I’m a sucker for Time Travel films, even the ones that are poorly done. Of all the Sci-Fi sub-genres, it’s my favorite, and includes the best film of all-time: Back to the future.

But not all time travel films are the same. There are those dealing with alternate timelines. Others tackle different paradoxes, the most famous of them being the grandfather paradox. Others are all about escaping a time loop which is reliving the same day over and over again.

Grandfather Paradox

Maybe the most famous of the cinematic time travel tropes: The time traveler goes back in time and kills his own grandfather. As a result, the time traveler is never born. But, if he were never born, then he would be unable to travel through time and kill his grandfather, which means the traveler would then be born after all — but then, if he were born, he would be able to travel through time and kill his grandfather. In Back to the Future, travelling back to 1955 causes Marty to interfere with his parents’ meeting, and then races to rectify the mistake before he’s erased from existence. But… if he is erased from existence, then he never would have traveled back in time and interfered with his parents’ first meeting, and thus would have been born after all.

Alternate Timeline

In the film Deja Vu there’s a perfect example of the alternate timeline facet of Time Travel in films. Someone affects the events of the past, creating an alternate timeline where his/hers present never occurred, or has been altered.

Predestination Paradox

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In this paradox, the protagonist travels back in time and starts a chain of events that underpin their own present, which is pretty much a chicken-and-egg scenario, with a loop sequence that doesn’t actually have a beginning. Interstellar is an excellent example of this paradox.

Time Loop

There are a few different kinds of time loops in film: In films like Edge of Tomorrow, Groundhog Day and Source Code, he same time loop is formed multiple times. The time period repeats over and over again, trapping the protagonist until he does it the “right” way.

In Looper and Frequency, it’s a one-and-done loop. It’s formed and then the timeline continues. In the lesser known Triangle, Timecrimes and The Route V50 there are multiple time loops overlapping each other.

Time Slip

The time slip is a little more subtle than the previous methods, as the protagonist or time traveller isn’t aware he has just made the change. Midnight in Paris is a fine example of this trope, as the time travel occurence isn’t explained. Unlike a lot of other films with this “time trick”, the character isn’t stranded in the new “time” he encountered but moves back and forth between “worlds”.

Ontological Paradox

A scenario where, through time travel, a future event is the cause of an earlier (past). The paradox usually involves the creation or transfer of an object, an idea, or information.

For more on time travel, check out these “actual” cases of time travel, or our ranking of the best films in this category.