Spoiler Alert: This article contains references to events that happen in the sixth season of Game of Thrones. There are also revealing references to the plot of The Terminator, but if you haven’t seen The Terminator by now then you deserve to have it spoiled.
A few weeks ago I was talking to my mom about Game of Thrones. We were discussing the merits of holding doors. More specifically, how could someone travel back in time and influence a person in the past by making him or her hold a door in the future? Like that sentence, my attempt to sketch the timeline and explain the show’s time travel theory was confusing and convoluted. If you’ve never seen Thrones, or aren’t caught up on the newest season, the above anecdote probably won’t make any sense. Whether you’re familiar with Thrones or not, you no doubt have read, watched, or heard of some work of fiction concerning time travel. More often than not, when examined under a microscope, these time-bending scenarios seem to result in some sort of mind-bending paradox. Lo and behold, people of science have spent a considerable amount of time discussing…time. So we can try to make sense of it all.
To solve the problem of time travel paradoxes, a very smart Russian physicist named Igor D. Novikov worked with some other very smart people to develop what is now called the Novikov self-consistency principle. The principle asserts that if an event exists that would give rise to a paradox, or to any “change” to the past whatsoever, then the probability of that event is zero. Well, what the fuck does that mean? I’ll do my best to explain it but first let’s get ourselves into a pickle, or a paradox in time travel.
The most common paradox in time travel is the “Grandfather paradox.” A time traveler goes into the past. She finds her grandfather and kills him, eliminating the possibility of one of her parents ever existing. As a result the time traveler is never born. This is where the paradox really kicks in. If the time traveler is never born, then she cannot go back in time to kill her grandfather. So the grandfather lives, the time traveler is born after all, she goes back in time to kill her grandfather, and the paradox continues. 
The Novikov self-consistency principle plainly says, assuming time travel were real, that there is zero probability this event can take place. It would be physically impossible for any of us to travel back in time and kill our grandfathers, or more specifically create a future in which we are never born. A less suicidal, and more altruistic, version of the “Grandfather paradox” is the “Hitler murder paradox.” A time traveler goes into the past, where she kills Adolf Hitler before he can instigate World War II. Novikov’s self-consistency principle is at work again. No matter whether the time traveler succeeds or fails at assassinating Hitler, World War II is a necessary event in order for the time traveler’s trip to occur in the future. Without World War II taking place, there would be no reason for the time traveler to even imagine going back in time to stop World War II.
Let’s assume our time traveler does succeed in killing Hitler, or her grandfather, or maybe Hitler is her grandfather. It doesn’t matter. According to Novikov, anything a time traveler does in the past must have been part of history all along. The time traveler can never do anything to prevent the time traveling trip from happening, since this would represent an inconsistency. So maybe Hitler gets killed before WWII breaks out. It doesn’t matter. Some other asshole would rise to power by feeding off the fears and struggles of the German people, and WWII happens anyway. Maybe the time traveler does kill her grandfather. Doesn’t matter. Her grandmother remarries, gives birth to one of her parents, and she’s got a new grandfather anyway. No matter what, probabilities would continue to bend in order to prevent paradoxes from happening. The closer the time traveler gets to the unpreventable act (i.e. the start of WWII or the time traveler’s own birth) the stranger the outcomes become.
My favorite cinematic depiction of the Novikov self-consistency principle is The Terminator. The film made Arnold Schwarzenegger a household name, launched James Cameron’s directing career, and set the bar for action movies thereafter. It’s also a great fucking story about time travel! A cyborg assassin is sent back in time from 2029 to 1984 to kill Sarah Connor and prevent her from giving birth to John Connor, the man who will lead a resistance against the machines that create the cyborg assassin in the future. Simultaneously, John Connor also sends back a soldier, Kyle Reese, in time to protect Sarah from the cyborg assassin. Sarah and Reese succeed in defeating the Terminator, but unknowingly set off a chain of events that ultimately lead to the terminator’s future creation anyway. Likewise, Reese and Sarah hit it off and Reese turns out to be John Connor’s dad. That’s about as Novikov self-consistency principle as it can get!
The insertion of futuristic technology into the past is what allows for the creation of the Terminator in the future. John Connor sends a soldier back in time, allowing for his own birth in the future. Once again we see that the time traveler can never do anything to prevent the time traveling trip from happening, even if that time traveler is a fucking machine programed to kill humans.
Game of Thrones currently employs the Novikov self-consistency principle and it’s pretty neat to think about what that means for Bran and his green-seeing ability. There are so many other examples of time travel in literature and film, from H.G. Wells’ The Time Machine to Twelve Monkeys to Primer (Spoiler at link). Some of them adhere to the Novikov self-consistency principle and some of them don’t **cough**Back to the Future**cough**. The ones that don’t adhere to Novikov’s principle are still fun to think about, but you could go mad trying to wrap your brain around infinite universes and divergent time lines. For now I’ll just confuse myself with simple causal loops like The Terminator and Thrones.