Schild's Ladder

by Greg Egan

  “What do we have to do to get through to them?”

  Mariama said, “We could push ahead with the mathematics leading to the GDL.”

  “Just like that? As a monologue?”

  “What choice do we have?”

  The toolkit had developed a Graph Description Language, a precise set of semantic conventions for talking about vendeks, Planck worms, and what would happen when they met. Given some moderately sophisticated mathematical concepts—which could be built up from elementary ideas based on integer exemplars—quantum graphs were far easier to talk about than anything as abstract and contingent as social structures.

  If the Colonists weren't going to degin to reply, though, there'd be no way of knowing if the dictionary of concepts was coming at them too quickly, or even whether the basic syntax was being understood. They manipulated vendeks with skills that no QGT theoretician would dare aspire to, but that didn't mean they understood them in the same way. Humans had tamed and modified dozens of species of plants and animals before they'd had the slightest idea what DNA was.

  Tchicaya started the program running. Without feedback along the lines of “Yes, we understood that, please skip ahead to something ten times harder,” it would take four ship-days to complete. He could choose sections to omit, himself—but which ones? What concepts were obvious to a xennobe?

  Mariama smiled tentatively. “They haven't left the room yet.”

  “It is an alien artifact. That in itself must merit some level of attention.”

  “They chose the primes,” she said. “They picked the language, and it was exactly what we would have picked ourselves.”

  Tchicaya scanned the room. “We're missing something here.” The Colonists had no faces, no eyes, and he had no way of telling what they were attending to, but they were far better positioned to observe the nucleon nugget than the banner.

  He said, “They're showing it the banner. They're not even trying to make sense of the message themselves. They expect their meteorite to react.”

  Mariama was skeptical, but not dismissive. “Why would they think that way? Some kind of category error? They're intelligent enough to figure out that both these things came from the near side, but they have no concept of inanimacy? Because...everything here is living?” She grimaced. “Are you going to stop me before I start talking complete gibberish? Whether vendeks count as living or not, random collections of them would make very bad translators between xennobe languages.”

  Tchicaya said, “So are the Colonists suffering from animist delusions, or is this not a random collection of nucleons?” He addressed the toolkit. “Can you make any sense of its structure? What are the odds that nuclear matter in a star or a planet could be in a state that could come through the border like this?”

  “Negligible.”

  “So someone wrapped it? Someone prepared it deliberately?”

  The toolkit said, “That's more likely than it happening by chance.”

  Mariama said, “Don't look at me. Maybe someone was running their own secret experiments, but this was not a Preservationist project.”

  “Then whose was it? And what has it been doing down here?” Tchicaya asked the toolkit, “Can you model its dynamics? Is there information processing going on in there?”

  The toolkit was silent for a moment. “No. But there could have been, once. It looks to me like it started out as a femtomachine.”

  Gooseflesh rose on Tchicaya's arms. Back on the Rindler, comparing their varied experiences of local death, Yann had definitively trumped him with tales of going nuclear.

  He said, “It's the Mimosans. They're buried in there.”

  Mariama's eyes widened. “They can't be. The Quietener blew up in their faces, Tchicaya. How much warning would they have had?”

  Tchicaya shook his head. “I don't know how they did it, but we've got to look for them.” He asked the toolkit, “Can you map the whole thing? Can you simulate it?” The crushed femtomachine was vastly larger than the Sarumpaet, but having started from merely nuclear densities, it would have made far less efficient use of its graphs.

  The toolkit said, “I'll try. It will take time to get the information out; the probes can only move it at a certain rate.”

  They waited. The mathematics lesson played on through the banner; the Colonists floated in place, patient as ever, expecting...what? The femtomachine had talked to them, once. It must have functioned long enough for its inhabitants to learn their language. Had it told them to make the signaling layer? Or had it commenced its own attempts to communicate with a sequence of primes, which they'd gone on to copy?

  After almost an hour, the toolkit declared, “I have a complete model of the structure inside the Sarumpaet. Now I'm trying to repair some of the damage.” It juggled connections, looking for gaps in information routes; it searched for redundancies that would allow it to reconstruct the missing pathways.

  “There's a simulation of something resembling a primate body. With standard representation hooks into the model.”

  “Show us,” Tchicaya said.

  A person appeared on the deck in front of them, standing motionless, arms raised as if in defense against a blow, or an impact. The body did not resemble anything Tchicaya had inhabited himself, but it was a piece of software that made no sense unless the femtomachine had contained a sentient inhabitant.

  “Can you trace back the sensory and motor hooks?”

  “I'm trying. Okay. I've found it.”

  “You've found the mind?”

  “Yes.”

  “What kind of state is it in?”

  “Wait. I'm computing integrity signatures.” Sentient software was always packed with check sums that would allow it to detect whether it had been corrupted. “Not scrambled, just frozen. Most of the physics that leaked in seems to have slowed down the strong force interactions, rather than damaging the quarks and gluons.”

  Tchicaya said, “Can you run it? Can you wake it?” He was shaking. He didn't know if he was digging a tenacious survivor out from beneath a rock slide, or breathing unwelcome life back into a mutilated castaway who'd escaped into a merciful local death. Too much was at stake, though, to let the Mimosan rest in peace until he learned the answer for himself.

  The simulation twitched, looked around the scape, then dropped to its knees, sobbing wretchedly. “I'm going mad! I'm going mad!” The body being simulated had been designed to function in vacuum; it was even pretending to speak in infrared.

  Tchicaya understood the words as they were spoken; his Me diator had turned the data into sounds in his head, and granted him the survivor's language immediately.

  He knelt beside her and wrapped his arms around her shoulders. “You're not going mad, Cass. We're real. You're not home yet, but you're very close now. And you're among friends.”

  Chapter 18

  Time was everything, and Tchicaya felt a streak of brutal pragmatism demanding that he press their only hope of a translator into service as rapidly as possible. It would be a false compassion that ended with all of them dead. But though Cass was undoubtedly sane, and increasingly lucid, she was still in shock. Before she could help them, she needed to make sense of her own situation.

  Tchicaya told her about the signaling layer, and how the Sarumpaet had been led to this place. He said nothing about the Planck worms; he and Mariama were explorers from the near side, that was all that mattered for now. He invited Cass to complete the account, to bridge the gap between the events at Mimosa and this extraordinary meeting. Seated on a couch they'd conjured up for her, she told them some of the history of her voyage.

  For the last of their experiments on the novo-vacuum, the Mimosans had sent clones into a femtomachine, in order to be closer to the event in real time. They had seen the nascent border expanding, and struggled to understand their mistake. In one branch of the femtomachine's uncontrolled superposition, they had reached Sophus's insight: the physics of the ordinary vacuum represented just one eigenstate for a quan
tum graph's dynamic laws.

  Working from that starting point, they had devised a plan to spare the inhabited worlds from destruction. By modifying the border so as to make the emission of light sufficiently asymmetrical, the difference in radiation pressure could be used to accelerate the whole system. While the far side remained small, its mass as an object in the near side would be tiny (in fact, tiny and negative, since it had started at zero and lost energy as radiation). If it was left to others to tackle the problem decades later, the far side would have swallowed entire star systems—at the very least, Mimosa itself. If they acted now, they could send it flying out of inhabited space even faster than it was expanding.

  When the border hit the femtomachine, they would have a chance to interact with it, but no fleeting, localized encounter would be sufficient to sculpt the borderlight into a propulsion system. They needed to buy themselves more time. Matching the border's velocity would have been ideal, but there was no prospect of achieving that. Their only hope was to find a way to keep working on the problem after the far side had swallowed them.

  The Mimosans had choreographed a bravura quantum maneuver that would allow the femtomachine to inject a partial clone of itself through the border, and rotate all of its amplitude into the successful branch at the same time. But the passengers couldn't all pass through. The bulk of the femtomachine would have to become a device whose sole purpose was to implement the move, and only the acorporeals were structured in a way that gave them the power to rewrite their minds right out of existence, converting themselves into pieces of the quantum catapult. All seven had been needed, to make it work. Cass had been left to go in alone.

  The first part of the plan had succeeded: the core of the original femtomachine had been re-created, in miniature, in the far side. But it had not been as mobile as its designers had hoped, and Cass had been trapped by changing conditions, hundreds of times. She had kept struggling to get the Oppenheimer into position, proceeding in fits and starts, but the vehicle's hull had become compromised, vendeks had flooded in.

  If this had happened in the ferment of the Bright, Tchicaya doubted that any trace of the crippled machine would have remained a picosecond later, but the massed invasion by a single, tenacious species had effectively fossilized it whole. An unknown time later—near-side decades, or centuries—a group of intelligent xennobes had found the wreck. Subject to the same infestation themselves, they had revived the Oppenheimer with a vendek bred specifically to reverse the effects of the first.

  Awake, but still trapped—nothing could remedy the fact that her vehicle was too primitive for the constantly evolving terrain—Cass had begun trying to communicate with her benefactors. Her own first message had taken the form of a layer population, vibrating, counting out the primes. From there, it had been a long, arduous process, but they'd eventually reached a point of limited mutual understanding.

  Then the xennobes had vanished, prey to some shift in climate or culture; she had never discovered the reason. After decades had passed, another, related group had appeared, aware of the previous encounter, but speaking a different language themselves, and too impatient to learn to communicate properly. They had tried to carry her toward the border—knowing that this had been her original goal—without really understanding her nature. Moving anything through the far side was a delicate process, and their technology had not been up to the task. The Oppenheimer had become trapped again, damaged again. Invaded, frozen, and abandoned.

  That was her last experience before waking on the deck of the Sarumpaet. She had no way of knowing whether the Oppenheimer had been towed here by the builders of the city, or whether the city had grown up around it.

  Tchicaya was humbled; everything he'd been through was a stroll in the desert by comparison. He couldn't even offer her the comfort of hearing that her own failed mission had been completed from the outside.

  But he had to press on. As gently as he could, he began explaining what had happened on the near side. Cass had long ago faced up to the likelihood that her actions had destroyed whole worlds, but she'd had no way of knowing how much time had passed, and he could see the wounds reopening as he described the numbers, the scale of the evacuation.

  He compressed the machinations of the factions on the Rindler to the briefest sketch, but he made one thing clear: the vast majority of people had never intended to destroy sentient life in the far side. Most still wanted the incursion to be halted, but not at the cost of genocide.

  For all the bad news that accompanied it, understanding the Sarumpaet's presence seemed to solidify Cass's sense of reality. She could connect herself to the near side again. She could imagine something other than exile, and madness.

  When Tchicaya finished speaking, she stood. “You want them to evacuate the Bright, so you can trap the Planck worms there?”

  “Yes.”

  “And you'd like me to translate that message?”

  “If you can.”

  “I'll need to be able to create vendeks,” Cass explained. She had invented her own terminology for everything, but Tchicaya's Mediator was smoothing over the differences. “I don't understand the perceptual physiology, but there's a family of short-lived vendeks related to the parasprites that my first xennobe tribe employed for communication. Though what their descendants will make of any of this, I don't know.”

  Mariama worked with the toolkit to sort out interfaces with the software Cass had used back on the Oppenheimer to create the communications vendeks. While this was happening, Tchicaya rehearsed scenarios with her, possible responses from the Colonists. He wasn't entirely sure why she wanted this, but she appeared to be afraid of being caught out, unprepared.

  “Everything's ready,” Mariama declared. “As much as it will ever be.”

  They moved the Sarumpaet right up to the ruins of the Oppenheimer. The Colonists were still patiently looking on as the banner flashed out its mathematical lexicon.

  Cass said, “I hope they really are expecting this. If I waved a papyrus at Tutankhamen and he started speaking to me, I'd probably run screaming from the room and never come back.”

  She sent the first vendeks out from the ship.

  The scape painted a burst of color spreading out around them, fading rapidly as it moved. These vendeks did not last long in the room's environment; to Tchicaya's eyes, the signal looked faint by the time it reached the Colonists.

  It was not too faint for them to notice. They sprang into action, gathering more equipment. If the Bright had made them feign constant excitement, this was the real thing; Tchicaya hadn't seen their bodies convulse so much since they'd descended from the surface of the outpost.

  Reassembled in a huddle, armed with their additional machinery—recording devices, translators?—they finally found a reason to talk back.

  Tchicaya wasn't privy to the exchange. Cass didn't talk aloud in her own native language, offering up sentences for direct translation, nor was there any running translation of the replies. She had never got far enough to integrate the xennobe language into the usual, Mediator-based scheme of things; she was working from her own mental dictionary of signals, memories of past conversations, brute-force software assistance, and guesswork. She made gestures with her body, frowned to herself, and emitted grunts and sighs, but most of the action was going on inside her simulated skull.

  After almost twenty minutes, she paused to give the two spectators a brief commentary. “They expected me to speak in an ancient language, but they weren't quite sure which one it would be. We've sorted that all out now.” She looked ragged, but she smiled.

  Tchicaya was about to launch into a stream of lavish praise, but Mariama replied calmly, “That's good.”

  Cass nodded. “I think they trust me, more or less. At least they're willing to listen.”

  She resumed the conversation. Vendeks washed back and forth between the Colonists and the flea masquerading as a resurrected mummy.

  More than four hours after the exchange had begun
, Cass sat down on the deck and cradled her head in her arms. Three of the Colonists left the chamber.

  Tchicaya waited. There'd be a reason for the hiatus: the Colonists were fetching another language expert, another translation device, a better dictionary.

  Cass looked up suddenly, as if she'd completely forgotten that she was no longer alone.

  “It's done,” she said. “They understood me.”

  The Bright itself was of little value to their hosts, she explained, but it did contain several outposts from which they'd been attempting to learn more about whatever lay beyond. They hadn't constructed the signaling layer; they'd heard stories about the artifact, which had supposedly been built by an earlier civilization, but they had never had the means to verify its existence. They couldn't quite comprehend the nature of the threat she had described, but they did believe that she came from the outer reaches, and they had decided that they had nothing to lose by erring on the side of caution.

  They would permit the creation of the tar pit. They would begin evacuating the Bright immediately.

  The Sarumpaet rode the highway loop back into the Bright, escorted by Tännsjö and Hintikka—Cass's names for two of the Colonists who'd traveled down from the outpost with the banner. She'd explained to them that she'd moved from the wreck of her old vehicle into this new, smaller model, brought here by two colleagues who'd traveled all the way from her home; they found many aspects of this account baffling, but didn't expect to make sense of it until they'd learned much more. The legends about her had been full of obvious nonsense that they'd hoped to dispel, but they were patient, and they could wait for a more complete understanding.

  “Do they know you're their creator?” Mariama asked.

  Cass snorted. “That would be an overblown claim for me to make, when I didn't have the slightest idea what I was creating. But I haven't told them anything about Mimosa. All I've ever said is that I came into their world to try to keep it from colliding with my own.”