A Taxonomy of Time Travel
The problem is that while time travel is an inherently fascinating component of a story, it is also a complex enough phenomenon that it often fails to retain any sort of logical consistency, falling apart upon closer examination. Time travel in fiction thus falls into a few loose categories. It's unclear yet whether it does in non-fiction, but I'll let you know if CERN gives us any updates.
There is of course what is simplest to dub as time travel for idiots. Back to the Future is a perfect example, and one painful to accuse given that as a fun story it just works so well. But it fundamentally cannot get a handle on the notion that cause and effect happen in a certain order. Go back, make a change, return to find the world different. It seems basically right except for two trip-ups. The fading photograph (and eventually fading Marty) is the worst offender that makes for a clever story mechanism but breaks temporal logic. Setting aside all of the other clutter and contradiction consider it in its simplest terms. The photograph taken back in time now exists before it was taken. Changing circumstances such that the photograph was not taken cannot change the photograph because that would make an effect appear before the cause.
Then there is time travel that works on a slightly smarter level, by at least appreciating the consistency of cause and effect. These are the most common stories: go back, change something, return to find the world different. I read a short story once that operated on this principle and proposed that the greatest crime was changing the past, because while you might have saved six million by killing Hitler as a baby, you murdered an infinite number of lives in the destroyed time line. But this of course sets up the paradox. If you go back in time and create a world in which you were never born, how do you exist? A paradox, you suggest? These stories are just smart enough to understand that effect must follow cause, but are not quite able to work out the math to a proper result. We'll return to this in a moment.
As a slight aside at this point, we should bring the many worlds hypothesis into play. The many worlds hypothesis is a loophole sometimes used to make the logical problems with the above setups resolve themselves. It posits that for every possible event, the universe branches into multiple universes, creating an infinite number of universes in which every thing that ever could happen, did happen. This allows story writers to get away with egregious offenses against the logic of cause and effect by merely waving a magic wand and declaring that the characters are now in a universe in which such and such did or did not happen. Go back in time and kill Hitler as a baby? The reason that you remember a future in which he lived is because you were in one time stream, but now that you changed the past you are in a different parallel stream.
While the many worlds hypothesis has the appeal of at least being logically consistent, it has a glaring problem. It's a brute force hammer of solving the problem, like multiplying by zero to demonstrate both sides of the equation are equal. It's just plain inelegant. But it also has the story flaw of essentially rendering time travel moot. If anything that can happen, has happened in an alternate timeline, then the actions of the characters do not matter one bit. Going back in time and killing Hitler as a baby doesn't change anything, because there is still an original timeline in which he doesn't die.
The gold standard of time travel stories is the construction of a four-dimensional knot. They are relatively rare, with Heinlein's All You Zombies taking the ultimate prize, and various "Twilight Zone" episodes sneaking into that territory as well. "Futurama" has also pulled it off multiple times. The key is in realizing that while it is possible to go back in time and change the past, there is no apparent change because you are already living in a world with that change as part of its timeline. Want to kill Hitler as a baby? You already failed. It's not that there's a lack of free will, or that changing the past is impossible, it's that you already did it. Or in other words, a proper time travel story has no paradoxes because working the math through, the remainders cancel out.
It is possible to go back in time and become your own grandfather, but only if you accept that you've already done so. Tying a knot in three dimensions is incomprehensible in two dimensions, the end result is an impossible construction when viewed by someone in a two-dimensional world. What we see as paradoxes in time travel are the knot in four dimensions.
Steven Lloyd Wilson is a hopeless romantic and the last scion of Norse warriors and the forbidden elder gods. His novel, ramblings, and assorted fictions coalesce at www.burningviolin.com. You can email him here.