Part II – The Philosophy of Time Travel

Lost, Fringe, Doctor Who, Star Trek, The Flash, Harry Potter, Legends of Tomorrow, even Game of Thrones. Why do so many of the stories we love involve time travel?

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What is time?

The concept of time is, in many ways, the feature that lends gravitas to every story ever told. The ticking of the clock follows us wherever— and whenever— we go. We consider time wistfully when looking to the past; anxiously or excitedly when anticipating the future; even contentedly, when meditating on a continuous present. Time is what makes life meaningful; memories across time are what make us, us.

Time defies all description and definition. In his book Physics of the Impossible, theoretical physicist Michio Kaku notes that time is “not possible” because “the past is gone, the future does not exist, and the present exists only for an instant.” As such, we are forced to resort to metaphor to explain time— and, interestingly enough, most (if not all) of these phrases normally refer to objects in space: time flies, time flows, time crawls. Time moves, tautologically, in respect to itself.

Time is what makes life meaningful.

But is time travel possible? Einstein’s theory of relativity proposed that time is a relative concept, inspiring the proliferation of the term “spacetime.” This, in turn, yielded a neat idea: If we can travel in space, and space shares similar properties with time, what’s to say we can’t travel in time?

Thanks to the theory of relativity, certain types of “time travel” are already a fait accompli. Time moves ever so slightly faster at the center of Earth’s gravitational pull, so astronauts in orbit far above Earth’s surface age more slowly than terrestrial-bound humans. Because of this time dilation effect, astronaut Sergei Krikalev, who spent 803 days, 9 hours, and 39 minutes in space, technically traveled 0.02 seconds into his own future over the course of his space travel. And in 2014, physicists successfully simulated photons traveling through time while experimenting with the intersection of Einstein’s cosmological theory of stars and planets and the nanoscopic world of quantum mechanics.

If we can travel in space, what’s to say we can’t travel in time?

The most promising time travel theory piggybacks on what Einstein proposed more than a century ago, positing that the hypothetical existence of wormholes could serve as a bridge not only through space, but through time as well. Kip Thorne was one of the first to devote serious research to the practical connection between wormholes and time travel.  Nowadays, most physicists concede that they have yet to discover a physical law that explicitly prohibits traveling in time— so it’s not impossible per se, merely very, very improbable.

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A model of “folded space” proposes a way to also “fold” time, making it possible to travel to a different time via a wormhole.

(For more on Kip Thorne, wormholes and the actual physics of time travel, check out Space.com.)

The Grandfather Paradox and the Philosophy of Time Travel.

While physicists attempt to tackle the overarching practicality of time travel, philosophers deal with matters of logical consistency. (For a deep dive into the philosophy of time travel, the Stanford Encyclopedia has got you covered.) David Lewis, known for his rendition of the time travel-centric “grandfather paradox,” was philosophy’s best-known champion of time travel until his death in 2001. In his famous paper “The Paradoxes of Time Travel,” Lewis spells out the paradox like so: Tim wants to go back in time to kill his grandfather as a young man. But if Tim succeeds and his grandfather dies before he meets Tim’s grandmother, then Tim will never be born, and therefore will not be able to go back in time and kill his grandfather in the first place.

This paradox has puzzled time travel enthusiasts for ages, but Lewis proposes a solution: Even if Tim can kill his grandfather— that is, he has a time machine, a rifle, two hands and a clear shot— he won’t, because he already didn’t. Infuriating, perhaps, but elegantly simple, hanging on two different meanings of the word “can.”

“‘Can’ is a word of possibility, and possibility comes in different grades,” Richard Hanley, professor of philosophy at the University of Delaware, explains. “We flip around between these grades without contradicting it.”

We experience this phenomenon even without the added wrinkle of time travel, Hanley notes. “There are certain things that you’ll try to do today that you’ll fail to do,” he says. “Why? Not because there’s some ‘time police’ to stop you from doing things, but because something ordinary stops you.” We may be in an unusual epistemological position in the case of time traveling Tim, but Hanley says that it would be something similarly mundane that foils Tim’s mission— his gun jams, he gets distracted, perhaps he develops a conscience. In short, he won’t kill his grandfather, because he didn’t.

The idea that space and time could be connected emboldened time travel enthusiasts within the physics community and had a revitalizing effect on philosophers who hold an eternalist view of time. Eternalists like Hanley and Lewis view all points in time— rather than just the present moment— as equally “real,” the same way that all points in space are considered equally real.

“In order for me to talk to you in California,” Hanley says, “it would be plausible to say that California has to be there! It’s not like, when I’m wandering around, the only place that exists is the place that I’m actually at.”

Even if Tim can kill his grandfather, he won’t, because he already didn’t.

The eternalist view of time, Hanley points out, gets a good deal of support from people in disciplines as seemingly disparate as modern physics and medieval theology. “The other nice thing about it,” he says, “is that it’s a particularly friendly view to time travel.”

Check back in next week for a discussion of two popular, plausible time travel theories and thoughts from Damon Lindelof (Lost, The Leftovers), Marc Guggenheim (Legends of Tomorrow) and Terry Matalas (12 Monkeys). Or, you know, just hop in your time machine and read it now. If you choose that option, hit me up, I have some questions…

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