There aren’t words correct enough to describe how much I love reading and writing time travel fiction. The appeal I find in it comes from how dramatically it can affect a story: under normal circumstances a story contained a fixed timeline of events, character reactions and irreversible decisions. The sheer idea that a character could jump forwards or backwards along the timeline and change the supposed events of the story creates so many possibilities that my fingers tingle at the sheer realms of possibilities that open up.
In a way, writing a piece of fiction that utilises the concept of time travel is similar to writing one containing magic. As a writer, it’s your responsibility to decide how, within the fictional universe, the time travel will work. You need to decide if it can be controlled (tip – yes, to avoid incredibly cheesy deus ex machinas), how reliable the process is (Doctor Who’s a good example of how imperfect time travel can twist the story) and paradoxes. You guys are lucky that I was in Belgium during P week, as I wrote an entire project dossier on the subject of paradoxes in time travel and it would have been really boring.
Long story short, a paradox is a conflict of events: the traditional example is the Grandfather Paradox, where if theoretically you went back in time and killed your own grandfather before your father was conceived, you would have never been born and gone back in time to kill your grandfather, etc, etc. As the creator of your fictional universe and individual time travel rules, you need to decide what would happen if a paradox were allowed to run free. The traditional favourite is the complete destruction of the universe, which as far as character peril goes is up there with stepping on Lego bricks. There’s also the option of having paradoxes resolve themselves but that’s complicated and I’ll revisit the subject another time.
I suggest reading The Time Machine by H.G. Wells if you’re new to the concept. That or Harry Potter and the Prisoner of Azkaban.