“All right.” Jasim reached into the model. “So where can we go hat’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: light-speed 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 ne.” He rephrased his question to the map. “For how many of those triples would the beam pass between them, intersecting the triangle hose 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 they moved apart again, the magnetic fields were switched from repulse 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.