by Greg Egan
Rakesh worked with Parantham to design a probe swarm to send into the ruins. Each probe would be about a micrometer wide, and would hop from asteroid to asteroid by riding the currents of the stellar wind — not the Interloper's feeble exhalation, but the overpowering breath of the neighborhood giants. On each rock they visited, the probes would gather energy from sunlight to feed a small band of exploratory nanomachines.
The wind couldn't carry the probes all the way from Lahl's Promise into the asteroid belt, so they had the workshop build half a dozen delivery modules, driven by ion thrusters, each carrying a kilogram or so of probes to scatter as they arced along the edge of the belt. These delivery modules would also act as information relays, with instruments to track the probes closely and elicit stored data from them.
The modules filed out of the workshop, flung away from the ship by centrifugal force before their thrusters lit up. Rakesh watched their blue exhaust trails through the cabin window. "Do you regret coming with me now?" he asked Parantham.
"Not at all!" she said. She seemed shocked by the question. "Why would I?"
"If the Steelmakers are dead, with no descendants. "
"Then that's sad," she said, "but history is full of sad stories. If there's no chance of meeting them face to face, I'll happily settle for archaeology. Archaeology in the disk is finished: every ruin has been tomographed down to the molecular level, every scrap of ancient language and every artefact has been interpreted to death. I was promised nothing but a rock full of microbes when I signed up for this, remember? And you expect me to be having second thoughts just because the sentient species we've discovered might have lasted less than one hundred and fifty million years?"
Rakesh couldn't argue with anything Parantham had said, but his own sentiments were very different. "Maybe at the back of my mind I thought the worst case scenario would be a thousand-year-long slog that ended with nothing but bacteria, while the best case would take us straight to the Planet of the Long Lost Cousins, who I could invite into the Amalgam to live happily ever after. Now that we've caught a glimpse of the real story, it seems that it's bacteria who would have had the best chance of living happily ever after."
He could easily picture his own village on Shab-e-Noor with a dark pinprick crossing the sky, the ground rumbling, an ominous lightness. Of course, that couldn't happen in the Age of the Amalgam; there was no conceivable cosmic threat out in the disk that could not be detected and neutralized. Such vulnerability had been relegated to history. Nevertheless, the image haunted him in a way that went beyond mere empathy for its putative victims. There was a chill in his bones at the recognition that, in the broadest sense, he'd stepped out from the shadow of the same kind of ax. His ancestors had been luckier than the Steelmakers, that was all.
The first wave of results from the probes came in while Rakesh was in the kitchen, cooking breakfast.
Dead microbes had been found in more than sixty per cent of the asteroids sampled so far. That figure was surprisingly high; either the biosphere of the Steelmakers' world had extended deep into the mantle, or the rubble that originated from the depths of the planet had been cross-contaminated by other debris, from closer to the surface.
The genome fragments and general morphology closely matched those of the microbes they'd found in the Aloof's meteor. Along with the isotope data, this left Rakesh with no doubt that they'd found their target. Half of the Interloper's asteroid belt consisted of rocks virtually identical to the one that had triggered their search.
"The Aloof should give us a treat and a scratch behind the ears now," he told Parantham as he filled her plate.
She stared at him as if he'd lost his mind.
"On my home world," he explained, "we have domestic animals that can find things by scent. You give them a whiff, then they go searching for something that smells the same."
"You don't have machines to do that?"
"Of course we do. But these animals enjoy it, it's part of their ancestry. If they don't get the chance to exercise their skills, they get sick with boredom."
"Like the gang back at the node?" Parantham suggested dryly.
"Well, yes." Rakesh hadn't intended the comparison to be taken literally, but he felt a momentary frisson of unease. "I suppose that's one theory we can't rule out: the Aloof took pity on us and offered us a chance to chase a strange new scent across their paddock."
"It doesn't show much pity if they only do it for a couple of people every million years." Parantham shook her head. "We're not their pets. They've kept a few secrets from us; good for them. It doesn't make them our superiors."
"A few secrets?" Rakesh laughed. "We mapped their gamma ray data routes. They get to read our minds, down to the last byte. And you're the one who told me that the gamma ray network was probably just a honey pot."
"I'm not saying that the relationship is symmetrical," Parantham conceded. "They've certainly out-somethinged us. "
"Outwitted?" Rakesh suggested. "Outsmarted? Outmanoeuvred?"
"Out-sphinxed us," she replied. "We stared into the bulge for a million years, trying to get a reaction, and they just stared back out at us, stony-faced. We did much more than blink; we gave up the game completely. I don't believe it's harmed us, though. I don't believe it's a loss on our part, or a victory on theirs. It's just a difference in our natures. We never wanted to keep our nature and our history secret. It's a game we never wanted to win."
Rakesh was woken by the next wave of results. He watched the data and images spinning in his skull as he walked down the corridor to the control cabin, where Parantham was already seated.
"It's alive!" she crowed. "DNA-based, multi-cellular, engineered. but then drifting genetically, running wild for tens of millions of years."
The probes had found a scum of fungus-like growth clinging to parts of some of the asteroids. These were not just colonies of microbes; the cells showed specialization, and were organized into distinct clusters. Though the anatomy of the clusters included a protective skin, all of the cells were individually tough enough to retain internal liquid water while exposed to vacuum, over a considerable temperature range, with antifreeze compounds and vapor-reducing soluble polymers augmenting the sheer strength of the cell walls. Their genome showed clear evidence of sophisticated engineering, and although they shared a common ancestor with the dead microbes, most of the traits that ensured their survival in their present harsh environment appeared to have been artificially introduced.
The creation of the species couldn't be dated exactly until mutation rates and generation times had been measured, but on general biochemical grounds it seemed likely that this fungus had been deliberately constructed at about the time the Steelmakers' world was torn apart.
Rakesh immersed himself in a diagram of metabolic pathways. "It lives on the stellar wind," he marveled. "That's its energy source. For raw materials, it's coping on the asteroids, but there are vestigial enzymes that suggest it might have thrived with a slightly different substrate. So it spread to the asteroids from somewhere else, and adapted to them over time, but the original species was happier in a different environment."
Parantham said, "You look up into the sky, and a neutron star is coming. There is no transport network to whisk you away to safety, and you can forget about deflecting this planet-killer. What do you do?"
"Build a spaceship."
"To go where? There are plenty of stars around, but they're all devoid of companions. A hundred million years ago your ancestors visited another planet, but the space program has grown a little rusty since then."
Rakesh grimaced. "So I give up on the idea of running, and make a fungus that will outlive me? I know I've been spoiled by high-tech immortality, but that doesn't sound like much consolation to me."
Parantham said, "Perhaps it's just the bottom of the food chain. Make a fungus that will outlive you, then a few species that can eat it, and so on. Then give birth to a child that can live on them."
"Maybe." Rake
sh ran his fingers through his hair. "Live on them where, though? Those old genes I mentioned were for enzymes that relied on elements that most of the asteroids don't have. If you know that your world is going to be torn apart, and there are no other planets in sight, where exactly do you expect to live, if not on the scrap heap that's left behind?"
A few hours later they had the answer, from their telescopes rather than from the probes. Near the edge of the belt, an object some six hundred meters across with a highly atypical spectrum had been found orbiting among the rocks. The telescope's image showed a gray ellipsoid, pitted and corroded, but clearly too regular to be an asteroid itself. Spectroscopy revealed that its surface contained molecular filaments, carbon nanotubes with elaborate chemical modifications that both strengthened them and protected them against the stellar wind. A variety of the vacuum-hardened fungus they'd detected in the asteroids could be seen in the indentations of small impact craters, where the wind couldn't reach in to scour it off.
"The material is advanced beyond the Steelmakers' technology," Rakesh mused, "but it's not one hundred million years ahead. They must have gone through a long Dark Age before they finally rose up again." Only to be cut down once more? That wasn't clear. Their home world was in ruins, but this artefact was in one piece.
Parantham said, "That surface looks as if it hasn't been repaired in fifty million years."
"Not everyone cares about surfaces," Rakesh replied. "There could still be someone home."
They sent a surveyor probe, which tomographed the artefact with ambient neutrinos. Inside was a maze of tunnels and caves. Apart from these empty spaces, there was an intricate pattern to the density of the structure itself: parts of the walls were solid as basalt, while others seemed as spongy and permeable as limestone.
Parantham beamed a radio signal from the surveyor probe down to the artefact, a simple message of greeting repeated across the frequency spectrum. The faint passive echo that came back suggested some long strips of conducting material, but no resonant circuits: electrical wiring, perhaps, but no obvious low-tech receivers or transmitters.
An analysis of the artefact's thermal emissions showed no significant amounts of heat being generated within, beyond what might be expected from a small amount of the fungus, and perhaps other species. There was no obvious stream of waste, organic or otherwise, leaving the artefact, though with the stellar wind as its only input any putative ecosystem would have to cling tightly to all of its materials.
Rakesh said, "It's time to send in the jelly babies."
"Ha! You were far more cautious with Steel Mountain," Parantham reminded him.
"If we trigger some elaborate defensive response," Rakesh said, "then at least I'll die happy. Knowing that this civilization survived."
There was no entranceway into the artefact, but the surveyor probe identified a system of narrow cracks in the exterior wall that ultimately led to one of the internal tunnels. If they made their avatars even smaller than before, about a fifth of a millimeter tall, they would be able to squeeze through.
Rakesh glanced up one last time into the sky full of hot blue stars before following Parantham into the chasm.
As the walls twisted around them, they soon reached a point where the stars were hidden and everything was swallowed by the deep shadows of vacuum; by switching to infrared vision, though, it was possible to grope their way down by the thermal glow of their surroundings. Their avatars sported adhesive pads on their hands and feet, tailored to the chemistry of the bare surface, but the infestations of fungus made their grip less secure.
Rakesh sent nanomachines from a stock in his avatar's arm into a patch of fungus, to sequence it. There were at least nine distinct species present, and they all showed marked differences from the kind found on the asteroids. The vestigial enzymes he'd noted there were being produced in far greater quantities here, and seemed to interact with several components of the wall material. As he pondered the modified diagram of metabolic pathways, he realized what was happening. The walls acted as a reservoir for the raw materials that the fungus needed, but the fungus did more than leech essential nutrients out of its environment. As part of its life cycle, it returned everything it took, with the added bonus that structural flaws in the wall were repaired in the process. The system wasn't perfect, but a few cracks after fifty million years wasn't bad.
It was a tortuous business navigating the fissure, but Rakesh wasn't tempted to disconnect his senses and leave his avatar on autopilot. He didn't know if he was entering a mouldy tomb or a thriving metropolis, but he had no wish to dilute, or distance himself from, the experience. As painful as he found it to be forced to confront the bleak prospects for life in the bulge, this expedition was exactly what he'd been seeking ever since he'd left his home world. Who else on Shab-e-Noor, who else in the whole disk, would be able to tell their descendants: "We climbed down through a gap in the wall, not knowing what we would find inside the structure after fifty million years"?
When they emerged into the tunnel, Rakesh found himself immersed in a featureless glow. The tunnel wall was so close to being uniform in temperature that its thermal emissions rendered everything in contrast-free monochrome. It was almost impossible to interpret what he saw, let alone navigate by it.
"Is it just me who's gone blind?" he asked Parantham.
"IR sensitivity isn't enough. We need to rewrite our whole visual processing system," she suggested.
Rakesh searched the library. Leaving aside olfactory and tactile modes — sniffing or groping your way through the dark — most underground species employed vibration sensors or sonar. The walls here were excellent sound conductors, but even so that would be of limited use. He found a mode of IR-based perception that some asteroid-mining robots and a few tunnel-dwelling species relied upon. It involved extracting and interpreting very small temperature differences from thermal emissions; it was exactly what he and Parantham needed.
The tunnel snapped into focus, decorated with elaborate patterns where the fungus in all its variety grew. Despite the strangeness of the view, the new system felt right: Rakesh knew where he was now, how to move, and what to expect to see when he did. It was unsettling to be reminded that vision was a highly refined form of knowledge, a set of propositions about the world that needed to be deduced, not some passive stream of data that simply flowed into his skull as effortlessly as light into a camera.
They set out along the tunnel, which loomed over them like some monumental feat of engineering. It was only about two centimeters wide, but Rakesh had no way of knowing whether its builders would have viewed it as a cramped passageway, a great highway, or something in between.
They'd chosen not to use the avatars' vibration sensors as their primary mode of perception, but that didn't stop them picking up a faint but rising beat conducted through the tunnel wall. "Should we go and explore that?" Parantham asked.
Rakesh said, "It sounds as if it's coming toward us already."
A giant creature came scurrying around a bend in the tunnel. It was moving on twelve legs like a busy arthropod, about a millimeter across. Their mode of vision rendered it translucent, revealing hints of membranes and chambers flexing and contracting within.
When it changed course to charge straight toward them Rakesh suppressed the urge to flee; their avatars were extremely robust, and in any case easily replaced. The creature halted and inclined the axis of its body toward him; it seemed presumptuous to assume that it was lowering its face, when Rakesh could make no immediate sense of the complex mass of bristles, knobs and tendrils that confronted him. A cluster of these organs suddenly sprang forward and made contact with his avatar, wrapping it and holding it firmly; he steeled himself for the shock of being vicariously swallowed, but after a moment the creature unwrapped him and disengaged. It stood motionless for a second or two, as if pondering the need for another taste, then it turned away and continued down the tunnel, as rapidly as it had approached.
Parantham said, "W
e should follow it."
"Yes."
The avatars had small fusion-powered ion thrusters attached like backpacks; with no gravity or air resistance to overcome, catching up with the creature and flying a few centimeters above it was easy. Having dismissed them as inedible once, the creature seemed untroubled by their presence, if it was aware of them at all.
The creature had shed cells on Rakesh's avatar, and he had the nanomachines sequence them as he flew. They shared the fungus's vacuum-hardening traits, and a large proportion of its other genes, both natural and introduced.
Parantham said, "I'd like to run a morphogenetic model. What do you think?"
"Coarse-grain it, and I think that would be ethical." Software could take the genome and use it to simulate a growing embryo. A fine-grained simulation would necessarily experience everything that a real organism would, but a coarse-grained simulation could provide information about the range of generic experiences that were possible, without anyone actually experiencing them.
"All right."
In a spare corner of Parantham's mind, a sketch of the virtual creature took shape. While Rakesh watched the adult below him scurrying along, pausing now and then to graze on patches of fungus, a second viewpoint showed him an annotated diagram of the developing embryo in its egg case. As morphogen gradients washed over the dividing cells, eight distinct segments formed, the middle six slowly sprouting a tightly folded pair of legs each. Mouthparts, excretory and reproductive organs were whittled out of the growing mass of cells. The developing nervous system was extremely simple, and by the time the egg hatched it was close to hardwired: a handful of innate drives and reflexes would enable this creature to move, feed and mate, but it had no potential to do anything more complex.
