Schild’s Ladder

by Greg Egan


B B   A A A A A

This is an excerpt from the novel Schild’s Ladder by Greg Egan, first published in the United Kingdom by Orion/Gollancz and in the United States of America by HarperCollins. Copyright © Greg Egan, 2002. All rights reserved.

Publication history


1

Diamond graph

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 vicinity 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 in 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 travelled in all but one favoured direction.)

Diamond graph

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 travelled to Mimosa Station, half a light-year from the blue sub-giant 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 kilometres away. Mimosa Station had so little room to spare that she’d had to settle for a body two millimetres 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 seven-hundred-and-forty-year delay between each stage of the argument. Or she could have sent a Surrogate, well-briefed but non-sentient, 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 favourite 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-metre 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 already 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 travelling 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 membrane-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, backwards. 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 realise 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 re-description 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 leant towards 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 civilisation 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 spacetime 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 travelled. Generalising this idea, criss-crossing 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 pre-existing 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 spacetime 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 towards 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; spacetime 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 millimetres 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 towards the illusory meadow, laser-painted 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 favour?”

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.

2

Riding her ion scooter the million kilometres to the Quietener, Cass found herself revelling 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 travelled 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 coloured 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 centre 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 spacetime 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 spacetime 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 vaporised, ionised, 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 centres 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 counter-sources, their orbits timed to stretch space at the centre of the device when the bodies they mimicked squeezed it, and vice versa.

As Cass passed within a few kilometres 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 per cent of light-speed. The Mimosans themselves had come from all over, travelling 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 travelled the width of a continent on Earth. Not to the moon and back, and more.

One wall of the bay had handholds. As she 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 rationalise 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 tantalisingly 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 swivelled 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 civilisation 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 judgement 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 anti-noise features herself, she had to agree to be snap-frozen to a few Kelvin, three minutes before each run. Apart from being immobilised, 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 left-over 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 grille 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 stabilised 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 circumnavigate 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 travelling slower than ninety-five per cent of light-speed. 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 favoured one direction over another, but the sum combined every possible bias, cancelling 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 kilometres 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 behaviour en masse had merely hinted at the detailed motion of its own atoms, then that behaviour 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 centre of the sphere would be equivalent to the novo-vacuum, and a new kind of spacetime would blossom into existence.

That was the idealised 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 metres, 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 misdemeanours she might commit, would be erased beyond recovery.

Rainzi looked sceptical, 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 armoured 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.”

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 any more. 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 would lead 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 centre of the Quietener, the sign of an unstable boundary between old vacuum and new.

The data had been coming in for only 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 materialise 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 savour 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 double-counting. (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 artefact 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 modelled 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 awakened 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 upwards 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, but 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 even-handed. 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 flavours 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 specialised 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 travelling at light-speed — or they wouldn’t have 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 clamouring 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, defenceless 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 back-ups 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 traveller she’d hoped to become was dead.

“I don’t know how to help you make peace with that,” Rainzi said. “But I can think of only one way to make my own peace with the people we’ve endangered.” Mimosa was remote from the rest of civilisation, 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 civilisations. 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 naïve. 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.”



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