Schild's Ladder

by Greg Egan

  “If we don't spot another xennobe before the next transition, that will be the last,” he declared.

  Mariama concurred, reluctantly.

  They stood together, staring into the haze. Tchicaya could think of no other strategy, once they abandoned this thread, than plunging straight down, hoping at least to hit bottom soon, yielding a purely physical measure of how much territory they'd be sacrificing if they built the tar pit for the Planck worms.

  And if they never hit bottom, if the Bright went on forever? Then there'd be nothing they could do, nothing they could save.

  Mariama said, “That's a sprite-shadow, isn't it? It's not just haze.”

  “Where?”

  She pointed. Tchicaya could see a tiny gray distortion in the light. “If it's another airflower, that doesn't count.”

  The shadow grew, but the probes were still not reaching it. The object was much further away than they'd realized, and it was definitely not an airflower.

  Tchicaya would have abandoned the vendek current to go after this new find, but the current itself was leading them straight to it. This, ultimately, was the source of the vendeks on which the airflowers had been feeding. And its sprite-shadow kept looming larger, while the probes remained oblivious to it.

  Mariama said, “If this is a single organism, we've just gone from rabbits to whales. I thought the current must have come from necrotic decay, but this is so huge it wouldn't need to be mortally wounded; it could urinate a river.”

  The flickering outline of the shadow was roughly circular. “I don't think it's one creature,” Tchicaya said. “I think we've found an oasis in the desert.”

  The shadow now dominated the view completely, a sight as overwhelming as the border from Pachner, but its exact form remained elusive. “We have to get those probes to go faster,” Mariama complained.

  A tiny patch of color and detail appeared suddenly at the center of the object, spreading slowly through the grayness. The framing effect was confusing; Tchicaya found it harder than ever to interpret the probe image. Things that might have been xennobes were moving around on a roughly spherical surface; the scape labeled them as being hundreds of times larger than the rabbits, but they looked like mites crawling over an elephant. The scale of the structure was extraordinary; if an airflower was the size of a daisy, this was a floating mountain, an asteroid.

  The window of detail grew, revealing thousands of xennobes streaming about below them—the alignment of the Sarumpaet's deck still made “down” point to the center of the far side, but it was impossible not to grant this minor planet precedence—and that was just the surface. Some xennobes were coming and going from the mouths of tunnels leading into hidden depths. As yet, the probes were spread too thinly to report on the new xennobes' anatomy in any detail, and like the others, their forms swayed wildly in the winds that wrapped around them, merging with their bodies. Yet some system at the core of each xennobe retained its integrity through all the changes sweeping over it, and the same organizational information was endlessly reencoded, endlessly preserved.

  As the probe image spread out to encompass the entire colony, Tchicaya's heart leaped. He struggled to temper his excitement. His intuition didn't count for much here, and everything he was witnessing was constantly deforming, as if the whole vision was a reflection in liquid metal. He couldn't even pin down the source of his conviction, the one regularity in all the busyness beneath them that struck him as the signature of artifice over nature. But all technology would be built from nature, here. Nothing entirely lifeless could endure.

  He turned to Mariama. “This is not an oasis. It's not a jungle. We've found the Signalers. This is their city.”

  Chapter 17

  The Sarumpaet circumnavigated the xennobe colony, reconnoitering, apparently unnoticed. Tchicaya kept the density of the probes low, lest the rain of inquisitive devices cross the threshold of perception—or some more sensitive, artificial means of detection—and alarm the denizens. He had no urgent need to study these creatures' internal anatomy, and the details of the colony itself were overwhelming enough.

  Veins and bladders and sheets composed of thousands of different vendek populations defined the structure, separated by an intricate warren of tunnels through which the free vendeks of the Bright continued to pass. The probes identified changes in the winds as they flowed through the colony; specialized vendeks were diffusing out of a multitude of reservoirs and modifying the raw weather, killing off some species, supplanting them directly, or interacting with them to create new variants. To Tchicaya, this looked exactly like air-conditioning for physics: the Colonists could probably cope with all but the most extreme natural changes in their environment, but it made sense that they'd find it less stressful to delegate some of their homeostatic efforts to their technology.

  Hundreds of vendek currents snaked out of the colony, presumably waste products from both the thing itself and its inhabitants. A few were so stable that they completely resisted the passage of both probes and sprites, and they appeared in the scape as gnarled black roots twisting away into the distance.

  Tchicaya saw nothing to dissuade him from his earlier conclusion, though everything was open to alternative interpretations. Termite mounds had air-conditioning, ants had mastered agriculture, and the Colonists might not have needed to expend even as much effort as social insects to bring their home into existence; it was possible that they were mere symbionts, mindlessly tending some giant natural organism. Mariama remained cautious, but she did not choose to play devil's advocate. They both had the same hopes now, and they both knew how easily they could be dashed.

  They spent half a day debating the level of caution they needed to exercise. Whether these xennobes were the Signalers or not, they were likely to have far more potent defenses than the rabbit. It would be difficult to supervise any kind of complex interaction from too great a distance, though; if they hung back in their present orbit and sent down a drone, it would need to be largely autonomous.

  The plan they finally settled upon was to send in a mobile form of their signaling banner, as large and obvious as they could make it, while following behind at a prudent distance. If the reception was violent, the tiny sprite-shadow of the Sarumpaet would be the less likely target.

  If their mimicry of the signaling layer produced a promising reaction, they would move on to more complex exchanges, playing it by ear, hoping that the banner itself would prompt their hosts to respond in kind. Nothing the probes had revealed had offered any clues about the Colonists' preferred mode of interpersonal communication; sprites and other potential information-carriers flooded the colony, but plucking messages in an unknown language from all the influences modulating these carriers was beyond the standard Mediator software they'd brought through the border. Given time, Tchicaya would have happily observed the Colonists from a distance until everything about them, right down to the subtlest cultural nuance, was absolutely clear. He and Mariama could have descended from the sky expecting compliments on their perfect local accents and unprecedented good manners, like a pair of conscientious travelers.

  It was not going to happen that way. The coming of the Planck worms would be unheralded, but the five percent error bars of the toolkit's best statistical guess had already been crossed. If the sky rained poison right now, as they rushed through their rudimentary preparations, they would not even have the bitter consolation of knowing that they'd been ambushed by unforeseeable events.

  They'd reached the end game, ready or not. They'd have to walk a knife edge between recklessness and caution, but they could not afford to take a single step back.

  The signaling banner spiraled down toward the colony, twisting and fluttering like an airborne tent in a hurricane, but pulsing steadily from translucent to opaque. The Sarumpaet followed, close enough to maintain a probe image of the banner that was only a fraction of a ship-second out of date. The probes could also ferry instructions from the ship to the banner, enabling the signal to be
modified on a similar time scale as soon as the need arose.

  From the deck of the Sarumpaet, Tchicaya formed a rapid series of impressions of the crowded world below, none of which he believed was worth trusting. The density and animation of the creatures made him think of the bustle of markets of festivals, of riots. Of the crew of some ancient, oceangoing vessel battling a storm. In fact, all this violent swaying in the wind was probably about as exciting for the Colonists as a terrestrial animal's endlessly beating heart. For all he knew, this was what they looked like at their most indolent.

  He searched for any hint of a flickering shadow from the banner on the surface below, but between the shimmering of the sprites and the erratic geometry of all the objects involved, that was too much to expect. Perhaps it was fortunate that the nearsiders would not arrive with anything like the drama of an artificial eclipse; even if these xennobes did belong to the same species as the Signalers, different cultures could still have varying degrees of sophistication, and an overblown spectacle might have terrified a group for whom the search for life beyond the border was a barely comprehensible endeavor, something that only an obscure, deranged minority would even contemplate.

  On the other hand, since the banner had no significant effect on the ground, it was possible that no one would even notice it. It wasn't clear that any of the Bright's inhabitants focused the sprites to form an image; the rabbit had been close enough to the banner it attacked to sense its presence through a drop in overall irradiation, like a chill on the skin. It made evolutionary sense to expect all mobile xennobes to possess a detailed knowledge of their surroundings, but a sufficiently unnatural object might still be as invisible to them as a burst of neutrinos to a human.

  The banner came to a halt at a predetermined altitude: some twenty times the Colonists' typical body size. Tchicaya gazed down at the crowd, wondering how he was going to distinguish panic from indifference. The Colonists weren't as shapeless as the airflowers; their network of vendek tubes bifurcated twice to give four distinct clusters of branches, and their geometry at any moment tended to reflect this. They looked like medical scans of the circulatory system of some headless quadruped, dog-paddling ineffectually in extremely rough seas. But if that intrusive probe image was unlikely to reflect the way they saw each other, by sprites alone they resembled tortured, mutilated ghosts, trying to break through into the world of the living.

  Mariama said, “I think it's been noticed.”

  “Where?”

  She pointed; a group of six Colonists had left the surface. As Tchicaya watched, they ascended rapidly, but as they grew nearer to the banner they slowed considerably. This cautious interest was not proof of anything, but it was an encouraging sign.

  The Colonists surrounded the device, then began spraying it with a delicate mist of vendeks. “That's cooperative sensing!” Mariama exclaimed. “One of them illuminates the object, the other looks at the transmitted pattern.”

  “I think you're right.” The group was arranged in pairs on either side of the banner, and the members of each pair took turns emitting the vendeks. The probes hadn't encountered this species of vendek before; perhaps nothing inside the colony warranted the same kind of scrutiny as this alien object.

  The Colonists retreated and formed a loose huddle away from the banner. “What now?” Tchicaya wondered. “How do you react to a mutated version of your own stratospheric beacon suddenly appearing on your doorstep?”

  Mariama said, “I just hope they realize they don't need to launch a new signaling layer in order to reply.”

  “Maybe we should try to make a more obvious proxy,” he suggested. “Something that resembles one of their bodies.”

  “How would we decide which features to include, and which to leave out? We don't even know the difference between their communications signals and their waste products. We'd probably come up with the equivalent of a glove puppet of a monkey that smelled exactly like human excrement.”

  She had a point; even the six Colonists high above the din—and/or stench—of the colony were now bathed in a confusing fog of vendeks, and it was beyond the Sarumpaet's resources to untangle their functions and meaning.

  Tchicaya felt a sudden stab of pessimism. He believed he'd finally reached the people he'd come to find—but he had days, at most, not only to describe the Planck worms to them, but to reach a level of communication and trust that would enable them to work together to deal with the threat. However many subtleties, abstractions, and courtesies he omitted along the way, even conveying the core of the message was beginning to seem hopelessly ambitious.

  He said, “Maybe we should change the signal right now, instead of waiting for them to reply? Just to make it clear that the banner's not passive?”

  Before Mariama could respond, the Colonists began regrouping around the banner. In unison, they released a stream of vendeks, denser than before; in the probe image, it looked as if the six veined bodies were blowing soap films. The individual sheets met at the edges and merged, forming a bubble, enclosing the banner.

  The Colonists retreated again, then sprayed a new mixture at the bubble. It began to drift after them, down toward the surface.

  Mariama said, “They've grabbed it! They're towing it!”

  The wall of the bubble was passing sprites, but it resisted the Sarumpaet's probes—the only means they had to get instructions to the banner. They'd lost control of the device completely, now; they couldn't even reprogram its message, let alone command it to try to break out of its cage.

  “We could make another one,” Tchicaya suggested. “Right in front of their eyes.”

  “Why not see what they do with this one?”

  “You think we should follow it?”

  Mariama nodded. “They might release it from the container, once they've got it where they want it. They might even have their own signaling device down there.”

  Tchicaya was not convinced. “If they think it's just a message in a bottle, they're not going to talk back to it. And if we can't regain control of it, the last place we want to try scribing a new one is in the middle of some chamber down there.”

  “We'll only find out what they think it is if we go after it,” Mariama replied. “Besides, we initiated contact with this object. We should stick to that, follow through with it, or we risk confusing them.”

  That did make sense. They had to be flexible, or they'd end up chasing their preconceptions down a cul-de-sac, but they also had to try to be consistent. Changing tack every time they feared that they might have been misinterpreted could bury any message beneath all the distracting shifts in strategy.

  Tchicaya said, “All right, we'll follow it!” He instructed the Sarumpaet to pursue the purloined banner.

  As they descended, it finally struck him just how extraordinary a sight they were witnessing. The banner was still flashing out its programmed sequence from within its container; the Colonists hadn't damaged it at all. Towing anything without destroying it, here, was a feat akin to putting a tornado on a leash. There were no simple analogs of the notions of pushing or pulling something, let alone any reason to expect it to respond by moving as a whole—like a nice near-side object made from atoms bonded together into a mildly elastic solid. You couldn't even rely on the local physics to permit something to behave, in uniform motion, as it had when it was stationary, however gently you conveyed it from one state to the other.

  He turned to Mariama. “This is proof, isn't it? They have to be more than animals, to be able to move it like that.”

  Mariama hesitated, no doubt pondering the evolutionary advantages of a delicate touch when kidnapping other species of xennobe to fill with your parasitic young.

  But she said, “I think you're right. I've been giving them the benefit of the doubt until now, but I think they've finally earned it.”

  The six Colonists touched down on the surface and proceeded along a narrow path that opened up in the throng ahead of them. The bubble appeared to be following a v
endek trail laid by its creators, and the Sarumpaet stayed close enough behind it to avoid the crowd as it reclaimed the ground in the wake of the procession. Rather than rendering the flight deck in proportion to the ship's actual physical dimensions, the scape was constantly making choices of scale to keep the view of their surroundings intelligible, and the Colonists on either side of the ship appeared roughly as large as giraffes. Absurd as it was, Tchicaya found it difficult to suppress the feeling that they might look in through the hull and see him standing on the deck gazing back at them; he kept wanting to avert his eyes, so as not to risk frightening or provoking them.

  Close up, the ship's probes revealed more of the Colonists' anatomy. Dwelling on the crude, wind-blown X of their overall shape was pointless; everything that mattered was in the vendek mixtures locked in the network of tubes. The toolkit struggled to annotate the images, hinting at the subtlety of the vendekobiology and the complexity of the network's topology. Tchicaya could only take in a fraction of what the toolkit was managing to glean, but the Colonists were manipulating their internal physics with as much precision as any animal controlling its biochemistry, juggling pH or glucose concentrations.

  He caught Mariama's eye, and the two of them exchanged giddy, fearful smiles. Like Tchicaya, she was enraptured by the beauty and strangeness around them, but more painfully aware than ever of the vast gulf they'd have to bridge in order to protect it. The closer they came to the possibility of success, the more vertiginous the fall if they lost their grip. To be overrun by Planck worms in the honeycomb would have meant nothing but a bleak local death; here, they would be witnessing a whole world dying.

  The procession entered a tunnel, angled steeply down into the colony's interior. As the density of sprites dropped, the scape experimented with the other ambient information-carrying vendeks. No single species could come close to matching the details of the probe images, but taken together they provided a fair description of the surroundings. From the Colonists' point of view, the Bright might well have been horribly misnamed; the conditions down here stood a far better chance of providing useful illumination, and the colony could have been perceived to lie in a somber landscape of permanent twilight.