Lightning

Chris said.
    Stefan asked for water and drank half a glass this time. He wanted to hold the glass in his good hand, but he was shaking too much; water slopped on the bedclothes, and Laura had to help him.
    When he spoke again, Stefan's voice wavered at times. "Because the time traveler exists
outside
of time during his journey, he is not only able to move in time but geographically, as well. Picture him hanging above the earth, unmoving, as the globe turns below him. That's not what he does, of course, but it's easier to see that image than to imagine him hovering in another dimension. Now, as he hangs above the world, it turns below him, and if his journey to the future is gauged properly, he can travel to a precise time at which he will find himself in Berlin, the same city he left years before. But if he chooses to travel a few hours more or less, the world will have turned that much more beneath him, and he will arrive at a different place on its surface. The calculations to achieve a precise arrival are monumentally difficult in my era, 1944—"
    "But they'd be easy these days," Chris said, "with computers."
    Shifting in discomfort against the pillows that propped him up, putting his trembling right hand against his wounded left shoulder as if to quell the pain by his own touch, he said, "Teams of German physicists, accompanied by Gestapo, were sent secretly to various cities in Europe and the United States in the year 1985, to accumulate vital information on the making of nuclear weapons. The material they were after was not classified or difficult to find. With what they already knew from their own researches, they could obtain the rest from textbooks and scientific publications readily available at any major university library in '85. Four days before I departed the institute for the last time, those teams returned from '85 to March 1944, with material that would give the Third Reich a nuclear arsenal before the autumn of that year. They were to spend a few weeks studying the material at the institute before deciding how and where to introduce that knowledge into the German nuclear program without revealing how it had been obtained. I knew then that I had to destroy the institute and everything it contained, key personnel as well as files, to prevent a future shaped by Adolf Hitler."
    As Laura and Chris listened, rapt, Stefan Krieger told them how he had planted explosives in the institute, how on the last of his days in '44 he had shot Penlovski, Januskaya, and Volkaw, and had programmed the time gate to bring him to Laura in present-day America.
    But something had gone wrong at the last minute, as Stefan was leaving. The public power supply failed. The RAF had bombed Berlin for the first time in January that year, and the U.S. bombers had made the first daylight runs on March 6, so the power supply had been interrupted often, not merely due to bomb damage but also because of the work of saboteurs. It was to guard against such interruptions that the gate itself was powered by a secure generator. Stefan heard no bombers that day when, wounded by Kokoschka, he had crawled into the gate, so apparently the power failed because of saboteurs.
    "And the timer on the explosives stopped. The gate was not destroyed. It's still open back there, and they can come after us. And… they can still win the war."
    Laura was getting another headache. She put her fingertips to her temples. "But wait. Hitler can't have succeeded in building atomic weapons and winning World War Two, because we don't live in a world where that happened. You don't have to worry. Somehow, in spite of all the knowledge they took back through the gate, they obviously failed to develop a nuclear arsenal."
    "No," he said. "They've failed so far, but we can't assume they will continue to fail. To those men at the institute in Berlin in 1944, their past is immutable, as I have said. They cannot travel backward in time and change their own past. But they can change their future and ours, because a time traveler's future is mutable; he can take steps to alter it."
    "But
his
future is my past," Laura said. "And if the past can't be changed, how can he change mine?"
    "Yeah," Chris said. "Paradox."
    Laura said, "Listen, I haven't spent the last thirty-four years in a world ruled by Adolf Hitler and his heirs; therefore, in spite of the gate, Hitler failed."
    Stefan's expression was dismal. "If time travel were invented now, in 1989, that past of which you