by Greg Egan
Jasim said, “It means that they don’t want us physically intruding on their territory, putting our machines right next to their homes, but I’m not convinced that it proves that they have no desire to communicate whatsoever.”
“We should leave them in peace,” Tim insisted. “They’ve seen the probes, so they know we’re here. If they want to make contact, they’ll do it in their own time.”
“Leave them in peace,” echoed another Snake. A chorus of affirmation followed from others in the chamber.
Leila stood her ground. “We have no idea how many different species and cultures might be living in the bulge. One of them sends back the probes, but for all we know there could be a thousand others who don’t yet even know that the Amalgam has tried to make contact.”
This suggestion set off a series of arguments, some between guests and hosts, some between the Snakes themselves. All the while, the Snakes kept circling excitedly, while new ones entered the chamber to witness the novel sight of these strangers.
When the clamor about the Aloof had quietened down enough for her to change the subject, Leila asked Sarah, “Why have you come to Nazdeek yourself?”
“It’s out of the way, off the main routes. We can think things over here, undisturbed.”
“But you could have the same amount of privacy anywhere. It’s all a matter of what you put in your précis.”
Sarah’s response was imbued with a tinge of amusement. “For us, it would be unimaginably rude to cut off all contact explicitly, by decree. Especially with others from our own ancestral species. To live a quiet life, we had to reduce the likelihood of encountering anyone who would seek us out. We had to make the effort of rendering ourselves physically remote, in order to reap the benefits.”
“Yet you’ve made Jasim and myself very welcome.”
“Of course. But that will be enough for the next twenty years.”
So much for resurrecting their social life. “What exactly is it that you’re pondering in this state of solitude?”
“The nature of reality. The uses of existence. The reasons to live, and the reasons not to.”
Leila felt the skin on her forearms tingle. She’d almost forgotten that she’d made an appointment with death, however uncertain the timing.
She explained how she and Jasim had made their decision to embark on a grand project before dying.
“That’s an interesting approach,” Sarah said. “I’ll have to give it some thought.” She paused, then added, “Though I’m not sure that you’ve solved the problem.”
“What do you mean?”
“Will it really be easier now to choose the right moment to give up your life? Haven’t you merely replaced one delicate judgment with an even more difficult one: deciding when you’ve exhausted the possibilities for contacting the Aloof?”
“You make it sound as if we have no chance of succeeding.” Leila was not afraid of the prospect of failure, but the suggestion that it was inevitable was something else entirely.
Sarah said, “We’ve been here on Nazdeek for fifteen thousand years. We don’t pay much attention to the world outside the nest, but even from this cloistered state we’ve seen many people break their backs against this rock.”
“So when will you accept that your own project is finished?” Leila countered. “If you still don’t have what you’re looking for after fifteen thousand years, when will you admit defeat?”
“I have no idea,” Sarah confessed. “I have no idea, any more than you do.”
5
When the way forward first appeared, there was nothing to set it apart from a thousand false alarms that had come before it.
It was their seventeenth year on Nazdeek. They had launched their own observatory – armed with the latest refinements culled from around the galaxy – fifteen years before, and it had been confirming the null results of its predecessors ever since.
They had settled into an unhurried routine, systematically exploring the possibilities that observation hadn’t yet ruled out. Between the scenarios that were obviously stone cold dead – the presence of an energy-rich, risk-taking, extroverted civilization in the bulge actively seeking contact by every means at its disposal – and the infinite number of possibilities that could never be distinguished at this distance from the absence of all life, and the absence of all machinery save one dumb but efficient gatekeeper, tantalizing clues would bubble up out of the data now and then, only to fade into statistical insignificance in the face of continued scrutiny.
Tens of billions of stars lying within the Aloof’s territory could be discerned from Nazdeek, some of them evolving or violently interacting on a time scale of years or months. Black holes were flaying and swallowing their companions. Neutron stars and white dwarfs were stealing fresh fuel and flaring into novas. Star clusters were colliding and tearing each other apart. If you gathered data on this whole menagerie for long enough, you could expect to see almost anything. Leila would not have been surprised to wander into the garden at night and find a great welcome sign spelled out in the sky, before the fortuitous pattern of novas faded and the message dissolved into randomness again.
When their gamma ray telescope caught a glimmer of something odd – the nuclei of a certain isotope of fluorine decaying from an excited state, when there was no nearby source of the kind of radiation that could have put the nuclei into that state in the first place – it might have been just another random, unexplained fact to add to a vast pile. When the same glimmer was seen again, not far away, Leila reasoned that if a gas cloud enriched with fluorine could be affected at one location by an unseen radiation source, it should not be surprising if the same thing happened elsewhere in the same cloud.
It happened again. The three events lined up in space and time in a manner suggesting a short pulse of gamma rays in the form of a tightly focused beam, striking three different points in the gas cloud. Still, in the mountains of data they had acquired from their predecessors, coincidences far more compelling than this had occurred hundreds of thousands of times.
With the fourth flash, the balance of the numbers began to tip. The secondary gamma rays reaching Nazdeek gave only a weak and distorted impression of the original radiation, but all four flashes were consistent with a single, narrow beam. There were thousands of known gamma ray sources in the bulge, but the frequency of the radiation, the direction of the beam, and the time profile of the pulse did not fit with any of them.
The archives revealed a few dozen occasions when the same kind of emissions had been seen from fluorine nuclei under similar conditions. There had never been more than three connected events before, but one sequence had occurred along a path not far from the present one.
Leila sat by the stream and modeled the possibilities. If the beam was linking two objects in powered flight, prediction was impossible. If receiver and transmitter were mostly in free-fall, though, and only made corrections occasionally, the past and present data combined gave her a plausible forecast for the beam’s future orientation.
Jasim looked into her simulation, a thought-bubble of stars and equations hovering above the water. “The whole path will lie out of bounds,” he said.
“No kidding.” The Aloof’s territory was more or less spherical, which made it a convex set: you couldn’t get between any two points that lay inside it without entering the territory itself. “But look how much the beam spreads out. From the fluorine data, I’d say it could be tens of kilometers wide by the time it reaches the receiver.”
“So they might not catch it all? They might let some of the beam escape into the disk?” He sounded unpersuaded.
Leila said, “Look, if they really were doing everything possible to hide this, we would never have seen these blips in the first place.”
“Gas clouds with this much fluorine are extremely rare. They obviously picked a frequency that wouldn’t be scattered under ordinary circumstances.”
“Yes, but that’s just a matter of getting the signal t
hrough the local environment. We choose frequencies ourselves that won’t interact with any substance that’s likely to be present along the route, but no choice is perfect, and we just live with that. It seems to me that they’ve done the same thing. If they were fanatical purists, they’d communicate by completely different methods.”
“All right.” Jasim reached into the model. “So where can we go that’s in the line of sight?”
The short answer was: nowhere. If the beam was not blocked completely by its intended target it would spread out considerably as it made its way through the galactic disk, but it would not grow so wide that it would sweep across a single point where the Amalgam had any kind of outpost.
Leila said, “This is too good to miss. We need to get a decent observatory into its path.”
Jasim agreed. “And we need to do it before these nodes decide they’ve drifted too close to something dangerous, and switch on their engines for a course correction.”
They crunched through the possibilities. Wherever the Amalgam had an established presence, the infrastructure already on the ground could convert data into any kind of material object. Transmitting yourself to such a place, along with whatever you needed, was simplicity itself: lightspeed was the only real constraint. Excessive demands on the local resources might be denied, but modest requests were rarely rejected.
Far more difficult was building something new at a site with raw materials but no existing receiver; in that case, instead of pure data, you needed to send an engineering spore of some kind. If you were in a hurry, not only did you need to spend energy boosting the spore to relativistic velocities – a cost that snowballed due to the mass of protective shielding – you then had to waste much of the time you gained on a lengthy braking phase, or the spore would hit its target with enough energy to turn it into plasma. Interactions with the interstellar medium could be used to slow down the spore, avoiding the need to carry yet more mass to act as a propellant for braking, but the whole business was disgustingly inefficient.
Harder still was getting anything substantial to a given point in the vast empty space between the stars. With no raw materials to hand at the destination, everything had to be moved from somewhere else. The best starting point was usually to send an engineering spore into a cometary cloud, loosely bound gravitationally to its associated star, but not every such cloud was open to plunder, and everything took time, and obscene amounts of energy.
To arrange for an observatory to be delivered to the most accessible point along the beam’s line of sight, traveling at the correct velocity, would take about fifteen thousand years all told. That assumed that the local cultures who owned the nearest facilities, and who had a right to veto the use of the raw materials, acceded immediately to their request.
“How long between course corrections?” Leila wondered. If the builders of this hypothetical network were efficient, the nodes could drift for a while in interstellar space without any problems, but in the bulge everything happened faster than in the disk, and the need to counter gravitational effects would come much sooner. There was no way to make a firm prediction, but they could easily have as little as eight or ten thousand years.
Leila struggled to reconcile herself to the reality. “We’ll try at this location, and if we’re lucky we might still catch something. If not, we’ll try again after the beam shifts.” Sending the first observatory chasing after the beam would be futile; even with the present free-fall motion of the nodes, the observation point would be moving at a substantial fraction of lightspeed relative to the local stars. Magnified by the enormous distances involved, a small change in direction down in the bulge could see the beam lurch thousands of light years sideways by the time it reached the disk.
Jasim said, “Wait.” He magnified the region around the projected path of the beam.
“What are you looking for?”
He asked the map, “Are there two outposts of the Amalgam lying on a straight line that intersects the beam?”
The map replied in a tone of mild incredulity. “No.”
“That was too much to hope for. Are there three lying on a plane that intersects the beam?”
The map said, “There are about ten-to-the-eighteen triples that meet that condition.”
Leila suddenly realized what it was he had in mind. She laughed and squeezed his arm. “You are completely insane!”
Jasim said, “Let me get the numbers right first, then you can mock me.” He rephrased his question to the map. “For how many of those triples would the beam pass between them, intersecting the triangle whose vertices they lie on?”
“About ten-to-the-sixth.”
“How close to us is the closest point of intersection of the beam with any of those triangles – if the distance in each case is measured via the worst of the three outposts, the one that makes the total path longest.”
“Seven thousand four hundred and twenty-six light years.”
Leila said, “Collision braking. With three components?”
“Do you have a better idea?”
Better than twice as fast as the fastest conventional method? “Nothing comes to mind. Let me think about it.”
Braking against the flimsy interstellar medium was a slow process. If you wanted to deliver a payload rapidly to a point that fortuitously lay somewhere on a straight line between two existing outposts, you could fire two separate packages from the two locations and let them “collide” when they met – or rather, let them brake against each other magnetically. If you arranged for the packages to have equal and opposite momenta, they would come to a halt without any need to throw away reaction mass or clutch at passing molecules, and some of their kinetic energy could be recovered as electricity and stored for later use.
The aim and the timing had to be perfect. Relativistic packages did not make in-flight course corrections, and the data available at each launch site about the other’s precise location was always a potentially imperfect prediction, not a rock-solid statement of fact. Even with the Amalgam’s prodigious astrometric and computing resources, achieving millimeter alignments at thousand-light-year distances could not be guaranteed.
Now Jasim wanted to make three of these bullets meet, perform an elaborate electromagnetic dance, and end up with just the right velocity needed to keep tracking the moving target of the beam.
In the evening, back in the house, they sat together working through simulations. It was easy to find designs that would work if everything went perfectly, but they kept hunting for the most robust variation, the one that was most tolerant of small misalignments. With standard two-body collision braking, the usual solution was to have the first package, shaped like a cylinder, pass right through a hole in the second package. As it emerged from the other side and the two moved apart again, the magnetic fields were switched from repulsive to attractive. Several “bounces” followed, and in the process as much of the kinetic energy as possible was gradually converted into superconducting currents for storage, while the rest was dissipated as electromagnetic radiation. Having three objects meeting at an angle would not only make the timing and positioning more critical, it would destroy the simple, axial symmetry and introduce a greater risk of instability.
It was dawn before they settled on the optimal design, which effectively split the problem in two. First, package one, a sphere, would meet package two, a torus, threading the gap in the middle, then bouncing back and forth through it seventeen times. The plane of the torus would lie at an angle to its direction of flight, allowing the sphere to approach it head-on. When the two finally came to rest with respect to each other, they would still have a component of their velocity carrying them straight toward package three, a cylinder with an axial borehole.
Because the electromagnetic interactions were the same as the two-body case – self-centering, intrinsically stable – a small amount of misalignment at each of these encounters would not be fatal. The usual two-body case, though, didn’t require the combined packag
e, after all the bouncing and energy dissipation was completed, to be moving on a path so precisely determined that it could pass through yet another narrow hoop.
There were no guarantees, and in the end the result would be in other people’s hands. They could send requests to the three outposts, asking for these objects to be launched at the necessary times on the necessary trajectories. The energy needs hovered on the edge of politeness, though, and it was possible that one or more of the requests would simply be refused.
Jasim waved the models away, and they stretched out on the carpet, side by side.
He said, “I never thought we’d get this far. Even if this is only a mirage, I never thought we’d find one worth chasing.”
Leila said, “I don’t know what I expected. Some kind of great folly: some long, exhausting, exhilarating struggle that felt like wandering through a jungle for years and ending up utterly lost.”
“And then what?”
“Surrender.”
Jasim was silent for a while. Leila could sense that he was brooding over something, but she didn’t press him.
He said, “Should we travel to this observatory ourselves, or wait here for the results?”
“We should go. Definitely! I don’t want to hang around here for fifteen thousand years, waiting. We can leave the Nazdeek observatories hunting for more beam fluorescence and broadcasting the results, so we’ll hear about them wherever we end up.”
“That makes sense.” Jasim hesitated, then added, “When we go, I don’t want to leave a back-up.”
“Ah.” They’d traveled from Najib leaving nothing of themselves behind: if their transmission had somehow failed to make it to Nazdeek, no stored copy of the data would ever have woken to resume their truncated lives. Travel within the Amalgam’s established network carried negligible risks, though. If they flung themselves toward the hypothetical location of this yet-to-be-assembled station in the middle of nowhere, it was entirely possible that they’d sail off to infinity without ever being instantiated again.
