Exultant

by Stephen Baxter

  The cadets seemed to understand that. If you were stuck in your skinsuit on a Rock falling into Xeelee fire, the great sweeping strategy of the war meant little. You were there to fight for your comrades. Very close bonds formed between the cadets—bonds that were strictly non-Doctrinal, as you weren’t really supposed to have loyalties to anything but the greater cause. But the instinct to fight for your comrades seemed as deep as humanity itself. It couldn’t be denied—indeed it had to be encouraged, quietly, whatever the Doctrines preached.

  Pirius tried not to think about his situation. He knew he wasn’t here to think. But there were obvious questions he couldn’t help asking. For instance, why use human muscles to dig when you could get machines to do it for you?

  He heard a whole series of rationales. Even after millennia of development it was difficult to shield equipment from the blistering radiation environment inside the Mass. Machines were liable to break down—and of course they drew the fire of the Xeelee. Humans were comparatively robust, at least for a while. Then there was the psychological factor: the trenches and foxholes were there to provide cover for the infantry, and nobody trusted a trench dug by a machine as much as one you dug out yourself. It was good for morale, then, to keep digging, digging.

  But his pilot’s training prompted more questions. Why stick to such a crude strategy? Even using ground troops you could imagine more subtlety. You could coordinate your forces, strike with precision, move on.

  He wasn’t about to ask such questions of Captain Marta, but, soaking up his training, he could figure out what the answers would be. A Rock offered shelter, so you had to stick to its ground. But in combat a Rock was drenched in firepower—and, even if the Xeelee didn’t show up to play, in the hard radiation of the Core. You couldn’t rely on communication, coordination, in such circumstances; you had to train for a worst case, in which every platoon, maybe every trooper, was cut off from everybody else, save for what she could see of the battlefield around her. In this ultimate war, only the crudest of tactics could be relied on to work.

  But, guided by his conversations with This Burden Must Pass, he began to suspect that the truth behind the strategy was ideological. Clinging to humanity was the essence of the Druz Doctrines, the principles that had kept mankind united across twenty thousand years and the span of a Galaxy. So humans had to wage this war, humans had to dig their trenches, and fight and die, not their machines.

  Pirius built up a new image of the Front in his head. It was a great shell enclosing the center of the Galaxy, and it was studded with worldlets like this one, and on every one of them there were human beings, digging and burrowing. They were digging for victory; that was what the instructors told them. And whether or not they ever achieved that victory, Pirius thought, with every spadeful of glistening asteroid dirt, the Druz Doctrines and the unity of mankind were reinforced that little bit more.

  Chapter 14

  Two weeks after Pirius Red’s first test flight of the Xeelee nightfighter, Nilis set up a briefing for the Minister. It was held on Enceladus, moon of Saturn. Minister Gramm attended with his peculiar Virtual “advisor” Luru Parz, and Commander Darc and one of his adjutants represented the Navy.

  And Nilis began to lecture. Even before this glowering crew, in his typical overly academic way, the Commissary would never just state his conclusions: no, that wasn’t his style. He had to establish the facts first; he had to educate his audience.

  Since analyzing the results of the tests on the Xeelee craft, Nilis said, he had hardened his ideas about the nature and origin of the Xeelee. He tried to talk his way through a very complex series of graphics which supported, he said, his hypothesis about the nature of the Xeelee nightfighter: that it was not just a machine.

  “Life on Earth is of course built on oxygen-carbon chemistry. But a wide range of such compounds are possible under chemical law. If you analyze the contents of carbon-compound material scraped from a lifeless comet, you get a broad, smooth distribution like this.” A flat, even curve. “An indiscriminate melange of many compounds. Whereas if you analyze a scraping of my skin, for instance, a sample from a living being, you get this.” A spiky distribution showing a heavy concentration of certain compounds, nothing of others. “We call this the building-block principle, and it’s believed to be a universal feature of life. There is a strong selection toward standard building blocks, you see: living things from Earth use the same handful of key components—amino acids, sugars—over and over, out of all the theoretically possible compounds—”

  “A Xeelee nightfighter isn’t made of amino acids,” Gramm growled.

  “No. But look here.” Nilis showed displays of substructures he had observed in the Xeelee’s design, in its condensate hull, even its spacetime-defect wings. The distributions were spiky. “You see? A characteristic building-block pattern. And that has certain consequences. Of course any life-form must have certain features—notably an information store.”

  He began to speculate about how a Xeelee genome might be stored. A genotype of an organism was the internal data store that defined that organism’s growth and structure; Nilis’s own genotype was stored in DNA. The phenotype was the expression of that data, like Nilis’s body. Nilis said that extended quantum structures had been discovered in the “spine” of the craft. So far it had only been possible to hack into the simpler communications loops that controlled the ship’s basic operations. But if he was right, somewhere in there was stored the equivalent of Xeelee DNA.

  “They may reproduce through some exotic principle, much more sophisticated than our own molecule-splitting. We know they use quantum entanglement to communicate. Perhaps for a Xeelee, giving birth is more like teleportation, making a copy of oneself outside the body.” He imagined what might be possible if human hackers could break into that genotype, how Xeelee technology could be hijacked… .

  His listeners took this in with resentment and impatience. Pirius thought it was remarkable how a genius like Nilis could continually misjudge the mood of his audience. Pirius himself was sanguine. He had become a veteran of incomprehensible technical briefings long before he left Arches Base, and he knew how to keep up a show of attentiveness while letting his thoughts wander.

  Pirius had picked up some gossip from the locals. An ice-coated ball of rock, Enceladus wasn’t even Saturn’s largest moon—that was called Titan. On Titan, vast factory ships cruised seas of hydrocarbon slush and processed it into nano-food, to feed ever-hungry Earth. Of course, these days all this was controlled by the agencies of the Coalition, but Titan had a racy history. Titan had once been the most populous human world beyond the orbit of Earth itself. Even now—so the locals informed him—in the great ports with their ice-carved harbors, where kilometer-long factory ships put in to offload their stores and hundred-meter-high waves lapped like dreams, there were exotic adventures to be had, if you knew where to look.

  But Pirius hadn’t seen Titan. He was stuck here on Ensh, as the locals called it, which was just another Navy base that could have been anywhere from here to the Prime Radiant itself. Once, it wouldn’t even have occurred to him to feel restless. But now he felt as if his curiosity had been opened up by his time in Sol system. What else was out there to be experienced—what else might he already have missed, if not for the strange irruption into his life of Pirius Blue?

  He tried to focus on the discussion.

  Commander Darc was out of his depth. “Forgive me, Commissary. I’m just a humble tar. Are you saying that the nightfighter is alive? That the Xeelee are their ships?”

  “I, I—” Nilis stumbled, wiped his face with the back of his hand. He was overworking, Pirius knew, stretching himself thin across Sol system. “Yes, if you want a short answer. But it isn’t as simple as that. I’m saying there is no distinction between the Xeelee and their technology.”

  Luru Parz seemed amused by all this. “But, Commissary, spacetime defects or condensate—neither seems very promising material to make a phenotype out of.
Unlike the carbon-compound molecules of which you are made, for instance, there simply isn’t much of it about.”

  “Quite true.” Nilis smiled at her. “Most life-forms we have encountered have a certain commonality. Space is full of prebiotic chemicals, the carbon-based chemistry that underlies our kind of life—stuff like simple amino acids, ammonia, and formaldehyde. This stuff is manufactured in interstellar clouds and rains down on the planets. Even today, thousands of tonnes of the stuff fall on Earth, for instance. So carbon-water chemistry is really an obvious resource for making life. Of course there is little in common in the detail between humans and, say, Silver Ghosts. But we derive from the same prebiotic interstellar chemistry; in a deep sense we are indeed cousins.

  “But, as you say, Luru Parz, the Xeelee are different. These spacetime defects of which they have been baked aren’t common at all. Or at least, not now … but there was a time in the universe’s complicated history when they were common. The Xeelee—or their progenitors—must surely have arisen in an earlier age of the universe, an epoch when spacetime defects proliferated. But that era was in the first moments after the singularity. If that’s true, the Xeelee have very deep roots in time.”

  Gramm made an explosive noise through his plump lips. “You goad me beyond endurance, Commissary. This is supposed to be a military briefing! Will—you—get—to the point?”

  Nilis leaned on a desk and glared at Gramm. “The point, Minister, is that we now may understand why the Xeelee cluster around Chandra, the black hole. They need its deep gravity, its wrenching spacetime curvature.”

  “Ah.” Luru Parz nodded. “To them, Chandra is like a last fire in a universe grown cold.”

  “But there’s more than that,” Nilis said. He started to describe the condensate superstructure of the craft. “Now, condensate matter was common at a certain stage in the early universe—but a different stage from that when the spacetime defects emerged. It was a cosmic age as alien to the first as will our own far future be to us, that age when all the stars have died, and dark energy dominates the swelling of spacetime… . But the Xeelee, or their forebears, managed to form a partnership, a symbiosis, with these remote beings. Through that symbiosis they have managed to survive the slow unravelling of the universe—and it persists still, in the fabric of their craft.

  “How do we compare, then, with the Xeelee? There are some who argue that there have been ten crucial steps in the evolution of humanity… .”

  The ten began with the development of a DNA-based genetic code, and continued with steps Pirius understood only a little: the exploitation of oxygen to provide free energy, the use of glucose in energy metabolism, the development of photosynthesis, and the incorporation of mitochondria—like miniature power plants—into complex cells. “The first great triumph of symbiosis,” Nilis said enigmatically. His remaining steps were the formation of a nervous system, the evolution of an eye, the development of an internal skeleton to allow the colonization of dry land, the evolution of the backboned animals.

  “And finally,” said Luru Parz dryly, “the magnificent emergence of Homo sap ourselves.”

  Nilis said, “You might get picky about some of the steps—and alien creatures, of course, would have their own set of developmental steps—but the idea is clear enough. And certainly, for better or worse, humanity has not progressed beyond my step ten.”

  “But perhaps in the Xeelee,” said Luru Parz, “we get a glimpse of what step eleven might be.”

  “Yes,” mused Nilis. “We see extensions of the possibilities of life. A deep merging with technology. And a symbiosis, not just with other denizens of the same biosphere, but with aliens, different biospheres altogether—even with creatures from different ages of the universe, creatures governed by different physical laws. It’s actually a remarkable vision,” he said, almost dreamily. “It’s as if the Xeelee are more deeply embedded in the universe than we are.”

  “Oh, but this is all—” Gramm seemed outraged. “Is that the message you want me to take back to the Grand Conclave, Commissary? That the Xeelee do not only possess better firepower, processing capability, tactics—they are also, in some sense, biologically superior?”

  Nilis sighed, the hollows around his eyes deepening. “Minister, to destroy something, you have to understand it. We now know that the Xeelee are far older than we are, that we are dealing with relics of the antiquity of the universe. This battle of ours concerns the past as much as the present, or future.”

  That hung in the air for a long moment. Then the meeting continued, even more stormily than before.

  Chapter 15

  On the Moon, Torec spent long days and sleepless nights researching, chairing meetings, forcing face-to-face confrontation with recalcitrant techs, and scouring over every centimeter of her prototype setup.

  Following Nilis’s advice, she tried to impose discipline on the project. She forced her warring bands of technicians to agree on the designs for the interfaces between their components, and to work to those designs. And she imposed a series of freeze points beyond which change outside certain boundaries wasn’t allowed. The techs grumbled, but they got on with the job. She even suspected they were glad to have her show a bit of toughness, as if this was how they had expected her to behave since the beginning.

  But it took three weeks before she was satisfied; three weeks that used up all her remaining time before Gramm’s deadline. The next test run would be the last, come what may. It had to work.

  This time she was the first on the viewing mound, not the last. Again Nilis’s anguished Virtual was here, and Gramm sent a copy of his advisor, the supercilious woman Pila.

  But this time Luru Parz showed up, too.

  Once more the monitor bots floated into their ready positions, and the technicians cleared away from the ungainly prototype. As numerals on a hundred glowing clocks counted down, Luru Parz came to stand beside Torec.

  There was an extraordinary stillness about Luru Parz, Torec thought; she was as still as the ancient Moon itself. And she was dark. The light was bright, for this was noon of the long lunar day. But Luru seemed to soak up the light; though her Virtual cast no shadow, she looked oddly like a shadow herself. Torec was given to understand that this Virtual was not an avatar, a semisentient copy of an original with whom its memories would be merged after it had fulfilled its function. This Virtual was a mirror of life, which must mean, given the lack of any perceptible time delay, that Luru Parz herself was somewhere within the Earth-Moon system—or else she was linked to the Moon by some FTL channel, which would be hugely expensive.

  Luru Parz said to Torec, “So you have codified Pirius’s time-hopping technique.”

  “Yes, ma’am.”

  “Describe your algorithm.”

  Torec took a breath. Despite the way she had hammered away at her techs to get them to talk to her comprehensibly, the theory of the CTC software was still her weakest point. “We give the system a problem to solve, in the case of our prototype to find a particular protein geometry. And we give it a brute-force way to solve the problem. In the case of protein folding, we instruct the processor simply to start searching through all possible protein geometries. And we have a time register, a special cache that stores a flag if a signal has been received from the future.

  “The basic CTC program has three steps. When the processor starts, the first step is to check the time register. If a signal has been received—if the solution to the problem is already in memory—then stop. If not, we go to step two, which says to carry out the calculation by brute force, however long it takes. When the answer is finally derived, we go to step three: go back in time, deliver the solution and mark the time register.”

  Luru nodded. “So the timeline is redrafted. In the first draft timeline, the problem is solved by brute force. In the final version of the timeline, the answer is sent back through time to the moment when the question is posed. So it isn’t necessary to run the computation at all.”

  “T
hat’s correct.”

  Luru sighed. “The joy of time-travel paradoxes. You can get the answer to a problem without needing to work it out! But there must be a good deal more to your design. Your closed-timelike-curves must be pretty short.”

  “Actually just milliseconds.”

  “Surely you can solve no problem which would take longer to solve than that length of time.”

  Torec smiled, her confidence growing. “No. By breaking a problem down into pieces you can solve anything.” She described how the problem was broken up into a hierarchy of nested subcomponents. At the base level were calculations so trivial they could be handled within the processor’s short CTC periods. The answers were passed back in time to become the input for the next run-through, and so on. That way an answer was assembled piece by piece and looped back repeatedly to the zero instant, until the overall problem was resolved. “The technical challenge is actually decomposing the problem in the first place, and controlling the information flow back up the line,” she said.

  Luru laughed, an odd, hollow sound. “You’re computing with multiple time loops, and you think that’s the only challenge? Ensign, you’re a true pragmatist… . I think it’s nearly time.”

  Over the glittering, much-patched array of the prototype processor, the bots hovered, utterly motionless against the greater lunar stillness. Behind the prototype, the blank Virtual screen hovered, waiting to display the solution.

  The last seconds wore away.

  And at zero, the screen filled with a molecular diagram. Just like that, with no time elapsed. It was almost anticlimactic, Torec thought.

  There was utter silence on the common loops; nobody moved, none of the techs or the observers from the Navy or the Ministry, not even a bot. But on the screen the diagram whirled, as it was ferociously analyzed for verification. After ten seconds, the screen turned green, and numerical results scrolled over its surface.