Greg Egan - The Eternal Flame

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.