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?”