Schild's Ladder

by Greg Egan

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

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

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

  Mariama's eyes widened, but then she protested, “Sophus never puts it like that.”

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

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

  Mariama said, “Give us time.”

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

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

  Tarek replied curtly, “You're free to instruct it again.”

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

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

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

  Branco completed his second recitation. Yann said, “Approved.”

  Tarek said, “Yes, go ahead.”

  Branco addressed the Scribe. “Execute that.”

  The Scribe remained silent, but a heartbeat later there was a sharp hissing sound from under the floor. Tchicaya had no idea what this could be, until he saw the realization dawning on Branco's face.

  A fine crack appeared in one window, then another. Tchicaya turned to Mariama. “You're backed up?”

  She nodded. “While I slept. You?”

  “The same.” He smiled uncertainly, trying to reassure her that he was prepared for whatever happened, without discouraging her from expressing her own feelings. They'd been through a lot together, but neither of them had ever witnessed the other's local death.

  “Yann?”

  “I'm covered, don't worry.”

  Branco and Tarek were in the same position: no one risked losing more than a day's memory. After his fourth local death, Tchicaya had ceased to feel genuine, gut-churning dread at his own fate—and he had some memories that led up to the moment itself—but in the company of others it was always more stressful. Wondering how much fear they felt, and how careful they'd been.

  The hissing beneath them intensified, and the room began to creak. The windows had healed themselves, and the whole structure would be capable of a certain amount of self-repair, but if the border was lapping up against the Scribe, the wound it made would be reopened with every advance. The microjets were designed to compensate for the effects of bombardment with interstellar gas; shifts measured in microns were the crudest adjustments imaginable. The Scribe was not going to whisk them away to safety.

  Tarek looked around nervously. “Shouldn't we head for the shuttle?”

  Branco said, “Yes.”

  The wall behind Tchicaya emitted a tortured groan. As he turned, it concertinaed visibly, the angle between two windows becoming impossibly acute. Tchicaya marveled at the sight. Air leaking from the Scribe couldn't be producing shear forces of that magnitude; the border had to be tugging on the structure beneath them. Nothing of the kind had ever been witnessed before. Beams constructed from a variety of substances, poked through the border, had always behaved as if the far-side portion had simply ceased to exist; there were no forces exerted on the remainder. Whatever Branco had triggered, he'd done more than displace the border by a few centimeters.

  The wall flexed again, and the pair of windows that had been squashed together separated. Instead of reversing their original motion, though, they parted at the seam, like doors swinging open.

  Tchicaya bellowed with fright, and reached out for something to stop himself. He succeeded only in clutching Yann's shoulder, and the two of them tumbled through the opening together.

  For several seconds, Tchicaya remained rigid, preparing himself on some instinctive level for intense pain and a swift extinction. When neither arrived, his whole body began shaking with relief. He'd known that his suit would protect him, but the understanding hadn't penetrated far. He'd skydived from altitudes where oxygen was needed, and swum at depths where the next free breath was hours away, but black and starry space had remained the quintessence of beautiful danger: pristine, indifferent to his needs, predating every form of life. Vacuum was not a word that offered hope. He should have been snuffed out in an eye blink.

  He looked around. The push of the escaping air had been firm but brief, so it was unlikely that they were moving very rapidly, but he was facing the wrong way to catch sight of the Scribe, the only meaningful signpost. The border itself offered no cues as to their velocity in any direction.

  He'd been holding his breath deliberately, as if he'd plunged into water, but he realized now that the urge to inhale had vanished as soon as the suit's membrane had sealed off his mouth and nose. His body had shut down its lungs; the Rindler's model could operate for days on anaerobic metabolic pathways. His skin felt slightly chilly, but he could see the exposed film of the suit on the back of his hand, silvered to retain heat. He extended his arm shakily so he could examine Yann, whose face had turned entirely metallic except for two holes for his pupils.

  “You should have known it was futile, Tin Man, trying to walk among us. Robot nature always shows through.” Tchicaya's teeth were chattering, but that made no difference; his Mediator grabbed his speech intentions and routed them away from his useless vocal cords, shunting them into a radio channel.

  Yann said, “Believe me, the effect looks much stranger on you.”

  They were rotating slowly together, around an axis roughly perpendicular to the border. As they turned, the Scribe came into view over Yann's shoulder. The lower half of the structure was buckled and twisted, but the control room was still safely clear of the
border. As far as he could judge, he and Yann were still four or five meters from the border themselves, and their trajectory was virtually parallel to it. This freakish alignment was sure to prove inexact, though, one way or the other.

  He spotted a shiny Mariama standing at the ruptured wall, watching him.

  “We're all right,” he said. “Get in the shuttle.”

  She nodded and waved, as if he'd be unable to hear a reply.

  Then she said, “Okay. We'll come and pick you up.” She vanished from sight.

  Tchicaya instructed his Mediator to make his next words private. “Are we all right? I don't have the skills to determine our velocity that accurately.”

  “We're moving toward the border, but it would take hours before we'd hit it.”

  “Oh, good.” Tchicaya shuddered. His right hand was still locked on to Yann's shoulder, the fingers digging in as if his life depended on it. He knew that wasn't true, but he couldn't relax his grip.

  “Am I hurting you?” he asked.

  “No.”

  Yann's metallic face brightened strangely, and Tchicaya glanced down. A patch of borderlight more intense than its surroundings drifted slowly by.

  “What do you make of that?” Tchicaya asked. He was suddenly light-headed, from more than the shock of ejection. The Doppler-shift tints aside, he'd known the border as a featureless wall for centuries. The tiniest blemish was revolutionary; he felt like a child who'd just watched someone reach up and scratch a mark into the blue summer sky.

  “I'd say Branco has succeeded in pinning something to the near side.”

  “We have physics? We have rules now?”

  “Apparently.”

  Mariama said, “We're in the shuttle. Everyone's safe here.”

  “Good. No rush; the view is wonderful.”

  “I won't hold you to that. We'll be there in a few minutes.”

  The strange patch of brightness had moved out of sight, but after a few seconds another came into view. They were fuzzy-edged ellipses, traveling from the direction of the Scribe.

  “They're like the shadows of reef fish,” Tchicaya suggested. “Swimming above us in the sunlight.”

  Yann said, “Do you think you might be coming slightly unhinged?”

  As Tchicaya swung around him in their involuntary dance, he caught sight of the shuttle rising from the ruined Scribe. He smiled at the memory of Mariama's voice, promising to rescue him. On Turaev, if they'd given in to their feelings, it would have ended badly, burning out in a year or two. When this was over, though—

  Yann said, “That's a bit ominous.”

  “What?”

  “Can you turn your head back toward the Scribe? That might be quicker than me trying to put it into words.”

  Tchicaya twisted his neck. The border had formed a bellshaped hillock, forty or fifty meters high, that had completely swallowed the Scribe. As his rotation forced him to stretch even more, he stopped fighting it and twisted his neck the other way, hastening the sight's return instead of trying to delay its departure.

  The hillock was collapsing now, but as it did, a ring around it was rising up. Suddenly, Tchicaya noticed a whole series of lesser rings surrounding the first, like concentric ripples in water. They were undulating out from the center at great speed: the leading edge, the fastest component, in some kind of surface wave. The bulk of the wave was spreading more slowly. But it was still traveling faster than they were.

  He searched for the shuttle, and found it, its exhaust a pale blue streamer against the stars. The thrust generated by the ion engine was very low; over time it could accumulate into a significant velocity, but the craft was about as maneuverable as a bathtub on ice. It might just reach them before the wave, and even accelerate away from the border again in time, but there'd be no margin left for any more surprises that might manifest themselves in the wake of Branco's intervention.

  Yann read his mind, and declared flatly, “They have to stay clear.”

  Tchicaya nodded. “Mariama?”

  “No!” she hissed. “I know what you're going to say!”

  “It's all right. We're backed up, we're calm. Don't even think about it.”

  “It's a wave. It's a predictable phenomenon! I've computed a trajectory that meets all the constraints—”

  “Predictable?”

  “We can do it!”

  “You've all voted on that, have you? Tarek? Branco?”

  Branco replied laconically, “It's all the same to me.”

  Tarek said nothing, and Tchicaya felt a pang of sympathy for him. No one could reasonably expect him to put himself at risk, merely to spare his two adversaries the loss of their replaceable bodies and a few hours' memories. Yet if he did, many people would respect him for it. You had to be a utilitarian zealot, rotted to the core by dogma, not to admire someone who was willing to jeopardize their own comfort and continuity to preserve another's. Whether or not this required courage, at the very least it was an act of generosity.

  Tchicaya said, “Stay clear! We can't afford to lose the shuttle!” This argument made no sense—the Rindler's stock of raw materials had not been depleted, and there were parts of the ship itself that could be cannibalized anyway, if necessary—but he wanted to offer them an unselfish-sounding alibi. “You have to gather all the data you can,” he added, a little more cogently. “With the Scribe gone, every observation you can make is invaluable.” The Rindler itself had powerful instruments trained on the border, but some crucial detail might conceivably depend on the shuttle's proximity.

  Mariama did not reply immediately, but in the silence that followed Tchicaya knew that he'd swayed her.

  “All right.” Her voice was still strained, but there was a note Tchicaya recognized from their days on Turaev: a rare concession, not so much of defeat, as the realization that they'd been struggling over the wrong thing altogether. She understood the tradeoff, and she knew that he and Yann were resolved. “Peace, Tchicaya.”

  “Peace,” he replied.

  Yann said, “You handled that well.”

  “Thanks.” Over Yann's shoulder, Tchicaya could see the wave closing on them. It was dropping in height as it spread out from the point where the Scribe had been, but it wouldn't fall far enough to miss them. Tchicaya wondered if Yann would want to be distracted, or to confront what was happening directly.

  “So well that I almost hate to do this. How strong do you think your legs are?”

  “What?” It took a moment for Tchicaya to understand what he was suggesting. “Oh, no. Please—”

  “Don't go squeamish on me; we don't have time. It would be hard to decide who to save if we were from the same modes, but I can start from backup with no delay. You'd be out of the picture for months.”

  That was true. The Rindler had run out of bodies, and there were currently about twenty new arrivals waiting. Tchicaya would have to join the queue. Normally, a delay like that would mean nothing compared to the centuries he'd lost to transit insentience, but Branco's experiment had just guaranteed that every day from now on would be unique.

  “I've never killed anyone,” he said. His stomach was knotted with revulsion at the thought.

  Yann didn't quibble over the hyperbole. “And I've never died, in a body. Sex and death, all in one day. What more could an acorporeal ask for?”

  The wave came into view again; they'd have a minute or less. Tchicaya struggled to clear his head. Yann was demanding no more of him than he'd demanded of Mariama. The sense of shame and selfishness he felt, at the thought of indulging his own visceral urge to survive at Yann's expense, was the right thing to feel, but that didn't mean he had to elevate it above every other consideration. Nor, though, did he have to annihilate the emotion in order to act against it. He would do what the situation required, because it would be a foolish waste for both of them to lose their bodies, but he wasn't going to pretend that he was happy, or indifferent about it.

  He took hold of Yann's left hand, then released his iron gr
ip on his shoulder so they could join right hands as well. He folded his knees up against his chest, then froze. The crest of the wave was thirty meters away. This was too complicated. They'd never have time.

  Yann said calmly, “Give me your body. I've worked out the steps.”

  Tchicaya surrendered motor control, and they began to move together in a perfect, symmetrical ballet. It was as if his limbs had been gripped by a dozen firm, invisible hands, manipulating him without resistance. His back arched, his arms stretched painfully, but their fingers stayed tangled in a monkey grip as their legs forced their bodies apart, until their feet met, sole to sole.

  Tchicaya said, “You made me an isotopy.”

  Yann laughed. “Nothing original, I'm afraid.”

  “It's the thought that counts.”

  Tchicaya had become disoriented, but as they swung around together his line of sight fell from the stars to the approaching wave. The muscles in his legs tensed, and the pressure against his feet grew until he felt as if his arms would be torn from his shoulders.

  Yann said, “See you later.”

  Their fingers parted.

  Tchicaya clutched at the emptiness between them, then stopped himself and wrapped his arms across his chest. He was ascending at a shallow angle, back toward the point where the Scribe had been. As the crest approached, he curled into a ball, and it raced past beneath him, a flash of silver licking at his heels as he tumbled.

  An elaborate grid of colored lines scarred the inside of the retreating wave, like the map of some kind of convoluted maze. The pattern shifted as he watched. There was a tantalizing logic to the changes—the lines weren't dancing about at random—but deciphering it on the spot was beyond him. All he could do was record the sight.

  Drained for a moment of every other concern, Tchicaya locked his gaze on the retreating enigma.

  Everything had changed, now. Whatever Branco had revealed, or created, the wall between the worlds had finally been breached.