A Concise Breakdown of How Time Travel Works in Popular Movies, Books & TV Shows

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.

Relat­ed Con­tent:

What’s the Ori­gin of Time Trav­el Fic­tion?: New Video Essay Explains How Time Trav­el Writ­ing Got Its Start with Charles Dar­win & His Lit­er­ary Peers

Pro­fes­sor Ronald Mal­lett Wants to Build a Time Machine in this Cen­tu­ry … and He’s Not Kid­ding

Mark Twain Pre­dicts the Inter­net in 1898: Read His Sci-Fi Crime Sto­ry, “From The ‘Lon­don Times’ in 1904”

What Hap­pened When Stephen Hawk­ing Threw a Cock­tail Par­ty for Time Trav­el­ers (2009)

Pret­ty Much Pop #22 Untan­gles Time-Trav­el Sce­nar­ios in the Ter­mi­na­tor Fran­chise and Oth­er Media

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.