The new time-travel film Looper involves mob hitmen contracted to blow away bad guys from the future who've been sent back in time to be terminated. It's not a happy duty: When the assassins' 30-year terms of service are up, they know they'll be sent back in time themselves to be capped, or "close the loop." Unsurprisingly, Bruce Willis is having none of that — possibly because he already did all this in 12 Monkeys, a movie that held fast to the idea of immutable timelines, meaning you can't really change the past — if you go back in time, you just put into place the past that has already happened. In Looper, past, present, and future are anything but stable. All of this is very complicated, but why obsess? As Willis tells his younger self (Joseph Gordon-Levitt) that if they were to start talking about timelines, "We'd be here all day drawing diagrams with straws." Still, there are some rules that have evolved over the course of innumerable time-travel films and TV series, and for those planning a trip, here they are.
Don't Mess With Yourself
As with everything else concerning time travel, there are different rules and different schools of thought on this subject. Even when it might not cause the universe to end (as in Southland Tales), it's never a good idea to have different versions of yourself running around, as this might put an end to your own existence (as in Timecop). According to the Pauli exclusion principle, no two identical particles can occupy the same space at the same time — everything will just go kablooey. Also, imagine the confusion — you can't meet a past version of yourself and not have a memory of it, whereas a future version can't be surprised to see you. (This is why in Looper, Bruce Willis complains about his foggy memory.) But what if you do have to engage your doppelgänger? Treat him or her as if you're someone else, like Biff Tannen does in Back to the Future II — give out instructions, but not your identity. Or be like Spock in Star Trek and just remain detached.
This is all, of course, assuming you're inhabiting two different bodies. Some time travel, however, only involves using one body — you find yourself to be older or younger, such as in The Butterfly Effect, Peggy Sue Got Married, Hot Tub Time Machine, and Click. Or you have no real physical body and can't interact with the environment, if you're traveling via someone's memories in a Pensieve (Harry Potter) or a hologram (Quantum Leap). If this is the case, feel free to revise our advice.
Don't Mess With Your Ancestors
Marty McFly famously disrupts the moment when his parents were to meet and fall in love, which is why he has to reunite them — or else he won't exist. Apart from the icky Freudian issues that arise in Back to the Future when his mom crushes on him instead of his dad (because, let's face it, Michael J. Fox is cuter than Crispin Glover), screwing up your own conception is a very serious possibility in time travel. It even has a name: the grandfather paradox. Fry in Futurama experiences an even creepier wrinkle — after his grandfather dies, he ends up sleeping with his grandmother, in effect becoming his own grandfather.
On the flip side, if you know someone needs to go back in time to father you, as was the case with John Connor and Kyle Reese in The Terminator, make sure he falls in love with your mom. As Sarah Connor muses, "If you don't send Kyle, you can never be. God, a person could go crazy thinking about this." (Especially if you start thinking about how if you succeed in changing the future, you will also never be.) We say just don't interfere with anything involving your conception, your birth (The Butterfly Effect), or any near-death experiences you've had (Donnie Darko), unless you want to disrupt your timeline big time.
Don’t Mess With Historical Figures (and Don't Kill Hitler).
Bill and Ted kidnap Socrates, Genghis Khan, Joan of Arc, Beethoven, Abraham Lincoln, Billy the Kid, and Sigmund Freud from their respective time periods for the sake of a history report (Bill & Ted's Excellent Adventure), but what happened to those folks when they were returned to their own eras? It might be tempting to interact with and even dare to inspire such luminaries as Salvador Dali, Josephine Baker, Gertrude Stein, Pablo Picasso, and Ernest Hemingway, et al. as Owen Wilson does in Midnight in Paris, but what if you wind up changing the course of history? What if your new friends start babbling about the future? That's the kind of crazy talk that got Bruce Willis locked up in 12 Monkeys.
But the historical figure everyone seems to want to tangle with the most is Adolf Hitler. Imagine having a chance to kill him as a child and avert World War II! This is more or less Bruce Willis's plan in Looper — he's out to nail the child version of a shadowy future crimelord known as the Rainmaker. Unfortunately, this sort of thing never works. Just as when Sayid tried to kill Ben on Lost, this is what is known as the Hitler Exemption. Whatever you do, despite your best intentions, can cause a worse outcome. If it didn't work for the Terminator, why would it work for you? Even Doctor Who argues against this plan, and Godwin's Law of Time Travel backs him up. Also, there's the annoying temporal paradox — if an evil is removed, then there will be no need for you to go back in time, and therefore the evil will not be averted.
You Can't Save Them All.
Unless you're Superman and have world-spinning capabilities. Just as with the temptation to kill Hitler, you must resist the impulse to save a loved one in another time zone. It won't work, according to a universe course-correction known as the self-consistency principle. If someone's death in the past motivates you to create a time machine (The Time Machine), then that time machine can only exist because the person is dead. If you go back in time, they will still die (The Time Traveler's Wife). Scientists who understand this principle in 12 Monkeys and Source Code don't attempt to undo the past — they don't try to stop the virus from spreading or the bomb from going off. Instead, they seek to identify causality, so they can create a vaccine in the future or track down the bomber before his next act of terrorism. But if you've got a hero complex, you might try snagging bodies who are about to die to serve as hosts (Freejack) or to repopulate the future (Millennium). Yeah, that might work.
Sweat The Small Things.
The butterfly effect is a term partly derived from a Ray Bradbury story (later turned into a really bad film) called "A Sound of Thunder," in which time-travel tourists accidentally step on a prehistoric butterfly, which changes the entire future. Little things matter, which means you'll have to pay extreme attention to detail. That can be tiring, so we advise keeping your trip short — maybe thirteen seconds (Galaxy Quest), eight minutes (Source Code), or just a few hours (Harry Potter and the Prisoner of Azkaban). Plan ahead, and pack wisely (or arrange to have someone leave items for you), because you might need keys (Bill & Ted's Excellent Adventure) or hard currency (Looper). As Safety Not Guaranteed suggested, you must bring your own weapons, and for that, we like the blunderbuss shotgun in Looper. That's assuming you can bring any non-organic material at all — you might be naked (Time Traveler's Wife, Terminator). Either way, your best advance strategy is great health insurance, because time travel is dangerous — you might get bleeds (Primer) and/or brain damage (The Butterfly Effect). Be prepared!
Use Your Time Wisely.
Hermione Granger in Harry Potter and the Prisoner of Azkaban used a Time-Turner to schedule extracurricular classes such as Divination and Muggle Studies. Phil Conners in Groundhog Day eventually used his extra days to learn how to play piano, speak French, and ice-sculpt. Erica in Being Erica uses time travel as a form of therapy to address the regrets in her life — but in the end learns it's not about changing the past as much as it is about changing yourself. Risk of paradoxes: zero.
Sometimes, you will have to convince someone that you really are from the future or the past, so having lots of information is good — a password (Doctor Who), the details of a baseball game that's about to be played (Frequency), and other events that are about to happen (Groundhog Day). Depending where you are, saying that Ronald Reagan became president could be laughable to some (Back to the Future) — as would any contention that the Terminator became governor of California.
Beware The Crono-Cops.
Be aware that time travel is usually illegal. Such is the case in Looper, which is why only the mob employs it. If it's available in your time, there may be restrictions, so check with your local chronoguard, be they Time Cops or Time Lords, and make sure you're within parameters. And happy travels!