(Editor's note: Season six Game of Thrones spoilers ahead.)
Since Game of Thrones' sixth season kicked off, fans have followed Bran Stark back in time to understand the history of Westeros. On last night's episode, Bran's storyline opened the gate to explain, albeit in a trippy, Inception-meets-Nightmare on Elm Street way, why Bran's companion Hodor can say only one thing: "Hodor."
The episode finds Bran and Meera Reed running from the White Walkers in a cave north of the Wall. In a vision of the past, Bran enters the mind of young Hodor and forces him, as old Hodor, to "hold the door" of the cave while Bran and Meera escape. The White Walkers' army of reanimated corpses kill old Hodor, and young Hodor is changed forever, screaming "Hold the door!" until it becomes simply "Hodor." Bran's created what looks like a rift in reality — a closed loop of time that requires Bran to shape the past in order to live to the present day.
Is a time paradox like this the stuff of nothing but science fiction and fantasy, or could this type of temporal loop ever happen in real life? It's not impossible.
Recent discoveries imply it might be possible to travel into the future
Time dilation, which was actually proven in a 2014 trial, is a theory that suggests time moves more slowly on a moving clock than on a clock in a fixed position, and that the amount of gravitational force can influence how much time elapses, depending on an object's distance from a gravitational mass.
As the theory goes, black holes, which create a tremendous amount of gravitational pull, would dilate time so much that instead of elapsed seconds, like the time tests run on the International Space Station, we'd be dealing with unknown amounts of time, like what's imagined in the movie Interstellar.
Provided a black hole wouldn't rip you to pieces, you could travel with great velocity through space and time in a black hole. This is the closest working theory of actual time travel, but it implies going into the future. It doesn't explain Bran's ability to change the present by visiting the past.
But going backward is much harder than going forward.
Using the theory of general relativity, scientist Kip Thorne wrote that wormholes, structures that connect one point in space to another distant point through a sort of "shortcut," could be turned into a time machine. But they're extremely unreliable for space travel — and even if they worked, they might not be big enough for anything larger than a photon to travel through.
Maybe the most damning — but disputed — theory of all: A wormhole can only take you back to the time it was created. Like backing up your computer, you can't see the files you altered before you began your backups. So unless Bran's guide, the Three-Eyed Raven, found a wormhole that was created around the time of a young Ned Stark storming the Tower of Joy, for example, the pair probably couldn't travel back in time to see what Ned was up to.
Time-traveling to the past isn't necessarily impossible, but it's loads more complicated than traveling into the future. Maybe it's far-fetched to say whoever created the weirwood tree also built a hamster maze of wormholes throughout the space-time of Westeros. But this is one of those Game of Thrones devices that actually has roots in physics.
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