by Greg Egan
“That would be good. You know the food hall, about halfway to the summit from here—?”
“Yes.”
“Can we meet there?”
“That would be fine.”
“I’ll see you then,” Livio said. He tipped his head in thanks to Ada, then returned to his workbench.
Tamara was silent most of the way to the observatory. Livio seemed charming, civilized and enlightened. He would send their children out across the void, reveling in the starlight. Even if it was all an act, it was a more appealing performance than Tamaro’s endless lectures on her familial obligations.
But she couldn’t help feeling a twinge of claustrophobia at the thought of where she was being led. However genuine Livio’s virtues might be, the ultimate purpose of their alliance would be the same: one day, they would come together to end her life. This charming man might never coerce her—but she would still be expected to make the choice herself before her co-stead grew too old to raise children, and before any other fate befell her own borrowed flesh.
31
Carla looked up from her desk to see Romolo approaching, dragging himself across the workshop. Shafts of light escaping from the sunstone lamps crisscrossed his path, bright enough to be delineated clearly by the dust in the air, flaring into dazzling patches of radiance as they fell on his skin. She had been told to stay alert for warning signs, defects in her vision that might presage a relapse, but how would she tell? Everyone working in this maze of light was blinded momentarily a dozen times a day.
“I think I might have found something,” Romolo said cautiously. He offered her a strip of paper from his spectrograph.
The single dark line standing out against the background was in the far ultraviolet. For an instant Carla came close to panic: if this was the signature of annihilating luxagens, whatever was behind it could kill them all. But the line’s position against the calibration marks made its wavelength even shorter than the UV Ivo had recorded on the Gnat—and the notion that Romolo might have shaken some dormant cache of negative luxagens out of a lump of clearstone would have been fanciful at any frequency. Sharp, monochromatic UV didn’t have to come from the destruction of matter. All it would take would be stimulated emission between two closely spaced energy levels.
Carla freed herself from her harness and followed Romolo back along the guide rope. Across the workshop, five other students were working on their own samples. The ancestors who’d stocked the warehouses of the Peerless had made an effort to be comprehensive; more than three gross varieties of tinted clearstone were represented. Under the microscope about half of these had turned out to be variegated, with the hue they presented to the naked eye just a blend of the colors of various inclusions, but the remainder, the apparently pure specimens, would still take years to test.
Romolo’s current candidate was a dark green cylinder the size of Carla’s thumb, carefully polished so the ends were flat and parallel, held in place between one full mirror and one thin enough to be partially transparent. Making the search even more laborious was the need to try out each sample of clearstone with a dozen different pairs of mirrors. The lowest grades of mirrorstone produced small but measurable alterations in the hue of the light they reflected, but the same effect had the potential to ruin a coherent light source even when it was too small to measure. For now, the only solution was sheer force of numbers, with the hope that random variations among the highest quality mirrors would produce a few that were good enough to allow the device to function.
Romolo had shut off the sunstone lamp before coming to fetch her, but at her urging he lit it again, then quickly pulled the housing closed around the apparatus, leaving most of the lamplight to fall upon the sides of the clearstone. If they really had stumbled on a substance with just the right pattern of energy levels, this light would be pumping luxagens from their natural state in the highest level, down to one from which three separate jumps would take them back to their starting point, completing the cycle.
From the end of the cylinder, a diffuse, wan green disk fell onto the screen at the entrance to the spectrograph. To the naked eye, there was certainly no sign here of a perfectly aligned coherent beam. “Why couldn’t it be a longer wavelength?” Romolo lamented.
“Ha! Don’t be ungrateful.” An invisible light source wouldn’t please the navigators, but Carla would accept vindication of the underlying theory in any form. “The spectrum shows a strong, monochromatic signal,” she said. “Stronger than the UV of the same frequency in the sunstone’s light. So at the very least, you’re shifting energy into that line somehow. What we need to check next is the coherence length.”
She reached into the cupboard below the bench top and found a suitable double-slitted screen. It was a nuisance not being able to see the results immediately, but Carla mounted the slitted screen and a UV-sensitive camera in the path of the beam.
The image from the camera showed a clear pattern of interference stripes, as expected from any monochromatic light source—even a single hue filtered from the chaotic mixture emitted by a lamp. So long as the tiny difference in travel time that arose when the wave passed through one slit or the other didn’t exceed the lifetime of each wave train, the light from the two slits would interfere this way.
“Now, what would clinch it?” Carla waited for Romolo; she hadn’t spelt out every possible confirmatory step in the protocol the students were following, but she was sure he could think of something.
“We delay one of the paths,” he suggested.
“Yes!”
He took a small rectangular slab of clearstone from the cupboard, its faces polished to optical flatness, and mounted it against the screen so that it covered just one slit. This setup would leave the geometry of the light paths much the same, but the extra travel time through the clearstone would be more than enough to destroy the interference pattern from any ordinary source.
Romolo loaded the camera and made the exposure. When he retrieved the paper, Carla’s skin tingled with excitement. The interference pattern was shifted off-center, but the stripes were almost as sharp as before. The wave’s oscillations were following a regular sequence of peaks and troughs that persisted for so long that the delay couldn’t scramble the smooth variation of phase shifts responsible for the pattern.
“Coherent light,” she said. “Invisible or not, the principle’s the same. Congratulations!”
Romolo seemed unsure what to make of his achievement. Carla said, “In all of history, no one’s seen light like this before.”
He managed a self-deprecating chirp. “But do I get to tell my grandchildren that I helped to solve the fuel problem?”
“Maybe. Let’s see where this takes us.”
Carla gathered the whole team to watch the next test, checking the beam’s collimation. A truly parallel beam was impossible, but images of the UV light emerging from the end of the green cylinder showed a disk with no detectable change in size across the entire width of the workshop.
“The wavefront speed of this beam will be tiny!” Patrizia enthused. “Trap some luxagens in the valleys, and we might even have time to watch them jump levels.”
Carla said, “Slow down. Anything trapped in these beams will only be confined in one direction. That’s not enough to force the luxagen waves to take on discrete energies.”
Patrizia hesitated. Romolo said, “Couldn’t you use three beams, for the three dimensions?” He gestured with his hand, sketching three orthogonal planes in the air. “Combine three waves, and you can hem the luxagens in on all sides.”
“Perhaps,” Carla conceded. If the wave trains from all three beams were long enough, the pattern they formed together could persist for a significant time. And as Patrizia had said, the wavefronts themselves would be moving relatively slowly. This weird array of hills and valleys—like the energy landscape of a solid, but floating ethereally in the void—would drift backward through the beams that created it, carrying any cargo they managed to load into i
t.
Carla set Romolo to work testing all the mirrors in the workshop with the same slab of clearstone. Seeing whether the device still worked after the substitution would finally reveal which of the mirrors were good enough for their purpose—sparing his colleagues years of wasted effort.
At the end of the day she took Patrizia and Romolo aside. “I think the beam trap is an idea worth trying,” she said. “If we can arrange things so the majority of the valleys end up containing at most one luxagen, that would be the simplest possible system to study—maybe even simple enough for us to map out a direct connection between its spectrum and its energy levels.”
Romolo looked daunted. “How will we know what energy state the luxagens are in to start with? I don’t see how we can control that.”
“I don’t think we can,” Carla agreed.
Patrizia said, “Suppose we feed the luxagens in at one end—we don’t just scatter them into the valleys everywhere. Then we can sample the light that’s emitted at various distances from that starting point, which will tell us what’s happening at various times after the luxagens are dropped in. Every transition will take place at a different rate, so at least that should spread things out, making it easier to untangle what’s going on.”
The three of them worked together, sketching a preliminary design for the apparatus they’d need. The new project would not be simple, but Carla hoped the detour would prove to be worthwhile. Luck had delivered them a coherent light source that nobody could see, but if they could leverage that into a deeper understanding of the rules that dictated the behavior of every solid, they’d have a chance to make the rest of the search far more systematic.
32
Carlo gazed down at the forest canopy, the light from the giant violet flowers beneath him struggling through the murk of dust, loose petals and dead worms.
“Don’t panic!” Amanda called up to him. “Once I see where you are, I’ll throw you the rope.”
“You can’t see me? I can see you!” They were both peering through the same detritus—but if the sight of her was easy enough for Carlo to fix against the variegated glow of the treetops, from her point of view he’d be drifting through a formless clutter, back-lit by the ceiling’s uniform red moss. Breezes stirred the airborne litter around him, each small gust creating a flurry of petals, and not even the worms remained undisturbed. If she caught a glimpse of him, then glanced away, there’d be nothing to guide her back.
“Ah, I’ve got you now!” Amanda replied. “Get ready.”
Carlo saw her fling the end of the rope up from the canopy. It was a good throw, and she managed the uncoiling well, leaving the hook following an almost straight trajectory as it ascended. He reached out hopefully, but the rope passed half a stride beyond his fingertips as it extended past him. A moment later it was fully uncoiled, and he strained toward it on the chance that it might yet come his way as the hook rebounded, but instead it whipped sideways before folding messily and drifting back toward the thrower.
“Sorry!”
“That was close,” Carlo called back encouragingly. He was ascending, though; they’d probably only get one more try. It was not as if he could be stranded here forever, like some fire-watcher lost in the void, but if Amanda had to come back with a rescue team the humiliation would take years to live down. No adult on the Peerless—save the most reclusive farmer, accustomed to living entirely under gravity—would misjudge a leap from a guide rope or a solid wall. But Carlo hadn’t been in the forest since he was a boy, and he’d lost the instinctive feel he’d once had for the complicated way a slender tree branch could recoil.
“Hey! I can see an arborine!” He regretted the words as soon as he’d spoken them; this was not the time to offer Amanda distractions. But their quarry was maddeningly close: the female was clinging to the very same branch from which he’d inadvertently launched himself above the forest. She was about his own size and slightly built, but if her physique was not intimidating her behavior was disconcerting. Lizards and voles mostly stared right through him, but this animal was gazing up at him attentively, and she seemed to have had no difficulty spotting him amongst the litter.
“Tell me later,” Amanda replied sensibly. She had gathered up the rope again, and now luck had granted them clear sight of each other through the forest’s detritus. She tossed the hook directly at him.
Carlo drifted aside as it ascended, but not so far that the rope went out of reach. He seized it before it was taut, then waited anxiously for the forces to be distributed, afraid that the far end might whip itself out of Amanda’s grip—or worse, that the struggle to hold on to it might dislodge her from her branch. But she held firm to both the rope and the tree.
Carlo dared fate with a cheer of jubilation. The arborine was still watching him. He wondered if it was worth trying to get a dart into her from his present vantage; all the crud in the air wouldn’t help, but he’d never get a clearer shot in the maze of the canopy. He reached into the belted pouch he’d made and retrieved the slingshot, but when he felt around for the darts his fingers instead found a tear in the material. One small item did remain: a sheath from one of the darts. He was lucky he hadn’t ended up paralyzed himself.
Amanda saw the slingshot in his hands. “Leave it!” she shouted. “We can come back tomorrow with an expert.”
Carlo’s pride was wounded, but she was the only one with any darts left. “You’re right,” he replied. He began dragging himself along the rope, back toward the canopy. He looked around for the arborine, but she had vanished into the forest.
Carlo had expected Lucia’s workshop to be full of lizards, but it seemed all her captives went straight to the breeding center. On the walls there were dozens of sketches of the creatures, along with botanical drawings, all keenly observed and skillfully rendered.
Lucia’s family had been supplying biologists with animals from the forest for three generations. She’d been a young girl the last time anyone had requested an arborine, but she claimed that her father had let her come with him to watch the procedure. “It’s pointless trying to pursue them,” she explained. “You might entertain them for days that way, but you’ll never catch them. All you can do is pick a good spot and wait.”
Carlo couldn’t see how that would work. “If they’re smart enough to stay ahead of a pursuer, aren’t they going to be smart enough to avoid a stationary threat?”
“They won’t come close,” Lucia replied, “but they won’t stay as far away as they need to. They won’t go near a net trap—they’ll smell it, even if they didn’t see you set it. But they don’t understand darts; people have only used them a couple of times since the launch, and arborines can’t pass knowledge like that on to their children.”
Amanda said, “We need a breeding pair, if that’s possible. Do you think you can recognize a pair of cos?”
“Not from appearance alone,” Lucia said. “But with luck, we’ll be able to tell from their behavior.”
The three of them met in the forest the next day. After they’d penetrated a short way into the undergrowth, Lucia told the biologists to wait and she clambered up into the trees.
Carlo clung to one of the guide ropes they’d tied between trunks on their last incursion. “We’re lucky these species are so long-lived,” he said, gesturing toward the tree roots that penetrated the netted soil and found purchase in the rock beneath. “That’s never going to happen again without gravity: no seedling is going to establish itself here. And no one’s going to give up their farms to make way for a new forest out on the rim.”
“You don’t believe they’re going to free up space from the engine feeds?” Amanda asked.
“Not in our lifetimes.”
Amanda looked around, puzzled by something. “The lizards haven’t exactly vanished, have they?”
“No.” Carlo had seen two or three the day before.
“If the lizard population hasn’t crashed,” Amanda reasoned, “the arborines shouldn’t be starving. B
ut when you look at the surveys of their numbers it’s pretty clear that they’ve mostly switched to biparity.”
Nobody had had the patience to try to observe any actual births among the arborines, but Carlo had seen the numbers too, and they were compelling. “Maybe the threshold is set differently for them,” he suggested. “They just don’t have to be as hungry as we do.”
“Maybe.” Amanda wasn’t convinced. “Or maybe it’s the fact that the males are struggling to feed themselves as much as the females.”
Carlo buzzed dismissively. “I hate to break the facts of life to a biologist, but it’s the female’s body that provides all the flesh.” Quaint folk tales notwithstanding, even the ancestors had weighed male animals before and after breeding and established that they made no measurable contribution to the blastula.
Amanda ignored the jibe. “Breeding is an exchange of information. The female has certain physical resources at her disposal in creating the offspring, but why wouldn’t she also make use of every available fact? Surely the male’s state of nutrition says as much about the scarcity of the food supply as the female’s own mass?”
Lucia called down to them, “I’ve found the right place! Come on up!”
When they reached her, Carlo could see what she’d been looking for. They were still below the canopy, but the branches protruded into an open space about six stretches wide. If the arborines were sufficiently curious, there was no reason they wouldn’t feel safe watching the intruders from across the gap. Carlo’s aim with a slingshot wouldn’t pose much of a threat at that distance, but Lucia had brought a dart gun powered by compressed air. It would have been insane to try to carry a bulky machine like that on a long chase through the treetops, but as a stationary weapon it wasn’t so impractical.
“Is there any behavior we need to avoid?” Amanda asked. Lucia had made no effort to keep them quiet; they were here to be noticed and attract a few onlookers.