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.

The teacher pointed at him. “Jeremy.”

“We aren't limited to the resources of one world any more,” he answered. “We can get food and raw materials and ideas from a lot of different places, a little from here, a little from there. We don't take enough from any alternate world to hurt it.” He'd known all that stuff long before he took this class. With his mom and dad both working for Crosstime Traffic, he had to.

Ms. Mouradian knew where his folks worked. Maybe that was why she'd picked him to answer. She nodded when he was done. “That's good,” she said. “And what are some of the problems we've had since we started traveling to the alternates?”

Jeremy raised his hand one more time. He didn't want Ms. Mouradian-or anybody else-to think he didn't know there were problems. She didn't call on him again, though. She picked Michael Fujikawa instead. His folks worked for Crosstime Traffic, too. He said, “Probably contamination is the worst one.”

“That's right,” the history teacher said. “Please look at your desktops again.” Jeremy looked down. He saw the video he thought he would. There were long lines of people waiting to get shots for Hruska's disease. An early explorer had brought it back from a world that was off-limits now. There were also pictures of the blank, idiotic stares on the faces of people who'd come down with the illness. Then the desktop showed some of the plant and animal diseases and parasites that had come back here from other alternates.

A girl named Elena Ramos raised her hand. When Ms. Mouradian called on her, she said, “The other big problem is keeping people in the alternates from knowing we're visiting them.”

“Oh, yes.” The teacher nodded again. “That is the other important one. Wherever we go where there's civilization, we have to keep the secret. That's why we always pretend to be part of the world where we trade. Some alternates are advanced enough that they might be able to use the technology if they got their hands on it. That could be very, very dangerous.” The desktop showed another clip from the world where the Nazis had won the Second World War. It wasn't pretty. Ms. Mouradian went on, “That rule is also why we drill for oil and do our mining on alternates where there are only hunters and gatherers, or else worlds without any people at all. On worlds like those, we don't have to hide.”

On the desktop, oil rigs stood like steel skeletons in the middle of a vast, golden desert. Antelope with enormous horns watched, wondering what the fuss was about. An oil worker in grimy coveralls walked up to one and stroked its nose. It stood there and let him. It had never learned to be afraid of men. In that alternate, there were no men to be afraid of.

The antelope disappeared from the desktop. Jeremy sighed, and he wasn't the only one. Ms. Mouradian said, “Now we're going to go over some of the Supreme Court decisions that center on crosstime travel.” Jeremy sighed again, on a different note. Again, he wasn't the only one.

Amanda Solters stood under the awning at Canoga Park High. She stayed out of the sun while she waited for the bus and for her brother to show up. She hoped Jeremy would get there before the bus did. His last class this year was on the far side of campus, so sometimes he cut things close.

While she waited, she checked her handheld to see what she had to do tonight. She made a face at the thought of algebra homework. That was old-fashioned, boring drill and practice. She had to understand what she was doing to get it right. It wasn't like a foreign language, where she could soak it up in a few sessions with the implant. She'd learned Spanish that way, and French, and neoLatin and classical Latin for trips out to the alternate with her parents and Jeremy.

Here he came, as usual half a head taller than most of the kids around him. He'd tried out for the basketball team the autumn before, but he hadn't even got onto the JVs. Being tall wasn't enough. You had to be able to run and shoot, too.

He spotted her and waved. Amanda was tall herself, for a girl-one meter, seventy-three centimeters. Her grandfather, who was old-fashioned as well as old, sometimes said she was five feet ten. That meant next to nothing to her, any more than pounds or quarts or degrees Fahrenheit did.

“We've got to stop at the store and get apples,” Amanda said importantly when her brother came up. He started to laugh. She scowled at him. “What's so funny?”

“Did Mom leave a note in your lunchbox, too?” he asked.

“She left me one, all right,” Amanda said. “You mean she gave 'em to both of us?”

Her brother nodded. “She sure did.”

“Why didn't she just carve the message on a rock and leave it here at the bus stop?” Amanda said. “Sometimes I think she's even more stuck in her ways than Grandpa is.”

“I wouldn't be surprised.” Jeremy pointed up the street. “Here comes the bus.”

It was old-fashioned, too. The school district couldn't afford anything newer and cleaner. It burned natural gas, which meant it spewed carbon dioxide into the air. Most vehicles, these days, were either electric or ran on fuel cells that gave off only clean water vapor. Global warming hadn't stopped, but it had slowed down.

They got on the bus. As soon as it was full, the driver pulled off the side street where she'd picked up her passengers and turned north onto Topanga Canyon Boulevard. The bus rattled almost enough to drown out the trills of telephones as friends on other buses and in cars started catching up with people here. Kids on the bus made calls, too. Back in the old days, Amanda's grandfather said, everybody could listen to everybody else talking. She had trouble imagining that. It sounded like an amazing nuisance. Throat mikes let people keep conversations private, the way they were supposed to be.