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.

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.

"Just don"t go dropping out on your own," he warned Mariama.

"To do what?" She gestured at the spartan scape. "This makes Turaev in winter look exciting."

Tchicaya gave the command, and the honeycomb blurred around them, the palette of false colors assigned to the vendeks - already recycled a dozen times to take on new meanings - merging into a uniform amber glow. It was like riding a glass bullet through treacle. Above them, the Planck worms retreated, crept forward, slipped back again. The Sarumpaet inched ahead, but in fast motion the race looked even closer than before, their advantage even more tenuous.

As the Slowdown deepened, their progress grew smoother. After a full nanosecond of near-side time, they appeared to be leaving the Planck worms behind. After a microsecond, the worms slipped back out of range of the probes, and there was nothing to be seem but the Sarumpaet itself, and the honeyed esophagus down which it was gliding.

At sixty microseconds, the toolkit signaled an alarm and the ship dragged them back to full speed.

The Sarumpaet had stopped moving, in the middle of a cell of pale blue vendeks. "The probes can"t go any deeper," the toolkit explained. "We"ve reached a new kind of boundary: whatever"s behind it is qualitatively different from all the vendek mixes we"ve encountered so far."

Tchicaya glanced down into the darkness, as if his eyes could reveal something that the probes, responsible for the entire scene, had missed.

Mariama frowned. "Different how?"

"I have no idea. The probes don"t even scatter back from the boundary. I"ve tried redesigning them, but nothing works. Anything I send down simply vanishes." For all its knowledge and speed, the toolkit had never been intended to act as much more than a repository of facts. It couldn"t begin to cope with novelty in the manner of the people who"d contributed to it.

They sat and discussed the possibilities. Tchicaya had learned quite a bit from his faction"s experts, and Mariama even more, but they needed a bigger group; on the Rindler, everyone"s ideas had sparked off someone else"s.

For weeks, they argued and experimented. They took turns sleeping for an hour each; even without any fixed, bodily need to recuperate, their minds were still structured to function best that way. The toolkit diligently analyzed vast lists of possibilities, sorting through the quantum states that might be swallowing all their probes without a trace, hunting for a new design that would avoid that fate and return with solid information.

Nothing worked. The darkness beneath them remained inscrutable.

They had no way of knowing how long it would be until the Planck worms came flooding down after them. On bad days, Tchicaya consoled himself with the thought that when they died, the Planck worms might be buried with them. On worse days, he faced the possibility that brute mutation would find a way through, where all their passion and borrowed ingenuity had failed.

On the thirty-seventh day, Tchicaya woke and looked around the scape. They"d tried all manner of distractions for the sake of inspiration, but no stroll through a forest, no mountain hike, no swim across a sunlit lake had led them to the answer. So they"d stopped ransacking their memories for places to camp, and returned to the unpalatable truth. They were stranded in an ugly, barren cave in the pockmarked rind of an alien universe, waiting to be corroded into noise by a billion species of ravenous sludge.

Mariama smiled encouragingly. "Any revelatory dreams?"

"I"m afraid not." He"d dreamed he was a half-trained Sapper from the legend, suddenly confronted by a new kind of bomb, falling beside it toward a landscape of shadows that might have been anything from a desert to a vast metropolis.

"My turn, then. Come on, get up."

"I will. Soon." She could just as easily conjure up a bed of her own, but taking turns with one imposed a kind of discipline.

Tchicaya closed his eyes again. Sleep had lost all power to assuage his weariness, but it was still an escape while it lasted. He"d understood from the start that their struggle was quixotic, but he"d never imagined such a dispiriting end. They"d spend their last days writing equations on paper planes, and tossing them into an abyss.

As he drifted back toward sleep, he pictured himself gathering up a mountain of crumpled paper and heaving it out of the Sarumpaet into the darkness below. If by chance some scrap went wafting through into another world, he"d never even know that he"d succeeded.

He opened his eyes. "We launch all our paper planes at once. Then we throw a message back, and use it to clear away all the garbage."

Mariama sighed. "What are you ranting about?"

Tchicaya beamed at her. "We have a list of the kind of states the region below us might be in, and we have strategies for dealing with them all. But we still haven"t found a probe that will cross through and return - giving us a definite answer, letting us know which strategy to use. Fine. We put the Sarumpaet into a superposition of states, in which it tries them all simultaneously."

Mariama was speechless. It took Tchicaya several seconds to interpret this response; he had rarely surprised her, and he had certainly never shocked her before.

She said, "Who cares about quantum divergence, if one world out of every quadrillion is the best of all possible worlds? That sounds like some desperate fatalist nonsense from the last days before the Qusp."

Tchicaya shook his head, laughing. "I know! But it"s not! Answer me this: a quantum computer does a search for the solution to an equation, testing a few trillion candidates simultaneously. In how many worlds does it fail?"

Mariama scowled. "None, if there"s a solution at all. But that"s different. The divergence is all internal and contained; it doesn"t split the environment into branches halfway through the calculation." A flicker of uncertainty crossed her face. "You don"t think we could - "

Tchicaya said, "We"re not in the near side anymore. Coherence is nowhere near as fragile here. Whatever this gulf is that we"re facing, there"s no fundamental reason why we shouldn"t be able to stretch a single quantum computer all the way across it. And if we handle all the strategies with sufficient care, we ought to be able to manipulate the whole coherent system so that the failures cancel out."

She nodded slowly, then broke into an astonished grin. "We reach out and swallow the problem; we internalize it completely. Then we can bludgeon our way through by trial and error, without the world ever seeing a single mistake."

They spent three days refining the idea, thrashing out the details with the toolkit and the ship. It was a complex maneuver, and it would require precise control over the ship"s environment, both before and after it crossed through the boundary. The toolkit had had plenty of time to study the surrounding vendeks, and it understood the physics of this obscure cul-de-sac as thoroughly as that of the near-side vacuum itself. The second half of the problem could not be dealt with by direct observation, but that didn"t mean they"d be taking a leap into the dark. Each strategy for making the crossing relied on a set of assumptions about the other side. Once they put the ship into a superposition of strategies, each component would know the kind of place it would end up in, if it ended up anywhere at all.

Tchicaya snapped awake, knowing the reason instantly. He"d been summoned to alertness by the tug of a trip wire that he"d installed, back on the near side, when he"d worked with the toolkit to construct a software container to sit between their minds and the raw quantum gates of the ship"s processor.

Mariama was seated a short distance away, gazing out into the vendek cell. Tchicaya said, "Do you want to tell me what you"re doing?"

She turned to him, frowning slightly. "Just rearranging a few things internally. I didn"t realize I had so little privacy."

"I own this whole setup," he said. "You knew that when you came into it."

Mariama spread her arms. "Fine. Rummage through my memories; see if I care."

Tchicaya sat up on the edge of the bed. "What were you trying to expel into the environment?" At the border of the simulated Qusp in which her mind was cocooned, he"d replaced some of the more arcane facilities of the standard hardware - things she"d have no good reason to want to use, under the circumstances - with fakes that merely rang alarm bells. It had been a last-minute decision; the toolkit would have happily simulated the Qusp in its entirety, as the simplest means of guaranteeing that everything worked smoothly when it was piped through.

"Nothing," she said. "It was a mistake. I didn"t even realize you"d put me in a cage, so I brshed against the bars by accident." She waved a hand at him irritably. "Go back to sleep."

He rose to his feet. "Are you going to tell me, or am I going to have to look for myself?" In an ordinary Qusp, the owner of the hardware could freeze the whole program and inspect its state at leisure. But the quantum gates here were implemented at too low a level; there was no room for that approach. All he could do was send in a swarm of utility algorithms to search for anything suspicious, while shuffling her working mind aside. That would do no lasting damage, but he had no idea how she would experience it. It could be extremely unpleasant.

Mariama regarded him calmly. "You do whatever you think you have to. I"ve already been flayed once."

Tchicaya hesitated. He did not want to hurt her, and if he was wrong, he"d never be able to look her in the eye again. There had to be another way to call her bluff.

"There"s no need," he said. "I know exactly what you were trying to do." He wasn"t certain of anything, but of all the possibilities he could imagine, one stood out sharply.

"Really? Do you want to enlighten me?"

"You brought in a stock of qubits entangled with the near side. You had to get rid of them now, or they would have shown up tomorrow when we prepared the ship." Anything that interacted with an entangled qubit would have its phase irretrievably scrambled. To a pure quantum system they"d be poison. They"d have to be carefully isolated, locked away somewhere inside her mind.

"You"re right," she admitted. The expression on her face barely changed, as if this amounted to a minor clarification of her original story. "But I wasn"t trying to use them. I was trying to get rid of them."

"Why don"t you use them right now? Kill us both, right now?" However many she was carrying, she could not have imagined they"d be enough to do real harm to the far side. So the poison could only have had one target.

"I don"t want to do that, Tchicaya. I want to go with you. Deeper in. As far as we can."

"Why?" Why had she dragged him down here at all? To give his version at the border an excuse to give up? Once he was also deep in the far side, battling the Planck worms like a valiant Lilliputian, it would be far easier to feel that he"d done all he could.

"To see what"s there," she said. "To help protect it, if it"s worth it."

"And help destroy it, if it isn"t?"

"I never lied about that," she insisted. "I never told you that I"d fight for some exotic wasteland, over the lives of real people."

That was true. She"d told him exactly what she believed, and he"d still wanted her beside him.

Tchicaya sagged to his knees. He had the means to kill her, or to leave her behind for the Planck worms. The ship"s processor would do whatever he asked. But nothing she had done was unforgivable. In her place, fighting for the same stakes, he would have lied, too, armed himself, too. How could he accuse her of betraying anything? For all he knew, if they"d taken different turns the last time they"d parted, they might have ended up in each other"s shoes.

She walked up to him and cradled his head in her arms. "I"ll get rid of them now," she said. "Will you let me do that?"

Tchicaya nodded. She took him by the hands and lifted him up. He constucted a safe route through the processor, and she ejected the tainted qubits, forming a tiny bubble of classical physics in the vendeks' quantum sea.

The toolkit completed its preparations for the Sarumpaet's second launch. In principle, this was just another quantum computation, no different from the commonplace operation of turning a string of zeros into a superposition of every possible binary number of the same length. Treating the entire ship as an operand, though, meant expanding the infrastructure that performed the computation far beyond the original hull, wrapping the Sarumpaet in a second computer. This processor would rotate the part of the ship"s state vector that described the propulsion system, giving it a small component in each of more than a quadrillion orthogonal directions. Then it would release the resulting superposition into the depths of the far side, and wait for the reply that would enable it to erase all its failures.

The scape made no attempt to portray the actual machinery in which they were embedded; an opaque shield moved into place around the hull, representing the fact that they"d ceased to exchange information with their surroundings.

The toolkit began a countdown from twenty.

"Give me liberty, and/or death," Mariama quipped.

Tchicaya said, "I"ll be happier when we can drop the and." He was more afraid of the possibility of a single success, diluted a quadrillion-fold, than he was of universal failure. "I don"t know if I should wish you a peaceful local death. Does this count, or doesn"t it?"

"Only if none of the strategies work."

"Then I won"t say anything."

The toolkit said, "Zero."

Tchicaya looked down through the panes in the floor into a borderless expanse of pale brightness, stretching out beneath the Sarumpaet like an inverted sky.

He turned to Mariama, relieved but confused. "That"s it? It"s over already?" The ship would not have sent out probes to explore their surroundings until the handshake across the boundary was completed.

The toolkit said, "No. The light represents information-bearing vendeks with which we"ve interacted, inadvertently. I"m afraid the shielding we emerged with was a bad choice; I"ve found something that works now, but they managed to crawl all over us first."

Tchicaya was horrified. "Catch them!"

"I"m trying. I"m weaving a net."

"Trying? You useless fucking machine!"

Mariama reached over and took him by the shoulders. "Calm down! We programmed a response to something like this, and it"s all happening, as fast as it can. There"s nothing more to be done."

When they signaled back through the boundary to consolidate their success, the Sarumpaet needed to be a complete quantum system, not part of something larger that included vendeks fleeing through the far side. The entire maneuver depended on it. If they could not catch the vendeks, their presence would become an insignificant statistical fluke: for every branch in which they"d succeeded, there"d be a quadrillion in which they"d vanished from the picture entirely.

"We should have covered this," he said. "We should have covered every eventuality."

"Covered it how?" Mariama retorted. "A superposition that included different shielding on emergence would still have emerged with the wrong shielding, some of the time. We were never going to banish every conceivable problem in advance."

She was right. They"d done as much as they could to prepare, and now they had no choice but to wait and see if the situation could be salvaged.

The light began to fade, slowly. The toolkit had netted a portion of the vendeks, trapping them in the structure it had woven and erasing their correlations with the ship. The light was only a metaphor; the task was not as hopeless as it would have been if they"d exposed a quantum processor to a random bombardment with photons. It was more like having a billion-piece jigsaw puzzle stolen by a swarm of flying insects: difficult to reverse, but not impossible.

The sky beneath them turned gray, then pitch black.

The toolkit said, "That"s all of them."

"How can you be sure?" Tchicaya asked.

"I can"t be, absolutely, but all the subsystems that were most likely to have been affected are displaying interference patterns as sharp as they"ve ever produced in isolation. Unless the vendeks that happened to escape also happened to interact with us in a way that could mimic that result, we"re in a pure quantum state."

Tchicaya could live with that much doubt.

The toolkit understood the physics on both sides of the boundary, now. As it exchanged information with the machinery that had launched them, the state vector for the ship was rotated into an eigenstate for a single strategy: the one that had succeeded. Give that they"d launched themselves toward the boundary at all, the probability that they"d failed to come through was zero.

Mariama exhaled heavily. "I think that"s the strangest thing I"ve ever been a part of." She held up her hands and inspected them. "You know, I half-expected to feel the amplitude come flooding into me. Moving from spine to fingertips, of course."

Tchicaya laughed, grateful that she"d found a way to break the tension. "We should have programmed in an oscillating factor, for that extra existential thrill." Not long after the Qusp had been developed, people had played around with all manner of quantum novelties, putting themselves into intentionally prolonged superpositions inside their skulls. But there was nothing even mildly strange to report about this: from the inside, each part of the state vector that described your mind experiencing something definite simply had that one, definite experience. Shuffling amplitude back and forth between two alternatives before finally letting one of them interact with the world could not be "sensed" as some kind of ontological ebb and flow.

As the shielding was removed from the hull, the bright expanse of vendeks reappeared beneath them. The inner workings of the ship still needed to be protected, just like the interior of any Qusp, but they could now live with the equivalent of sunlight on their faces. Sunlight, or a swarm of gnats. The Sarumpaet would keep sending out probes, but in this region some information would come to them for free.

"What now?" Mariama asked.

Tchicaya looked up at the bottom of the honeycomb; it appeared as black and fathomless here as it had from the other side. It would hold back the Planck worms for a while, but it would be hoping for too much to assume that they"d all dash lemminglike into oblivion. "We need to find out how deep this region goes, and exactly what it contains. Maybe we can build some kind of firebreak here, something that will stop the Planck worms once and for all."

They descended through the Bright as fast as they could, but their progress was erratic. The number of different vendeks here was thousands of times greater than in any cell of the honeycomb, and though there were no abrupt transitions, the environment was constantly changing. Currents of different physics flowed around them as the vendeks intermingled in new proportions and combinations. Umrao had largely anticipated the structures in the honeycomb, but these strange tides would probably have been too complex to show up in his simulations. Tchicaya could not decide if this place would be more hostile or more amenable to higher forms of life: the vastly greater diversity of the vendeks made it seem richer, but the honeycomb cells had offered a kind of stability that was entirely absent here.

The scape showed nothing beneath the ship but a distant haze, constantly retreating. The information-bearing vendeks - which Mariama dubbed sprites - seemed to pass intact through all the changing conditions, but they were refracted and scattered to varying degrees, so the visibility they provided was limited. The Sarumpaet's artificial probes became lost in the currents even sooner; beyond about half a micron, only a tiny fraction managed to return.

It was impossible to guess how deep this region might be. Though the border was advancing through the near side relentlessly at half the speed of light, the precise meaning of this for the far side remained unclear. Viewed from either side, the border itself had to be expanding in a consistent fashion, but that left open the question of whether all, or most, structures in the far side sat motionless while the edge of their universe rocketed away from them, or whether the relationship was more like that of the cosmic expansion of the near side, where relative velocities grew slowly with distance. The honeycomb was certainly clinging to the border, but that was not a good enough reason to believe that everything else in the far side would be following close behind. Sweeping principles of homogeneity were wishful thining here.

There was something deeply restful about moving through the Bright. With the scape"s fake gravity insulating them from the ship"s actual, bumpy passage, the Sarumpaet might have been a glass gondola hanging from an invisible hot air balloon, drifting through a planetary atmosphere after a volcanic eruption had shrouded the world in dust. Although there was nothing to see but the shimmering of the sprites, Tchicaya resisted the lure of Slowdown, and instead of retreating into virtual landscapes from their memories, they sat and talked about their travels. Mariama described the renaissance on Har"El, the excitement of the changes that had percolated up from nowhere. Tchicaya told her more about Pachner, and the similar vitality he"d seen at the approach of the border.

They were beyond arguing, beyond accusing, beyond holding up each other"s earlier ideals as some standard against which they"d fallen. They had seen different things, lived different lives, and they had allowed it to change them. All they could do now was keep on climbing Schild"s ladder.

Five tranquil days into the Bright, just as Tchicaya was beginning to fear that they risked being lulled into an irreversible torpor, they spotted a small, translucent structure drifting by at a leisurely pace. The sprites that the object modified and deflected reached them long before the ship"s probes could journey out to form their own impression, and for nearly an hour it was not at all clear that this was anything more than an unusually stable and localized feature of the shifting currents. The sprite-image looked like an eddy of some kind, and if no circulating winds could be detected brushing across the Sarumpaet as it approached, the rules governing vendek flows didn"t bear much resemblance to fluid dynamics.

Once they were close enough, the probes gave a more detailed picture. There were veins and pockets of vendeks inside the eddy that were like nothing they"d seen floating free here. Some of the mixes were similar to honeycomb populations; others were different again.

They tracked the thing for hours, and watched it negotiate the currents. As the free vendeks flowed over it and through it, the interior structures deformed wildly; these were not the kind of breezes that could stir a few leaves, they were shifts in the fundamental dynamic laws. Some species of interior vendeks died before their eyes; others seemed to be leached out, carried off into the wind. It was like witnessing an animal being sandblasted with bacteria and assorted foreign cells, fighting off some, incorporating others, surrendering whole lineages of its own. Twisting and reeling beneath the onslaught, but all the while continuing to function.

After eight hours of watching these feats of persistence, with neither of them willing to put it into words, Mariama finally declared, "This has to be alive. This is our first xennobe."

Tchicaya agreed. "What do you want to call it?"

"I named the sprites," she said. "It"s your turn."

The internal structures that the probe revealed looked like knots of offal caught in a tornado, but not many creatures were beautiful to behold once you dug that deep. The sprites' gentler scrutiny gave an impression of something woven from the winds.

"An airflower."

Mariama was amused, but she didn"t object. If the Bright was not actually much like air, nor did anything here lie within reach of one-word descriptions in near-side language.

They continued to follow the airflower, though it was drifting upward, back toward the honeycomb. The toolkit ventured no opinion on the question of whether or not this system was alive, but its observations had already yielded dozens of new methods for easing the Sarumpaet's way through the currents of the Bright.

"Could it be sentient?" Mariama wondered. The airflower had shown no obvious reaction to their presence, but it wasn"t actively probing its environment, and the ship was a fraction of its size. The tiny distortion in the sprite flow around the Sarumpaet's hull would barely be distinguishable from the background shimmer.

Before crossing the border, they"d planned to initiate contact with the builders of the signaling layer by a simple act of mimicry: scribing a layer of vendeks of their own which beat out the same sequence of primes. Back in the honeycomb, that would have been straightforward; here, it would have been like trying to communicate by waving a white silk banner in a blizzard.

They consulted with the toolkit, and eventually settled on a reasonable compromise. They unfurled a sturdier kind of banner, flexible enough to cope with the vendek flows. Its precise geometry remained prey to the weather, but instead of encoding anything in its position, its degree of transparency to the sprites flickered between two states, flashing out the primes like a shutter held up to the light.

The airflower drifted on, apparently indifferent to the signal. They could only guess as to how it might pursue a conversation with its own kind, but if this creature had constructed the signaling layer in the alien environment of the far side"s shallows - with the intention that it be noticed by beings from an even stranger realm - why would it remain oblivious to a version of the same message suddenly appearing in front of it?

It was possible that it was completely blind to the sprites. They seemed like the obvious basis for perception here, but the airflowers might have evolved before them. If that was the case, it could take months of painstaking work to discover the creature"s actual sensory modalities.

Tchicaya had asked the toolkit to run simulations of the known species of Planck worms interacting with the bottom of the honeycomb, and as he pondered his next move, the verdict arrived. By sheer force of numbers, the worms would almost certainly stumble upon the necessary mutations to find their way through. Once they managed that, they"d bring the near-side vacuum into play against the Bright, unraveling the intricate tapestry of vendeks into isolated deserts of homogeneous physics.

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.

"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."

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.