As least since H.G. Wells' 1895 novel The Time Machine, time travel has been a promising storytelling concept. Alas, it has seldom delivered on that promise: whether their characters jump forward into the future, backward into the past, or both, the past 125 years of time-travel stories have too often suffered from inelegance, inconsistency, and implausibility. Well, of course they're implausible, everyone but Ronald Mallett might say — they're stories about time travel. But fiction only has to work on its own terms, not reality's. The trouble is that the fiction of time travel can all too easily stumble over the potentially infinite convolutions and paradoxes inherent in the subject matter.
In the MinutePhysics video above, Henry Reich sorts out how time-travel stories work (and fail to work) using nothing but markers and paper. For the time-travel enthusiast, the core interest of such fictions isn't so much the spectacle of characters hurtling into the future or past but "the different ways time travel can influence causality, and thus the plot, within the universe of each story." As an example of "100 percent realistic travel" Reich points to Orson Scott Card's Ender's Game, in which space travelers at light speed experience only days or months while years pass back on Earth. The same thing happens in Planet of the Apes, whose astronauts return from space thinking they've landed on the wrong planet when they've actually landed in the distant future.
But when we think of time travel per se, we more often think of stories about how actively traveling to the past, say, can change its future — and thus the story's "present." Reich poses two major questions to ask about such stories. The first is "whether or not the time traveler is there when history happens the first time around. Was "the time-traveling version of you always there to begin with?" Or "does the very act of time traveling to the past change what happened and force the universe onto a different trajectory of history from the one you experienced prior to traveling?" The second question is "who has free will when somebody is time traveling" — that is, "whose actions are allowed to move history onto a different trajectory, and whose aren't?"
We can all look into our own pasts for examples of how our favorite time-travel stories have dealt with those questions. Reich cites such well-known time-travelers' tales as A Christmas Carol, Groundhog Day, and Bill & Ted's Excellent Adventure, as well, of course, as Back to the Future, the most popular dramatization of the theoretical changing of historical timelines caused by travel into the past. Rian Johnson's Looper treats that phenomenon more complexly, allowing for more free will and taking into account more of the effects a character in one time period would have on that same character in another. Consulting on that film was Shane Carruth, whose Primer — my own personal favorite time-travel fiction — had already taken time travel "to the extreme, with time travel within time travel within time travel."
Harry Potter and the Prisoner of Azkaban, Reich's personal favorite time-travel fiction, exhibits a clarity and consistency uncommon in the genre. J.K. Rowling accomplishes this by following the rule that "while you're experiencing your initial pre-time travel passage through a particular point in history, your time-traveling clone is also already there, doing everything you'll eventually do when you time-travel yourself." This single-time-line version of time travel, in which "you can't change the past because the past already happened," gets around problems that have long bedeviled other time-travel fictions. But it also demonstrates the importance of self-consistency in fiction of all kinds: "In order to care about the characters in a story," Reich says, "we have to believe that actions have consequences." Stories, in other words, must obey their own rules — even, and perhaps especially, stories involving time-traveling child wizards.
Based in Seoul, Colin Marshall writes and broadcasts on cities, language, and culture. His projects include the book The Stateless City: a Walk through 21st-Century Los Angeles and the video series The City in Cinema. Follow him on Twitter at @colinmarshall or on Facebook.