Schild's Ladder

by Greg Egan

  He said, “That's not particle propagation, but it's something I've seen before, in simulations. It's persistence, and replication, and interdependence. It's not a superposition of a billion different vacua—or if it is, that's only one way to describe it, and I don't believe it's the best.

  “It's a biosphere. It's an ecology. Right down at the Planck scale, the far side is crawling with life.”

  Chapter 11

  Tchicaya said, “We should tell them, now! Take them all the evidence. No, no—better, teach them Yann and Branco's method, and let them probe the far side for themselves. Then they'll know they're not being cheated with some kind of elaborate simulation.”

  Hayashi groaned. “And then what? They convince themselves that they're now facing the Virus That Ate Space-Time. While we've surrendered our sole advantage.”

  Pacing the ship, unable to sleep, Tchicaya had run into Suljan and Hayashi. When a casual exchange of views in the corridor had come perilously close to disclosing all the latest discoveries, he'd accompanied them to the Yielders' cafeteria, which was supposedly secured against listening devices. Other people passing through had become entangled in the debate.

  Rasmah said, “I agree. This isn't going to sway anyone. Even if they're willing to interpret this as evidence for Planck-scale biota, and even if that destroys all their preconceptions about the ‘Mimosan vacuum’...if you didn't care that much about far-side physics, why should you care about far-side microbiology?”

  Yann's icon appeared, seated beside her. “Microbiology? These organisms are a few hundred Planck lengths wide: about ten-to-the-minus-thirty-three meters. This is vendekobiology.”

  Suljan picked up a mug and raised it threateningly. “What are you doing here? This is where the real people come, to metabolize in peace.”

  Yann said, “My mistake. I thought you might be sitting around singing the praise of everyone who helped win you a glimpse of the far side. But I can see you're more interested in getting in some valuable belching and farting time.”

  Hayashi reached over and slapped Suljan on the back of the head. “You're an oaf. Apologize.”

  “Ow. It was a joke!” He turned to Yann. “I apologize. I'm in awe of your accomplishments. I'm already working on an ode to your sacred memory.”

  Umrao looked embarrassed by all the bickering going on around him. He said, “I suppose we need more evidence if we're going to convince the skeptics, but for what it's worth, I've been doing some simulations.” He summoned graphics, floating above the table. “The mix of replicators is probably not the same throughout the far side. There are other possible equilibria, other population mixtures that look more or less stable—and that's just changing the relative numbers of the species we've seen, not accounting for entirely different ones.” The images showed both a graph-level view of these teeming communities of organisms, and a higher-level map of a possible set of neighboring regions.

  “The transition zones tend to be quite sharp, and sometimes they just advance relentlessly in one direction at a constant velocity, like the border itself. But there are other situations where an intermediate mix of species forms in a narrow layer, and it stops either side from invading the other.”

  Tchicaya seized on this. “A kind of internal freezing of the border?”

  Umrao nodded. “I suppose you could think of it like that. Except that our side of the border is completely sterile, so it's not really subject to the same effects.”

  “You don't think we could create a layer population like these, that worked with one side unpopulated?”

  Umrao thought for a while. “I couldn't say. For a start, these are simulations, so I'm not even sure that any of this happens in reality. And we'd need to understand many things much more thoroughly before we set out to engineer a layer population with particular properties.”

  Suljan said, “Screw it up, and the border might just move faster.”

  Tchicaya gazed into the simulation. Our side of the border is completely sterile. All these millennia looking for life, scratching around on rare balls of dirt for even rarer examples of biochemistry, only to find that the entire substrate of the visible universe was a kind of impoverished badlands. Life had still arisen here, thirty orders of magnitude up the length scale, as heroic and miraculous as some hardy plant on a frozen mountain peak, but all the while, infinitely richer possibilities had been buzzing through the superposition that the dead vacuum concealed.

  He said, “Keeping this quiet is insane. People have evacuated whole planets for fewer microbes than there are in one atom-sized speck of the far side.”

  “Not always enthusiastically,” Rasmah replied dryly.

  For a moment, Tchicaya was certain that she knew what he'd done. Mariama had revealed their secret, whispered it in a few well-chosen ears, to punish him for his hypocrisy.

  That was absurd, though. It was common knowledge that compliance with the ideal of protective isolation had often been begrudging, and everyone suspected that there'd been cases where the evidence had been ignored, or destroyed.

  “This could win us the Wishful Xenophiles,” he persisted. “One glimpse of this, and they'd desert en masse.” Not all Preservationists shared the view that cultural upheavel was the worst consequence of Mimosa; a sizable minority were more afraid that it might obliterate some undiscovered richness of near-side alien life. Four known planets dotted with microbes—whatever potential they offered for evolutionary wonders in a few hundred million years' time—might not be worth fighting for, and most people had abandoned hope that the galaxy contained other sentient beings, but unexplored regions could still be home to alien ecologies to rival Earth's. Now, that uncertain possibility had to be weighed against life-forms by the quadrillion, right in front of their noses.

  “These aren't sophisticated creatures,” Hayashi pointed out. “We can quibble about the definition of life in different substrates, but even if that's conceded, these things really aren't much more complex than the kind of RNA fragments you find in simulations of early terrestrial chemistry.”

  “That's true,” replied Suljan, “but who says we've seen all the life there is to see?” He turned to Umrao. “Do you think these could just be the bottom of the food chain?”

  Umrao spread his hands helplessly. “This is very flattering, but I think some of you are beginning to ascribe oracular powers to me. I can recognize life when I see it. I can extrapolate a little, with simulations. But I have no way of knowing if we're looking at the equivalent of Earth in the days of RNA, or if this is plankton on the verge of disappearing into a whale.”

  Yann said, “Now we're talking xennobiology!” Tchicaya shot him a disgusted look, though on reflection the hideous pun seemed inescapable. A complex organism based on similar processes to the primitive ones they'd seen probably would be about a xennometer in size.

  Suljan wasn't satisfied with Umrao's modest disclaimer. “You can still help us take an educated guess. Start at the bottom, with what we've seen. I don't think we should try to imagine evolutionary processes; we don't know that these things are primeval, we just know that they seem to be ubiquitous. So we should ask, what else can fit in the same picture? The vendeks don't really prey on each other, do they?”

  “No,” Umrao agreed. “Where they coexist in a stable fashion, it's more like exosymbiosis. In totality, they create an environment in the graph where they can all persist, taking up a fixed share of the nodes. A given vendek in a given place in the graph will either persist or not, depending on the surrounding environment. At least in the sample we've seen, most do better when surrounded by certain other species—they don't flourish in a crowd of their own kind, but they can't make do with just any sort of neighbor. In microbiology, you get similar effects when one species can use the waste of another as food, but there's nothing like that going on here—there is no food, no waste, no energy.”

  “Mmm.” Suljan pondered this. “No vacuum, no timetranslation symmetry, no concept of energy. So even i
f there's another level of organisms, there's no particular reason why they should eat the vendeks.”

  “They might have subsumed them, though,” Hayashi suggested. “Imagine the equivalent of multicellularity. A larger organism might have different vendeks playing specialized roles. Different ‘tissues’ of a xennobe might consist of—or be derived from—some of the species we've seen.”

  “I suppose so,” Umrao said cautiously. “But remember, these things are much, much simpler than single-celled organisms. They don't have anything remotely akin to genomes. In most multicellular creatures, all the cells in all the tissues share their full genome, with different parts of it switched on and off. It's hard to see how vendeks could be regulated with the necessary precision.”

  Rasmah frowned. “Maybe multicellularity's not the right analogy. What's it actually like, on a larger length scale, to be immersed in these different vendek populations?”

  Umrao shrugged. “For what to be immersed? I don't know what kind of organized patterns of information can persist, apart from the vendeks themselves. If we're going to model the behavior of some object, we need to know what it's made from.”

  Tchicaya took a stab at this. “Different vendek populations, with stable layers between them? A kind of honeycomb of different heterogeneous communities?”

  Suljan said, “Hey, maybe they're the cells! Vendeks themselves are too small to play tissue types, but certain communities of them can be maintained within intact ‘membranes,’ so maybe our xennobes could regulate the population mixtures as a surrogate for cell differentiation.” He turned back to Umrao. “What do you think? Could you look for a form of motility in these walled communities?”

  “Motility?” Umrao thought for a moment. “I think I could build something like that.” He began tinkering with the simulation, and within minutes he'd produced an amoebalike blob moving through a sea of free vendeks. “There's one population mix for the interior, and a layer around it that varies as you go from the leading surface to the trailing one. The leading surface acts like an invasion front, but it decays into the interior mix as it travels. The trailing surface does the reverse; it actually ‘invades’ its own interior, but it lets the external population take over in its wake. Perpetual motion only, though: this cell could never stand still. And it's a contrived setup. But I suppose there are all kinds of opportunities to modulate something like this.”

  Tchicaya looked away from the simulation to the mundane surroundings of the cafeteria. He was beginning to feel more optimistic than he had since he'd arrived, but this was all still speculation. To build a machine, a body, from anything like these “cells” was going to be a dauntingly complex endeavor.

  He said, “We have to win time from the Preservationists. There has to be a truce, a moratorium, or this could all be wiped out before we learn anything.”

  Rasmah said, “You think they could make effective Planck worms, without knowing what they're dealing with?”

  “You're the one who's convinced that they have spies.”

  “If they have spies, why should telling them anything buy us more time?”

  “When did spies ever share their intelligence with the masses?” Tchicaya countered. “Suppose Tarek was looking over our shoulder right now, but everyone else remained in the dark?” He turned to Umrao. “I don't suppose you've investigated the possibility of Planck worms? A plague that kills the vendeks, and leaves a sterile vacuum in its wake?”

  Umrao glanced around the table warily. “If any of what you just said was serious, I don't think I should answer that question.”

  Suljan groaned. “Forget about politics. We need more data!” He slumped down across the table, drumming his fists on the surface. “I was playing around with something last night, before I stepped out for a snack and ended up mired in this discussion. I think I might have found a way to extend Yann and Branco's technique, pushing the range about ten thousand times further.” He looked up at Yann, smiling slyly. “The only way I could make any progress with your work, though, was to translate it all into my own formalism. Everything becomes clearer, once you express it in the proper language. It only took me a few hours to see how to scale it up, once I'd dealt with the mess you left us.”

  Rasmah asked sweetly, “So what was the great conceptual breakthrough, Suljan? How did you sweep our Augean stables clean?”

  Suljan straightened up in his seat and beamed proudly at them all. “Qubit network theory. I rewrote everything as an algorithm for an abstract quantum computer. After that, improving it was simplicity itself.”

  On his way to the Blue Room, crossing the observation deck, Tchicaya spotted Birago standing by the starside wall. His first thought was to walk on by; minimizing friction by minimizing contact had become an unwritten rule of shipboard life. But the two of them had got on well enough before the separation, and Tchicaya was sick of only talking to Preservationists at the interfactional meetings, when the entire discussion was guaranteed to revolve around a mixture of procedural issues and mutual paranoia.

  As Tchicaya approached, Birago saw him and smiled. He looked slightly preoccupied, but not annoyed at the interruption.

  Tchicaya said, “What are you up to?”

  “Just thinking about home.” Birago nodded vaguely in the direction of the blue shift, but Tchicaya knew which star he meant. It had been chosen by the people on Viro before they were scattered, and Tchicaya had had it pointed out to him by the evacuees he'd encountered on half a dozen worlds. The spore packages had already been launched from Gupta, and the evacuees—who'd spread out to many different intermediate destinations, to avoid overtaxing the hospitality of the locals—would follow within a couple of centuries. “We're not losing this one,” he said. “Not until the sun burns out.”

  Tchicaya had heard the slogan many times before. Whether it was a matter of being the oldest community of evacuees, or some other factor in the original culture, people from Viro always appeared more focused on their new home than on the loss of the old. Birago himself had no clear memories of Viro—he'd left as an infant, and moved from world to world a dozen times—but if his family had wrapped him in any vision of permanence, any sense of belonging, it was anchored to their future, not their past.

  Tchicaya said, “There's good reason to be hopeful now.” That wasn't giving anything away: the Preservationists would know, at the very least, that his side had had a series of breakthroughs. Their understanding was snowballing; a concrete plan for some form of stable compromise could only be a matter of time now.

  Birago laughed. “Hope is for when you have nothing else. When I was a child, no one around me would ever look up at the border and say, ‘It's too big. We're too late. It's unstoppable.’ We had no plans, we had no remedies; the only strength we had came from refusing to give up. Which was all very laudable...but you can't go on like that forever. There has to come a time when hope turns into something more tangible.”

  “Honey or ashes.”

  “Ah, know-it-all travelers.” Birago smiled, but there was an edge to his voice. Picking up a few idiomatic phrases in passing didn't mean you understood anything.

  “We'll both have certainty soon,” Tchicaya insisted. “I can't believe it will be much longer now.”

  “We? What counts as certainty for you?”

  “Safeguarding the far side.”

  Birago was amused. “And you think that could ever be part of certainty for us?”

  Tchicaya felt a chill of disappointment, but he persisted. “I don't see why not. Once we understand this thoroughly, we'll know what is and isn't safe. No one runs around extinguishing stars out of fear that they might go supernova.”

  Birago gestured with his right hand, “There are tens of billions of stars to learn from”—then with his left, toward the border—“but there's only one Mimosa.”

  “That doesn't mean it will remain a mystery forever.”

  “No. But no one's patience lasts forever. And I know where the benefit of the dou
bt belongs.”

  Tchicaya arrived late in the Blue Room, missing the start of Suljan's experiment. Many more people had chosen to avoid the crush and watch from their cabins, so the place was far less crowded than before, to the point where there was space for furniture.

  As Tchicaya joined Rasmah, Yann, and Umrao at a table not far from the console, Rasmah was saying, “I'm not optimistic about seeing anything new, such a short distance in. If the outermost mixture of vendeks is converting our vacuum at the fastest possible rate, there could be light-years of them behind the border.”

  “‘Light-years’?” Yann regarded her with amusement, as if she'd made some kind of category error: a liter of energy, a kilogram of space. The normal geometrical meaning of a quantum graph was intimately bound up with the presence of particles, and they were yet to unravel any simple notion of distance for the far side.

  “You know what I mean,” Rasmah retorted. “Ten-to-the-fiftieth nodes' worth.”

  Umrao said, “The hardest thing for me to wrap my mind around is the complete lack of Lorentz invariance. If you picture the graph's history as a foam—the edges all extending into surfaces, the nodes all extending into lines—you'd actually see different vendek populations if you re-sliced that foam in a different way.”

  Tchicaya grimaced. “Doesn't that imply that there's a preferred reference frame? Couldn't you assign yourself an absolute velocity, just by seeing what kind of vendeks you were made from?”

  Umrao gestured with his hands in a fashion that Tchicaya's Mediator translated as negation. “Without any external cues to guide you, you'd always slice your own world foam the same way, and see yourself as being made from the same vendeks. Other people moving past you might see your constituents change, depending on their velocity relative to you, but you'd see them change in the same way. And both of you would be entitled to claim that you were the best judge of your own composition.”

  Tchicaya pondered this. “So everything ends up on the same footing as rest mass? It's as if speeding past an electron fast enough could make it look like any other particle at all—but in its own reference frame, it's still an electron?”