"Wait a minute, Doc. Ah... Are you telling me that you built a time machine... out of a DeLorean?" says an awestruck Marty McFly in the iconic film, "Back to the Future." While we probably won't be visiting our hormone-charged teenage parents in souped-up DeLoreans, there are a few natural phenomena which could transport us into the future and, maybe, even into the past.
Hollywood has long fantasized about time travel and its seemingly endless possibilities, from thrillers like "The Time Machine" and "12 Monkeys" to over-the-top comedies like "Bill and Ted's Excellent Adventure" and "Hot Tub Time Machine."
But how can time travel reach beyond the realm of science fiction? Technically, it already has, as astronauts travel a few nanoseconds into the future every time they travel into space. In fact, just by driving in your car, riding an airplane, or climbing a ladder, you change the rate at which time flows. Einstein's Theory of Relativity established that both motion and gravity could slow down the progression of time. The closer an object--let's say, a space shuttle--travels to the speed of light, the slower it travels through time and the slower the pilot ages.
There have been many fictional ideas for time travel devices--phone booths and hot tubs come to mind--but in reality time travel does not require a machine at all, except perhaps a space ship. According to Dr. Max Tegmark, professor of physics at MIT, the most plausible way to travel into the future is by orbiting a black hole. Flying just outside of the black hole's event horizon--the point at which nothing can escape its gravitational pull--would allow you to travel close to the speed of light. But Tegmark points out that we would need to find a suitable black hole close by, which is no easy feat. Theoretically, black holes from other galaxies consumed by our own could be orbiting nearby and these would be our best bet for traveling into the distant future.
So far, we've only been talking about going forward in time. Could a black hole also give us access to the past? In the newest "Star Trek" movie, Spock's vessel and the Romulan ship get caught in the event horizon of an artificial black hole and are transported 129 years into the past. In reality, Spock would be torn apart by the intense gravitational pull before he could reach the center of the black hole. However, some theorists postulate that a rotating black hole called a Kerr black hole could be traversable. So, Spock could theoretically travel to the past by exiting through a "reverse black hole" called a white hole on the other side, where he'd find himself at a different point in space and, perhaps, in time.
A hypothetical spacecraft warps space-time. Image courtesy of NASA/Les Bossinas.
Another possibility: Just build a wormhole. Wormholes are shortcuts in space-time that connect two distant points, like a train tunnel cutting through a mountain. But scientists aren't sure how to create a wormhole--or how to keep one open.
Time travel to the past may never be a practical reality. If, someday, it were possible, wouldn't we have at least a few visitors from the future? Stephen Hawking posed this question in his chronological protection conjecture, and he concluded that the laws of physics must ultimately prevent time travel. But astrophysicist Carl Sagan disagreed, suggesting numerous possibilities which could account for the missing time-travelers. For instance, we may be able to go backwards in time, but only as far back as the moment time travel was invented. Or time travelers may be here, but hidden from view.
There are also many mind-bending paradoxes that challenge the concept of time travel to the past. Several of these are addressed in the "Back to the Future" trilogy. In the first film, Marty travels back in time to 1955 and accidentally interferes with his parents' relationship, so they never get married and have kids. This would mean Marty would never have been born, so how could he have prevented his parents from getting together in the first place? Hollywood's solution: Marty watches his two older siblings slowly disappear from a photo, as his parents grow further apart. If he doesn't get them together for their first date at the Enchantment Under the Sea dance, he'll be next!
Many physicists have theorized that time paradoxes would not be possible, as nature would somehow run interference to keep the "present" intact: Marty could affect the past, but the results would be the same. It's also been suggested that the time traveler would be transported to an alternate history, which would be one of many universes branching out in different directions.
None of these ideas has been tested. Time travel has been worked out mathematically, but reputable empirical research is almost nonexistent. Most scientists agree that, if time travel to the past really is possible, the necessary technology hasn't been invented yet and won't be for a very long time. As to whether these methods could ever prove stable enough to allow for actual time travel--only time will tell.
Samantha Johnston is currently studying broadcast and print journalism at UCLA Extension. She has traveled through Greece, Ecuador and Peru studying anthropology and archaeology, and earned her BA from the University of California, Santa Barbara, in 2007. On clear, moonless nights, you'll find her stargazing on Southern California's Mount Pinos.