Schild's Ladder

by Greg Egan

  As he launched the last probe, he turned to Mariama. “If you gave me all the details of the work you did with Tarek, there'd be no need for you to hang around.”

  She emitted a disgusted wheezing noise, the first real sound he'd heard her make. “Is that some kind of childish comeback, because I didn't want to waste fuel on making you cozy?”

  “No. But I'm the one who came to the Rindler to protect the far side. There's no reason for you to keep crawling over broken glass for the sake of someone else's agenda.”

  Mariama searched his face. “You really don't trust me, do you?”

  “To do what? To betray your own ideals? You always wanted to wipe this thing out.”

  “I never thought that would involve genocide.”

  “We're still not certain that it would.”

  She sighed, bodily. “So you're afraid that if we find a natural explanation for the signaling layer, my presence might suddenly become embarrassing?”

  “Wouldn't it?”

  “I voted for the moratorium,” she said. “I voted to do nothing but look for signs of life, for a full year. Whatever happens, I'll honor that commitment.”

  Tchicaya experienced a twinge of shame, but he didn't back off. He said, “Make up your mind. Are you here to protect the far side? Or are you here to relaunch the Planck worms in a year's time, if the far side proves to be sterile?”

  Mariama shook her head. “Why do I have to choose? If there are sentient creatures in there, they deserve our protection. If there's nothing but an exotic ocean full of different kinds of Planck-scale algae, then the sooner it's rendered safely back into vacuum, the better. Is that distinction really so hard to grasp? What did I ever do to get lumped in with the rebels, in your head? When's the last time I displayed nineteenth-century morality?”

  “Twenty-third.”

  “That just shows how little history you know. Most people who left Earth in that era did so precisely because they were out of step with contemporary mores. In this case, I'd say they were about four centuries behind the times.”

  Tchicaya looked away. Was she protesting too much? But she was just as entitled as he was to be contemptuous of the anachronauts' views. Being wise after the fact about the complexity of the far side, and the unwitting genocide the Preservationists might have committed, was like blaming the Mimosans for failing to anticipate the failure of the Sarumpaet rules.

  The probes began returning. The Planck worms they revealed were dauntingly complex structures, at least as elaborate as the vendeks themselves. And Mariama had been right: they'd begun to mutate, to try out variations. The software counted thousands of strains.

  Even if they were capable of adaptation, though, they were too simple to achieve it through anything but trial and error. Their designer had left them to fend for themselves, and in the end that would leave them as vulnerable as any other dumb pathogen.

  Tchicaya addressed the toolkit, allowing Mariama to listen in. “Find a graph we can scribe that will wipe these things out—without moving deeper and damaging the native vendeks.” As he spoke the words, this sounded like a breathtakingly optimistic request, but the Planck worms themselves had been seeded from a single point, so there was no reason why the antidote couldn't be introduced the same way.

  There was a perceptible delay while the toolkit explored the problem. “I don't believe that's possible,” it declared. “The Planck worms are exploiting the ordinary vacuum behind them: they set up correlations across the border that cause the vendeks to decohere. I'm unable to find a method of attacking the Planck worms that wouldn't also destroy the whole vendek population in which they're immersed.”

  Mariama said, “What if the vendek population changes, deeper in?”

  “Anything might be possible then, but until I know the details, there are no guarantees.”

  Tchicaya scribed probes to look deeper.

  The second change swept the border as swiftly as the first. Through the windows of the shuttle, they saw the smooth gray plain transformed into a complex, striated pattern of dozens of bright hues. Tchicaya's heart raced; it was like watching a pool of acid eat its way down through featureless rock, exposing thousands of delicately layered sediments.

  Mariama said, “The border must be motionless again, or we'd see the pattern changing. So the Planck worms have hit more obstacles. We might have killed them off, if we'd burnt away this whole layer first.”

  “Including whatever it contained,” Tchicaya countered. “We have no idea what might have been there.”

  Mariama replied flatly, “Whatever was there, it's gone now anyway.”

  Tchicaya said nothing, but she was right. If he'd acted more swiftly, they might have cauterized the wound. If he was going to refuse to make decisions with imperfect knowledge, he might as well give up intervening and simply leave the far-siders to protect themselves.

  The Left Hand had launched fresh fireflies immediately, but he wasn't going to wait for them. He told the shuttle to follow them down, keeping just enough distance to be sure it could decelerate in time.

  The new border lay some sixty kilometers down, but its altitude was no longer constant; the shuttle came to a halt in the middle of a sinuous valley. The borderlight around them revealed the striations they'd seen from afar to be just one level of structure: the bands were crossed with networks of fine, dark lines, super-imposed over shifting waves of increased luminosity. And this was just the naked-eye view of a ravaged landscape, exposed to the vacuum and thick with alien marauders. What the pristine depths contained on a xennometer scale, Tchicaya couldn't begin to imagine, but between these macroscopic structures and the vendeks themselves, the opportunities for complex life were greater than ever.

  While they waited for the stylus to realign itself, Mariama said, “Can I ask the toolkit something?”

  Tchicaya nodded warily.

  “How complex an algorithm could you inject into the far side?” she said.

  The toolkit replied, “On what time scale? If you give me long enough, there are no limits.”

  “How long would it take to inject yourself?”

  “Scribing all the data directly with the Left Hand? About a hundred thousand years.”

  Mariama laughed in infrared. “What about other ways of doing it? What's the most efficient method that would be achievable with the hardware at our disposal?”

  The toolkit fell silent, conducting an exhaustive search.

  Tchicaya said, “What's this about?”

  “We're blind up here,” she replied. “All our time and effort is going into shuttling information back and forth across the border. Yann and the others have given you a lot of valuable knowledge, but the place where it needs to be applied is the far side.”

  The toolkit said, “I could scribe a series of graphs that would give rise to a far-side structure that would let me send data through the border as modulated light. That would take seventeen minutes. The total bandwidth would then be about one zettabyte per second. I could send myself through in a millisecond.”

  “In a form that could then travel deeper, away from the border?”

  “Possibly. I could wrap the basic quantum processor in a shell of motile vendeks. It still might not be able to survive in every environment it encountered, but it could send out probes to explore its surroundings, and it could tweak the vendek populations in the protective shell as it moved.”

  “What about communications with the near side?” Mariama asked.

  “I could try to maintain a shielded data cable back to the border, but the prospects for that look much poorer. The Planck worms are going to attack the border interface, and anything else that isn't moving faster than they are.”

  “Okay. But you could operate autonomously, once you were in there?”

  “Sure.”

  Tchicaya said, “You want to just drop it through and tell it to improvise from there?”

  “Why not? What's it up against? It's a lot smarter than the Planck wor
ms. It would know exactly what it was doing.”

  “On one level.” Tchicaya asked the toolkit, “How would you go about recognizing sentient life?”

  “I have no idea,” it admitted. “I have no information about that concept, beyond the rudimentary epistemological sketch that's stored in the conversational interface you're now addressing.”

  Tchicaya said, “I've spoken to cribs with more sense than that. We can't unleash it on the far side as a free agent.”

  Mariama closed her eyes. Clear fluid was spilling from fissures in her scalp and running down her face. She said, “My Exoself now tells me that this body's packing up. It thought it could repair itself, but there's too much damage. I'm afraid you're about to be stuck with a corpse.”

  Tchicaya reached over and took her hand, gently. “I'm sorry.”

  “It's all right,” she said. “I've never gone acorporeal before, but I'm not a fanatic. A few days without flesh won't kill me.” She smiled, splitting the skin on her face. “If you live long enough, you get to compromise on everything.”

  As Tchicaya watched, she let go of her body. Her breathing halted, and she slumped sideways. The flesh of her hand became rigid beneath his fingers; the individual cells had given up trying to maintain the integrity of the tissues they comprised, and had started to encyst, protecting themselves as best they could in case they were of any use for recycling.

  Tchicaya felt tears spilling down his face. “Fuck.” Mariama could no longer hear him; the IR link to her Mediator had worked via nerve and skin cells, and that was the only functioning route into her Qusp. She was deaf, dumb, and blind now, until he dug her out.

  He made his way to the shuttle's tool bin, and selected something long and sharp. Then he strapped himself into the seat beside her, to keep himself from being pushed away by the force he applied.

  Tchicaya knew that she was beyond harm, but he couldn't stop weeping as he cut into her flesh. He was not an acorporeal. He had never found a way to love her that entirely surrendered the notion that her body was the thing to cherish and protect.

  He got the three devices out: three small, dark spheres chained together with optical cables. The Mediator and the Exoself both bore a fuzz of fine gray wires that had tapped into the body's nervous system.

  Tchicaya consulted his own Mediator; it wasn't a great resource compared to the Rindler's library, but it knew all about its own design. Given a disembodied version of the same hardware, with the radio transceiver fried, how could he reestablish contact?

  His Mediator described the specialized hardware that could do this. The shuttle was carrying nothing even remotely similar.

  Tchicaya contemplated the bloodied parts in his hand. He'd asked her once to leave him, so he could complete this task alone. Now he appeared to have had his request granted.

  “There are no other ways to make contact?” he asked his Mediator.

  “Not if the device remains disembodied.”

  He couldn't grow her a new body from scratch; there was no time. And the cells of the old one had already done their best; they would not be coaxed back into operation.

  Tchicaya said, “What if it was inside someone else's flesh? Inside a body with another Mediator?”

  “Where, exactly?”

  “Where would it have to be?”

  “Inside the skull. Or very close to the spinal cord.”

  That was the solution, then. Tchicaya steeled himself. He still wasn't certain where her loyalties finally lay, but he was even less certain that he could go on without her.

  He stripped of his bloodied clothes, and peeled away his suit. Then he asked his Exoself to guide him. It knew the position of every nerve and blood vessel in his body, and it could move his hands with great precision.

  The stylus came into alignment with the border. Tchicaya launched a swarm of probes, then instructed the toolkit to start work automatically as soon as the echoes began returning: designing a replicator that would burn away all the current strains of Planck worms, whatever the cost to the vendeks around them.

  Mariama spoke. “What's happening?”

  Tchicaya said, “You're behind my right kidney. My nervous system's just managed to link up with your Mediator.”

  This revelation only fazed her for a moment. “I didn't even think about communication. That body failed so suddenly, I didn't have time to make plans.”

  “Are you okay?”

  “Absolutely.”

  “What are you simulating?”

  “Nothing, yet. I've just been thinking in the dark.”

  “Do you want to share my senses?” It was what he would have asked for, himself, if their roles had been reversed: anything to anchor his mind to reality, even if it was secondhand.

  Mariama hesitated. “I'd like access, thanks, but I'll make myself an icon with a viewpoint in a scape, and put your vision up on a screen. I don't want to start pretending that I'm inhabiting your body. Since I can't actually control it, that would just make me feel trapped.”

  “Right.” Tchicaya felt a frisson of anxiety, but the notion that he'd invited in a guest who could mount a coup was pure fantasy. Every connection between his nerve cells and her Mediator was entirely under the control of his Exoself; right down to the molecular level, this body would only take instructions from the matching hardware.

  “Keep talking while I do that,” she said. “What's the situation with the border?”

  Tchicaya brought her up to date.

  Mariama was puzzled. “You're not scribing the interface?”

  “What's the use?” he replied. “That would only tie up the stylus. We're better off trying to kill the Planck worms from the outside. That way, we can use their own trick against them: we can correlate them with the vacuum, make them decohere. It's a simpler problem. All we have to do is scribe something aggressive enough to take them on, but with a dead-end design that will fail completely at the next change of vendeks.”

  “You might be right,” she conceded. “I hope it is that simple.”

  Tchicaya looked out across the rainbow-hued landscape. Everything that happened here—all the destruction wrought by the Planck worms, and by their putative remedy—would spread out at the speed of light across the entire border. The vendeks' diversity seemed to have acted as an effective barrier so far, but there could be gaps in that defense, threads or channels of identical populations running deep into the far side. He was gambling on a dizzying scale, like some dilettante ecologist in Earth's colonial era, trying to balance one introduced predator against another.

  The toolkit spoke. “I'm afraid the Planck worms have been sneakier than I expected. The need to attack a new mix of vendeks hasn't filtered out any of the old mutations; they've all hitched a ride down with their successful cousins. So there are more than ten million different variants now. I can scribe seeds for individual replicators that would wipe out all of them, but that's going to take more than nine hours.”

  “Start doing that immediately,” Tchicaya said, “but also start thinking about a single seed that could do the same job.”

  The toolkit pondered his second request. “I can't see a way to do that without scribing something every bit as virulent as the Planck worms. It would have to mutate, itself, in order to deal with all the variants, and there's no guarantee that it wouldn't either burn out prematurely, or not at all.”

  Mariama said, “We can't count on nine hours at the border. And if it falls again before we've finished the job, the next time can only be harder.”

  “So what do you suggest?”

  “I've told you what I think we have to do,” she said.

  “Drop something through that can work from the inside? And I've told you what's wrong with that. There are no magic bullets so smart that you can fire them into an uncharted world and expect them to repel an invader without destroying everything they're meant to be saving.” He laughed bitterly. “It's hard enough believing that I can make those judgments myself.”

 
“I know. Which is why you need to start making them from the other side of the border.”

  Tchicaya had suspected that this was where she was heading, when death interrupted her train of thought. He'd hoped to render the whole idea superfluous before she got around to putting it into words.

  “You think I should send myself in?”

  “The data rate would be fast enough. Seventeen minutes to build the interface, then about an hour to get you through.”

  “And then what? All our strategies for dealing with the Planck worms rely on correlating them with the vacuum. You can't do that from the inside.”

  “So you look for other strategies,” Mariama insisted, “once you've gone deep enough to have a better idea of what's safe and what isn't. I'm not saying we should give up working from this side, but there are advantages to both. A two-pronged attack can only improve our chances.”

  Tchicaya had run out of arguments. He looked up at his reflection in the window, knowing she could see it. “I can't do this alone,” he said. “I can't go in there without you.”

  He waited for some scathing rebuke. This was even more self-indulgent than demanding that she pluck him from the vacuum, when he should have been willing to drift stoically into oblivion. The worst of it was, he still harbored doubts about her. How many chances to rid himself of her presence was he going to turn down?

  Mariama said, “Joined at the hip, after four thousand years?”

  “Joined at the kidney.”

  “I take it you won't let me go in by myself?”

  “No. Think of this as extending the old protocols for the Scribe. There always had to be an observer from the other faction, to keep everyone honest.”

  Tchicaya tried to keep his voice lighthearted, but this felt like the final recognition of the way it was between them. He had always followed her, every step of the way. Out of Slowdown, away from Turaev. Even in the centuries they'd spent apart, his own travels, his own adventures, had only seemed possible once she'd blazed the trail. He was not ashamed of this, but he wished he'd faced it squarely much sooner. He wished he'd told Rasmah, when the rebels first showed their hand: I am not the one to leave behind here. You head for the shuttle, I'll head for the hub. Anyone can toss saboteurs from the scaffolding. But not everyone could walk into the far side alone.