Again and again, people keep coming back to time travel. Stories involving time travel are popular because, in part, they tap into our common desire to fix the mistakes of our past, to take what we know now and get 100% completion. The TV series Quantum Leap, where Scott Bakula leapt from life to life striving to set right what once went wrong, is a sterling example of this. The Back to the Future trilogy leans more toward the temptations and dangers inherent in that premise. Yesterday, I saw the trailers for a new time travel movie, Hot Tub Time Machine, which - and I know this will shock you - is about a hot tub that is also a time machine.
This movie, incidentally, looks like it will be worth seeing, if only for the time travel and massive '80s-retro factor. Nevertheless, I can't help but ignore that with the trailer taglines "forget the present, change the future," it sort of rubs me the wrong way. Whether it's the guys of Hot Tub Time Machine ("We could combine Viagra with Twitter! Twittagra!"), Marty McFly with Gray's Sports Almanac in hand, the heroes of Chrono Trigger fighting to save the world from Lavos across sixty-five million years, or the time traveller of the day simply resolving to take advantage of future knowledge to reshape the way it all unfolds, all too often there's a massive elephant in the room that the writers either don't notice or refuse to address.
There are two fundamental theories for how time travel could work. The first, which is my personal preference, is that time travel would create a branching of timelines; at the point the time travellers arrived, a new timeline would split off from the one they left, allowing them to manipulate history and the future to their heart's content while their original timeline goes chugging on. The second, which was explicitly used in Back to the Future, postulates one single timeline. In this theory, time travellers actually enter their own history, and in so doing put themselves in a positions where their actions could delete their present and replace it with another, stemming from their actions in the changed past.
Where the elephant comes into play is that, if the second theory is the one that's used in the story, through the act of changing history the time traveller will effectively become history's worst mass murderer. The actions of Hitler, Stalin, Pol Pot, and all the rest are nothing compared to what one guy with a time machine in a single-timeline universe could do without even realizing it. Sometimes, of course, the time travellers do realize this - thus making this the Time Traveller's Dilemma.
Maybe it's because I don't put any stock in the concept of a "soul" that this rubs me the wrong way so much. Ultimately, though, I think this is a function of a rational outlook on the universe. Take even a situation where time travel is utterly necessary - say, without going back a few decades for some reason, the universe will be destroyed. In a single-timeline world, everyone but the time travellers would be killed regardless of whether they are successful - the only difference would be if they're killed by the event the time travel was meant to avert, or killed when the successful temporal intervention removes them from time.
The fact of the matter is, time travel is not only dangerous, it can easily cause headaches. For the good of everything, it's best to leave the time circuits off and to keep the flux capacitor from fluxing. Besides, if you're a really unlucky time traveller, you might avoid changing the future by stumbling into something that's far, far worse...
...a predestination paradox. I hate predestination paradoxes.