Then the desktop cut away from the chronophysicists. It showed some of the worlds they'd found-worlds where things had gone differently from the way they'd happened here.

Jeremy had seen a lot of these videos, too. Here was footage from a world where the Vikings had settled North America. Here was one where successors of Alexander the Great ruled half a dozen empires that stretched from Spain to the borders of China. Here were gaudy pictures from a world where civilization in the Old World had got off to a later start than it had here, so the Native American cultures were the most advanced anywhere.

Here was a triumphal procession through the streets of Rome in a world where the Roman Empire hadn't fallen. Jeremy smiled when that one came up. His folks spent a lot of their time trading there. He and his sister went there, too. Sometimes the locals needed to see a whole family. It added realism.

And here, quickly, one after another, were worlds with breakpoints closer to here-and-now. Here were Spaniards with bayoneted flintlocks swaggering through a town on the border between their empire and Russia in a world where the Armada conquered England. Here was a race riot in a town that didn't look too different from the ones Jeremy knew, but where the Confederate flag flew. And here was one in a world where nobody had discovered atomic energy. The United States and the Soviet Union were fighting World War VI there right this minute.

The desktop went blank. Jeremy knew how many more alternates it might have shown: the one where the Chinese had discovered the Americas; the one where the United States was a contented part of a British Empire that covered three-quarters of the globe; the nasty one where the Germans had won World War I; the even nastier one where they'd won World War II; and on and on.

Ms. Mouradian said, “How did finding the alternates change things for us?” Again, a lot of hands went up. Again, Jeremy's was one of them. Nobody yelled out the answer this time, though. It wasn't so simple.