Time travel shares a dubious distinction with faster-than-light travel: they are two of the most popular ideas in all of science fiction, but also two of the most implausible from a scientific point of view. And both of them have become so ingrained in the culture of science fiction that they no longer require detailed justification when introduced into a story. As I noted earlier, the very first time travel novel, The Time Machine, begins with a lengthy explanation of the workings of time travel, while in a modern story Ann and Bob would just jump into their time machine and take off for the 13th century.
Anyone writing about time travel must choose one of two very different possibilities:
1. Allow the past to be changed
2. Assume the past is frozen and cannot be altered
This choice produces starkly different types of stories. Today I want to talk about the first possibility -- I'll save the second for a later post.
Allowing the past to be changed is a much more "fun" possibility, but it leads to the famous grandfather paradox. If you went back in time and killed your grandfather before he had children, then you would never have been born. Which means that you never went back in time and killed your grandfather. So he did have children. So you were born after all, which means.... you get the picture. (I have my own personal version of the grandfather paradox. Several of my relatives fought for the Union in the Civil War, and one died in the Battle of Chattanooga. But my only direct ancestor involved in the war seems to have spent the entire war guarding St. Louis from invasion by the Confederates -- not a bad place to be! I have met very few people with a direct ancestor who enlisted in the Union Army before having children and died in battle...)
Faced with the grandfather paradox, what's a science fiction writer to do? Ignore it, of course! There are countless stories in which the intrepid time travelers romp through the past, leaving a trail of mangled reality behind them. Two early stories of this kind, both quite famous, are "The Brooklyn Project" by William Tenn (which is extremely funny), and "A Sound of Thunder" by Ray Bradbury. In Bradbury's story, a time-traveler steps on a bug back in the Mesozoic, producing huge consequences in the present.
Other authors have tried to find loopholes in the grandfather paradox. One possibility, explored by Gregory Benford in Timescape, is to make use of the many-worlds interpretation of quantum mechanics. Benford's time travelers are able to change the past, but only in a different branch of the universe -- their own personal timeline is unaffected.
I also wrote a (not entirely serious) story to evade the grandfather paradox -- it's called "A Flash of Lightning" [and now you know where I got the title!] I'll post it next time.