by Greg Egan
Carla wasn’t entirely sure that his advice had been misguided. Whatever authority she had in her students’ eyes came from her ability to explain the phenomena she chose to put in front of them. This is where the lens focuses its image—just as our equations predicted! This is the angle at which the light comb diffracts red light—in perfect agreement with Giorgio’s formula! Talking to the class about her tarnishing experiments might have been a good way to assure them that the field was far from moribund—that new discoveries were still being made, and if they persisted with their studies they could be part of the vanguard themselves—but now here they were chasing free luxagens, and she had absolutely no idea what they’d find.
But it was too late to cancel the experiment. All she could do was try to get through the session without making a fool of herself.
Carla joined the students, called them to order, and began allocating tasks, starting with the polishing of the mirrorstone they’d use as a luxagen source. “We don’t have a lot of space here, so please move slowly and carefully. If you break something, tell me straight away. And if anyone touches the sunstone, they’re going straight out the airlock.”
The experiment they’d designed required a simple variation on the tarnishing apparatus: since they were aiming to maximize luxagen production while minimizing stray visible light, the mirrorstone surface would be exposed to nothing but infrared. A second beam from the same lamp—this one undivided by color, in order that it remain as bright as possible—would be directed across the vacuum above the mirrorstone, and an eyepiece on a semicircular rail would be used to check for light scattered from the beam at various angles.
Carla stood back and watched as everything came together, only having to intervene physically when Azelia became confused by the vacuum supply. “The low-pressure chamber we use is shared by other workshops and factories,” she explained. “It’s vented after each use—that’s why the access valve is locked now. If you’d managed to force it open, you would have made a direct path between the interior of the Peerless and the void, which is something we try to discourage.”
When all the apparatus was finally in place, Carla approached and double-checked the alignment of the optics. “Good job, everyone!” She managed to ignite the sunstone without flinching, then she called on Patrizia to extinguish the firestone lamp in the corner. They had taken care to block most of the spillage, and the beam that crossed the evacuated container ended up striking an unreflective black screen, so the moss-free workshop was in almost total darkness now.
Romolo was already in place beside the swiveling eyepiece, ready to do the honors. When Carla heard no movement from his direction she urged him to go ahead. He was probably as anxious as she was, having put his pride at stake with such a bold prediction. Light blasting luxagens out of a solid and into the void.
“First observation, three arc-bells from the beam axis,” Romolo began. There was a long silence. “I can’t see anything,” he said.
“Adjust the focus on the eyepiece, very slowly,” Carla suggested. “When your eyes have nothing to look at, they can end up focused beyond the point where the eyepiece is presenting the light. You can stare right through a weak image without even knowing it’s there.”
She waited while Romolo tried this. If there were luxagens in the container they should be scattering light in all directions, and the view perpendicular to the beam would be unlikely to include any stray reflections from the container walls. The primary lens of the eyepiece was as wide as the beam itself, so it could gather light over a much greater area than the pupil of an unaided observer, but if there were simply too few luxagens for the scatter to be visible, that was that.
“Still nothing,” Romolo admitted.
“All right,” Carla said. “Change the angle.” She couldn’t see how that would make any difference, but having gone to so much trouble it would be absurd not to collect a full set of observations.
The class stood in the dark, listening patiently as Romolo announced negative result after negative result. According to calculations that stretched all the way back to Nereo, any luxagen jiggling back and forth at a suitable frequency should live up to its name and create light. Individually, each particle would emit a bit more light parallel to the axis of its vibrations than in other directions—but if those vibrations were being driven by randomly polarized light all the individual biases would average out, so whatever pale glow the thin gas of luxagens produced, it should have been visible from any angle.
“Ah, I can see something! There’s a reddish light!” Romolo sounded even more surprised than Carla. He was down to an angle of six arc-chimes, almost staring into the beam itself, so he was probably just seeing light scattered by the container’s walls, rather than by anything in its interior.
Carla said, “Reach out and pull the lever that brings the shutter down over the infrared.” If the glow persisted, then it was nothing to do with any hypothetical luxagen wind rising off the mirrorstone.
Carla heard the click of the lever. “The red light’s gone,” Romolo said. “There’s nothing.”
“Lift the shutter again,” Carla suggested.
“Yes. Now the light’s back.”
“You must be blocking the visible light, not the IR!” Carla declared. She slipped past the students in front of her, then felt her way around the edges of the bench. She could see a faint splotch of gray where the beam came to an end, and once she was oriented she knew where everything was.
She put one hand on the lever that would bring the shutter down over the visible beam, then reached for the IR lever; Romolo’s hand was still on it. He buzzed in surprise and pulled his hand away. “Did I have the wrong one?” he asked, embarrassed.
“No,” Carla replied. “You didn’t.”
She asked Romolo to move aside, then she peered through the eyepiece herself and tried blocking each beam in turn. Shutting off either one made the reddish glow disappear. There was no escaping the conclusion, then: something that the infrared light was driving off the mirrorstone into the vacuum was scattering the visible light through a small angle—and showing a preference for red in the process.
Luxagen scattering was predicted to be stronger at the red end of the spectrum, but the small angle made no sense. Perhaps the mirrorstone was giving off a very fine dust, reactive enough to be absorbed by the container walls as soon as the IR was shut off. If these dust particles were transparent they could be refracting some light away from the beam axis.
Carla explained her hunch to the students, then swung the eyepiece around by almost half a revolution, in the hope of seeing some backscatter reflected off the dust. There was nothing. She went back to the light Romolo had found; as she moved the eyepiece even closer to the beam axis, the red tinge became less pronounced while the overall brightness grew a little.
But it was hard to quantify the changes in this complex mixture of hues. Carla asked Patrizia to relight the firestone lamp. “I don’t know what we’re seeing here,” she admitted, “but I think it will be easier to study if we try scattering a single color at a time.”
Following her instructions, Palladio and Dina fitted a prism and a color-selecting slot into the visible beam. “Let’s start with green,” Carla suggested.
With the workshop in darkness again, Carla bent down and looked through the eyepiece. She’d left it in the position where the scatter had first appeared, as far from the axis as you could go while still seeing anything at all. It took almost a lapse for her eyes to adapt sufficiently to pick up the weaker glow now that most of the beam was being blocked, but the glow was still there.
And it was red. Pure red. The green light crossing the container was being scattered—and in the process it was turning red.
Carla felt utterly lost. If nature had deliberately set out to mock her—to prove to her students once and for all that their optics teacher knew nothing about light—it could not have done a better job.
She steadied herself. This would
make sense, somehow; she just needed to be patient. “Who’s got good vision in low light?” she asked. After a moment Eulalia replied, “I’ve been doing fire-watch shifts lately, if that’s any help.”
“Perfect.”
Carla had Eulalia take her place at the eyepiece.
“What do you see?” she asked.
“Red light,” Eulalia confirmed.
Carla found the lever for the visible light shutter and closed it about halfway. “What now?”
Eulalia was silent for a pause or two. “A dimmer red light.”
“Is the color any different?”
“Not as far as I can tell.”
Carla addressed the students in the darkness. “Why did I reduce the intensity?” she asked.
Patrizia replied from the corner of the workshop. “If the luxagens were getting trapped in the light wave’s energy valleys, they’d be rolling back and forth in those valleys—giving off light of their own at a different frequency to the frequency of the beam.”
“So what does it mean that the scatter remained red when I made the beam dimmer?” Carla pressed her.
Patrizia said, “It means that explanation can’t be right. The exact shape of those valleys would depend on the strength of the light. A weaker beam would have made the valleys shallower… making the luxagens roll back and forth more slowly, reducing the frequency of the light they emitted.”
“Exactly,” Carla replied. But she knew of no other way that one pure color could give rise to a completely different hue. White light could end up being filtered selectively, changing its appearance in all kinds of ways, but when you started out with a wave of a single frequency it was supposed to make everything it touched oscillate at the very same pace, generating more light of the very same hue.
Carla opened the shutter fully again. Then she groped her way around the bench and adjusted the slotted screen in front of the prism that determined the color of the visible beam, changing it from green to blue.
“What do you see now?” she asked Eulalia.
“The light’s turned green.”
She pushed the slot back in the other direction, until the beam was yellow.
“And now?”
“Nothing,” Eulalia replied. “It’s gone dark.”
Carla buzzed, delighted in spite of herself. “Blue becomes green, green becomes red, yellow becomes infrared.” At least the shift was in the same direction each time. She’d given up all hope of impressing the class with a simple explanation for these strange results. They’d found a completely new anomaly, a mystery to rank with the stability puzzle itself. There was nothing to be done now but to accept that.
And to gather more data.
She called for the workshop to be lit again, and asked Palladio and Dina to add a second prism to the light path, this time directly behind the eyepiece. Then for each color beamed across the container, she had the students take turns measuring the frequency of the light that was scattered at a variety of angles.
The experiment had one more surprise for her. At the smallest angles, violet light produced two distinct colors in the scatter: one only slightly altered in hue, the other shifted far toward the red. At larger angles the two colors moved closer together—just before the scatter disappeared completely. Blue light showed signs of doing something similar, though in that case the second color moved beyond the visible range, at a point not far below the maximum scattering angle.
Carla plotted all the measurements on her chest, then dusted her skin with dye and made copies for the students to keep. “Think of this as a souvenir,” she told Romolo. “Maybe by the time your grandchildren are studying optics, this experiment will be as famous as those Sabino did to measure Nereo’s force.”
“I’m confused,” Romolo said. “Did we find free luxagens in the container, or didn’t we?”
Carla said, “Ask me that again in six years’ time.”
10
Carlo stiffened his tympanum to keep himself silent, then plunged the probe deep into the flesh of his wrist. As he struggled to force the needle all the way down to the calibration mark the pain became excruciating, but once the thing was in place and motionless the sensation was tolerable.
“The voles of the Peerless thank you for your sacrifice,” Amanda said wryly.
Carlo managed a dismissive buzz. Loath as he was to inflict needless suffering on the animals, he was stabbing himself more out of expediency than compassion. The current version of the probe was so large that he could not have expected the creatures to endure it without an elaborate routine of anesthesia and recovery—and by the time he’d also trained the voles to perform specific movements on cue he would have ended up with a protocol where every trivial experiment took half a dozen stints to complete.
He waited a lapse or so for his skewered flesh to recover from the shock, then wiggled his fingers cautiously. He hadn’t paralyzed any of them. The question now was whether he’d erred in the other direction; if the probe was too far from the bundle of motor pathways he’d have no chance to spy on its traffic.
Amanda was harnessed to the bench beside the light recorder. Carlo gestured to her to look through the eyepiece, then he moved all his fingers at once.
She said, “Nothing.”
“All right. Let me turn it a little.”
The hardstone tube protruding from his wrist had a cross-hatched ring at the top, attached to the inner sleeve that held the primary mirror within the probe’s clearstone tip. Above the ring, the same sleeve slotted into the side of a much longer tube that carried the light across into the recorder. Carefully, Carlo began to turn the ring, aiming the mirror below in a new direction. Since none of the moving parts were in contact with his flesh the adjustment ought to have been painless, but in fact there was enough friction between the sleeve and the outer tube to make the whole probe start twisting, so he had to stop and sprout a new hand to hold the thing steady.
He wiggled the fingers of his impaled hand again. Amanda said, “Yes! There’s light now!”
He tried the six fingers one by one. Amanda could catch glimpses of his brain’s messages to all of them, but the second finger from the right gave the best results. Carlo adjusted the mirror further, turning it back and forth by ever smaller angles until the light coming through was as bright as he could make it. He might have been able to do better still if he’d been willing to yank the probe out and reinsert it closer to the pathway, but that didn’t seem worth the pain. So long as he had a visible signal, that would be enough to tell him whether or not the machine was going to be useful.
He began tracing out a circle with the tip of the chosen finger, repeating the motion as smoothly as he could. “Can you see that?” he asked Amanda.
“Yes. Don’t ask me what the sequence is, but I’d swear it’s periodic.”
This was not an unbiased judgment: she was watching his hand with her rear gaze even as she peered through the eyepiece. But with luck they’d soon have a more objective way of assessing the signal’s properties.
“Start the recorder,” he said.
Amanda flipped the lever to retract the mirror that was diverting light into the eyepiece, then she disengaged the brake on the drive wheel. Carlo tried to keep his mind on his rotating finger and ignore the machine’s whir, surprised by the strength of his urge to hold his whole body expectantly motionless. When he’d first been testing the recorder—with a slab of lamp-lit translucent resin taking the place of his flesh—he’d usually ended up waiting, tensed, for the sound of tearing paper.
“It’s finished,” Amanda announced. She opened the device and extracted the spool, then stretched out a portion of the strip so they could both see it.
It was blank.
Carlo was disappointed, but not greatly surprised. With every trained instrument builder busy on the Gnat or some ancillary project, he’d had to use salvaged components for most of the optics and clockwork, while the way the parts had been brought together was unmistakably a pro
duct of his own inexpert hands. The failure he’d been dreading most was that the temperamental system for dispensing the activation gas might stop working again, leaving the paper to run through the machine without being properly sensitized.
“Maybe the viewing mirror’s stuck,” he suggested hopefully.
Amanda bent toward the eyepiece. Carlo was still mechanically twiddling his finger. She said, “Not unless it stuck halfway, because I can’t see anything.” She pushed the lever to reinsert the mirror. “And I still can’t.”
“So I’ve been torturing myself for nothing?” Carlo joked. “The voles will be pleased. Maybe something else has slipped out of alignment.”
“Wait, what was that? You stopped—”
He’d stilled his finger. “Yes.”
Amanda said, “When you stopped, there was a burst of light.”
Carlo started up the motion again, slowly and deliberately. “What do you see now?”
“That’s… back to how it was at the start.”
He said, “Show me the rest of the recording strip.”
Amanda unspooled it completely. At the start of the recording the paper displayed a long sequence of dark streaks, the density of the pigment rising and falling in a complex pattern. Only the last quarter of the strip was blank.
Carlo said, “Check the eyepiece again.”
Amanda complied. “The signal’s still visible.”
Carlo tried to distract himself, to think of anything but his rotating finger. “How’s your co?” he asked.
“He’s fine,” Amanda replied, surprised by the question. “He’s just switched to a new job, doing maintenance on the main cooling system—ah, the signal’s gone again.”
“Either there’s an intermittent fault in the optics,” Carlo said, “or my finger doesn’t really need to be told what to do all the time. If the instructions follow a simple pattern, the flesh soon gets the message and the brain stops repeating itself. Until—”
He halted the twirling.