by Greg Egan
Carlo watched her eating, Zosimo beside her. For the first few days both of the males had tried to help their cos work the light probes out of their flesh, but the tubes were hardstone, impossible to bend or break, and Carlo had melded the females’ skin together around half a dozen loops set into the plinth. No ordinary deformation of the flesh could free them, and even if they’d grown desperate enough to bite or scratch themselves loose there was no access for teeth or claws.
The females had been unconscious for the surgery, and the aftermath should not have been painful, but Carlo still felt a twinge of revulsion at the fate he’d imposed on them. They would divide, or they would stay trapped: that was the verdict he’d written in stone.
Zosima had finished eating. She called out with an elaborate sequence of chirps, and when she received no reply she repeated herself; the pattern was almost identical, but some notes now rose more emphatically above the rest. After a moment Zosimo responded.
The exchange continued, longer and more elaborate than any Carlo had heard before. He’d been taught that the arborines lacked a true language, but he doubted that anyone was in a position to know. In the old reports there had been a crude attempt to classify the cries, but no systematic annotation of their structure. If the day ever came when future biologists were free of more desperate concerns, it might be worth someone’s time to spend a year in the forest, just watching these animals and listening to their calls.
Zosima stretched a hand up from the plinth; her co took it and she drew him toward her. Carlo hesitated, afraid for a moment that he was deceiving himself and misinterpreting the encounter. But Zosimo had released his hold on the branch and the two were maneuvering into a tight embrace. Carlo scrambled quickly along the cross rope and reached the lever under the front of the cage.
He tugged on it; it stuck. Trying not to panic, he pushed back on the lever and worked it past the obstruction, then he succeeded in disengaging the brake. The sudden clatter of the six light recorders was so loud that he feared it would startle the arborines into changing their minds, but when he looked up they were undeterred, oblivious to the machinery.
As Carlo watched them he was unable to dismiss a shameful sense of voyeurism, though it was doubly absurd when the recorders were capturing far greater intimacies than his gaze. But the arborines’ posture had never looked more like their cousins’, and the shape they made together was disturbingly close to the vision of Carla and himself that years of anticipation had burned into his brain. This was not like breeding voles.
Zosima grew still, but Zosimo continued to stroke her face as if to comfort her. A yellow glow shone out from the skin where they were joined: the male’s promise in light, committing him to care for the children. Carlo couldn’t untangle how much of his discomfort at the sight was guilt at forcing the arborines’ choice and how much was a rebuke from the part of his mind that would judge his own life worthless until he’d made the same promise himself.
Abruptly, the noise from the recorders took on a new component: the sound of paper being shredded. “No, no, no!” Carlo dragged himself under the cage into the dimly lit equipment hatch. As he approached the offending machine the tearing was replaced by a rhythmic thwack; the tape had broken completely and the loose end from the driven reel was whipping against the chassis. He shut off the motor and quickly removed both reels, but it took him another few lapses to tug all the fragments of damaged tape from between the capstans, then load a fresh reel and restart the machine.
As he emerged from the hatch and climbed toward the observation post, he saw that Zosimo had broken free of his co and moved back to the branch that overhung the plinth. Zosima was limbless now, and Carlo watched, disquieted, as her anatomy began to surrender features that no conscious effort could have changed. The ridges of her tympanum sunk into the membrane, then the whole glorious structure merged with the top of her chest, resorbed as easily as an impromptu arm. Her closed mouth, the dark lips already strangely flattened, lost its usual contrast with the skin of her face until it faded entirely from view. Her eyelids were the last aspect to go, the pale slitted ovals rotated and stretched out vertically, distorted like the remnants of insignificant wounds as the head itself began to reform. It was as if an unseen sculptor, having crafted this body in resin years ago, had returned to swipe her thumbs across the remains of the ruined face before squeezing the entire figurine into an undifferentiated lump, mere material now, ready to be reused.
Zosimo emitted a long, mournful hum. Carlo looked up at him, struggling to retain a sense of distance, but then the arborine’s whole body began to shake with anguish. Nature had bribed both participants in this metamorphosis, imbuing the act with an incomparable sense of joy, but while a vole could pass untroubled from the raw pleasure of triggering fission to the compelling obligations of nurturing his young, this arborine understood what he had lost. The companion who had loved and protected him all his life had just been erased before his eyes. What else should that elicit but grief?
Carlo averted his gaze and tried to regain his composure. How had he imagined it would feel, when he and Carla finally brought her life to an end? Had he ever really fooled himself into believing that he’d be borne through it all in a daze, anesthetized by the biological imperative, untouched by the gravity of what he’d done? No man ever told his children anything but anodyne lies, but if he could forgive his father for sparing him as a boy he could not forgive his own cowardice since. All his training, all his animal experiments, had only helped him bury the truth under a mountain of facts. He had to accept that his life’s greatest purpose, the one role that would make him complete, could never be right, could never be bearable, could never be forgiven. It wouldn’t matter how long they waited, it wouldn’t matter what plans they’d made, it wouldn’t matter how willingly they took the final step. In the end he would know exactly what he’d done, with only the children to keep him sane.
And that only if their number was right.
Carlo looked up. Zosimo was huddled against the branch, swaying, his upper arms wrapped around his head. He had fallen silent, but now Benigno and Benigna were howling in reply. But through all the arborine sorrow, Carlo finally had something to celebrate himself.
The surface of the blastula was marked with its first partition—and it was transverse, not longitudinal. Zosima would divide into just two children, and there was a chance that the light recorders had captured the signals underlying this result.
35
“It’s working!” Romolo announced gleefully. He held a mirror in the beam and wiggled it, sending a dazzling red spot careering across the walls of the workshop. “Visible at last!”
Everyone gathered around to play with the new device. Carla watched, delighted by the spectacle, but she hung back herself and let the rest of the team enjoy it.
It would take some effort to scale up this humble red spot into a light for the beacons, but in time the navigators would have what they needed. Ivo was already working on a machine for capturing samples of orthogonal matter with a coherent UV source. It was beginning to look as if the Gnat’s successor could be flying to the Object within a couple of years.
“We should use these for our engines,” Eulalia enthused. “No more exploding sunstone, just a photon rocket with an exhaust of pure light!”
“Er—” Romolo pointed to the lamp that was powering the device. “This beam’s only carrying a tiny amount of the energy from the sunstone. The rest is being wasted. If we tried to drive the Peerless using something like this, we’d need to burn sunstone so much faster than the original engines that it would vaporize the whole mountain with waste heat.”
“And the light source would stop working,” Patrizia added.
“Why?” Eulalia demanded. “From the heat?”
“No. From the acceleration.”
Romolo turned to Patrizia. “What do you mean, from the acceleration?”
“The light source only works at a particular frequency,” Pat
rizia replied. “If the Peerless is accelerating, then while the light’s traveling from one end of the source to the other, the clearstone will end up moving faster than it was when it emitted the light. Any change in the relative velocity between the light and the clearstone will change the apparent frequency of the light—so it won’t be at the right frequency to stimulate any more transitions.”
Romolo was at a loss for words, so Carla intervened. “She’s joking! Any frequency shift would be extremely small. Even at one gravity there’s no chance at all it could stop the light source from operating.”
“I was joking,” Patrizia admitted. “But maybe we could design a system that’s deliberately sensitive to the shift, and use it as an accelerometer—as a kind of navigational aid.”
Carla couldn’t think of any objection to that in principle. “Why not?” she said. “Another project for our grandchildren.”
Romolo angled the reflected beam onto Patrizia’s chest. The red disk looked like a hole in her skin, revealing the realm of light within.
Carla woke, her gut in spasms. She turned to the clock by the bed and waited for her vision to come into focus. Breakfast was still more than two bells away.
She lay beneath the tarpaulin, humming softly. She wondered if it would help if she made some kind of promise to herself, to end her hunger if it became too much to bear. But end it how? She couldn’t take Silvana’s way out, even if she’d wanted to: Carlo was so convinced that he could rescue her from the famine that he’d rather fend her off with his ridiculous knife than cure her of her misery. She wasn’t going to go off holin, or step out of an airlock. There was nothing to be done but to endure it.
She tried to sleep again, but it was impossible. She pulled herself out of bed and left the apartment. If she followed the corridor around in a circle until she was too weary to continue, she could drag herself back to bed with some hope of losing consciousness.
The precinct was quiet, the moss-lit corridor deserted. What did other women do, she wondered, when their hunger became unbearable? Did they lie beside their cos and fantasize about the day it would finally end, until one by one they abandoned all the plans they’d made for their lives and gave in to that glorious vision?
Carla searched for something cheerier to occupy her thoughts. Romolo’s new light source was a striking vindication of the whole theory of energy levels… but when she thought about the journey the device would enable, the prospect filled her with dread. Without a deep understanding of the annihilation reaction, any plans for an engine that burned orthogonal matter would be nothing but whimsy. But was it really her duty to face the risk of becoming fuel for that fire herself, not just once, but over and over again?
If she refused, there would be plenty of volunteers to take her place. She could still work on the theory underlying the reaction, but she would probably slip behind the researchers with first-hand knowledge of the new results. If Patrizia flew on the second Gnat and returned with a triumphant discovery of her own, it would finally place her reputation unambiguously beyond Carla’s.
Would that be so intolerable, though? Would it be unjust? They had both made contributions, but the most powerful ideas had been Patrizia’s. Looking back, it seemed to Carla that the best thing she’d done had been to impose some discipline on Patrizia’s wilder speculations and then tidy up the details of those that worked out. So perhaps she should reconcile herself to that role. If it was to be her legacy, better to value it than resent it.
What was left for her, then? More tidying up? Turning throwaway lines about accelerometers into real devices? If she could come up with a design for a light-based accelerometer that actually worked, there’d be nothing dishonorable in that. On the scale of a small craft like the Gnat it might be fanciful, but over greater distances there’d be more time for the acceleration to reveal its effect.
How long would it take the slowest detectable infrared light to run the full length of the Peerless, from the tip of the mountain to the base, and back? Still just a fraction of a flicker. In which time, at one gravity’s acceleration, the velocity of the mountain would have changed by… a few parts in the fifth power of a gross.
Carla dragged herself faster along the guide rope, determined to complete her first circuit of the corridor and get past her apartment while she was still distracted. As she pondered the problem she realized that she’d been careless: it was reasonable to assume that the light’s frequency would be unchanged when it bounced off the mirror that sent it back toward its source—that was the definition of a good mirror, after all—but she’d ignored the fact that the mirror would be accelerating along with the Peerless. On the finicky level of detail required to keep track of the tiny effects she was hoping to measure, that would be enough to change the result.
She worked through the geometry more carefully, sketching the history of the mountain’s extremities and the light moving between them. The frequency measured for any given pulse of light depended solely on the relative velocity between the apparatus doing the measuring and the light pulse itself—which in turn came down to nothing more than the angle between their histories. Those angles were easy enough to find, and four of them told the whole story.
The mirror’s acceleration into the oncoming light would mean that the light struck it a little faster than the speed with which it had left the source. But the light source, in turn, would be accelerating away from the reflected light. By the time the light came back to the source, the relative velocity between the two would be reversed but otherwise unchanged—and the net result would be that there was no frequency shift at all.
In principle the blue shift could be measured by comparing the light at the base of the mountain with a reference beam produced locally by a second light source. But the ideal method would involve a direct comparison between the shifted light and the original beam. Carla hunted for a way around the problem, but the geometry always led back to the same result: the beam would suffer a blue shift traveling down the mountain, and a red shift traveling back up. And so long as the light was reflected unchanged, on a round trip the two effects would cancel out. It was just a form of conservation of energy.
What about Eulalia’s flight of fancy, then: a photon rocket? Would frequency shifts disrupt the light source there, or not? If a beam of light was powerful enough to be the cause of the mountain’s acceleration, it would be imparting momentum to the mirrors it struck, and losing some of its own. It would no longer be reflected unchanged; it would have to experience a red shift.
How much, though?
That depended on the mass of the object each photon effectively bounced against. In the experiments with free luxagens, the light had been scattered back with a huge red shift; because the individual luxagens were less massive than the photons that struck them, their recoil had carried off a lot of momentum. In the inferior grades of mirrorstone that could ruin a coherent light source, the luxagens were still mobile enough to recoil significantly before they transferred their momentum to the bulk of the material. In the highest quality mirrors, the luxagens were bound so tightly to their neighbors that each photon was effectively colliding with a significant portion of the mirrorstone—a portion heavy enough to be unmoved. But there were limits to this collective inertia: a single photon could never bounce off an entire mountain, as if it were a rigid, indivisible whole. So it would be the material properties of the mirror itself, not the acceleration of the mountain, that determined the frequency of the reflected light.
Carla had lost track of her surroundings. She paused, clinging to the guide rope, and looked around the corridor at the doors ahead and behind her. She had passed her apartment twice, she realized, and she was now a short way into a third circuit. The reminder that her food cupboard was only a few stretches behind her was enough to make her gut start twitching again, but she resolved to complete a couple more circuits in the hope that it would be enough to let her sleep.
She took up the thread of her argument once more
. A poor quality mirror would reflect light with a small red shift, bouncing back photons that were no longer tuned to the gap between the energy levels that produced them. And perhaps a beam intense enough to be part of a photon rocket would exacerbate the effect, with the stronger light fields effectively “softening” mirrors that were perfectly adequate at a lower power. Was there any way around that? A red shift meant an increase in true energy: each reflected photon would be carrying too much energy to stimulate the emission of another photon from the original transition. But then, why not give it a different task? If its energy matched another gap between levels, maybe the whole system could be made to do something useful nonetheless.
Carla tinkered for a while, and came up with one possibility.
The luxagen started in the lowest of three levels; it would have to be pushed there by an external light source. From there, it jumped spontaneously to a higher level, emitting an infrared photon. Then it moved to a higher level still, emitting an ultraviolet photon.
Both photons were reflected back, red-shifted by their encounter with the mirror. But if the mirror’s properties and the spacing of the energy levels were related in just the right manner, the reflected IR photon would be able to push the luxagen back to the lowest level: exactly where it had started.
And then the cycle could begin again.
In each cycle, two photons were created and one was never recaptured. To balance that photon’s true energy, the clearstone and the mirrorstone would need to gain conventional energy; in principle, any mixture of kinetic, thermal and potential energy would do. But to balance the photon’s momentum, the device as a whole would need to accelerate, so the energy gained couldn’t be entirely in the form of heat. When a fuel met its liberator, the creation of light was accompanied by the creation of heat—and this device would certainly heat up to some degree. Over time, it might also suffer some degradation, some chemical change. But unlike burning fuel, it would not disintegrate in a flash, it would not turn to smoke.