Schild's Ladder

by Greg Egan

  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 le
ft, 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.