What do Charlton Heston, the Battle of Shiloh, and stories about time travel have in common?
Union General Lew Wallace, who played a significant role at the Battle of Shiloh, went on to write the novel Ben-Hur, which was made into the famous movie starring Charlton Heston. And what does Ben-Hur have to do with time travel? I am actually going somewhere with this -- just bear with me.
Suppose that time travel is possible, but the past is frozen and can never be changed. This eliminates the paradoxes that inevitably follow if time travelers can change the past, but it raises a problem of its own: it's really boring. How can a writer put together an interesting story about a past that can never be altered?
One common plot line has our intrepid hero going back in time to stop Lincoln's assassination/kill Hitler/save the Titanic, only to be thwarted by some trivial event. Or (IRONY ALERT) actually causing the undesirable event through his own actions. An unsuccessful attempt to stop John Wilkes Booth was the plot of a Twilight Zone episode. And in Jack Finney's From Time to Time, the protagonist tries to stop the sinking of the Titanic, only to cause it instead. The problem with stories of this kind is that they are extremely predictable (hmm, I wonder how the time traveler will get thwarted this time) and, frankly, a bit depressing.
Another possibility is to confine the action to a place or era in history that we know very little about, allowing the time traveler a bit more free rein. This is where my allusion to Ben-Hur comes in. That movie is, at the end, associated with the story of the New Testament. But a straight retelling of the New Testament leaves little to the imagination, since we all know the story. That's why Ben-Hur concentrates on someone completely outside the main events and follows his life story instead. You see a lot of this "Ben-Hur effect" in other Biblical films of that era, such as Quo Vadis and The Robe. By concentrating on peripheral figures, they're free to take the plot anywhere. Science fiction stories in this vein often verge on historical fiction, but some of them are very well done, particularly the short stories of Poul Anderson and the "Company" series by Kage Baker.
Finally, one can assume a fixed past, but use this to introduce a clever "twist" in the plot. The grandfather of this approach was Robert Heinlein, with his short stories "By His Bootstraps" and "All You Zombies." (Heinlein is clearly the subject of that famous quote, "All my best thoughts were stolen by the ancients"). In "By His Bootstraps," the protagonist is recruited to assist a mysterious world leader from the future. Although the hero initially resists, he eventually comes to discover that he is the future leader himself. "All You Zombies" concerns a character who, through the use of time travel, ends up being his own mother and his own father. You might think that the possibilities for this kind of story would have been exhausted by now, but these "twist" stories still come up every so often and are very satisfying when they're done well. A more recent example is "Scherzo for Tyrannosaur" by Michael Swanwick. Although they avoid the most obvious time-travel paradoxes, stories of this kind still present a problem. Imagine that my future self shows up and presents me with a time machine. I then hold onto it for ten years, and go back in time to present it to my past self. The past has not been altered, and there's no grandfather paradox. But where did the time machine come from? I'll talk about this a bit more in my next post, when I discuss the science of time travel.