Greg Egan - The Eternal Flame

Carla allowed herself a quick glance at Silvano. His expression was grim, but she believed she was about to improve his mood considerably.

“There is one thing, though, that can safely interact with the material of the Object: light. Light does not itself produce light, so its history isn’t marked with Nereo’s arrow: there is no ‘positive’ light and ‘negative’ light. The starlight from ordinary and orthogonal stars falls on both us and the Object alike, causing no damage to either. So if we hope to make tools that will allow us to acquire and manipulate samples of the Object, I believe that will require a new source of light.”

Councilor Giusta interjected, “So you want to drill into the Object with the beam from a giant sunstone lamp?”

“No,” Carla replied. “Brute force like that wouldn’t get us very far. What we need is a light source whose output is more orderly than the beam from any kind of lamp.”

She summoned a sketch onto her chest.

“The luxagens in every solid are restricted to certain energy levels,” she said. “These levels are arranged in bands. The bands themselves are so widely spaced that a luxagen can only jump between them by creating several photons at once. That’s a very slow, inefficient process—which is what makes solids stable in the first place.

“Within each band, though, the levels are close enough for luxagens to move between them by emitting or absorbing single photons. Left to themselves, most luxagens will occupy the highest possible levels in their bands, because those at lower levels will spontaneously emit a photon and move up.” Carla hesitated, half expecting someone to challenge her on these events without a cause, but either the Councilors had heard of Assunto’s strange “zero-photon” light that filled the cosmos and shook every luxagen at every imaginable frequency… or they were simply willing to accept that there would always be some kind of disturbance pushing luxagens from the most precarious states into more stable positions.

“When a photon enters a solid, it can affect the luxagens in two different ways,” she continued. “If there are luxagens in a high energy level, they can absorb the photon and move down to a lower level. And if there are luxagens in a low energy level, they can emit another photon exactly like the first, and move up to a higher level. For either of these things to happen, the energy of the photon must match the difference in energy between the two levels.”

“The two processes push the luxagens in opposite directions, and all things being equal they’d both take place at exactly the same rate. But with luck, it might be possible to find a solid where things can be kept very far from equal. Suppose we have four different energy levels, and we illuminate the solid with light of the right frequency to push luxagens from the highest level down to the very lowest.”

“If nothing else was going on, the same light would also give rise to the opposite process: stimulating luxagens in the bottom level to jump back up to the top. But suppose the spontaneous jump from the bottom level to the one just above happens very quickly—so quickly that most luxagens end up at that second lowest level instead. From there, the luxagens will spontaneously jump up one more level, and then one more again. Once they reach the top, our light will push them to the bottom again.

“Now, the photons that are emitted spontaneously will have random phases and directions, just like the light from a lamp. But suppose we put this solid between a pair of mirrors that send the light from the middle transition back and forth through the material.”

“Each time the light is reflected back through the solid, it will prompt more luxagens from the second lowest level to jump up one step, emitting photons of the same phase and in the same direction as the one passing through. But we can avoid the unwanted reverse process—where the photons we send back get absorbed, instead of duplicated—if the transition from the third level to the top one happens fast enough. If that third level is kept almost empty, there’ll be almost no luxagens in a position to absorb the light.

“By separating the mirrors by an exact multiple of the wavelength of the light we’re trying to multiply, we can reinforce a mode that is entirely parallel and in phase. If we make one of the mirrors only partially reflective, the beam that emerges from this device should be aligned like nothing we’ve seen before. A lens will focus it down to a spot whose size is limited only by the laws of optics—allowing all the power in the beam to be concentrated on a target a fraction of a scant across. What’s more, the light itself will be comprised of an orderly series of wave fronts, maintaining a regular pattern over vastly longer times and distances than the jumble of waves that we get from a lamp. So instead of the light field changing direction at random, partly canceling out its own effects, the full strength of the field can be brought to bear at the focal point.

“The stronger the light field, the greater the pressure it exerts. So I believe there’s a chance that this ‘coherent light’ could be used in much the same way as we originally hoped to use air jets: to manipulate small samples of orthogonal matter without touching them.”

Carla spread her arms: she was finished. For a pause or two all the Councilors were silent, then Silvano asked, “How far does this take us toward a new engine design?”

Carla forced herself to meet his gaze. Engine design? She thought she’d just offered him a chance to salvage some dignity after his overblown claims for the Object, but apparently he wouldn’t be satisfied with anything less than an instant answer to the fuel problem. “If we can build this tool it might let us conduct preliminary experiments,” she said. “We could study the annihilation reaction between very small samples, under controlled conditions. But any talk about a new engine design would be premature, until we’ve done those experiments.”

Councilor Massimo said, “Would you ever seriously envision bringing orthogonal matter into the Peerless at all?”

“Not unless we’d developed an extremely reliable technology for handling it,” Carla replied.

“Reliable?” Massimo buzzed softly. “You’re holding a universal liberator in place using nothing but light . What happens when your light source fails?”

“If the sample was kept under weightless conditions in an evacuated container, there could be days to recover safely from the failure,” Carla said.

“But the whole point would be to use this in our engines!” Massimo reminded her. “When we’re firing the engines, we’re not weightless, are we?”

Hadn’t she just said that any talk of engine design would be premature? Carla struggled to find an answer that wouldn’t sound discourteous or uncooperative. “I can imagine an arrangement where a failure to contain the orthogonal matter would see it vented behind us—falling straight out of the engines into the void. But for now, all I’m talking about is a research program to try to build a coherent light source.”