We live in four dimensions: three spatial dimensions plus a fourth dimension, time. Of these, time is the most fluid, the one that is most influenced by personal experience. It goes too fast, or not fast enough. Time gets to the very heart of what it means to be human since our experiences, which take place in time, are what shape our lives and decisions in the present and the future. And yet it is the only dimension in which we are restricted to moving in only one direction: forward.
Or are we? And if we can move backward in time, how would we prove it?
These are the kinds of questions my research group and I were discussing one day over pizza and poker. Time travel: Literature and movies are full of references to it. The stories may be fictional, but much about the nature of time is still unknown, and science thrives in these gaps in knowledge: as we try to fill the gaps, we discover how many more there are. Yet for as long as scientists and philosophers have been contemplating time travel, no one seems any closer to having an answer.
Since Einstein’s Theory of Relativity, time travel to the future has stood on firm scientific footing. To slow down the rate at which time passes, all you must do is travel close to the speed of light. This phenomenon, called time dilation, is famously illustrated by the Twin Paradox, in which a space-traveling astronaut returns to Earth to find that her homebody twin sister has aged decades while she’s aged only a year. It is possible to test and confirm this effect even without the aid of a near-light-speed journey into interstellar space: You just need a pair of very precise clocks and an airplane. Indeed, time dilation has been tested by placing one such clock on a plane and leaving another on the ground. When the flying clock landed, it showed a time that was about 10 the other clock. -7 seconds behind
That discrepancy includes the effect of a second, competing “time travel” phenomenon: time dilation due to gravity. Time slows down in the presence of a gravitational field, so a clock on an airplane, at the top of a mountain, or on the top floor of a skyscraper will run just a bit faster than a clock closer to the Earth’s surface. Even just a meter off the ground , time runs measurably faster.
While these are very small differences, they prove a point: Time travel is well within our grasp. Of course, here we are discussing extraordinary time travel. Technically, we all travel to the future every day at the rate of one second per second. We are interested in what happens when that number is greater or less than one.
But that’s only half of the story. Traveling back to the past is where things start to get interesting, and the science starts to become more speculative. Special Relativity allows travel into the past—but only if you move faster than the speed of light. Though this is traditionally forbidden, some theorists have found mathematical solutions in General Relativity that allow faster-than-light travel. The mathematician Kurt Gödel devised one such solution to Einstein’s field equations, and theorists have conjured scenarios in which time-travelers might use wormholes and black holes to exceed the universal speed limit. However, these solutions all involve what physicists call “ closed time-like loops ,” anomalies in space-time in which, instead of continuing into the future, one always returns to the same starting point in time. Most scientists consider these to be unphysical, meaning that they do not accurately describe reality and lead to philosophical and historical problems like the so-called Grandfather Paradox: What would happen if you went back in time and killed one of your grandparents before you or your parents were born? Would you still be alive to travel through time and commit the crime?
Finding the right search term was the first order of business. It had to be a word or phrase that was “born” on a specific date, before which it had never been used, and which would continue to be significant into the distant future. We decided on two such terms, one historical and one astronomical: “Pope Francis” and “Comet Ison.” We then began to methodically search for evidence of prescient mentions of these events in search engines and on social media sites. Many of our attempts to scour search engines were futile. Google searches yielded results that turned out to be ads on random pages and Google+ was generally unreliable. Facebook was too easily tampered with, as it allows users to pre-date their posts as far back as the day the account was opened.
Twitter, however, turned out to be a practical venue, though our search came up with nothing we could identify as a time travelers’ post. We also searched through the log files of a popular astronomy site (apod.nasa.gov) to see if anyone had presciently searched for our terms, but to no avail. We decided to try a more direct appeal, and broadcast a request on a popular astronomy forum that any time-travelers in the audience transmit an email sent before we made the post, but no email arrived. Not that we were expecting one. We were really more interested in the process: As far as we know, no one has carried out a search this extensive, verifiable and reproducible.
So do our results mean that there are no time-travelers in our midst? Not necessarily. Our results verify the general consensus that time-travel does not exist, but do not rule out the possibility. We only searched for two terms, neither of which may hold any interest for time-travelers. Maybe they do not want to be discovered and have covered their tracks. Or perhaps it isn’t backwards time travel that is prohibited, but the discovery of such a phenomenon. Maybe some law of physics makes finding time travelers impossible.
Our exercise in time travel is over, but there are plenty of other ways one could look for evidence of time-travelers: uncanny lottery picks and eerily accurate March Madness brackets, just to name a few. It may sound like a ridiculous idea, but you’ll never find anything if you don’t look!
Editor’s picks for further reading
Stephen Hawking: How to build a time machine
In this whimsical essay, Stephen Hawking describes how wormholes could be used as “time tunnels” to travel through time.
Relativity for the Questioning Mind
In this friendly introduction to Einstein’s special and general relativity, Oberlin College physicist Daniel Styer provides a rigorous but non-technical look at time dilation, the twin paradox, and more.
How does relativity theory resolve the Twin Paradox?
Ronald C. Lasky, a lecturer at Dartmouth College’s Thayer School of Engineering, explains.