wanted to be at a performance where people were hanging from the rafters."

Sophus cleared his throat, and the audience fell silent almost immediately. Tchicaya was impressed; even if he’d known everyone on the ship personally, he would probably have asked his Mediator to plead on his behalf for their attention.

Sophus began. "We’ve been scribing probes and gathering data now for more than two hundred and fifty years, trying to understand what’s going on behind that wall." He motioned with a raised fist, as if pounding against the border. "The results are there for everyone to see. Theories come and go, and all we have gained is the ability to rule out ninety-nine percent of new models without performing a single new experiment, because we already have enough data to kill off most of our ideas at birth.

"To some people, it’s beginning to look hopeless. How can the laws we’ve failed to understand be so difficult to grasp? It only took three and a half centuries to get from Newton to Sarumpaet. What’s wrong with us? We have the mathematical tools to model systems far more arcane than anything nature has ever actually thrown at us, before. The acorporeals grew bored with physics ten thousand years ago; expecting them to live with such meager intellectual stimulation was like asking an adult to spend eternity playing with a child’s numbered blocks. But even their boundlessly flexible minds can’t make sense of the new toy they’ve come here to admire."

Tchicaya glanced at Yann, who whispered plaintively, "Maybe I should be grateful whenever it slips someone’s mind that acorporeals were running the Quietener."

"The Sarumpaet rules survived twenty thousand years of scrutiny!" Sophus marveled. "How flawed, how misguided, could they possibly be? So we began with the sensible, conservative approach: we’d find a new set of rules that extended the old ones, very slightly. The smallest change we could possibly make, the tiniest correction, or expansion, that would encompass all their past successes — but also explain what happened at Mimosa.

"Fine. That’s a simple enough piece of mathematics; people solved the equations within days of hearing the news. Then we built the Rindler…and that minimal extension didn’t quite fit what we found. So we tweaked the rules a little more. And a little more.

"In essence — and I know this is unfair to some of you, but I’m going to say it anyway — most of what’s been done here has consisted of repeating that process, over and over, for a quarter of a millennium. We’ve raised ever more elaborate theoretical towers on the same foundations, and most of them have been toppled by the very first prediction they made."

Sophus paused, frowning slightly. He looked almost apologetic, as if he’d been surprised by the tone of his own rhetoric. When he’d spoken to Tchicaya earlier, he’d appeared casually optimistic, but now his frustration was showing through. That sentiment was understandable, but it risked undermining the reception of whatever he said next: to claim any kind of fundamental new insight now would sound like arrogance, after so many people before him had struggled and failed. Still, if he honestly believed that they’d all been misguided, and that progress would come not from standing on their shoulders but from digging in the opposite direction entirely, there was a limit to how graciously that opinion could be expressed.

He collected himself and continued, loosening his posture, visibly striving to make light of his subject, however many worlds, and egos, were at stake.

"Sarumpaet was right about everything that happened before Mimosa. We have to hold on to that fact! And in one sense, we were right, to aim to tamper with his work as little as possible. But what we shouldn’t have done was paint ourselves into a corner where we just kept building ever more baroque and elaborate refinements of the original rules.

"What do the Sarumpaet rules really say?" Sophus looked around the theater, as if expecting volunteers, but he’d caught everyone off-balance, and there were no takers. "We can write them half a dozen ways, and they’re all equally elegant and compelling. A combinatorial recipe for transition amplitudes between quantum graphs. A Hamiltonian we exponentiate to compute the way a state vector evolves with time. There’s a Lagrangian formulation, a category-theoretic formulation, a qubit-processing formulation, and probably a hundred more versions cherished by various enthusiasts, who’ll never forgive me for leaving out their favorite one.

"But what do they all say, in the end? They say that our vacuum is stable. And why do they say that? Because Sarumpaet required them to do so! If they’d implied anything else, he would have considered them to be a failure. The stability of the vacuum is not a prediction that emerges from some deep principle that had to be satisfied, regardless; it was the number one design criterion for the whole theory. Sarumpaet certainly found some simple and beautiful axioms that met his goal, but mathematics is full of equally beautiful axioms that don’t get to govern everything that happens in the universe."

Sophus halted again, arms folded, head inclined. To Tchicaya he seemed to be pleading for forbearance; what he’d just stated was so obvious and uncontroversial that half the audience had probably found it baffling, if not downright offensive, that he’d wasted their time spelling it out for the thousandth time.

"Our vacuum is stable: that was the hook on which Sarumpaet hung everything. So why did he have such unprecedented success, despite basing his entire theory on something we now know to be false?"

Sophus let the question hang in the air for a moment, then changed tack completely.

"I wonder how many of you have heard of superselection rules? I only learned the phrase myself a month ago, while doing some historical research. They’re an arcane notion from the dawn of quantum mechanics, and they only persisted in the vocabulary for the first couple of centuries, before people finally got things straightened out.

"Everyone knows that it’s an axiom of quantum mechanics that you can form superpositions of any two state vectors: if V and W are possible physical states, then so is aV + bW, for any complex numbers a and b whose squared magnitudes sum to one. If that’s true, though, then why do we never see a quantum state with a fifty-percent probability of being negatively charged, and a fifty-percent probability of being positively charged? Conservation of charge is not the issue. Long after people could routinely prepare photons that were equally likely to be on opposite sides of a continent, why couldn’t they manage to prepare a system that was equally likely to be an electron here and a positron here" — Sophus held up his left hand, then his right — "or vice versa?

"For a hundred years or so, most people would have answered that question by saying: Oh, there’s a superselection rule for charge! You can usually combine state vectors…but not if they come from different superselection sectors of the Hilbert space! Apparently there were these strange ghettos that had been cordoned off from each other, and whose inhabitants were not allowed to mix. Cordoned off how? There was no mechanism, no system; it was just an inexplicable fact dressed up in some fancy terminology. But people went ahead and developed methods for doing quantum mechanics with these arbitrary borders thrown in, and the lines on the map became something to be memorized without too much scrutiny. If some innocent novice asked a jaded elder student, Why can’t you have a superposition of different charges? the reply would be, Because there’s a superselection rule forbidding it, you idiot!"

Sophus lowered his gaze slightly before adding acerbically, "We’re far more sophisticated now, of course. No one would tolerate mystification like that — and besides, every child knows the real reason. An electron and a positron in the same position would be correlated with vastly different states for the surrounding electric field, and unless you could track all the details of that field and incorporate them into your observations, you’d have no hope of recognizing the state as a superposition. Instead, the two different charge states would decohere, and you’d be split into two versions, one believing that you’d detected an electron, the other that you’d detected a positron. So although there are no superselection rules, the world still looks so much like the way it would look if there were that all the mathematics that revolved around the term lives on, in various guises."

Tchicaya sensed a sudden change in the atmosphere around him. When he’d glanced at people before, most had seemed puzzled that they were being offered such mundane observations. Tolerant, and prepared to go on listening for a while, thanks to Sophus’s reputation, but clearly not expecting much from yet another tortured reexamination of their field’s basic assumptions. Now there was a shifting of bodies, a creaking of seats, as people felt compelled to transform their postures of indifference or mild disappointment into something altogether more vigilant.

As this mood swept the room, Tchicaya felt gooseflesh rise along his spine. He couldn’t claim to have anticipated the words he heard next, but they thoroughly merited his body’s reaction.

"I believe there are no Sarumpaet rules," Sophus proclaimed. "Not the originals, and not some grander, more perfect version that will explain what happened at Mimosa. But the world still looks so much like the way it would look if there were that we couldn’t help but think such rules existed."

In the silence that followed, Tchicaya turned to Mariama, wondering if she’d picked up more from Sophus’s earlier remarks than he had, but she appeared to be equally stunned. Tchicaya was beaming with delight at the audacity of Sophus’s claim. Mariama looked dismayed, almost fearful.

Sophus continued. "How can the Sarumpaet rules seem to be true, when they’re false? How can our vacuum seem to be stable, when it isn’t? I believe that the right way to answer these questions is virtually identical to the resolution of another paradox, one that was dealt with almost twenty thousand years ago. How can the universe appear to obey classical mechanics, when it really obeys quantum mechanics?

"What creates the illusion of classical mechanics is our inability to keep track of every aspect of a quantum system. If we can’t observe the whole system — if it’s too large and complex in itself, or if it’s coupled to its surroundings, making them part of the system — we lose the information that distinguishes a genuine superposition, where alternatives coexist and interact, from a classical mixture of mutually exclusive possibilities.

"I believe the same effect is responsible for the Sarumpaet rules. How can that be? The Sarumpaet rules are quantum rules. They apply to systems that have not been rendered classical by decoherence. How can interaction with the environment explain anything wholly quantum-mechanical?"

Sophus smiled wearily. "It’s been staring us in the face for twenty thousand years. An electron — a charged particle, which transforms the ordinary vacuum around it into an entirely different state — still obeys quantum mechanics in all of its other degrees of freedom. Its position is quantum-mechanical, its charge is classical. Even when we do our best to isolate an electron from its surroundings, we actually fail miserably at half of the task, while succeeding at the other half. So decoherence hides superpositions of different charge states from us, but not different position states. Our failure looks classical, our success is quantum-mechanical.

"We thought the Sarumpaet rules were pure quantum mechanics: the final story, the lowest level, the rules that held for a system in perfect isolation. Of course, we accepted the fact that, in practice, we could never isolate anything from its surroundings completely, but that wasn’t the point. The universe itself, the total system, was assumed to be obeying the Sarumpaet rules — because whenever we did our best to examine any small part of it, separated out as scrupulously as possible, those were the laws that held.

"That was the wrong conclusion to reach. The electron shows how quantum and classical properties can coexist. The fact that you can demonstrate some quantum behavior in a system doesn’t mean you’ve uncovered all that there is to be found.

"I believe that the Sarumpaet rules are classical rules. Part of the total state vector of any system obeys them, but not the whole. The part that does follow the Sarumpaet rules interacts with the environment one way: transforming its surroundings into what we think of as our own vacuum. But there are other parts that interact differently, creating other states. Because we can’t begin to track what’s really happening to the environment on the Planck scale, what we see is a single, certain, classical outcome: the Sarumpaet rules hold absolutely true, and our vacuum is absolutely stable."

A member of the audience stood, and Sophus acknowledged the request. "Tarek?"

"You’re claiming that the vacuum has been stabilized by something like the quantum Zeno effect?"

Tchicaya craned his neck to observe the questioner more closely. Tarek was the Preservationist who’d been trying to scribe Planck worms to devour the novo-vacuum, without waiting to discover what it was, or what it might contain. There was nothing fanatical about his demeanor, though; he merely radiated an impatience that everyone in the audience shared.

"It’s similar to that," Sophus agreed. "The quantum Zeno effect stabilizes systems through constant measurement. I believe that part of the total graph in which everything’s embedded measures the part we see as the vacuum, which also determines the dynamic laws that govern matter moving through that vacuum. It’s like the vapor in a cloud chamber, condensing in droplets around the path of a subatomic particle. The particle only appears to follow a definite trajectory because each path is correlated with a particular pattern of droplets — and the droplets have too many hidden degrees of freedom to exhibit quantum effects themselves. But we know there are branches where the particle follows different paths, surrounded by different trails of droplets."

Tarek frowned. "So why can’t we discover the path, the rules, that are holding sway behind the border?"

Sophus said, "Because what lies behind the border is not another vacuum, another set of rules. It has no classical properties like that to discover. It’s not that it couldn’t be divided up — formally, mathematically — into a sum of components, each obeying a different analog of the Sarumpaet rules. But we’re not correlated with any particular component, the way we are with our own vacuum, so we can’t expect to uncover any particular set of rules."

Tchicaya was exhilarated. It was too soon to take Sophus’s idea seriously, but there was something deeply appealing in the simplicity of the notion. Behind the border was a superposition of every possible dynamic law.

Tarek said, "We can’t measure those properties? Make them definite, if only for different branches of ourselves? When we interact with the novo-vacuum — or whatever you now wish to call it — shouldn’t we end up as a superposition of observers who each find definite laws?"

Sophus shook his head firmly. "Not by dropping a few Planckscale probe graphs into a system six hundred light-years wide. If there were preexisting laws behind the border, we might hope to discover them that way, but that’s not what we’re dealing with. On our side of the border, there’s a tight correlation stretching across all of space-time: the dynamics being followed at different times and places has become a tangle of mutual interdependence. What lies behind the border isn’t correlated from place to place, or from moment to moment. What we’re sampling with our probe graphs might as well be random noise at every level."

Rasmah stood, just ahead of a dozen other people. The others resumed their seats, and Tarek begrudgingly followed.

She said, "This is wonderful speculation, Sophus, but how do you plan to test it? Do you have any solid predictions?"

Sophus gestured at the space behind him, and a set of graphs appeared.

"As you see, I can match the borderlight spectrum. That’s not claiming much. I can match the half-c velocity of the border, which is slightly harder. And I can match the pooled results of all the experiments performed here so far: namely, their complete failure to identify anything resembling a dynamic law.

"So much for retrodiction. I’m making the following prediction: when we repeat the old experiments, re-scribe the old probe graphs, and monitor the results with your new spectrometer…we’ll find exactly the same thing, all over again. No patterns will emerge, no symmetries, no invariants, no laws.

"We’ve already discovered that there’s nothing to be discovered. All I can predict is that however hard we look, that absence will be confirmed."

Chapter 8

Yann rolled off the bed and landed on the floor, laughing.

Tchicaya peered over the edge. "Are you all right?"

Yann nodded, covering his mouth with a hand but unable to silence himself.

Tchicaya didn’t know whether to be annoyed or concerned. Acorporeals taking on bodies often mapped them in unusual ways. Perhaps laughter was Yann’s only available response to some terrible psychic affront that Tchicaya had unwittingly inflicted.

"You’re sure I haven’t hurt you?"

Yann shook his head, still laughing helplessly.

Tchicaya sat on the edge of the bed, struggling to regain his own sense of humor. "This is not a reaction I’m accustomed to. Rejection and hilarity are perfectly acceptable responses, but they’re supposed to occur much earlier in proceedings."

Yann managed to regain some composure. "I’m sorry. I didn’t mean to offend you."

"I take it you’re not interested in finishing what you started?"

"Umm." Yann grimaced. "I could try, if it’s important to you. But I think it would be very difficult to take seriously."

Tchicaya planted a foot on his chest. "Next time you want an authentic embodied experience…just simulate it." He still felt a pang of lust at the touch of skin on skin, but it was fading into a kind of exasperated affection.

He crouched down and kissed Yann on the mouth, meaning it as a gesture of finality. Yann smiled, puzzled. "That was nice."

"Forget it." Tchicaya stood and started dressing.

Yann lay on the floor, watching him. "I think I’m getting all the signals you talked about," he mused. "But they’re so crude, even now. And before, it was just a single message, repeating itself endlessly: Be happy, be happy, be happy! Do you think there’s something wrong with this body?"

"I doubt it." Tchicaya sat cross-legged on the floor beside him.

"You expected more?"

"I was already happy, so it was a bit redundant."

"How happy?"

"As happy as it’s possible to be, for no particular reason."

"I have no idea how to interpret that. What gets to count as a particular reason?"

Yann shrugged. "Something more than being told by my body: Be happy. Be happy…why?"

"Because you’re with someone you like. And you’re making them happy, too."

"Yes, but only if they accept the same reasoning. That’s circular."

Tchicaya groaned. "Now you’re being disingenuous. It’s a tradition, passed down from reproductive biology. Every tradition’s arbitrary. That doesn’t mean it’s empty."

"I know. But I still expected something more subtle."

"That takes time."

"What, hours?"

"Centuries."

Yann narrowed his eyes with suspicion.

Tchicaya laughed, but made a face protesting his honesty. "On Turaev, it takes six months of attraction before anything’s physically possible." Like most generic bodies, the Rindler's were promiscuous: any two of them could develop compatible sexual organs, more or less at will. You could wire in your own chosen restraints while you inhabited them, but since leaving home, Tchicaya had never felt the need to delegate the task. "The waiting was nice, in its own way," he admitted. "You might think it was risking an awful anticlimax, but I think the buildup improved the sex itself almost as much as it raised expectations. Acting on the spur of the moment is more likely to be disappointing."

Yann protested, "I’ve been contemplating this for almost six months."

"Since I arrived? I’m flattered. But then, who else would you dare to ask?"

Yann smiled abashedly. "How could I not be curious? It’s what flesh is famous for. However undeservedly." He watched Tchicaya carefully, serious for a moment. "Have I hurt you?"

Tchicaya shook his head. "That usually takes longer, too." He hesitated. "So what do acorporeals do, instead? When I was a child, I used to imagine that you’d all have simulated bodies. Sex would be just like embodied sex, but there’d be lots of colored lights, and cosmic bliss."

Yann guffawed. "Maybe twenty thousand years ago there were people that vacuous, but they must have all decayed into thermal noise before I was born." He added hastily, "I’m not saying you’re wrong to continue the tradition. You’ve mapped some stable mammalian neurobiology, and it’s not too pathological in its original form. I suppose it still serves some useful social functions, as well as being a mild existential placebo. But when you have a malleable mental structure, intensifying pleasure for its own sake is a very uninteresting cul-de-sac. We worked that out a long time ago."

"Fair enough. But what do you do instead?"

Yann sat up and leaned against the side of the bed. "All the other things the embodied do. Give gifts. Show affection. Be attentive. Sometimes we raise children together."

"What kind of gifts?"

"Art. Music. Theorems."

"Original theorems?"

"If you’re serious."

Tchicaya was impressed. Mathematics was a vast territory, far more challenging and intricate than physical space. Reaching a theorem no one had proved before was a remarkable feat. "That’s positively…chivalric," he said. "Like a knight riding off to the edge of the world, to bring back a dragon’s egg. And you’ve done that, yourself?"

"Yes."

"How often?"

"Nine times." Yann laughed at Tchicaya’s expression of astonishment, and added, "It’s not always that serious. If it was, it really would be as daunting as winning the hand of medieval royalty, and no one would bother."

"So you start with something easier?"

Yann nodded. "When I was ten years old, all I gave my sweet-heart was a pair of projections that turned the group of rotations in four dimensions into principal bundles over the three-sphere. Ancient constructions, though I did rediscover them for myself."

"How were they received?"

"She liked them so much, she extended them to larger spaces and gave me back the result."

"Can you show me?"

Yann sketched diagrams and equations with his hands; through their Mediators, Tchicaya saw them painted in the air. To make sense of the group of four-dimensional rotations, you could project it down to the three-dimensional sphere of directions in four dimensions, by mapping each rotation to the direction to which it took the x-axis. All the rotations that treated the x-axis in the same way then differed from each other by rotations of the other three directions. This effectively sliced the original group into copies of the group of three-dimensional rotations — which was just a solid sphere with opposite points on its boundary glued together, since any pair of rotations around opposite axes became equal once you reached one hundred and eighty degrees. Like an artful rendering of depth in a painting, these striations made the topology of the larger group much clearer.

"The other projection inverts all the rotations first, so it turns the whole construction inside out." Yann demonstrated, smiling nostalgically. "I know it’s sentimental, but the first time always stays with you."

"Yeah." The mathematics was simple, but it struck Tchicaya as having all the charm of an embodied child’s handmade gift.

"So what about you?"

"I’ve generally had more success with flowers."

Yann rolled his eyes. "Your own first love. What was that like?"

Tchicaya contemplated lying, but he usually did it badly. And what would he say? He wasn’t going to substitute someone else, writing Mariama out of his life.

He said, "I can’t tell you."

"Why not?" Yann was twice as eager for the details, now. "How embarrassing can it be, four thousand years later?"

"You’d be surprised." Tchicaya struggled to think of a way to deflect the inquiry without piquing Yann’s curiosity further. "There’s much better story I can tell you," he said. "About my father’s first love. Can I trade that instead?"

Yann agreed, reluctantly.

"When my father was fourteen," Tchicaya began, "he fell in love with Lajos. It started in winter, when they used to sneak into each other’s houses at night and sleep together."

Yann said, "Why did they have to sneak? Would their parents have stopped them?"

Tchicaya was momentarily at a loss for an answer; he’d never had to explain this before. "No. Their parents would have known. But it’s more enjoyable to pretend that it’s a secret."

Yann seemed slightly bemused by this claim, but willing to take his word for it. "Go on."

"By summer, they were giddy with it. They could touch and kiss, nothing more, but they knew it wouldn’t be much longer. They’d go swimming together, walking together, waiting for it to happen. Aching this wonderful ache." Tchicaya smiled, hiding a sudden upwelling of sadness. He doubted he’d ever return to Turaev, to talk to the stranger his father had become.

"At the height of summer, they were walking on the outskirts of town. And my father witnessed the strangest, most terrifying event that had happened on Turaev for a thousand years. A spaceship descended from the sky. An ancient engine, spouting flames, burning up crops, melting rocks."

Yann was outraged. "And Lajos — " He struggled with his emotions. "Your father saw Lajos — "

"No, no!" Tchicaya was amused at the preposterousness of this suggestion, but he still warmed to Yann’s response. He’d met bigots who would have assumed that an acorporeal would shrug off the notion that witnessing the local death of your first love would be of any consequence at all.

"Not even anachronauts land their spacecraft on top of people," he explained. "They do have instruments."

Yann relaxed. "So your father and Lajos got to meet the anachronauts. What were they like?"

"They’d left Earth fourteen thousand years before. Pre-Qusp. They used biological techniques to keep their flesh viable, but they spent a lot of time cryogenically suspended."

"Cryogenically suspended." Yann was mesmerized. "I always knew they were out there, but I’ve never met anyone before who’s spoken to someone who’s seen them in the flesh." He shuddered with vicarious otherworldliness. "What did they want?"

"When they left Earth, they knew they’d be overtaken by newer technologies; they knew they’d be traveling into the future. They knew there’d be established societies along their route. That was why they left. They wanted to witness what humanity would become."

"I see." Yann appeared to be on the verge of raising another objection, but then he let it pass.

"They had one particular interest, though," Tchicaya continued. "They told my father that they wanted to know what stage his people were in, in the eternal struggle between women and men. They wanted to hear about the wars, the truces. The victories, the compromises, the setbacks."

"Wait. How old is your father now?"

"About six millennia."

"So…" Yann rubbed his neck, perplexed. "Turaev was the very first planet they’d visited? After fourteen thousand years?"

"No, they’d made planet-fall six times before."

Yann spread his arms in surrender. "You’ve lost me, then."

"No one had had the heart to tell them," Tchicaya explained. "When they first made contact with a modern society, on Crane, it took a while before they were sufficiently at ease to reveal their purpose. But by the time they got around to asking questions, the locals had already gained a clear sense of the kind of preconceptions these travelers had. They’d been in cold storage for millennia, and now they were finally beginning the stage of their voyage that would justify the enormous sacrifices they’d made. Nobody could bring themselves to break the news that the sole surviving remnant of human sexual dimorphism was the retention, in some languages, of different inflections of various parts of speech associated with different proper names — and that expecting these grammatical fossils to be correlated with any aspect of a person’s anatomy would be like assuming from similar rules for inanimate objects that a cloud possessed a penis and a table contained a womb."

"So they lied to them?" Yann was horrified. "On Crane? And on all the other planets?"

"It must have seemed like the kindest thing to do," Tchicaya protested. "And when it started, no one seriously expected them to reach another planet. When they did, though, word had gone ahead of them, so people were much better prepared."

"And this happened six times? Even if they were fed the same story on every planet, by the time they’d had a few chances to compare it with reality — "

Tchicaya shook his head. "They weren’t fed the same story on every planet; that would have defeated the whole point. They’d traveled into the future in the hope of being entertained in a very specific way. On Crane, they’d revealed a lot about the kind of histories and practices they expected to encounter on their voyage, and so people played along with their expectations. The locals there told them that all the men had been wiped out by a virus shortly after settlement, and made a big song and dance about the struggle to adapt: one faction trying to reinvent the lost sex; another, bravely pursuing monosexuality, finally triumphant. The anachronauts lapped it up, oohing and aahing over all the profound things this told them about gender. They made notes, recorded images, observed a few fake ceremonies and historical re-enactments…then moved on."

Yann buried his face in his hands. "This is unforgivable!"

Tchicaya said, "No one lied to them about anything else. They had some equally bizarre notions about the future of physics, but the people on Crane gave them an honest account of all the latest work."

Yann looked up, slightly mollified. "What happened next?"

"After Crane? It became a kind of competition, to see who could Mead them the best: make up the most outlandish story, and get the anachronauts to swallow it. A plague wasn’t really barbaric enough. There had to be war between the sexes. There had to be oppression. There had to be slavery."

"Slavery?"

"Oh yes. And worse. On Krasnov, they said that for five thousand years, men had slaughtered their own firstborn child to gain access to a life-prolonging secretion in mother’s milk. The practice had only ended a century before."

Yann swayed against the bed. "That’s surreal on so many levels, I don’t know where to begin." He regarded Tchicaya forlornly. "This is really what the anachronauts expected? No progress, no happiness, no success, no harmony? Just the worst excesses of their own sordid history, repeated over and over for millennia?"

Tchicaya said, "On Mäkelä, the people insisted that their planet had been peaceful since settlement. The anachronauts were terribly suspicious, and kept digging for the awful secret that no one dared reveal. Finally, the locals reviewed the transmission from Crane describing the first contact, and they realized what was needed. They explained that their society had been stabilized by the invention of the Sacred Pentad, in which all family units were based around two males, two females, and one neuter." Tchicaya frowned. "There were rules about the sexual relationships between the members, something about equal numbers of heterosexual and homosexual pairings, but I could never get a clear description of that. But the anachronauts were thrilled by the great cultural richness they had finally uncovered. Apparently, their definition of cultural richness was the widespread enforcement of any social or sexual mores even more bizarre and arbitrary than the ones they’d left behind."

Yann said, "So what happened on Turaev?"

"The ship had been tracked for centuries, of course, so the mere fact of its arrival was no surprise to anyone. My father had known since early childhood that these strangers would be turning up, somewhere on the planet, at about this time. A variety of different hoaxes had been advocated by different groups, and though none of them had gained planet-wide support, the anachronauts rarely visited more than one place, so it would only require the people in one town to back each other up.

"My father wasn’t prepared at all, though. He hadn’t kept up with news of the precise timing of the ship’s arrival, and even though he’d been aware that it would happen soon, the chance of planet-fall outside his own town had been too microscopic to worry about. He’d had far more important things on his mind."

Yann smiled expectantly, despite himself. "So when the flames died down, and the dust settled, and your father’s Mediator dug up the visitors' ancient language from its files…he had to stand there and insist with a straight face that he knew nothing whatsoever about the subject of their inquiries?"

"Exactly. Neither he nor Lajos had the slightest idea what they were supposed to tell these strangers. If they’d read the reports on the anachronauts, they’d have realized that they could have claimed all manner of elaborate taboos on discussing the subject, but they weren’t in a position to know that and invoke some imaginary code of silence. So all they were left with was claiming ignorance: claiming to be both prepubescent, and stupid." Tchicaya laughed. "After six months of longing for each other? Within days, or even hours, of consumation? I don’t know how to translate that into terms you’re familiar with — "

Yann was offended. "I’m not an idiot. I understand how much pride they would have had to swallow. You don’t need to spoonfeed me similes."

Tchicaya bowed his head in apology, but he held out for precision. "Pride, yes, but it was more than that. Claiming anything but the truth would have felt like they were renouncing each other. Even if they’d known their lines, I’m not sure that they could have gone through with the charade." He held a fist against his chest. "It hurts, to lie about something like that. Other people might have been swept up in the excitement of the conspiracy. But to Lajos and my father, that was just noise. They were the center of the universe. Nothing else mattered."

"So they told them the truth?"

Tchicaya said, "Yes."

"About themselves?"

He nodded. "And more."

"About the whole planet? That this was the custom all over Turaev?"

"More."

Yann emitted an anguished groan. "They told them everything?"

Tchicaya said, "My father didn’t come right out and state that all their earlier informants had lied to them, but he explained that — apart from a few surviving contemporaries of the travelers themselves — there’d been nothing resembling sexual dimorphism in the descendants of humans, anywhere, for more than nineteen thousand years. Long before any extrasolar world was settled, it had gone the way of war, slavery, parasites, disease, and quantum indecisiveness. And apart from trivial local details, like the exact age of sexual maturity and the latency period between attraction and potency, he and his lover embodied a universal condition: they were both, simply, people. There were no other categories left to which they could belong."

Yann pondered this. "So did the intrepid gendographers believe him?"

Tchicaya held up a hand, gesturing for patience. "They were far too polite to call my father a liar to his face. So they went into town, and spoke to other people."

"Who, without exception, gave them the approved version?"

"Yes."

"So they left Turaev none the wiser. With an unlikely tale from two mischievous adolescents to add to their collection of sexual mythology."

Tchicaya said, "Perhaps. Except that since Turaev, they haven’t made planet-fall anywhere. They’ve been tracked, the ship’s still functioning, and they’ve had four or five opportunities to enter inhabited systems. But every time, they’ve flown on by."

Yann shivered. "You think it’s a ghost ship?"

Tchicaya said, "No. I think they’re in cold sleep, with their bodies frozen, and tiny currents flowing in their brains. Dreaming of all the horrors they’d wished upon us, in the name of some crude, masochistic notion of humanity that must have been dying right in front of them before they’d even left Earth."

As Tchicaya boarded the shuttle ahead of Yann, Mariama looked back and flashed him a brief smile. Her meaning was unmistakable, but he pretended not to notice. He didn’t mind her knowing what he and Yann had attempted, or even how it had ended, but it drove him to distraction that she could deduce at least half the story just by watching them together.

He could have instructed his Exoself to embargo whatever small gestures were giving him away. But that was not how he wanted to be: hermetically sealed, blank as a rock. For a moment, Tchicaya contemplated reaching over and putting his arm across Yann’s shoulders, just to devalue her powers of observation. On reflection that would have been petty, though, and likely to cause Yann all kinds of confusion.

Mariama sat beside Tarek. In the unlikely event that the two of them were lovers, Tchicaya would be the last to know. Behind him, the fifth passenger, Branco, strapped himself in place. Tchicaya turned to him and joked, "It doesn’t seem right that you’re outnumbered. You should at least have brought an observer along."

Branco said pleasantly, "Fuck that. The last thing I want to do is start mimicking all your paranoid games."

Branco had been part of the original coalition who’d designed and built both the Rindler and the Scribe. Yielders and Preservationists had arrived over the decades, exuding a kind of bureaucratic fog through which he was now forced to march, but as he’d explained to Tchicaya earlier, he’d become inured to the squatters and their demands. The Scribe was still available to its creators, occasionally, and with patience he could still get work done. The factions made a lot of noise, but in the long run, as far as Branco was concerned, they’d be about as significant as the vapid religious cults who’d once squabbled over contested shrines on Earth. "And you sad airheads can’t even slaughter each other," he’d observed gleefully. "How frustrating that must be."

As they fell away from the Rindler, Tchicaya barely noticed the weightlessness, or the strange doll’s-house/termite-colony view some of the modules offered as they shrank into the distance. The trip hadn’t quite become as unremarkable to him as air travel in a planetary atmosphere, but on a planet even repeated flights along the same route were never as unvarying as this.

Tarek said, "Actually, we’re outnumbered, three to two. If you’re neutral, you’re a Yielder. There is no difference."

"Oh, here we go!" Branco chuckled and settled back into his couch. "It’s a short trip, but please, entertain us."

"You’re not fooling anyone," Tarek insisted heatedly.

"It’s not important," Mariama said. Tchicaya watched her, wondering if she’d make eye contact with Tarek as she spoke. She didn’t. "There are observers here for both sides. It doesn’t matter how many there are." Her tone was calm, neither argumentative nor imploring.

Tarek dropped the subject. Tchicaya was impressed; she’d defused the situation without alienating Tarek, or incurring any debt to him. She hadn’t lost her touch, she’d only grown more subtle. When Tchicaya had trailed after her as a tortured, infatuated child, it must have perplexed and frustrated her to find that she couldn’t hone her skills on him. Anything above and beyond mere hormonal effects had been superfluous; she might as well have tried to learn martial arts by practicing on a rag doll.

Branco sighed with disappointment, then closed his eyes and appeared to doze off.

Most of the Rindler's passengers had watched with a mixture of denial and dismay as Sophus’s predictions had been borne out, and all their ingenious models had been dashed to pieces, once again, by the new spectrometer. Branco, however, had embraced the No Rules Theory wholeheartedly, and managed to extract predictions that went far beyond Sophus’s gloomy verdict. Just because there were no preexisting correlations between the dynamics on the far side of the border, that didn’t mean none could be created. Branco had designed an ingenious experiment that aimed to use the near side of the border as a kind of intermediary, to entangle different regions of the far side with each other. The dynamics revealed would still be a random choice from all the possibilities — or, strictly speaking, the near-side universe would split into decoherent branches, and in each, a different result would be observed — but at least the result would apply across more than a few square Planck lengths.

As they docked with the Scribe, Yann mused, "I think this is the first time I’ve come here with any possibility of being disappointed."

Tchicaya was taken aback. "You never had your hopes pinned on any of the old models? You never even had a favorite?"

"There were some esthetically pleasing ones," Yann conceded. "I certainly would have been happy if they’d survived testing. But I never had a good reason to expect it. Not until now."

"That’s very touching," Branco said dryly, "but I see no reason why you should abandon your earlier stance."

Tchicaya challenged him, "You have no emotional stake in the outcome at all?"

Branco regarded him with amusement. "You’ve been here how long?"

Tarek went through the tunnel first, then Mariama. Tchicaya followed her. "Do you remember that playground?" he whispered. "With all the pipes?" She glanced back at him, puzzled, and shook her head. Tchicaya felt a stab of disappointment; he’d assumed that the sight would have triggered the same memory in her.

In the control room, Branco instructed the stylus. With his gravelly voice and deliberate singsong intonation, he succeeded in making every word drip with contempt, like a kind of sardonic poetry. "The phase relationships between the twelve TeV and fifteen TeV beams will be as follows." They really are making me read this aloud.

Tchicaya looked out the window, down at the immutable plane of light. He’d had vivid dreams about the border, imagining as he slept that the wall of his cabin was the thing itself. He’d hold his ear against it, listening for sounds from the far side, straining with his whole body, urging the signal across.

Sometimes, the instant before he woke, he’d see an iridescent film blossoming on the wall, and his heart would race with joy and fear. Did this new infestation mean that he’d been found out? Or that his crime had never really happened?

Branco looked up and announced with mock astonishment, "Am I finished already? Is that all I have to do?"

Tarek said, "For now. But I’m invoking my right to a functional audit."

"Hooray," said Branco. He pushed himself away from the control panel and floated by the window with his hands on his head.

Tarek took his place, and instructed the stylus to rise from the border. Tchicaya had heard about functional audits, but he’d never witnessed one before. A package of detectors, verified by the faction invoking the audit, was placed under the tip of the stylus, and the particles emitted were scrutinized directly, to be sure that they conformed to the agreed sequence.

Tchicaya was tempted to say something derisive, but he held his tongue. Whatever made Tarek believe that this was necessary, complaining about the procedure would do nothing to lessen his suspicions.

He used the handholds beneath the windows to drag himself closer to Mariama. "Where have you been hiding? I haven’t seen you for weeks."

"I have a lot of meetings."

"I go to meetings, too."

"Not these ones," she said.

She didn’t need to spell it out. She’d come to the Rindler hoping to work with Tarek on Planck worm design, and apparently the notion still wasn’t dead.

The novo-vacuum was already the largest object in the galaxy, and it was growing so rapidly that its surface area would increase almost forty-fold while it was encircled at the speed of light. Even if the Preservationists discovered a potential method for dealing with it, there was no prospect whatsoever of surrounding the entire thing with conventional machinery to administer the cure. The only practical tool would be a self-replicating pattern embedded at the level of quantum graphs, able to "eat" novo-vacuum and excrete something more benign.

To supporters of the idea, these hypothetical Planck worms would do no more than reverse the disaster of Mimosa. To Tchicaya, the symmetry was false. The places lost to Mimosa — ordinary planets, unique as they were — had already been thoroughly understood. Learning just enough about the novo-vacuum to infect it with a kind of fungal rot struck him as a corruption of every impulse that made intelligence worthwhile. He had enough trouble forgiving that kind of cowardice in a child.

"So what do you think the prospects are?" He meant those for Branco’s experiment succeeding, though if she cared to disclose her thoughts on anything further down the line, so much the better.

Mariama thought carefully before replying. "I’m almost persuaded that Sophus is right, but I’m not certain that Branco’s ideas follow. When we have no access to any particular far-side dynamics, even plucking out a random correlated state seems like too much to ask."

Yann had been floating a polite distance away, but the room was too small for any real privacy, and now he gave up pretending that he couldn’t hear them. "You shouldn’t be so pessimistic," he said, approaching. "No Rules doesn’t mean no rules; there’s still some raw topology and quantum theory that has to hold. I’ve reanalyzed Branco’s work using qubit network theory, and it makes sense to me. It’s a lot like running an entanglement-creation experiment on a completely abstract quantum computer. That’s very nearly what Sophus is claiming lies behind the border: an enormous quantum computer that could perform any operation that falls under the general description of quantum physics — and in fact is in a superposition of states in which it’s doing all of them."

Mariama’s eyes widened, but then she protested, "Sophus never puts it like that."

"No, of course not," Yann agreed. "He’s much too careful to use overheated language like that. The universe is a Deutsch-Bennett-Turing machine’s is not a statement that goes down well with most physicists, since it has no empirically falsifiable content." He smiled mischievously. "It does remind me of something, though. If you ever want a good laugh, you should try some of the pre-Qusp anti-AI propaganda. I once read a glorious tract which asserted that as soon as there was intelligence without bodies, its 'unstoppable lust for processing power would drive it to convert the whole Earth, and then the whole universe, into a perfectly efficient Planck-scale computer. Self-restraint? Nah, we’d never show that. Morality? What, without livers and gonads? Needing some actual reason to want to do this? Well…who could ever have too much processing power?

"To which I can only reply: why haven’t you indolent fleshers transformed the whole galaxy into chocolate?"

Mariama said, "Give us time."

"The equipment seems to have passed inspection." Tarek pocketed the detector package and began lowering the stylus.

Branco folded his arms and pondered this announcement. "Seems? I’ll take that as a general statement of Cartesian skepticism, shall I?"

Tarek replied curtly, "You’re free to instruct it again."

Branco began repeating the sequence. Tchicaya was expecting him to rush through it this time, but instead he took pains to reproduce the same pacing and intonation as he’d employed originally.

Tchicaya caught Tarek’s eye and said, "You know, you have as much to gain from this experiment as anyone."

Tarek frowned, as if the implication was not merely unjust but completely surreal. "You’re right. That’s why I’m taking it seriously." He hesitated, then added defensively, "Don’t you think I’d prefer to believe that everyone was acting in good faith? I’d like to assume that. But I can’t; there’s too much at stake. If that makes me look petty to you, so be it. I’ll answer to my descendants."

Branco completed his second recitation. Yann said, "Approved."

Tarek said, "Yes, go ahead."

Branco addressed the Scribe. "Execute that."

The Scribe remained silent, but a heartbeat later there was a sharp hissing sound from under the floor. Tchicaya had no idea what this could be, until he saw the realization dawning on Branco’s face.

A fine crack appeared in one window, then another. Tchicaya turned to Mariama. "You’re backed up?"

She nodded. "While I slept. You?"

"The same." He smiled uncertainly, trying to reassure her that he was prepared for whatever happened, without discouraging her from expressing her own feelings. They’d been through a lot together, but neither of them had ever witnessed the other’s local death.

"Yann?"

"I’m covered, don’t worry."

Branco and Tarek were in the same position: no one risked losing more than a day’s memory. After his fourth local death, Tchicaya had ceased to feel genuine, gut-churning dread at his own fate — and he had some memories that led up to the moment itself — but in the company of others it was always more stressful. Wondering how much fear they felt, and how careful they’d been.

The hissing beneath them intensified, and the room began to creak. The windows had healed themselves, and the whole structure would be capable of a certain amount of self-repair, but if the border was lapping up against the Scribe, the wound it made would be reopened with every advance. The microjets were designed to compensate for the effects of bombardment with interstellar gas; shifts measured in microns were the crudest adjustments imaginable. The Scribe was not going to whisk them away to safety.

Tarek looked around nervously. "Shouldn’t we head for the shuttle?"

Branco said, "Yes."

The wall behind Tchicaya emitted a tortured groan. As he turned, it concertinaed visibly, the angle between two windows becoming impossibly acute. Tchicaya marveled at the sight. Air leaking from the Scribe couldn’t be producing shear forces of that magnitude; the border had to be tugging on the structure beneath them. Nothing of the kind had ever been witnessed before. Beams constructed from a variety of substances, poked through the border, had always behaved as if the far-side portion had simply ceased to exist; there were no forces exerted on the remainder. Whatever Branco had triggered, he’d done more than displace the border by a few centimeters.

The wall flexed again, and the pair of windows that had been squashed together separated. Instead of reversing their original motion, though, they parted at the seam, like doors swinging open.

Tchicaya bellowed with fright, and reached out for something to stop himself. He succeeded only in clutching Yann’s shoulder, and the two of them tumbled through the opening together.

For several seconds, Tchicaya remained rigid, preparing himself on some instinctive level for intense pain and a swift extinction. When neither arrived, his whole body began shaking with relief. He’d known that his suit would protect him, but the understanding hadn’t penetrated far. He’d skydived from altitudes where oxygen was needed, and swum at depths where the next free breath was hours away, but black and starry space had remained the quintessence of beautiful danger: pristine, indifferent to his needs, predating every form of life. Vacuum was not a word that offered hope. He should have been snuffed out in an eye blink.

He looked around. The push of the escaping air had been firm but brief, so it was unlikely that they were moving very rapidly, but he was facing the wrong way to catch sight of the Scribe, the only meaningful signpost. The border itself offered no cues as to their velocity in any direction.

He’d been holding his breath deliberately, as if he’d plunged into water, but he realized now that the urge to inhale had vanished as soon as the suit’s membrane had sealed off his mouth and nose. His body had shut down its lungs; the Rindler's model could operate for days on anaerobic metabolic pathways. His skin felt slightly chilly, but he could see the exposed film of the suit on the back of his hand, silvered to retain heat. He extended his arm shakily so he could examine Yann, whose face had turned entirely metallic except for two holes for his pupils.

"You should have known it was futile, Tin Man, trying to walk among us. Robot nature always shows through." Tchicaya’s teeth were chattering, but that made no difference; his Mediator grabbed his speech intentions and routed them away from his useless vocal cords, shunting them into a radio channel.

Yann said, "Believe me, the effect looks much stranger on you."

They were rotating slowly together, around an axis roughly perpendicular to the border. As they turned, the Scribe came into view over Yann’s shoulder. The lower half of the structure was buckled and twisted, but the control room was still safely clear of the border. As far as he could judge, he and Yann were still four or five meters from the border themselves, and their trajectory was virtually parallel to it. This freakish alignment was sure to prove inexact, though, one way or the other.

He spotted a shiny Mariama standing at the ruptured wall, watching him.

"We’re all right," he said. "Get in the shuttle."

She nodded and waved, as if he’d be unable to hear a reply.

Then she said, "Okay. We’ll come and pick you up." She vanished from sight.

Tchicaya instructed his Mediator to make his next words private. "Are we all right? I don’t have the skills to determine our velocity that accurately."

"We’re moving toward the border, but it would take hours before we’d hit it."

"Oh, good." Tchicaya shuddered. His right hand was still locked on to Yann’s shoulder, the fingers digging in as if his life depended on it. He knew that wasn’t true, but he couldn’t relax his grip.

"Am I hurting you?" he asked.

"No."

Yann’s metallic face brightened strangely, and Tchicaya glanced down. A patch of borderlight more intense than its surroundings drifted slowly by.

"What do you make of that?" Tchicaya asked. He was suddenly light-headed, from more than the shock of ejection. The Doppler-shift tints aside, he’d known the border as a featureless wall for centuries. The tiniest blemish was revolutionary; he felt like a child who’d just watched someone reach up and scratch a mark into the blue summer sky.

"I’d say Branco has succeeded in pinning something to the near side."

"We have physics? We have rules now?"

"Apparently."

Mariama said, "We’re in the shuttle. Everyone’s safe here."

"Good. No rush; the view is wonderful."

"I won’t hold you to that. We’ll be there in a few minutes."

The strange patch of brightness had moved out of sight, but after a few seconds another came into view. They were fuzzy-edged ellipses, traveling from the direction of the Scribe.

"They’re like the shadows of reef fish," Tchicaya suggested. "Swimming above us in the sunlight."

Yann said, "Do you think you might be coming slightly unhinged?"

As Tchicaya swung around him in their involuntary dance, he caught sight of the shuttle rising from the ruined Scribe. He smiled at the memory of Mariama’s voice, promising to rescue him. On Turaev, if they’d given in to their feelings, it would have ended badly, burning out in a year or two. When this was over, though —

Yann said, "That’s a bit ominous."

"What?"

"Can you turn your head back toward the Scribe? That might be quicker than me trying to put it into words."

Tchicaya twisted his neck. The border had formed a bellshaped hillock, forty or fifty meters high, that had completely swallowed the Scribe. As his rotation forced him to stretch even more, he stopped fighting it and twisted his neck the other way, hastening the sight’s return instead of trying to delay its departure.

The hillock was collapsing now, but as it did, a ring around it was rising up. Suddenly, Tchicaya noticed a whole series of lesser rings surrounding the first, like concentric ripples in water. They were undulating out from the center at great speed: the leading edge, the fastest component, in some kind of surface wave. The bulk of the wave was spreading more slowly. But it was still traveling faster than they were.

He searched for the shuttle, and found it, its exhaust a pale blue streamer against the stars. The thrust generated by the ion engine was very low; over time it could accumulate into a significant velocity, but the craft was about as maneuverable as a bathtub on ice. It might just reach them before the wave, and even accelerate away from the border again in time, but there’d be no margin left for any more surprises that might manifest themselves in the wake of Branco’s intervention.

Yann read his mind, and declared flatly, "They have to stay clear."

Tchicaya nodded. "Mariama?"

"No!" she hissed. "I know what you’re going to say!"

"It’s all right. We’re backed up, we’re calm. Don’t even think about it."

"It’s a wave. It’s a predictable phenomenon! I’ve computed a trajectory that meets all the constraints — "

"Predictable?"

"We can do it!"

"You’ve all voted on that, have you? Tarek? Branco?"

Branco replied laconically, "It’s all the same to me."

Tarek said nothing, and Tchicaya felt a pang of sympathy for him. No one could reasonably expect him to put himself at risk, merely to spare his two adversaries the loss of their replaceable bodies and a few hours' memories. Yet if he did, many people would respect him for it. You had to be a utilitarian zealot, rotted to the core by dogma, not to admire someone who was willing to jeopardize their own comfort and continuity to preserve another’s. Whether or not this required courage, at the very least it was an act of generosity.

Tchicaya said, "Stay clear! We can’t afford to lose the shuttle!" This argument made no sense — the Rindler's stock of raw materials had not been depleted, and there were parts of the ship itself that could be cannibalized anyway, if necessary — but he wanted to offer them an unselfish-sounding alibi. "You have to gather all the data you can," he added, a little more cogently. "With the Scribe gone, every observation you can make is invaluable." The Rindler itself had powerful instruments trained on the border, but some crucial detail might conceivably depend on the shuttle’s proximity.

Mariama did not reply immediately, but in the silence that followed Tchicaya knew that he’d swayed her.

"All right." Her voice was still strained, but there was a note Tchicaya recognized from their days on Turaev: a rare concession, not so much of defeat, as the realization that they’d been struggling over the wrong thing altogether. She understood the tradeoff, and she knew that he and Yann were resolved. "Peace, Tchicaya."

"Peace," he replied.

Yann said, "You handled that well."

"Thanks." Over Yann’s shoulder, Tchicaya could see the wave closing on them. It was dropping in height as it spread out from the point where the Scribe had been, but it wouldn’t fall far enough to miss them. Tchicaya wondered if Yann would want to be distracted, or to confront what was happening directly.

"So well that I almost hate to do this. How strong do you think your legs are?"

"What?" It took a moment for Tchicaya to understand what he was suggesting. "Oh, no. Please — "

"Don’t go squeamish on me; we don’t have time. It would be hard to decide who to save if we were from the same modes, but I can start from backup with no delay. You’d be out of the picture for months."

That was true. The Rindler had run out of bodies, and there were currently about twenty new arrivals waiting. Tchicaya would have to join the queue. Normally, a delay like that would mean nothing compared to the centuries he’d lost to transit insentience, but Branco’s experiment had just guaranteed that every day from now on would be unique.

"I’ve never killed anyone," he said. His stomach was knotted with revulsion at the thought.

Yann didn’t quibble over the hyperbole. "And I’ve never died, in a body. Sex and death, all in one day. What more could an acorporeal ask for?"

The wave came into view again; they’d have a minute or less. Tchicaya struggled to clear his head. Yann was demanding no more of him than he’d demanded of Mariama. The sense of shame and selfishness he felt, at the thought of indulging his own visceral urge to survive at Yann’s expense, was the right thing to feel, but that didn’t mean he had to elevate it above every other consideration. Nor, though, did he have to annihilate the emotion in order to act against it. He would do what the situation required, because it would be a foolish waste for both of them to lose their bodies, but he wasn’t going to pretend that he was happy, or indifferent about it.

He took hold of Yann’s left hand, then released his iron grip on his shoulder so they could join right hands as well. He folded his knees up against his chest, then froze. The crest of the wave was thirty meters away. This was too complicated. They’d never have time.

Yann said calmly, "Give me your body. I’ve worked out the steps."

Tchicaya surrendered motor control, and they began to move together in a perfect, symmetrical ballet. It was as if his limbs had been gripped by a dozen firm, invisible hands, manipulating him without resistance. His back arched, his arms stretched painfully, but their fingers stayed tangled in a monkey grip as their legs forced their bodies apart, until their feet met, sole to sole.

Tchicaya said, "You made me an isotopy."

Yann laughed. "Nothing original, I’m afraid."

"It’s the thought that counts."

Tchicaya had become disoriented, but as they swung around together his line of sight fell from the stars to the approaching wave. The muscles in his legs tensed, and the pressure against his feet grew until he felt as if his arms would be torn from his shoulders.

Yann said, "See you later."

Their fingers parted.

Tchicaya clutched at the emptiness between them, then stopped himself and wrapped his arms across his chest. He was ascending at a shallow angle, back toward the point where the Scribe had been. As the crest approached, he curled into a ball, and it raced past beneath him, a flash of silver licking at his heels as he tumbled.

An elaborate grid of colored lines scarred the inside of the retreating wave, like the map of some kind of convoluted maze. The pattern shifted as he watched. There was a tantalizing logic to the changes — the lines weren’t dancing about at random — but deciphering it on the spot was beyond him. All he could do was record the sight.

Drained for a moment of every other concern, Tchicaya locked his gaze on the retreating enigma.

Everything had changed, now. Whatever Branco had revealed, or created, the wall between the worlds had finally been breached.

Chapter 9

"Everyone complains about the laws of physics, but no one does anything about them."

Tchicaya turned away from the control panel. He hadn’t heard Rasmah entering the Blue Room.

"It’s an old joke they used to tell, back on Maeder," she explained, crossing the wide, empty floor. "Which just goes to show how much work it takes to send a bad meme off to smallpox heaven."

"Don’t count on having done that," Tchicaya warned her. "I believe the original version was Everyone complains about human nature. When the second half became patently false, the meme just shifted context. You can tear the meaning right out of these one-liners, and they’ll still find a way to keep propagating."

"Damn." She sat beside him. "So what are the laws, right now?"

"As far as I can tell, we have a macroscopic SO(2,2) symmetry, and E7 as the gauge group." He gestured at the display. "Nothing we haven’t grabbed before, generically, though the details of the Lagrangian are unique." Tchicaya laughed. "Listen to me. I really am getting blasé about this."

"Seen one universe, seen them all." Rasmah leaned closer to examine the symmetry diagrams that the software had guessed from some partial results, and was now proceeding to test further with the Left Hand.

She glanced at the endurance clock. "Thirteen minutes? That’s close to the record. You think this might — " Tchicaya glowered at her, and she laughed. "Don’t tell me: I’m jinxing the result."

"Hardly. I’m just growing a little impatient with the idea that we keep grabbing dynamics, over and over, in the hope that one of them will turn out to be stable. It’s never going to happen."

"You think not?" Rasmah pursed her lips. "Okay. It’s no use just complaining, though. What do you want to do about it?"

Tchicaya made a gesture of helplessness.

She regarded him with disappointment. "Are you this lazy about everything?"

She was only teasing, but the accusation stung. Rasmah had been on the Rindler just six months longer than he had, but she’d already contributed substantially to several projects. Having helped to design the spectrometer that had been lost with the Scribe, she’d gone on to improve the design still further for the models used in both the Left and Right Hands. The Scribe’s replacement had been planned as a single machine, but when attempts to renegotiate the protocols for its shared use collapsed for the seventh time, even the most ecumenical researchers had lost patience, and agreed to the duplication.

Tchicaya stretched his arms. "I’ve certainly had enough of staring at this for one day. Are you here to take over?"

"Yes." She smiled and added, "But I’m early, so I’m afraid you can’t actually leave yet."

The destruction of the Scribe, and the end to cooperation between the factions, had delayed follow-ups to Branco’s experiment, but once the two Hands were in place and gathering data, everybody on the Rindler had been riveted by the results. For months, the Blue Room — where the Left Hand’s data was displayed, now that trips to the border were considered imprudent — had been packed with people twenty-four hours a day, and it was no secret that the Preservationists had reacted in the same way.

Branco’s technique appeared to have confirmed Sophus’s original assertion: the novo-vacuum did not obey any single analog or extension of the Sarumpaet rules. It was possible to correlate a macroscopic portion of the near side of the border with parts of the total far-side state that did obey specific rules, but each time the experiment was repeated, the rules were different. All of Sarumpaet’s carefully reasoned arguments about which patterns of nodes in a quantum graph could persist as particles had been revealed as utterly parochial; the larger truth was, the ordinary vacuum that dominated the near side was correlated with sequences of graphs that behaved in that particular fashion, so it hid the fact that they were really just part of a superposition of countless other possibilities. The quantum subtleties that could, in principle, render the whole superposition visible were buried in the sheer number of details that would have had to be tracked in order to observe it.

The far side lacked the means to conceal its quantum nature in the same fashion, but if the view was less misleading, it remained confusing. Interpreting the new experiments was like trying to make sense of a jungle by watching an endless parade of exotic creatures cling briefly to the windows of a vehicle, stunned by the light, curious, or angry, but always flying off a moment later, never to return.

At first, every new set of laws had had their fifteen minutes of fame, but since none of them could be pinned to the near side for much longer than that, the novelty had begun to wear thin. Exhilaration at the cornucopia had given way to frustration. The experiments continued, but it had become a struggle to maintain even the symbolic presence of one sentient observer around the clock. Tchicaya supposed that this was fair enough: all the theorists were drowning in data already, and they had better things to do than sit and watch more come pouring in. For a week or two, he’d hoped that patient observation might actually lead him to a worthwhile discovery himself, but that was beginning to sound as crazy as looking for patterns in any other set of random quantum results.

"Oh, there it goes!" Rasmah wailed, as if she’d seriously expected otherwise. The patch of the border they’d pinned to the latest set of laws had just reverted to the old inscrutable glow. "What do you think would happen," she mused, "if we scribed some device that could function under the far-side dynamics, before we lost the correlation?"

Tchicaya said, "Even if it survived, what good would that do us? We’ve never been able to grab the same dynamics twice."

"What if we scribed a Scribe?"

"Ha! Like that Escher drawing?"

"Yeah." Rasmah pulled a face, suddenly aghast. "Though…that’s a left hand drawing a right, and vice versa. We can’t have that, can we?"

"Are you serious, though? Do you think we could insert a