Time travel is a staple of science fiction films - but that doesn't indicate it always makes a ton of sense. As far as anyone knows, time travel isn't technically possible, which means all the ways it's depicted in films are dreamed up by talented writers, not physicists. The time travel scenarios concocted by screenwriters often lead to plot holes and strange situational paradoxes, which might make for a fun and entertaining movie, but don't do much in the way of realism.
Some of the plot holes involve what is called the "predestination paradox," when the time traveler becomes a player in past events, and they might even instigate something that leads to the need in the future to go back in time.
Granted, realism isn't always what the audience is looking for in a sci-fi flick involving time travel, but some people still insist they fully understand the time-bending trajectory in a movie, even when it isn't so apparent to the rest of us. Everybody knows somebody who claims to comprehend movies like Inception.
Pointing out problematic paradoxes doesn't ruin the quality or draw of these films, which are classics with adoring fan bases. But scrutinizing their time travel mechanics can at least get your brain working and give you something to point out when a know-it-all claims to have the contradictions all figured out.
The Rules: The rules of time travel in Looper are never fully explained. All that is known is time travel was/will be invented about 20 years in the future, and the people there use the past as a dumping ground for bodies because disposing of them in the future is nearly impossible.
The Issues: Contract slayers who want to work for one of the future syndicates are required to off their future selves to claim their final payment of one gold bar at the end of their career. This effectively closes the "loop," but also offers something of a predestination paradox, which is eliminated through the course of the film. Essentially, the actions of the main character that close out his loop paradoxically rewrite the entire movie, preventing any of the action from happening in the first place. Because the film doesn't explain or allow for a bifurcation of the timeline, the time travel elements fall apart under scrutiny.
The Rules: The only thing revealed about time travel in Kate & Leopold is that a person has to fling themselves off a specific spot of the Brooklyn Bridge at a predetermined time. Doing so sends them to April 28, 1876.
The Issues: The most glaring issue is how somehow can survive the jump into the water from such a height, even if time travel somehow lessens the blow. If you look past this daring feat of acrobatic diving, there is a time loop that doesn't hold up. Liev Schreiber's character, Stuart, is dating Meg Ryan's Kate before going back in time to meet up with his great-great-grandfather, Leopold, played by Hugh Jackman.
By the end of the film, Kate goes back in time to get with Leopold, which means she was Stuart's great-great-grandmother the whole time they dated. But even if you look beyond that detail, the fact that Kate ends up with Leopold only because her great-great-grandson figured out time travel and sent her back in time is a massive causality loop resulting in a universe-ending paradox.
The Rules: Time travel in Source Code doesn't function like most movies. A person enters the Source Code to tap into another person's life within their alternate timeline via a neural interface.
The Issues: The film is about trying to ascertain the identity of a subversive by placing one person's consciousness into that of another from the recent past. There isn't anything about this that makes any sense, especially when you consider the movie concludes with an alternate timeline. Enjoying this film requires a complete suspension of disbelief.
The Rules: You can only travel through time if you are comprised of organic material - and you can't wear clothing.
The Issues: People have dissected the Terminator franchise for years. The first film is a sort of reverse grandfather/predestination paradox in that John Connor couldn't have existed to send Kyle Reese back in time unless he had done so before his conception, which doesn't make sense. If you allow that to happen, the rules get thrown out the window in the second movie, when a Terminator made of liquid metal - not organic material - finds its way to the past.
The movies that followed manage to muck up the Terminator timeline by overwriting what came before them. Terminator: Genisys rewrites everything back to before the first movie even took place in the past, which effectively wipes out everything that happened in the sequels. The method of time travel isn't fully explained, which is a good thing. Diving too deep into that level of confusion would leave more than a few in the audience lost and angry.