Pirius glanced back over his shoulder. Saturn, the only object in his universe large enough to show a disc, had been reduced to a tiny yellowish spot. He should have felt even more isolated, he thought; exposed. But all he felt was power. With this ship he could go anywhere, do anything.

And the test had barely begun.

“Sir, do you want me to bring her back?”

“You sit tight,” Darc snapped. “That fly is going to get a thorough checkout before it moves another centimeter — as are you. We’re coming as fast as we can.” And so they were, Pirius saw. Staring toward Saturn, he made out a small flotilla of ships, gradually drifting across the background stars.

He pressed his hands to his thighs, resisting the temptation to take off once more.

Nilis loomed huge over the nightfighter. With its wings furled, the ship would have rested in the palm of the hand of this kilometers-high Virtual, Pirius thought.

It ought to have been an absurd sight, even a faintly revolting one; Nilis’s head was the size of a Spline starship, every blocked pore in his aged skin a pit like a weapons emplacement. But Pirius was back in orbit around Saturn now, and the planet’s subdued, golden light oddly filled up the Virtual image. And wonder was bright in Nilis’s tremendous eyes.

“Defects in spacetime,” Nilis said. “That’s what the wings of a nightfighter are. Flaws in the structure of spacetime itself. And look here.”

He waved his immense hands and produced another gigantic Virtual. This one showed the Xeelee nightfighter in flight, the beautiful, elusive, bafflingly complex motions of its wings of flawed spacetime. Nilis replaced the true image with a schematic. The ship was overlaid by a framework, a kind of open tetrahedron, with bright red blobs at its four corners. The tetrahedron went through a complex cycle of deformations. It closed like an umbrella, its legs shortening as they moved; then they would lengthen before the “umbrella” opened again and the frame returned to its starting configuration.

“This is a schematic of the wings’ motion,” Nilis said. “See the way the wings change their shape. You have to think of spacetime as the natural medium of the craft. It is like — like a bacterium embedded in water. To a small enough creature, water is as viscous as treacle, and in such sticky stuff swimming is difficult, because if your recovery stroke is the same as your impulsive stroke you pull yourself back to where you started. So what bacteria do is adopt different geometrical shapes, during the first and second parts of the stroke, to pull themselves forward. It’s called a geometric phase, a closed sequence of different shapes.

“Pirius, the nightfighter is embedded in spacetime as surely as any bacterium in water. By pulsing through their sequence of shapes, the wings of the nightfighter are clearly using a geometric phase to control and direct the ship’s motion. It’s a shape-shifting drive — nothing like a rocket, no need for anything like reaction mass to be thrown out the back of your ship — really quite remarkable. And quite unlike the principles on which human sublight drives are based.”

Pirius understood, if vaguely. Human-designed drives pushed, not against spacetime itself, but against the vacuum, the seething quantum foam of virtual particles that pervaded space. At the heart of such a drive was an extended crystalline substrate, made to vibrate billions of times a second. As the substrate passed through the quantum foam, electric fields were induced in its surface by the foam’s fluctuating forces, fields dissipated by spraying out photons. If you arranged things right, so Pirius had been told in cartoon-level lectures, you could use those shed photons to push you forward.

“Our drives work all right,” Nilis said. “But they are slower than the Xeelee drive. And they break down constantly. Those crystals are expensive, and they shatter easily.”

Pirius knew that. You had to carry a rack of spares for a journey of more than a few light-hours. “And besides,” he said slowly, “the Xeelee method sounds more…” He couldn’t think of the word.

Nilis smiled hugely. “Elegant?”

“I guess so.”

“Thanks to your brave work today, we understand the source of that elegance a great deal better. But still there are questions. Swimming in spacetime is an odd way to do things. This is a method that would work best in regions of highly curved spacetime, where you can get more traction — say, around a black hole.”

“We know the Xeelee infest Chandra.”

“Yes, and that offers us all sorts of clues about them. But they also have to operate in environments like this, far from any dense concentrations of matter, where spacetime is all but flat. In fact, if the spacetime were perfectly flat, the drive couldn’t work.

“And why use spacetime defects as the basis of your drive in the first place? There was a time, in the moments after the Big Bang singularity, when such things were common, for the orderly structure of the swollen spacetime we inhabit was still forming. There were points, loops, sheets—”

“The point defects are monopoles.”

“Yes. That’s why monopoles are useful weapons — one defect can interfere with another. Spacetime was heavily curved, too. I suppose if you were designing a drive system, then you might naturally pick defects and spacetime-swimming as your way to work. It isn’t nearly so obvious now — and hasn’t been since microseconds after the singularity. So why use it? And then there is the question of the Xeelee themselves. Where are they?”

That leap confused Pirius. “Sir?”

“No matter how closely I inspect this craft I can only find machinery, layer upon layer of it. No sign of a crew!”

“I don’t know what that means.”

“Nor do I — not yet.” The immense ghostly Virtual leaned forward, and a glistening eye the size of a Conurbation loomed eagerly over Pirius. “Still, I do think we’re getting somewhere. The word ’Chandra,’ you know, is very ancient — pre-Occupation. Some say the black hole is named after a scientist of antiquity. Others say that the word means luminous. Well, if luminous it is, I don’t think Minister Gramm is going to enjoy the sight of what Chandra is beginning to illuminate for us!”

Alone, her skinsuit stained dark gray by moondust, Torec clambered through the remains of an exotic matter factory.

This place had been built twenty thousand years ago by the Qax, alien occupiers of Earth. After the rebellion that had forced the Qax out of Sol system, all the equipment had been stripped out, and the roof smashed open to the black sky. You could still see small blast craters and bits of wreckage left over from that ancient turmoil. And the gaunt walls remained, broken sheets of lunar concrete that cast long, sharp shadows over the undulating dust of the Moon ground.

While Pirius had been flying a nightfighter among the moons of Saturn, she had been stuck here a month already. There were three weeks left before the deadline Gramm had set, and the final demonstration of Nilis’s prototype CTC processor, his time-travel computer, would be due. Torec longed for those weeks to be up.

Torec was always aware of Earth, high in the sky. The great arc of the Bridge was easily visible — stunning, unnatural, disturbingly defying logic. And she could see the layers of defenses that surrounded the home planet: the circling Snowflakes, the crawling specks of patrolling warships. Even seen from its Moon, Earth bristled with fortifications.

It was only in these ruins, half an hour’s hike from the development base, that she felt at ease. When she walked here, Earth’s bright blue was eclipsed by the broken masses of the walls, and she could imagine she was far away from here, not in Sol system at all. And the best thing was that the earthworm techs she had to work with never came here.