Schild's Ladder

by Greg Egan

  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 sk
in, 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.