Greg Egan - The Eternal Flame

“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 it.

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.