The third plot type in science fiction is the time travel story. Ever since H. G. Wells created the form with The Time Machine, readers have evidenced a continuing interest in the subject. One reason for this popularity is that the science of time-space is so esoteric, so intangible, that a writer can formulate a "wonderful new discovery" to justify the existence of the time machine and place his story at any point in history: today, tomorrow, next week, a hundred years from now, or a hundred years ago, making for varied and vivid backgrounds and plots. Too, because time travel stories deal with a quantity which people are familiar with—minutes, hours, memories—it seems more real than a story based on science beyond their understanding.
A time machine can operate in several ways. If the story purpose is best suited by a machine that will only carry passengers into the past (Harry Harrison's The Technicolor Time Machine), the writer need only say so. If he wants a machine that only travels into the future (Wilson Tucker's The Year of the Quiet Sun) and then back to the starting point, or if he wants a machine capable of forward and backward time travel, he need only inform the reader, briefly, of the machine's limitations or abilities.
Trips into the future require the same extrapolated background we have discussed. Trips into the past require a background for the proper period; this can easily be researched in any library with a good selection of historical reference texts.
Only one main brand of error is endemic to the time travel tale: the time paradox. The term is best explained through examples, which are limitless. For instance:
If you traveled back to last Thursday morning in a time machine and met yourself back then and told yourself to invest in a certain company because their stock would soar during the next week, what would happen if the Early You did as the Later You wished? When the Later You returned to the present, would he find himself rich? Or perhaps, while the Early You was running to the stock broker, he was stricken by an automobile and suffered two broken legs. When the Later You returned to the present, would he find himself with two broken legs? Perhaps you would end up hospitalized, never having been able to make the trip in the first place because your legs were broken a week ago. Yet, if you had never taken the time trip, you wouldn't have sent your Early Self into the path of the car and would not have broken legs. Yet, if you did make the trip, and had the broken legs, you couldn't have made the trip because of the broken legs and...
Do you see what a time paradox is?
Suppose your hero went back in time and killed the villain ten years in the past. The villain would cease to exist at that point. Any children he had fostered would cease to exist if they had not been fathered before that day ten years ago. Did you really mean to kill his innocent children as well as him?
Or suppose you traveled back in time and married your great-grandmother. Would you be your own great-grandfather?
If you returned again and again to the same general period in time, wouldn't there be a whole crowd of you walking around?
If you go into the future and see something unpleasant in your own life, and you come back to the present to make sure that the future thing never happens—can you really hope to change the future? If you've already seen yourself dying in a wrecked automobile, can you return to the present and avoid that accident? If you've seen it, isn't it already predestined?
To better understand the complications you must deal with in the time travel story, read Up the Line, a modern classic of the form by Robert Silverberg, published by Ballantine Books. Silverberg purposefully generates every conceivable time paradox and carries them all to their wildly absurd and fascinating conclusion.
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