Schild's Ladder

by Greg Egan

  The toolkit had found no certain way to prevent this, but it was studying one possibility. It looked as if it might be feasible to transform the whole region into a kind of tar pit, deep enough to trap and drown every last species of Planck worm. The worms acted as conduits for correlations with the vacuum, but not every interaction with them induced decoherence. The honeycomb vendeks had made short work of some of the earlier would-be invaders, and a sufficiently diverse mixture of vendeks, tailor-made for the purpose, would have a chance of dealing with the entire current wave in the same fashion.

  Along with every native inhabitant of the Bright.

  “Would you sacrifice all of this,” he asked Mariama, “to save whatever lies beneath it?”

  She said, “Ask me that again when we know ten times more.”

  Tchicaya shook his head. “That's always going to be the right answer. Until it's too late for anything we do to make a difference.” The toolkit's simulation was riddled with uncertainties, but to the extent that the risk could be quantified at all, within a few ship days it would cease to be insignificant.

  “Don't be so pessimistic,” she countered. “Don't assume that we're going to have to choose between utter recklessness and some paralyzing quest for perfect knowledge.”

  “Perfect knowledge? There could be a billion times as many sentient beings beneath us as the rest of the galaxy has ever contained, or we might already be looking at the pinnacle of far-side life—which might be a miracle of xennobiology but dumb as a cactus, or might be conscious in ways we're too stupid and parochial to fathom. How do you cope with that kind of ignorance?” Dwelling on it was enough to make his faithfully simulated body sick to the stomach. Part of him screamed that the only thing to do in the face of such barely comprehensible stakes was to bow out, to withdraw from any possibility of intervention—as if showing the appropriate humility was more important than the outcome.

  But Mariama refused to be cowed by the gravity of the situation. “We keep exploring,” she insisted. “We keep narrowing the gap between what we know and what we need to know.”

  “What I need to know is when we have no choice but to stop gathering information and make a stand.”

  Tchicaya gazed into the strange machinery of the airflower. This creature was a thousand times more sophisticated than anything that had been found away from Earth before, but if the signaling layer was an artifact at all, he did not believe that he was looking at its maker.

  He said, “We need to go deeper.”

  With the refinements to its hull, the Sarumpaet traveled faster. For half a day they were alone in the Bright again, but then they began to spot more of the airflowers. The sightings became more frequent as they descended; at first they were seeing one or two an hour, but it soon reached the point where half a dozen were always in view.

  Mariama suggested that they try to follow the path of the migration back to its source. “That could lead nowhere, but it's the only clue we've got as to where other life might be concentrated.”

  This made sense to Tchicaya. They moved the ship closer to the airflowers, and descended along the sparse trail.

  Within an hour, the creatures were crowded around the Sarumpaet like coral spawn. When the toolkit probed the Bright itself, it appeared that the airflowers had latched on to a particularly stable current of vendeks; if this broke apart higher up, the specimens they'd encountered earlier might have pursued it as far as it went, and then scattered. The current was useless for transportation—you couldn't ride it like a thermal updraft, in a world without conservation of momentum—but whether the airflowers were using it as a navigation aid, as a feature to congregate around for breeding purposes, or merely as something to graze upon was impossible to say. The vendeks certainly diffused into the airflowers' bodies, but they still might have been anything from valuable symbionts, sought out by their hosts, to burdensome parasites that came with the territory.

  “Can vendeks ever really be prey?” Tchicaya wondered. “They're the smallest stable objects, so there's no point seeking them out just to break them down into their constituent parts.”

  Mariama said, “There are no subunits that you can extract from them and treat as nutrients—nothing analogous to vitamins or amino acids—so when you eat for the sake of eating, you're infecting yourself. All food works like yogurt. But that doesn't mean that the only reason to seek out a particular kind of vendek would be to give it a new home. Nothing that crosses your path is going to move aside for you automatically here, so you have no choice but to convert whatever you encounter into a part of yourself. Sometimes the vendeks around you can be incorporated unchanged, but other times you need to have your own tame vendeks invade the graph ahead of you, chewing up whatever's there as they propagate through—in which case, you want them to be taking on adversaries that they can conquer easily, even if you're not planning to pillage the corpses for specific spare parts. Whether you call that predation or not is a moot point.” She smiled. “Assuming that all this talk about larger organisms makes sense at all, and we're not just watching a few vendeks traveling in packs, lording it over the rest.”

  “I wish you hadn't said that.” Tchicaya already found it eerie enough contemplating the identity of these xennobes. Humans had been nothing but a colony of specialized cells, but at least those cells had all been related to each other, and subdued to the point where they could pursue a common genetic goal. In the airflowers, there seemed to be as many vendeks plucked into service from the surroundings as there were specialized ones that appeared only in the creatures' tissues.

  “What's that?” Mariama had spotted something through the floor. She gestured impatiently to the scape, transforming the checkerboard beneath their feet into a completely transparent surface.

  A dark shape was spiraling up around the column of airflowers, a sprite-shadow that the probes were yet to fill in. Seconds later, it began to take on details, the colors shifting wildly as the scape improvised palettes to encode the information, then judged them inadequate and started again from scratch.

  The probe image showed a dense, branched network of tubes filled with specialized vendeks, cloaked in a more complex version of the eddies that wrapped the airflowers. The tube walls were layer populations, but they extended fine tendrils out into the trapped currents of the Bright. Controlling them? Feeding off them? The scape was unable to track all the dynamics; too much was happening for the probes to capture it all, and many of them were being captured themselves, lost among the vendeks they'd been sent to map.

  The new xennobe was ten or twelve times larger than a typical airflower. As it soared past the Sarumpaet, Tchicaya instructed the ship to follow it. Going into reverse was disturbingly easy; the only thing resembling inertia that the ship possessed was the precise distribution of the hull vendeks that chewed their way through the Bright.

  When they caught up with the xennobe, it was circling the airflowers closely, moving in on one target. As it struck, the probes showed the two cloaks of entrained Bright vendeks merging; it was impossible to tell if the airflower's covering had been stripped away or whether the creature pursuing it had deliberately exposed its own inner organs. As the process continued, though, neither party remained shielded from the other. Veins became entangled, endogenous vendeks flowed between the two. The airflower had made no attempt to flee, so it was either insensate, too slow, or a willing participant in the exchange.

  Tchicaya said, “I don't know if I'm watching a wolf tearing open a lamb's throat, or a hummingbird drinking nectar.”

  “It might even be sex,” Mariama suggested.

  “Urk. I've heard of dimorphism, but that would be ridiculous. Besides, what are the gametes they're meant to be exchanging?”

  “Who said anything about gametes? The mix of specialized vendeks inside the xennobes must control all their morphology. Animals share beneficial symbionts with each other, and pass them on to their young—but in this case, there's nothing else to pass on. Instead of
having a genome, your heritable traits are defined by a unique blend of gut flora.”

  When the larger xennobe moved away from the airflower to which it had attached itself, and the remnant disintegrated into random currents in the Bright, Tchicaya said, “Wolf and lamb it is—or maybe rabbit and lettuce. And don't start reminding me about male spiders that die after mating; if there's no genome and no gametes, why call one creature a sexual partner of another, when at most it's really just a specialized dietary supplement?”

  Mariama conceded the point, begrudgingly. “So do we follow the rabbit?” It had moved up along the column, outpacing the airflowers, apparently finicky about its next choice of meal.

  Tchicaya glanced after it, then he looked down along the plume of airflowers vanishing into the haze. As much as anything, he wanted to know where the Bright ended. “Follow the food chain to the top of the pyramid? Or is that just naive?”

  “There's no energy here,” Mariama mused, “but there might be a hierarchy of concentrations of the most useful vendeks. Maybe airflowers strain some valuable species from the winds, or make them for themselves, and everyone else steals them from each other.”

  “Or goes straight to the airflowers. The Signalers could be herbivores, not rabbit hunters.”

  “That's true.”

  Tchicaya sent the ship in pursuit of the rabbit. When they finally caught it between meals, he unfurled the signaling device.

  The rabbit froze in midflight. When the sequence was completed, it remained motionless.

  Tchicaya waited hopefully for some kind of response. “Do you think we've frightened it?”

  “It might just be wondering how to reply,” Mariama suggested. “Some encounters must put you on the spot, even when you're half-expecting them. Like your father, cornered by anachronauts.”

  “I hope it's not trying to decide how to Mead us. But why would it need to lie, when it knows nothing about our expectations?”

  “Maybe the airflowers are sentient, too,” she joked, “and we caught it doing something that it senses we might not entirely approve of.”

  After fifteen minutes with no change, Mariama suggested repeating the sequence. Tchicaya started the banner flickering again.

  The probes showed a series of topological changes spreading rapidly through the rabbit's plumbing. The process was too fast to follow in detail, but it culminated in the release of a rich brew of vendeks from deep within the rabbit's body. Most of the discharge flowed over the banner, but the portion that reached the Sarumpaet's hull worked its way all around the ship, blocking out probes and sprites alike. The last thing the scape portrayed was the rabbit fleeing into the Bright.

  Tchicaya addressed the toolkit. “What's happening? Is the hull intact?”

  “It hasn't been breached, but it's not going to take us anywhere for a while. The foreign mixture has invaded a short distance, but it's not aggressively replicating or advancing.”

  “Can't you tweak the hull vendeks to break through?”

  “I'm looking for ways to do that, but this mixture seems to have been optimized to make the problem as difficult as possible.”

  Mariama started laughing. “This is what you get for flashing your Rosetta stone at randomly chosen strangers. They glue you to the spot and run away.”

  “Do you really think that was more than a frightened animal?”

  She shrugged. “Wouldn't it be wonderful if it was a shy cousin of the Signalers, out plucking fruit, who'll run home and tell the rest of the clan to come and take a look? But you're right; it was probably just a squid spraying ink in our faces.”

  They waited for the toolkit to find a way out. If the situation became desperate they could always try the superposition trick again, but the fact that they were hemmed in on all sides would complicate the maneuver: they'd have to leave part of the ship behind to clean up the failures of the part that escaped.

  After almost two hours, the toolkit spoke. “We should be free soon.”

  Tchicaya was relieved. “You found vendeks for the hull that could invade through the glue?”

  “No, but the weather is doing the job for us, from the outside. The glue is moderately stable, but it's not taking any kind of action to remain impervious to changing conditions in the Bright.”

  Mariama made a sound that was equal parts delight at this revelation, and disgust at her own slowness. “Of course! Anything static is doomed here. Stable mixtures of vendeks can endure for a while, but in the long run you need all the flexibility and organizational powers of a higher organism, just to keep up with the Bright. An entire xennobe might have managed to cling on to us indefinitely, but it would be a bit much to have to give birth to a dedicated assassin every time someone frightens you.”

  Tchicaya nodded appreciatively. “That must make technology difficult to get started. Vendeks are the material from which everything is made, so all engineering is bioengineering, but you probably couldn't expect any artifact less sophisticated than the most primitive xennobe to survive for long.”

  A crack of sprite-light appeared through the glue. Mariama sighed wistfully and leaned against him, wrapping an arm around his neck. It was the kind of unself-conscious physicality she'd often displayed when they were very young, before they'd even heard of sex.

  She said, “Don't you wish we could have come here with nothing to do but understand this place?”

  “Yes.” Tchicaya felt no desire whatsoever to add a retort about her old allegiances. The factions belonged to another universe.

  “For a thousand years.”

  “Yes.” He put his arm across her shoulders.

  Mariama turned to him. “Can I ask you something?”

  “Sure.”

  “Do you think you would have traveled to the Rindler at all, if it wasn't for the power station?”

  “I don't know. I can't answer that.”

  “But you still feel bad about it?”

  Tchicaya laughed curtly. “It's not relentless crushing guilt, if that's what you mean. But I knew it was wrong, even when I did it, and I haven't changed my mind about that.”

  She said, “You know, I actually expected you to be grateful, because you got what you wanted. That's the last time I made that mistake with anyone.”

  “I bet it was. Ouch!” She'd punched him on the arm.

  “But you just blamed me for everything, because I didn't fight hard enough against you.”

  “I didn't blame you,” he protested.

  Mariama gazed back at him neutrally.

  Tchicaya said, “All right, I did. That was unfair.”

  “You made me feel like a murderer,” she said. “I was just a child, the same as you.”

  “I'm sorry.” Tchicaya searched her face. “I didn't know it still—”

  She cut him off. “It doesn't. It doesn't still hurt me. It hadn't even crossed my mind for centuries. And it had nothing to do with me coming to the Rindler. I would have done that anyway.”

  “Right.”

  They stood for a while without speaking.

  Tchicaya said, “Is that it? Are we at peace now?”

  Mariama smiled. “Not histrionic enough for you?”

  “The less catharsis I can get these days, the better.” She'd smuggled in a weapon, she'd been prepared to kill him, and they'd still found a way to go on. But it had taken them until now to speak a few words and untangle the oldest, simplest knot.

  “I think we're at peace,” she said.

  They continued down along the airflowers' crowded highway. Eventually the creatures began to thin out; presumably the Sarumpaet was approaching the bottom of the vendek current that had attracted them in the first place—or at least the end of the weather conditions that rendered the current detectable from afar.

  After the last airflower had disappeared into the haze above them, they tracked the current itself for another hour. When it finally came to an end, there was nothing. Just the Bright itself, empty and shimmering.

  Mariama
said, “I don't believe it! A river like that can't appear out of nowhere.”

  “We haven't seen any other currents as long,” Tchicaya said cautiously. “But what does that prove? We don't know the limits of ordinary weather.”

  “I suppose some vendek mixes are just stable because they're stable,” she conceded. “But xennobes have particular uses for stable combinations. I was expecting at least a pile of decaying xennobe corpses.”

  They circled around, examining the region with the probes. There was another persistent current, feeding into the first; it hadn't been obvious immediately, because the transition zone between them was far less orderly than the currents themselves. The vendek mix in the deeper current appeared to be decaying into the mix that had attracted the airflowers, catalyzed by a shift in the ambient weather; as they watched the probe image, they could see the transition zone drifting back and forth.

  Tchicaya said, “Well, it's coming from deeper in. And I'm not going back up to try chasing rabbits.”

  They followed the river back toward its source. Within an hour, they'd hit a second transition zone—this time forking into two different upward flows.

  A third transition.

  A fourth.

  Mariama said, “At least we're learning a lot of vendekobiology. Can you imagine the kind of diagrams it would take to describe the Bright? I used to think the fusion reactions in a star were complicated.”

  “Students will curse our names. What more can anyone hope for?”

  A fifth transition.

  A sixth. Here, the current was flowing down to them, making a U-turn. If they were going to trace it to its origin, they would have to travel an unknown distance back toward the honeycomb.

  Tchicaya was torn. They didn't know if this was an offshoot of a mighty river, the backbone to an entire xennobe ecology, or just a meaningless cobweb drifting through the Bright. They could end up chasing it back and forth, like a cat stalking a feather, until the Planck worms came raining down.