Schild's Ladder (32 page)

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
.

Mariama said, “All right, I'll go with you. We can keep each other honest. But the process has to be set up so it doesn't jeopardize everything. If the border starts falling while only one of us is through, the vehicle will have to be programmed to interrupt the transfer, and dive without the second passenger.”

“That makes sense,” Tchicaya conceded.

“Which only leaves one thing to be decided.”

“What's that?”

“Who goes first.”

Chapter 15

Tchicaya looked out from the
Sarumpaet
into a lime-green sea. In the distance, glistening partitions, reminiscent of the algal membranes that formed the cages in some aquatic zoos, swayed back and forth gently, as if in time to mysterious currents. Behind each barrier the sea changed color abruptly, the green giving way to other bright hues, like a fastidiously segregated display of bioluminescent plankton.

The far side here was a honeycomb of different vendek populations, occupying cells about a micron wide. The boundaries between adjoining cells all vibrated like self-playing drums; none were counting out prime numbers, but some of the more complex rhythms made it seem almost plausible that the signaling layer had been nothing but a natural fluke. Even if that were true, though, Tchicaya doubted that it warranted relief at the diminished prospect that sentient life was at stake. The signaling layer might have brought him this far, but with millions of unexplored cubic light-years beneath him, judging the whole far side on that basis would be like writing off any possibility of extraterrestrial life because the constellations weren't actually animals in the sky.

The view he was looking at was a construct, albeit an honest one. The
Sarumpaet
was constantly “illuminating” its surroundings with probes, but they were more like spy insects than photons, and they had to return in person with the details of everything they'd encountered, rather than radioing back images from afar. His body, the vehicle itself—a transparent bubble like a scaled-down version of the
Rindler
's observation module, with an added checkerboard of windows in the floor—and the gravity he felt, were all pure fiction.

He turned to Mariama's icon-in-waiting, complete up to the shoulders now. Her body was rendered as a transparent container, slowly filling with color and solidity from a trickle of light flowing down through a glassy pipe that ran all the way to the border. Tchicaya looked up along the pipe to the roiling layer of Planck worms, inky violets and blacks against the cheerful false pastels of the vendeks. Every few seconds, a dark thread would snake down toward him, like a tentacle of malignant tar invading a universe of fruit juice. So far, the vendeks had always responded by pinching off the thread and extinguishing the intruders. The
Sarumpaet
avoided sharing this fate by wrapping itself in a coat that mimicked the stable layers it saw around it, but though the Planck worms could only hope to achieve the same kind of immunity by stumbling on it blindly, once they did, they'd put it to a far less benign use.

Tchicaya was running his own private Slowdown, to keep the wait from being unbearable; the Planck-scale quantum gates of the
Sarumpaet
could have made the hour stretch out into an eternity. The toolkit was using its enhanced speed to broaden its search for new strategies, though as yet this had yielded nothing promising. The ten million individual Planck-worm-killers it had designed on the near side could have been scribed here in a fraction of a microsecond instead of the original nine hours, but most of them would have consumed the
Sarumpaet
itself in an instant. Tchicaya would not have minded mimicking the anachronauts and going out in his own blaze of glory, but only if he was unleashing a fire that was certain to be both effective and self-limiting.

Mariama was beginning to develop a chin. Tchicaya asked the icon if it was representing the proportion of data received through volume, or height.

“Volume.”

The crisp image of her body began to soften, but it was the scape's lighting that was changing, not the icon itself. Tchicaya looked up to see a dark, fist-shaped protuberance pushing its way through the vendeks. An instinct from another era tensed every muscle in his simulated body, but he wouldn't need to make a split-second decision, let alone act on it physically; the
Sarumpaet
itself would determine when it had to flee. Dropping out of Slowdown to monitor events at a glacial pace would only be masochistic; he would speed up automatically as soon as the flight began.

The infestation of Planck worms spread out like a thundercloud. As the dark layer brushed the tube that represented the link across the border, the
Sarumpaet
launched itself down into the far side.

The single, brooding cloud exploded into a storm of obsidian, rushing toward the ship like a pyroclastic flow. Tchicaya had sprinted down the slopes of a volcano on Peldan, racing hot gas and ash, but the effortless speed of the
Sarumpaet
made this dash for safety even more nerve-wracking. The risk of being overtaken on foot was only to be expected, but the ship's pattern of data was propagating at close to the maximum rate the environment permitted. There was no such thing as lightspeed here, but he was nudging a barrier that was just as insurmountable.

As he glanced down, he saw that the visibility had diminished; the probes were traveling as far ahead as ever, but the
Sarumpaet
was racing forward to meet them. The toolkit would still have the crucial information it needed to adapt the ship's harnessed vendeks to changes in the environment, but the faster they fled, the less time it would have to cope with any surprises.

The first boundary was almost upon them, but they'd probed this one thoroughly in advance. As the ship crossed through the glistening membrane—an act portrayed as a simple mechanical feat, but which amounted to redesigning and rebuilding the entire hull—a motion within the scape caught Tchicaya's eye.

Mariama turned to him with a triumphant smile. “That's what I call an amphibious vehicle: glides smoothly from microverse to microverse, whatever their dynamic spectra.”

He stared at her. “You weren't—”

“Complete? Ninety-three percent should be good enough. I packaged myself very carefully; don't take that decapitated progress icon literally.” She looked up. “Oh, shit. That wasn't meant to happen.”

Tchicaya followed her gaze. The Planck worms had already crossed the boundary. Some freeloading mutation, useless against the earlier obstacles, must have finally proven its worth. Their adversary was not dispersing, weakening as it spread; it was like an avalanche, constantly building in strength. If the Planck worms retained every tool they tried out, whether or not it was immediately successful, their range of options would be growing at an exponential rate.

“You have to hand it to Birago,” Mariama observed begrudgingly. “The killer twist was his, not Tarek's or mine. We were too hung up on the notion of mimicking natural replicators—as if nature ever made plagues that were optimized for destroying anything.”

“Humans did. He might have had some tips from the anchronauts.”

They crossed into another cell of the honeycomb, as smoothly as before. Tchicaya wasn't entirely sure what would happen if the
Sarumpaet
failed to negotiate a population transition, but whether it was the Planck worms or some hostile strain of vendeks that rushed in and consumed them, they wouldn't have much time to dwell on their fate before they blinked out of existence. As local deaths went, he'd had worse.