“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.