Looper is all about returning to the past. It twists time to send Bruce Willis back to meet his younger self (Joseph Gordon-Levitt). Willis has dipped into his cinematic past in his return to time travel as a plot device. Like Twelve Monkeys, Looper’s story considers a fascination of modern physics: the problem of time travel. Science fiction is rarely constrained by the bounds of current science fact, but time travel might actually be possible.
In Einstein’s model of general relativity, time is one co-ordinate in the four-dimensional fabric of “spacetime.” Using the equations that describe spacetime, Willem Jacob van Stockum theorized the existence of “closed timelike curves” (CTCs) in 1937. CTCs travel through spacetime but end at the place where they began. In short, a CTC is a time machine. An extremely massive object or intense gravitational field, like a black hole, could potentially warp the fabric of spacetime such that a CTC occurs. Theoretically, a CTC could send somebody back in time.
If we could send Willis back to the time of Gordon-Levitt, could Willis make significant changes to the timeline? Other films have explored the possibility of timeline alteration and the time paradoxes that might result. In Back to the Future, Michael J. Fox nearly creates a grandfather paradox and becomes his own ancestor, nearly erasing himself from existence. Paradoxes confuse the relationship between cause and effect, essentially breaking fundamental laws of physics. Physicists have struggled to align the possibility of time travel with the impossibility of breaking the laws of causality.
In a paper called the “Chronology Protection Conjecture,”. Steven Hawking writes, “It seems there is a chronology protection agency, which prevents the appearance of closed timelike curves and so makes the universe safe for historians.” Hawking’s paper argues mathematically that causality violations could be the root cause to prevent the formation of CTCs. If time travel is not already inherently impossible, then it is made so by the laws of causality. He then notes, “There is also strong experimental evidence in favor of the conjecture from the fact that we have not been invaded by hordes of tourists from the future.”
Others believe that time traveling tourists are possible, but that meddling time-traveling tourists are not. In 2005, a team of scientists led by Dan Greenberger and Karl Svozil used quantum theory to explore CTCs. Under their proposed quantum model, CTCs can exist, but the past cannot be changed. Quantum theory uses probability to distinguish between different outcomes. Outcomes that are certain — that are known to have happened — are immutable. Thus, if a person traveling in the past knows that an event has already happened, they cannot stop or modify that event. If it is known that in the future person A is unscarred, quantum theory prevents time-manipulating person B from any action in Person A’s past that would cause scarring. Anyone that has seen Looper will recognize the problem that this model poses to the movie’s plot.
It is impossible to know if the theory proposed by Hawking, or the theory proposed by Greenberger and Svozil, or if one of the other many scientific theories on time travel is correct or incorrect. Until time-travelling tourists show up, or until functional time travel exists, the theories on the physics of time travel remain theories. If Looper is correct, we’ll get an answer in 2074, but it’s entirely possible that the debate on time travel and CTCs may last until the clock runs down.
(Author’s Note: Julianna Kettlewell of the BBC reported on the Greenberger and Svozil model in an article titled “New model ‘permits time travel’” on June 17th, 2005. For more information on CTCs and time travel, the About.com Physics section is recommended.)
