The bartender says, “No time travelers allowed in this bar!”
Two time travelers walk into a bar…
In literature and culture, the concept of time travel goes back … a long time. References to time travel can be found in ancient Buddhist and Hindu texts as far back as 300 B.C. Proto-sci-fi writer Samuel Madden conjured the idea in his 1733 novel Memoirs of the Twentieth Century, detailing a series of diplomatic letters written in 1997 and 1998. H.G. Wells’ 1895 novel The Time Machine popularized the modern conception of time travel, and it’s a staple in movies from The Terminator to Hot Tub Time Machine.
There’s something deeply compelling about subverting the tyranny of time. But in our fast-forward 21st century, is any real-world science being done that could make time travel a reality?
“The short answer is no,” says physicist and philosopher Tim Maudlin, one of the world’s experts on the concept of time travel as both physics and metaphysics. Maudlin is a professor at New York University and founder and director of the John Bell Institute for the Foundations of Physics. And he’s not trying to be a bummer.
“I just don’t think there is any science underneath it right now,” Maudlin says. “Time travel stories that are used in movies and fiction are mostly incoherent.”
Maudlin says that such stories are simply incompatible with real science, and that’s okay — they’re not really meant to be compatible. Fictional time travel is a narrative device, not a depiction of proposed technology.
“Time isn’t like space — time has a direction.”Tim Maudlin, physicist and philosopher
As to the potential real science, it’s useful to think of time travel in two categories — moving forward and moving backward.
Moving forward in time is easy enough, of course — we do it every day on a 1:1 minute ratio. And it’s just possible that we could fast-forward that process through the phenomenon of time dilation. According to Einstein’s theory of special relativity, time passes at different rates depending on relative motion. A spacefarer moving at close to the speed of light — 90 percent, say — would experience time passing about 2.5 times slower than back on Earth. Take a 10-year roundtrip, and you’ll arrive home 25 years into the future. (Approximately; the math is tricky.)
This isn’t just conjecture. Time dilation on a micro scale has been measured using atomic clocks on jet aircraft, although the time travel is extremely modest: measured in nanoseconds, or billionths of a second. Variations on this basic time-dilation dynamic can be spun out into more theoretical fields, involving black holes, wormholes and quantum physics. While we don’t currently have the technology to make practical use of these principles, forward-facing time travel is technically a possibility.
The idea of moving people backward through time, though — Maudlin says that simply isn’t in the cards for our species, in the universe as we know it.
“Time isn’t like space — time has a direction,” Maudlin says. “Time passes, you know; we’re always getting older and there’s nothing we can do about it. The idea of somehow going from the future to the past is inconsistent with the nature of time itself.”
The key to thinking clearly about time travel as a real-world possibility, he says, is to acknowledge that it’s not just a matter of technology. Unlike tractor beams or holograms, backward time travel would require a fundamental rethinking of observed reality, which is why it’s a matter for both physics and philosophy.
The hard truth is that there is no legitimate real-world science being done toward time travel — forward or backward — excepting some extremely peripheral studies, like the computer scientist who’s working with artificial intelligence to study time travel’s famous grandfather paradox. The dilemma: If you go back in time and kill your own grandfather, do you even exist to go back in time in the first place? The algorithms offer some technically logical resolutions to the causal paradox, but it gets weird — like becoming your own ancestor and siring yourself.
So is there any hope for future era-hopping technology? Maudlin concedes that there are mathematical models in which various kinds of time travel are potentially workable. In fact, Maudlin and a colleague wrote the official entry on the topic in the prestigious Stanford Encyclopedia of Philosophy. (If you speak physics, you can find it here.) Maudlin is quick to note that equations are not reality, but still — the math works.
And we may have other options if we’re willing to open up our definitions just a little. For instance, fiction has long toyed with the idea of time travel by way of cryonics or suspended animation: Freeze your brain for 100 years and you wake up in the future, in all the ways that count. Virtual reality and digital brain emulation offer a similar path. What if you were to die, then upload your digital consciousness into a computer model of, say, 13th century Scotland?
In any case, Maudlin believes that the truly important part of the time travel concept will always be a part of our future: As a storytelling tool, it’s just too good.
“You can see why novelists are attracted to time travel, especially travel back in time,” Maudlin says. “It’s about regrets and these very human emotions. It’s a way to make vivid that thought we’ve all had: If only this little thing had gone another way, then everything would be different.
“It’s just great science fiction.”