At some point every geek franchise goes for time travel. Superman circles the globe at high speeds to make sure he can be in two places at once. Captain Kirk needs to find some humpback whales. I dream of dumping Barney back in the paleolithic age where he belongs. Time travel is what every geek has considered at some point.
But, how exactly does it work?
I’m not speaking in the scientific sense now. I’m speaking of the mechanics involved when its used in fiction. Time travel has been around at least since H.G. Wells wrote The Time Machine and used time travel as a way to represent a 19th century version of Occupy Wall Street. Wells’ traveler went to the far future where the privileged evolved into a bunch of wimpy wisps of a thing, while the working class were literally forced underground to continue to work the machines for the pretty people above ground, where they became the light-sensitive, cannibalistic Morlocks. The traveler sympathized with the wisps, but really, there’s some strong social commentary there the likes of which could just as easily been Snowpiercer.

Most fictional time travel, broadly speaking, works in one of two ways. I’ll connect them to some pop culture as a means to explain them. First, there’s the idea that time travel can be used to alter the flow of time and change history. We’ll call this the Back to the Future theory. Second, there’s the idea that no matter what you do or when you travel to, you can’t change what happens. We’ll call this the Babylon 5 theory.
In the world of Back to the Future, it is remarkably easy to change the course of time. Marty McFly in his first ten minutes in 1955 has already caused a minor ripple in the form of the name of the Twin Pines Mall becoming the Lone Pine Mall back in 1985. While he does manage to keep his parents together, he also manages to improve their lives in ways that it’s probably better not to think about, like how they hire an attempted rapist to wash and wax their cars. Had Marty sought out Doc first, Doc probably would have isolated him from everybody for a week before sending him back to 1985, just to be on the safe side. Instead, the whole plot revolves around fixing the time disruption Marty accidentally caused, and the two sequels do much the same thing, showing Marty and Doc having to consider almost nonstop the effects of them being anywhere or at anytime.

The syndicated sci-fi TV series Babylon 5 did things a bit differently. With only one or two exceptions, any glimpse of the future was going to come true, and since psychic ability was something many characters could lay claim to, there were many glimpses of the future. Even a trip to the future couldn’t actually affect changes. Series creator and writer of most episodes J. Michael Straczynski decided early on that the idea of changing things that were going to happen was out, but the viewer might have fun seeing how things turned out rather than what would happen. One example was a character having a vision of his own death. He knew he was going to die at the hands of another character, an enemy of his on the station, and he would kill his enemy at the same time. Eventually, the audience learned that the character’s death was a request for a mercy killing to his former enemy, and some other complications arose that made sense in context. The audience was told something would happen. It did happen. What they were not told was the context for it happening, and nothing anyone could do would change that event from happening. And while Babylon 5 did not use time travel as much as it could, travel to the past was a vital component to the show’s mythology, an event that could not be changed because it had already happened and had to happen again.

Most time travel stories work with one of these two ideas, or come up with novel ways to get around them. Stephen King wrote in his novel 11/22/63 that minor changes could be made to the past without too much trouble, but attempting to make major changes like stop the Kennedy assassination would result in all manner of unexpected interference and could end with the near destruction of the Earth. The Terminator films, at least in the original three films, bounce all over the place. The first movie goes by the Babylon 5 rules and the Terminator cannot change history, and, in fact, Skynet’s attempt to monkey with the past only results in John Connor’s conception. The first sequel showed the actions of the characters changed the future and Judgement Day would not come. The second sequel, not a very good movie, showed that Judgement Day was only postponed and nothing John Connor could do would change that. Marvel Comics states that traveling to the past actually creates an alternate timeline, so nothing you do can change a dang thing from the time you came from, and a frequent time traveler like Kang has plenty of other copies of himself running around. Quantum Leap assured us that changing the lives of little people for the better (or the worse if the Evil Leaper was around) was very possible, but only forgot that and other rules, like how Sam could only leap within his own lifetime, when the show was in its final years and running our of ideas.

Of course, sometimes time travel isn’t thought out very well. I doubt anyone watched Bill and Ted’s Excellent Adventure for hard science (or history or…well, anything serious), but the movie seemed to play with the idea that Rufus had to leave the future at a very specific time in order to get to Bill and Ted at the right moment to guide them towards their destiny. This doesn’t make sense. Rufus has access to a time machine. He should be able to leave at any time. Bill and Ted treat time travel as more like hopping between dimensions than actual time travel, where things occur at the same time and the phone booth just goes there instead of to a specific moment. Why does time continue to move forward in the present when Bill and Ted jump into the past anyway?

Time travel is used often enough, my non-geek wife assumed there was some in Captain America if he went from World War II to the present. She just gave me a skeptical look when I explained that, no, he was just much older than he looked.
And sometimes, time travel is just a gimmicky excuse. The Doctor can and does travel anywhere through time, but this seems to be done more in service to various sci-fi ideas. No matter what the time period, its generally safe to say there are aliens involved somewhere. Early comic adventures from DC character Rip Hunter worked off the same principle.

Now, I could do some kind of looping thing here and end my essay where I started it, but instead, I’ll just add this:
I loved the post-crisis, Pre-Jimmy Rip Hunter’s limitation of only being able to use methods time travel ONCE. If he wanted to go back again, he needed to discover a new form of time travel.