Грег Иган - Schild’s Ladder

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.

He watched the Planck worms as they reached the partition; this time, they appeared to be trapped. However many mutations were part of the throng, they couldn’t include an exhaustive catalog of all the possibilities. The toolkit was X-raying each gate and designing the perfect key as they approached; that strategy had to win out some of the time.

If not always by a wide margin. Tchicaya was just beginning to picture the Sarumpaet streaking ahead triumphantly, when the second barrier fell to the Planck worms.

He addressed the toolkit. "Is there anything we can throw in their way? Anything we can scribe that would act as an obstacle?"

"I could trigger the formation of a novel layer population. But that would take time, and it would only stretch across a single vendek cell." However long the artificial barrier held, the Planck worms would still percolate down along other routes.

They glided through a dozen more cells, maintaining a tenuous lead. Even when they appeared to be widening the gap, there was no guarantee that they wouldn’t plunge into a cell to find that the Planck worms had reached the same point more quickly by a different route.

The honeycomb stretched on relentlessly; the Sarumpaet gained and lost ground. After eight hours of nominal ship time, they’d crossed a thousand cells. In near-side terms, they were a millimeter beneath the point where the border had last rested, and the chase had gone on for mere picoseconds. The Planck worms had spent more than two hours diversifying before they’d learned to penetrate these catacombs, but having found the basic trick they appeared to be unstoppable. So much for the strategy of burning away one vendek population and the predators trapped within it; that would have been like trying to cure a victim of bubonic plague by sterilizing a single pustule.

Tchicaya said, "If this goes on for a hundred kilometers, I’m going to lose my mind."

"We could go into Slowdown," Mariama suggested. "We wouldn’t risk missing anything; the ship could bring us up to speed in an instant."

"I know. I’d rather not, though. It just feels wrong."

"Like sleeping on watch?"

"Yeah."

Three days later, Tchicaya gave in. The honeycomb could prove to be a centimeter thick, or a light-year; the probes could barely see half a micron ahead. They had no decisions to make; until something changed, all they were doing was waiting.