Lightning

literally could have been called
Zukunftstrasse
, or Future Road, Stefan explained, for Vladimir Penlovski had been unable to discover a way to send men backward in time from the gate he had invented. They could travel only forward, into their future, and return automatically to their own era.
    "There seems to be some cosmic mechanism that prohibits time travelers from meddling with their own pasts in order to change their present-day circumstances. You see, if they
could
travel back in time to their own past, there would develop certain—"
    "Paradoxes!" Chris said excitedly.
    Stefan looked surprised to hear the boy speak that word.
    Smiling, Laura said, "As I told you, we've had rather a long discussion about your possible origins, and time travel turned out to be the most logical. And in Chris here, you're looking at my resident expert on the weird."
    "Paradox," Stefan agreed. "It's the same word in English and German. If a time traveler could go back in time to his own past and affect some event in history, that change would have tremendous ramifications. It would alter the future from which he had come. Therefore he wouldn't be able to return to the same world he'd left—"
    "Paradox!" Chris said gleefully.
    "Paradox," Stefan agreed. "Apparently nature abhors a paradox and generally will not permit a time traveler to create one. And thank God for that. Because… suppose, for example, Hitler sent an assassin back in time to kill Franklin Roosevelt and Winston Churchill long before they rose to high office, which would have resulted in the election of different men in the U.S. and England, men who might have been less brilliant and more easily dealt with, leading to Hitler's triumph by '44 or sooner."
    He was speaking now with a passion that his physical condition would not allow him to sustain, and Laura could see it taking a toll of him word by word. The perspiration had almost dried on his brow; but now, although he was not even gesturing, a new thin film of sweat silvered his pale forehead again. The circles of fatigue around his eyes appeared to grow darker. But she could not stop him and order him to rest, because she wanted and needed to hear everything he had to say—and because he would not have allowed her to stop him.
    "Suppose
der Führer
could send back assassins to kill Dwight Eisenhower, George Patton, Field Marshal Montgomery, kill them in their
cradles
, when they were babies, eliminating them and others, all the best military minds the Allies possessed. Then most of the world would have been his by '44, in which case time travelers would have been going back in time to kill those men
who had already long been dead and posed no threat
. Paradox, you see. And thank God that nature permits no such paradox, no such tampering with the time traveler's own past, for otherwise Adolf Hitler would have turned the entire world into a concentration camp, a crematorium."
    They were silent a while, as the possibility of such hell on earth struck each of them. Even Chris responded to the picture of an altered world that Stefan painted, for he was a child of the eighties, in which the villains of film and television melodramas were usually either voracious aliens from a distant star or Nazis. The Swastika, the silver death's-head symbol and black uniforms of the SS, and that strange fanatic with the small mustache were to Chris especially terrifying because they were part of the media-created mythology on which he had been raised. Laura knew that real people and events, once subsumed by mythology, were somehow
more
real to a child than the very bread he ate.
    Stefan said, "So from the institute we could go only forward in time, but that had its uses too. We could leap forward a few decades to discover if Germany had held on in the dark days of the war and had somehow turned the tide. But of course we found that Germany had not done any such thing, that the Third Reich had been defeated. Yet with all the knowledge of the future to draw from, could not that tide be turned, after all? Surely there were things Hitler could do to save the Reich even as late as '44. And there were things that might be brought back from the future with which the war might be won—"
    "Such as," Chris said, "atomic bombs!"
    "Or the knowledge of how they were built," Stefan said. "The Reich already had a nuclear research program, you know, and if they'd had a breakthrough early enough, had split the atom…"
    "They'd have won the war,"