Nilis had continued to analyze the data he had gathered on Chandra, the monstrous, enigmatic black hole at the center of the Galaxy, and hypothesized on its nature and what the Xeelee were doing with it.
He knew now that the Xeelee used Chandra to make nightfighters. Somehow, Nilis had deduced from remote images, they peeled spacetime-defect wings and other structures out of the distorted environment of the black hole. That much had long been suspected by Navy intelligence. But Nilis said he suspected the Xeelee had a more profound use for the black hole.
It was all to do with computing. There were fundamental limits to computing power, he said. The processing speed and memory of any computer were limited by the energy available to it.
He picked up a data desk and waved it around in the air. “This is a sophisticated gadget, the result of twenty-five thousand years of technological progress. But what does it weigh — around a kilogram? From the point of view of the gadget’s purpose, which is computation, almost all of this mass is wasted, just a framework. This desk would be able to achieve a lot more if all of its mass-energy were devoted to computing. In the form of photons, say, this kilogram of stuff could process at the rate of ten to power fifty-one operations per second. That’s a million billion billion billion billion…” Similarly, the memory capacity of a computer depended on how many distinguishable states its system could take. If Nilis’s inert kilogram were converted to a liter of light, the capacity would become some ten thousand billion billion billion bits.
“In fact, our most advanced computers have a design something like this,” he said. “Perhaps you know it. At the core of the ’nervous system’ of a greenship is a vat of radiant energy, much of it gamma-ray photons, but some of it more exotic higher-energy particles. Energy is bled off from the ship’s GUT generator, to keep the photon soup at around a billion degrees. Information is stored in the positions and trajectories of the photons, and is processed by collisions between the particles. To read it, you open up a hole in the side of the box and let some of the light out.”
There were limitations with such a design, because the rate at which information could be extracted, limited by lightspeed, was much less than the computer’s storage capacity. “You only get a glimpse of what’s going on in there,” said Nilis. “So our best computers are massively parallel, with subsections working virtually independently.” The input-output rate could be increased if the computer were made smaller, because it took less time for information to be moved around. But as the size was reduced, the energy density would increase. “You encounter more and more exotic high- energy particles,” said Nilis, “until you pass the point at which you can control them. Gamma-ray processing is the limit of our technological capabilities right now. But of course that’s not the physical limit. If you keep crushing down your computer, keep increasing its density, you finish up with—”
“A black hole,” said Torec.
“Yes.” He beamed, and plucked at a thread dangling from the sleeve of his battered robe. “And then the physics becomes simple again.”
Pirius began to see it. “And Chandra is a black hole — the biggest in the Galaxy.”
“Exactly,” Nilis whispered. “I thought the Xeelee were using Chandra to power their central computing facility. Now I believe that the Xeelee are using Chandra itself, a black hole with the mass of millions of suns, as a computer. The audacity!”
Torec asked, “How can you use a black hole as a computer?”
Nilis said that information could be “fed” to a black-hole computer during the hole’s formation, or by infalling matter later. “The data would be stored on the hole’s event horizon in the form of impressed strings.”
Pirius was becoming baffled. “Strings?”
All of reality could be looked on as an expression of vibrating strings. Invisibly small, these loops and knots shimmered and sang, and their vibration modes, the “notes” they sounded, were the particles of the universe humans could discern. Pirius took in little of this, but he liked the idea that the universe was a kind of symphony of invisible strings in harmony.
“A black hole’s event horizon is a terminus to our universe, though,” Nilis said. “Strings can’t extend beyond it. So they become embedded in the surface — like wet hair plastered over your head. The strings bear information about how the hole was formed, and how it grew.” To get at the information you had to let the hole evaporate, as all black holes did, by emitting a dribble of “Hawking radiation.” The smaller the hole the more rapidly it evaporated.
“You won’t be surprised that the Silver Ghosts once dabbled with this kind of technology,” Nilis said ruefully. “They created microscopic black holes, with information and processing instructions encoded in the formative collapse. Small enough holes evaporate very quickly — they explode, in fact. The computation’s output is encoded in the radiation they emit in the process. You can solve some spectacularly hard problems that way.” He sniffed. “The Ghosts did get it to work. But each micro-hole computer was a one-off; you could only run one program, because it blew itself up in the process! Even the Ghosts couldn’t find a way to make the technology practical.”
“And,” Torec prompted, “this is what the Xeelee are doing?”
“Yes. But they don’t restrict themselves to mere microscopic holes.”
He showed them data extracted from Pirius Blue’s jaunt into the Cavity. They still had no good close- up images of Chandra, but somehow the Xeelee were controlling the inflow of matter to the event horizon. And through that control, they were “programming” the monstrous black hole. They allowed the Planck scale dynamics of the event horizon to process the input information, and were “reading” the results by analyzing the Hawking radiation the black hole gave off.
“At least that’s what I think they are doing,” Nilis said. “There is still a great deal we don’t know about black holes. For instance, there must be structure in the deep interior, close to the singularity. There the strings and membranes that underlie subatomic particles must be torn and stretched, perhaps reaching dimensions comparable to the black hole itself. Can this ’fuzzball’ be used for computing purposes? I don’t know — I can’t rule it out. Or perhaps the Xeelee work on some other principle entirely…
“It’s remarkable,” he breathed. “A black hole is a convergence of information and physics, a junction in the structure of our universe. And the Xeelee are using this miracle as a tactical computer!” He grinned. “No wonder they were able to fend us off so easily. But now, thanks to your CTC computer, we’ve changed the rules — eh, Ensigns? Even a black hole computer can’t beat that.
“But I don’t think I’ve got to the bottom of it yet.”
“Of what, sir?”
“Of Chandra.”
His proximity to the Galaxy’s center had inspired him to start a whole new line of research, he said. He would go to the Navy’s archives for tactical material from three thousand years’ worth of scouting missions, and perhaps even hunt out scientific data from more innocent times, when Chandra had been thought to be a mere astrophysical marvel, not a military target. “In the quagmites and the Xeelee we have already found two layers of life, from quite different cosmic epochs — and a third, if you include us! I’m beginning to wonder what else is there in Chandra’s nested layers, waiting to be uncovered. Perhaps there is more life to be found in there, still more ancient and strange, perhaps permeating the singularity itself.
“We understand so little,” he said, “even now. If you look back at the theories of the ancients you can see how they groped for understanding. Their physics made them capable of recognizing a black hole, say, and describing its broad features. But their science gave them no real understanding of what it is. Some of what we are capable of today would have seemed impossible to the ancients, as if we were defying the laws of physics themselves! But you have to wonder how incomplete our own precious theories may be.”