I was just reading a really fun time-travel story (Heather Albano's Timepiece), and a thought that had been bouncing around in my head for a long time came clear to me: from a purely narrative point of view, time travel is prophecy's long-lost (and possibly evil) twin.
Don't get me wrong -- they're clearly very different plot devices, and stories that include one or the other tend to play out in somewhat different ways. But they do essentially the same thing to and for a plot. They bend narrative logic so that it can snap in interesting and unexpected ways.
If you want to look at it this way, stories are machines for challenging and reinforcing a reader's understanding of causality: action A leads to result B which leads in turn to new action C. Even the most anarchic, apparently plotless story -- the Marx Brothers' movies or Douglas Adams's books, for example, or Beckett's plays -- uses the reader or audience's expectations of causal relationships to create a tension that relieved only in absurd laughter.
Prophecies are a time-honored storyteller's cheat, dating back as far as we have records of stories, to Gilgamesh. When you have a prophecy in a story, it a) will turn out to be true; but b) won't mean what you think it means. The plot is given away, so that story-machine seems to have been broken, but somehow the way that events play out always lead to things happening very differently from the natural assumption. Think about it: if a prophecy revealed early in a story exactly how everything was going to happen, where would the pleasure be?
Time travel generally takes one of two tacks: either the time traveller changes something in the past that has unexpected consequences (the so-called butterfly effect), or tries to alter a known event, yet the time-stream leads inexorably back to the original end-point. (I'm talking here about travel to the past. Travel to the future most often takes the form of social commentary, or is simply prophecy in a technological guise.)
There are all sorts of scientific theories that might be trotted out to show the reasons for this, but really, this is just an inversion of the narrative imperatives that spring from including a prophecy in a story. By including a plot device that seems to explode causality, the author needs either to change history in a way that has a narrative payoff that makes sense in terms of where we started, or to show that things were always going to be the way that they turned out -- though as with prophecy, this result feels like a surprise.
One example that makes excellent (if occasionally mind-bending) use of both of these devices is J. K. Rowling's Harry Potter and the Prisoner of Azkaban. The apparently incompetent Divination professor Sybill Trelawney makes a prophecy that appears on the surface to refer to Sirius Black, the person we believe to be the villain, but actually refers to Black's supposed victim Peter Pettigrew. Of course, the prophecy turns out (over the course of the next two books) to be precisely true, but not at all the way we might have expected.
Later in the book, Sirius is captured and sentenced to the Dementor's Kiss, a fate worse than death. Having learned the truth, Harry and his friend Hermione travel back in time to rescue Sirius before he has been captured. Everything that they do along the way -- freeing the unjustly condemned hippogriff Buckbeak, sending the Patronus that gets rid of the Dementors that were about to attack Sirius and Harry, and helping Sirius fly away on Buckbeak's back -- all of this plays out exactly as they had already experienced it, though not in the ways that they expected. Harry can only cast the difficult Patronus charm, for example, because he's already seen himself do it.
When done less effectively, the effect can be confusing and unsatisfying. The causal knots that tangle the on-going plot threads in recent seasons of Dr. Who spring to mind.
It might make one sympathize with Deep Space Nine's Chief O'Brien: "I hate temporal mechanics." Yet when well employed, time travel and prophecy can both increase a story's effectiveness, even as they seem to be short-circuiting it.
Image by Rachel Caitlin (TrueLovesKiss) on flickr.com. Used under a Creative Commons license.