Reading the scholarly theories on going back in time—which, by the way, might be possible
TO THE NAKED EYE, IT MAY APPEAR THAT: Looper, the Bruce Willis-Joseph Gordon-Levitt vehicle that opened last weekend, is a pretty nifty film with an impossible, imaginary premise: It's possible to travel back in time and change the past.
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BUT ACCORDING TO SOME EXPERTS WHO THOUGHT VERY HARD ABOUT THIS: Looper is definitely unrealistic, but not for the reason we might think. Time travel, as it turns out, may not be impossible at all—it's the alteration of the course of history that's the dubious part.
Yes, activate suspension of disbelief now. But the maybe-viability of time travel has kept logicians, physicists, philosophers, and other academics squabbling among themselves for decades. Several modern theories of time travel are built on Albert Einstein's vision of the universe: a model that imagines a four-dimensional spacetime continuum with a curving shape. Einstein's model, according to University of Connecticut theoretical physicist and aspiring time-machine builder Ron Mallett, allows for the possibility of warps, twists, and manipulations of space and time, which would bend the continuum into a closed loop that would enable time travel. Dr. Mallett explained some of his theories to CNN in 2007:
Mallett's lifelong dream has been to build a time machine in order to go backward in time and tell his father, who died of a heart attack when Mallett was 10, not to smoke. As Mallett points out, though, science is still a long way away from sending an actual person—or even an object or piece of data—to a different point in the future or the past. The discussion of time travel's feasibility rests squarely on the theoretical possibility at this point.
MORE THEORIES, MORE PROBLEMS: Looper, set in a bleak vision of 2044, pits young Joe (Gordon-Levitt), a killer of future criminals sent backwards in time to be executed, against an older version of himself (Willis) who's been exiled to the past. But time travel into the past is where the argument for real time travel becomes the most problematic.
According to the Internet Encyclopedia of Philosophy, the famed theoretical physicist Stephen Hawking and other theorists believe backward causation—a retroactive influence on events that have already happened—may be logically possible, but it's physically impossible. Hawking's belief is that if time travelers could travel back in time, certain forces collectively known as a "Chronology Protection Agency" would prevent him or her from interacting at all with the events of the past.
So at the crux of the time-travel question sits the "grandfather paradox"—the notion that the past cannot be changed because its consequences have already manifested themselves in the future, making it impossible for the time traveler to return "home" to the same future. In 1976, the American philosopher David Lewis explained the grandfather paradox at length in a parable about "Tim" the time-traveler, who travels back to 1921 in his time machine to dispose of his much-hated granddad.
Tim cannot kill Grandfather. Grandfather lived, so to kill him would be to change the past. But the events of a past moment are not subdivisible into temporal parts and therefore cannot change. Either the events of 1921 timelessly do include Tim's killing of Grandfather, or they timelessly don't. We may be tempted to speak of the "original" 1921 that lies in Tim's personal past, many years before his birth, in which Grandfather lived; and of the "new" 1921 in which Tim now finds himself waiting in ambush to kill Grandfather. But if we do speak so, we merely confer two names on one thing. ... If Tim did not kill Grandfather in the "original" 1921, then if he does kill Grandfather in the "new" 1921, he must both kill and not kill Grandfather in 1921—in the one and only 1921, which is both the "new" and the "original" 1921. It is logically impossible that Tim should change the past by killing Grandfather in 1921.
In Looper, young Joe's mission is to kill the elder Joe, while the elder Joe is determined to stay alive and influence the decisions of his younger self. So Looper actually riffs on (and inverts) some variants of the grandfather paradox: the auto-infanticide paradox and the causal loop.
As the University of Sydney philosophy lecturer Nicholas J.J. Smith puts it, the auto-infanticide paradox ensues when a time traveler attempts to kill a younger version of him- or herself. "If time travel were possible, the objection runs, there would be nothing to stop a time traveler from traveling back and killing her younger self," Smith wrote in The British Journal for the Philosophy of Science in 1997. "However, the fact that the time traveler was present in such a case would entail that she had died young. Hence her killing her younger self would involve a contradiction: She both would and would not survive into adulthood." Smith added that, because of the contradictions that would inevitably exist as a result, theorists have a long-standing disagreement over whether backward time-travel is truly impossible or just highly improbable.
MORE ON TIME TRAVEL
Looper's plot also flirts with another much-bickered-about subject that philosophers call the "causal loop." Causal loops are cycles of events that cause one another but have no beginning or end. They ensue when an older, time-traveling version of a person—like Looper's older Joe—visits his or her younger self—like younger Joe—and influences the younger self's decisions in a way that will alter the future for the better. Eventually, younger Joe will reach the age at which older Joe hopped in a time machine to go back in time. So when he gets there, will he hop aboard the time machine again to visit his younger self and do the same damage control? Scholars say he will, and that the cycle will continue on after that.
According to the IEP, theorists like New York University analytic philosopher Paul Horwich argue that causal loops aren't impossible, but they're certainly improbable—and that the paradoxes presented by the causal loop suggest that spacetime wouldn't allow a time traveler to revisit his or her own past.
Without ruining too much of Looper's well-crafted suspense, the movie ends with a drastic altering of events in 2044 that creates a "new" version of the future. The creation of a causal loop is avoided, but the principle that auto-infanticide is impossible in time-traveling situations pretty much falls by the wayside.
AND THUS, WE CAN CONCLUDE THAT: Looper is completely implausible, but not because its characters cavort about through time in a time machine. One day, that part might not be impossible at all. Its portrayal of characters who alter their own lives' trajectories, though—that's science fiction.