The solar system wasn’t entirely inhospitable, despite the swollen sun. The Jovian and Saturnian moons were relatively warm and stable. Vox could have sailed indefinitely on the blue-gray seas of Europa, for instance, under an atmosphere no more toxic than Earth’s.

“Mars,” Allison said suddenly. “If you’re serious, I mean if we can really travel between planets—there’s an Arch on Mars—”

“No, not anymore.” The Hypotheticals had protected Mars as long as there was a human presence there. But the last native Martians had died centuries ago and their ruins had been thoroughly scavenged; in recent decades the Arch had been allowed to decay and collapse. (I extracted this knowledge from the Hypotheticals’ pool of data, which had become my second memory.) Mars was a closed door.

“But you’re saying Vox Core can function as a kind of spaceship,” Allison persisted. “How far can it go, how fast?”

“It can go almost any distance. But only at a very small fraction of the speed of light.”

She didn’t have to explain what she was thinking. The planets that comprised the Ring of Worlds were linked by Arches but separated by vast physical distances. Some of these distances had been calculated by astronomers even in Turk’s time. The nearest human world was more than a hundred light-years from Earth. Reaching it would take several lifetimes. “But I can modify the passage of time so that it would seem like much less. A few hundred days, subjectively.”

“But it won’t be the same Ring of Worlds when we get there,” Allison said.

“No. Thousands of years will have passed. It’s impossible to predict what you might find.”

She looked out at the forest. Beams of artificial sunlight raked through the trees like bright, vague fingers. The tier’s high ceiling was the color of cobalt. No birds or insects lived here. There was no sound but the rustling of leaves.

After a while she turned back to Turk. He nodded once. “All right,” she said. “Take us home.”

*   *   *

I let my physical body sleep while I defined a sphere to contain Vox Core and part of the island beneath it. That sphere constituted our border with the exterior universe. Spacetime curved around us in a complex new geometry. Vox Core soared away from the dying Earth as if it had been fired from a gun, though we didn’t feel it: I further modified the local curvature of space to create the illusion of gravity. After a few hours we had passed beyond the orbits of Uranus and Neptune.