Incandescence

by Greg Egan

  Roi pondered Zak's mundane analogy for the arcane symmetry he was proposing. She could see the appeal of it, but was that really enough to determine all the laws of weight and motion?

  She said, «What if the reality, with spin included, is two, not three? Then without spin, all the weights would be equal in size, but the garm-sard weight would be opposite the others. Wouldn't that be simple, too?»

  «Perhaps,» Zak conceded. «Perhaps it's too much to hope for the geometry of weight to match the geometry of resin.»

  «What we need to do,» Roi said, «is find some way to check. The map told us one thing, but our own weight measurements disagreed. We have to find another test, another measurement we can perform that will settle the question.»

  Zak made a sound of concurrence, then sank into contemplation. Roi looked around the chamber. How long had passed since she'd entered? A whole shift? She was hungry, but reluctant to move, reluctant to break her connection to Zak. The most important thing now was their work.

  He'd done it, she realized. All alone, without team-mates, with nothing but words, a couple of machines, and some simple ideas.

  She was not going back to the crops on the edge. He'd hijacked her loyalty. He'd recruited her.

  7

  The first thing Rakesh saw upon opening his tent was Parantham, seated in a chair, in human form. Her detailed appearance was not the same as that which he'd assigned to her back in the node, but her identity signal ensured instant recognition. As he stepped out of the tent he tensed his forearm; his body believed it was real flesh. A moment's further introspection told him that he was not modifying his perceptions in any way. As far as he could tell he was simply seeing her as she was.

  Parantham said, «Welcome to the bulge.» She was even speaking in his own native tongue.

  «Thanks.»

  She must have noticed his puzzlement, because she explained, «I thought it would make things simpler for our hosts if they only had to deal with one phenotype and one language.» She gestured at the instruments around them, which Rakesh had barely begun to take in. «Lots of hand-and-eye-driven interfaces, so it looks as if I made the right choice.»

  Rakesh told his tent to fold itself. They were in a large cabin, inside some kind of space habitat; a window looked out on to a densely packed field of stars, slowly turning, suggesting a centrifugal origin for the gravity he felt. They'd requested exactly the same destination address as Lahl, but her metabolic and ergonomic needs would have been very different, so their hosts must have undertaken some extensive reconstruction. Rakesh had no idea what the Aloof would have done if Parantham had asked to be embodied as a blind limbless blob: maybe piped all the data straight into their minds, which would have been useful. Then again, maybe their hosts would have split them up, requiring them to take turns to examine the meteor with different instruments tailored to their different bodies.

  The meteor itself was prominently displayed in the middle of the cabin, encased in a transparent enclosure, protected from contamination. As Rakesh walked over to it Parantham joined him. The object that had brought them all these thousands of light years was a dark gray slab of basalt about four meters across, its surface pitted with small impact craters.

  He said, «What do the Aloof think we can do with this, that they can't do themselves?»

  «Give a damn?» Parantham suggested.

  «They cared enough to summon us here.»

  «That wasn't difficult,» she said. «Though it might not be a question of effort; it might be a matter of what they see as appropriate. They might believe that they have no right to mess with this themselves, but we're entitled to know about it, and make of it what we will.» She smiled. «Though maybe that only applies to you, as molecular next-of-kin.»

  They left the cabin and circumnavigated the habitat, a spinning ring some two hundred meters across. The main corridor led them to a kitchen, storerooms, a bathroom, two bedrooms, an exercise room, and a workshop. It was both gratifying and slightly chilling to see how well the Aloof understood the human phenotype's needs. The fixtures all had a generic quality, rather than the look of something made by humans for humans, but many cultures within the Amalgam would not have done a better job. Rakesh had swallowed a library before they'd left Massa, so it was a moot point as to whether his hosts had read his mind as the source of all this, or had studied other unencrypted human travelers on their way through the bulge, but they certainly hadn't shaped this place from his own memories; there was nothing specific to the culture of Shab-e-Noor, and they hadn't covered the walls with portraits of his family or lovers. They really couldn't win, though, because such tact itself invited its own creepy sense of invasiveness: they'd peered inside him deeply enough to understand how wrong that would have been.

  If Rakesh felt naked, he had nobody but himself to blame. He'd known from the moment Lahl had offered him the key exactly how vulnerable he'd be, and he'd poured scorn on his friends' concerns. These were the terms, this was the deal; it was too late to have second thoughts. In principle, the possibilities for abuse were endless: the Aloof could be systematically torturing a billion helpless Rakesh-clones at this very moment. When he'd mentioned this primal fear to Parantham back on Massa, she'd pointed out that, while she'd regret the Aloof making anyone suffer, they could easily construct de novos of their own from scratch in order to mistreat them; sufficiently deranged sadists could always manufacture someone to torture, removing any need to lure their victims into a trap. In any case, Rakesh decided, there was nothing to be gained now from such paranoid speculation. Having handed their minds and bodies to their hosts as open books, the only sensible strategy that remained was to take their pleasant surroundings at face value and assume that the Aloof's hospitality, however narrowly defined, was genuine.

  Back in the meteor room, they set to work. Rakesh had never had reason to be much of a materials scientist or ejecta expert before, and as he invoked the aid of the library the knowledge that flowed into him brought a thrill of discovery, a sense of new vistas opening up before him, that stretched far beyond his immediate needs. Imbibing a massive bolus of pre-digested information was not his usual means of educating himself — he much preferred the slow process of building incrementally on his own prior knowledge, testing and interpreting every assertion before accepting it — but there was no denying the rush of suddenly having thousands of new facts and insights jostling in his skull.

  The equipment the Aloof had given them could probe the meteor's surface down to an atomic level; elicit and analyze emissions across the spectrum from gamma rays to microwaves; tomograph it in a thousand different ways; strike it, tap it, pound it, tickle it, and listen to the harmonics as it rang like a bell. Its gross chemical composition and its rarest impurities, its crystalline microstructure and the subtlest deformations thereof, were there for the asking. This rock, Rakesh thought, was as naked to them as they were to the Aloof.

  He and Parantham collaborated efficiently, discussing the best strategies for the investigation, speaking a dense specialist lingo that would have been foreign to them both just minutes before. The primary interface to all of the instruments was a touch-screen console, but mercifully they weren't limited to reading the screen and tapping menus; the Aloof had tailored the interface to their detailed embodiments rather than a generic notion of the ancestral human phenotype, and the console could exchange data with the infrared ports in their fingertips.

  Tomography alone was enough to locate the dead microbes, but it was necessary to send nanomachines crawling through the crevices to extract reliable DNA sequences. A dose of paleogenetic expertise from the library left Rakesh with no doubt that Lahl had been correct: these were not the corpses of any micro-organism, from any epoch, from any of the known DNA worlds. Their ancestors had probably been blasted off one of those planets billions of years before, on an entirely different piece of rock; that earlier meteor must have fallen to the ground somewhere in the bulge, and seeded a whole new biosphere. A bi
llion or so years later this lump of basalt had been flung into the sky; with better luck it might have contributed to the DNA panspermia itself, but it was a dead seed now. At least, no pristine world could have revived these desiccated, shocked, radiation-fried microbes, though perhaps if they'd achieved the unlikely fate of landing on a planet already awash with DNA-based life, the right species of distant cousin might have scavenged a few of these corpses' gene fragments and tried them out for new ideas.

  «The question now,» said Parantham, «is how do we find the parent world?»

  The DNA sequences were enough to assign probabilities to the meteor's «grandparent world»: the planet out in the disk whose ejecta had seeded the world from which this rock had been blasted. Even those probabilities were not sharp, though; there were seven candidates that were almost equally likely. Given the chaotic dynamics of the bulge, this did not do much to narrow the search.

  If the DNA couldn't help them, what of the rock itself? Three billion years before, lava flowing to the surface of the parent world had cooled into crystals of olivine, a magnesium-iron silicate, and augite, in which calcium, aluminum and titanium were also present. Subtle deformations in the structure of these crystals offered a partial history of the temperatures and pressures experienced by the rock since then.

  The sudden heat and shock of the impact that had thrown the rock into space had left distinctive chemical fingerprints as well as physical dislocations. Over time, in the cool of the interstellar vacuum, some of the substances forged during the rock's fiery ejection had slowly decayed, hinting at a date for the event: fifty million years before. At the same time, the high-energy cosmic rays that flooded through the bulge from a myriad of sources had corroded the meteor's surface, left chemical deposits of their own, scoured tracks deep inside the rock, and created trace amounts of new isotopes. As Lahl had claimed back in the node, both lines of evidence converged on the same date: the rock had apparently drifted through the bulge, unprotected by any atmosphere or planetary magnetic field, for about fifty million years.

  The same console that allowed them to control the analytic instruments provided access to a star map. When Parantham activated it, it opened with a view showing their own path and present location (labeled with a stylized picture of the habitat's ring), along with several hundred billion kilometers of the meteor's trajectory before capture (labeled just as clearly with an image of the rock). In fact, the meteor had barely been «captured» — it looked as if the habitat had been constructed around it, more or less matching its original velocity — but the map was careful to delineate between the undisturbed object and its present state.

  The map labeled the stars of the region solely by their physical characteristics, and despite phrasing this information in Rakesh's language, declined to adopt the catalog numbers or coordinate system that he would normally have used. Nevertheless, by invoking the library he could match the Aloof's descriptions with his own sources. The Amalgam's maps of the bulge were somewhat patchy, but there was more than enough overlapping data to establish a reliable fit.

  For the first time, now, they knew exactly where they were. They had traveled some thirteen thousand light years from Massa, and while part of that journey had taken them «west» — clockwise around the galaxy, looking down from galactic north — they had also penetrated deep into the bulge, and had ended up less than a thousand light years from the galactic center. Lahl had reached roughly the same conclusions, though she hadn't been carrying star maps of her own to compare with the Aloof's.

  This central region was distinctly more crowded and violent than the outer reaches of the bulge. Packed with massive gas clouds that periodically burst into life with episodes of star formation, as well as a varied population of older stars that had drifted in from the rest of the bulge, it was as different from the galactic disk as a teeming metropolis was from a rural backwater.

  Rakesh said, «Where do you think Csi and the gang are now?»

  «Dead to us,» Parantham replied bluntly. «And dead to each other as well.»

  «I was inviting light-hearted speculation,» Rakesh said dryly, «not baleful philosophical pronouncements.»

  «Then I'm sure they're having a wonderful time somewhere, sailing the high seas together.»

  It was true that they were unlikely to have much in common any more; they were not part of a synchronization clan, they had made no appointment for a reunion. They had probably spent most of the past twenty-five thousand years as insentient data traveling through the Amalgam's network, but even if by some extraordinary coincidence they had crossed paths again, the chances were that their various measures of the time that had passed would have been millennia apart, placing the memories of their shared experiences into very different perspectives.

  «So long as they're not still stuck at the node, then I'm happy,» Rakesh declared.

  He shifted his attention back to the map — constructed inside his skull, but shared with Parantham — which pooled data from the Aloof and the Amalgam and annotated it according to the explorers' own priorities. It was easy to rule out all the stars that were younger than the rock itself; after that, the next obvious step was to try to account for the direction in which the rock was traveling.

  The Aloof's map provided current velocities for the stars in the region — and like the stars' positions, these would have to be theoretical extrapolations from the latest data that could reach them at light speed — but it offered no past trajectories, either observed or computed. Rakesh wondered if this omission was a kind of strategic self-censorship; perhaps the Aloof considered that revealing just how long they'd been tracking these stars would grant some insight into the history of their civilization that they did not wish to disclose to outsiders. It could hardly have escaped their notice that the information would have been useful to their guests.

  «Do they want us to find this planet, or not?» he muttered.

  Parantham was undaunted. «They've had this meteor for at least fifty thousand years now. If their priority was making things easy for us, they could have located the planet themselves long ago, and sent us straight to it the moment we arrived. But that's not the deal. We're going to have to work for this. We knew that.»

  The best dynamical model in the library couldn't wind back time fifty million years without generating uncertainties many times greater than the average distance between stars. Lahl had mentioned six hundred candidate stars; Rakesh couldn't whittle this down to less than five hundred with celestial mechanics alone.

  Factoring in the chemistry of the rock made a difference. The Aloof's map included high-resolution spectra of each star, revealing the chemical composition of its outer layers precisely. Using a model of planetary-system formation it was possible to compute the probability of the rock's parent world being born from the same nebula as any given star. This reasoning was subject to its own uncertainties; nevertheless, the results allowed them to eliminate more than three hundred of their original candidates, and re-rank the two hundred that remained.

  Before Rakesh could invoke any kind of high-powered statistical analysis, Parantham said, «That can't be right.» The chemistry-based ranking was not at all what might have been expected, with some candidates shuffled slightly down the list while others were promoted a few places. Rather, the second list more or less turned the first one on its head. The chemical profile of the region's stars placed the rock's origins in a completely different direction than that from which it seemed to have come.

  «It must have undergone a sharp course change,» Rakesh suggested, «maybe even passing through another planetary system on its way.»

  «Either that, or its chemistry's distorted for some reason,» Parantham said.

  «So which trail do we follow?»

  «Both, I suppose.»

  Rakesh groaned. «So instead of halving our short list, we've just doubled it?»

  Parantham said, «We haven't finished yet.»

  «Of course not. I'm sure we can add
another thousand candidates if we keep trying.»

  Parantham selected close-up views of one star after another, but the Aloof's map displayed no planets around any of them. The data simply wasn't included, as if mere balls of rock were as irrelevant here as an anthill on a roadmap. Rakesh hadn't seriously expected to find the parent world in all its glory, teeming with long-lost DNA cousins, just by sitting here and zooming in on a map of the bulge, but a little more detail might have helped. The Amalgam's maps showed what was known, given the constraints, but if any world within the bulge had screamed «life» loudly enough for an observatory out in the disk to detect it, that would have been old news.

  The gene fragments they'd found in the rock gave some tantalizing hints of the kind of proof-of-metabolism signature that the parent world's atmosphere might contain, though as ever there were uncertainties; these rock-dwelling microbes didn't have to be typical, let alone dominant on the planet as a whole, fifty million years later.

  Rakesh said, «We need to make direct observations of our own.» The workshop had facilities that would allow them to construct a reasonably powerful telescope, but they lacked the raw materials to make anything big enough to analyze a planetary atmosphere from hundreds or thousands of light years away. They would need to travel further; they had no choice.

  The console's main menu did not include any category for travel. It occurred to Rakesh that Lahl had never explained to them precisely how she'd got the message through to her hosts that she'd spent as much time as she could with the meteor, and wanted to move on.

  After exploring every option pertaining to the habitat itself — including the ability to remodel the bathroom on command — Parantham finally realized that selecting a star on the map enabled a sub-menu with the unassuming option «Go to star». Choosing this did not change the map's viewpoint or magnification; rather, it caused the map to inquire politely, «Are you sure you wish to travel to this star?»