They toured the network of catwalks and ladders that had been laid throughout the ship, from the command deck with its fifteen-foot-wide display screen, through the control rooms, living quarters, and hospital, to the cargo holds and the tiers of cages that had contained the animals. The primary energy-converter and generator section was as imposing and as complex as the inside of a thermonuclear power station. Beyond it, they passed through a bulkhead and found themselves dwarfed beneath the curves of the exposed portions of a pair of enormous toroids. The engineer leading them pointed up at the immense, sweeping surfaces of metal.

"The walls of those outer casings are sixteen feet thick," he informed them. "They're made from an alloy that would cut tungsten-carbide steel like cream cheese. The mass concentration inside them is phenomenal. We think they provided closed paths in which masses of highly concentrated matter were constrained in circulating or oscillating resonance, interacting with strong fields. It's possible that the high rates of change of gravity potential that this produced were somehow harnessed to induce a controlled distortion in the space around the ship. In other words, it moved by continuously falling into a hole that it created in front of itself—kind of like a four-dimensional tank track."

"You mean it trapped itself inside a space-time bubble, which propagated somehow through normal space?" somebody offered.

"Yes, if you like," the engineer affirmed. "I guess a bubble is as good an analogy as any. The interesting point is, if it did work that way, every particle of the ship and everything inside it would be subjected to exactly the same acceleration. Therefore there would be no G effect. You could stop the ship dead from, say, a million miles an hour to zero in a millisecond, and nobody inside would even know the difference."

"How about top speed?" someone else asked. "Would there have been a relativistic limit?"

"We don't know. The theory boys up in Jupiter Four have been losing a lot of sleep over that. Conventional mechanics wouldn't apply to any movement of the ship itself, since it wouldn't be actually moving in the local space inside the bubble. The question of how the bubble propagates through normal space is a different ball game altogether. A whole new theory of fields has to be worked out. Maybe completely new laws of physics apply—as I said before, we just don't know. But one thing seems clear: Those photon-drive starships they're designing in California might turn out to be obsolete before they're even built. If we can figure out enough about how this ship worked, the knowledge could put us forward a hundred years."

* * *