The physics of time travel isn’t just the stuff of science fiction

The distant past might be in our near future.
The distant past might be in our near future.
Image: Danielle Krysa
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We’re all time travelers nowadays. We project ourselves mentally into the misty past and the risky future. We revisit and revise our memories. We invent possible futures and explore them with anticipation and desire. We’re temporal experts and manipulators; we have flashbacks and flashforwards and stop motion and instant replay and time gates and time shifting and time forks. Not content with nostalgia for the past that never was, we begin to feel nostalgia for futures that will never come.

We time travel in our imaginations. We time travel in our storytelling. But we still want more. Would an actual time machine be too much to ask for? Can’t science come to our aid?

Science-fiction authors have a lot of experience in turning our time-twisting desires into realities. They naturally rely on a certain amount of—for want of a better term—mumbo-jumbo. That comes with the territory. But the best time-travel stories respect real science, too. The problem sometimes is telling which is which.

The Time Machine

H. G. Wells was the first. He built a vehicle. “Parts were of nickel, parts of ivory, parts had certainly been filed or sawn of rock crystal… Quartz, it seemed to be.” His contraption had a bronze frame and a saddle for its rider. It looked closer to a bicycle than a sedan, which isn’t surprising: In 1895, the automobile barely existed.

Was it scientific? The Wells time machine has no evident source of fuel, or even pedals, but it did have some mysterious dials. You just throw the lever and off it goes. As Wells tries to persuade the skeptical reader in his pseudoscientific introduction, in order to believe in the time machine, you have to reconsider everything you thought you knew about time itself. You have to recognize that time is the fourth dimension.

“Well, I do not mind telling you I have been at work upon the geometry of Four Dimensions for some time,” the Time Traveller, the book’s nameless protagonist, solemnly explains. “There is no difference between Time and any of the three dimensions of Space except that our consciousness moves along it.”

We modern folk tend to agree. Nowadays everyone knows time is the fourth dimension—only no one knew it when Wells penned this tale, in 1895, while Albert Einstein was just a schoolboy in Munich. Wells thought his four-dimensional patter was in the category of mumbo-jumbo, little realizing that it was about to become real science.

Time for the Stars

Thanks to Einstein, we have learned how to accomplish a certain kind of time travel: All you have to do is accelerate to near light speed or through a powerful gravitational field, and—for you—time will slow. Einstein called this Zeitdilatation: time dilation. It’s not as handy as a time machine; for one thing, there’s no going back. Instead, you could think of time dilation as an anti-aging technique. It leads to the so-called twins paradox thought experiment: The twin who zooms off at near-light speed returns home younger than the one who stays behind.

In his 1956 novel Time for the Stars, Robert Heinlein put time dilation to work as a plot device employing literal twins, Tom and Pat. Humanity urgently needs to explore the stars. (By the way, it needs to do so because Earth is grievously overpopulated: It has grown to the gargantuan level of 5 billion inhabitants.) So off Tom goes on a spaceship while Pat stays behind. When Tom returns, a 22-year-old by his reckoning, Pat is a codger of 89, because 71 Earth years have passed. “Let me look at you,” says Pat. “I knew intellectually that you would not have changed with the years. But to see it, to realize it, is quite another thing, eh? The Picture of Dorian Gray.” Tom then marries his great-grandniece, and they live relativistically ever after.


Christopher Nolan’s 2014 blockbuster movie takes time dilation to a new level and adds some weird science of its own—and Matthew McConaughey. When our hero Coop (McConaughey) blasts off for the stars, he leaves behind his 10-year-old daughter, Murph (Mackenzie Foy). “Time will change for me,” he explains. “It’ll run more slowly. When I get back we’ll compare.” She is a budding scientist herself. “You say science is about admitting what we don’t know,” she remarks wisely. By the time Cooper returns from his near-light-speed and black-hole travels, having aged hardly at all, Murph will have grown up to be Jessica Chastain, and then, at the end of the film, an even older Ellen Burstyn.

Interstellar deploys its own kinds of mumbo-jumbo—some ghosts from a hypothetical fifth dimension, for example—but it’s full to the brim with real science. They’ve got cryo-beds for deep sleep (speculative, but maybe possible); they’ve got counter-orbital slingshots for fancy gravitational dynamics; and, best of all, they’ve found what seems to be one end of a wormhole, which is a disturbance of spacetime.

Are wormholes real? Yes and no. The great theorist John Archibald Wheeler, who also named black holes, invented the word to describe a possible shortcut through the warped fabric of spacetime—a “handle of multiply connected space” that can be visualized as a tube or a tunnel. Later theorists, like Kip Thorne of Caltech, who won the Nobel Prize in physics this year, have speculated that a “traversable” wormhole could be used as a sort of time machine: If one end (“mouth”) of a wormhole could be accelerated by hypothetical “advanced beings” to near light speed, a hypothetical spaceship entering that mouth and exiting the other could be hurled backward to the past.

Thorne himself signed on to the movie as a producer and scientific advisor and worked hard to make everything theoretically plausible—up to a point. After all: How do you illustrate an event horizon? The script featured factual descriptions that likely challenged the special-effects team:


Cooper PLUNGES INTO THE WORLD SPHERE … As he falls his SINGLE WORLD LINE stretches behind him – the INFINITE FUTURES OF HIS WORLD LINE splitting ahead to all the different possibilities in spacetime.

Already sucked in by plausible physics, we may be too distracted to notice when the science gives way to plain old magic. To make the plot work out, for example, Coop needs to get a message back from the future to young Murph. He accomplishes this by reaching across time and flicking the second hand of an old wristwatch with the help of some hypothetical, invisible, presumably benign beings who live in the fifth dimension. Science!

The Garden of Forking Paths, Sliding Doors,


The Peripheral

Alternate lives playing out over different dimensions are another important offshoot of time travel. Perhaps our universe—like Coop’s timeline—is merely one among many. Perhaps you could even hop from one to another.

If you look up multiverse in the Oxford English Dictionary, you learn that it was “orig. Science Fiction”—and now it’s Physics, too: a putative ensemble of many universes, of which our own little home is merely one. The great Argentine writer/philosopher Jorge Luis Borges offered his conception in a 1941 short story, “The Garden of Forking Paths”: Rather than unidirectional Newtonian time, he proposes “an infinite series of times, in a growing, dizzying net of divergent, convergent and parallel times…times which approached one another, forked, broke off, or were unaware of one another.”

That wasn’t science to Borges, but it became science 15 years later. A physics graduate student named Hugh Everett, trying to resolve the quantum puzzle that causes Schrödinger’s cat to be both alive and dead, proposed that every quantum measurement is a branching of the entire universe. In one fork the cat is alive; alas, it is dead in another. This is now known as the many worlds interpretation of quantum mechanics. As the theorist David Deutsch explains, “Time does not flow. Other times are just special cases of other universes.”

We’re surprisingly comfortable with this idea of multiple lives being played out simultaneously in multiple realms. In his 2014 novel The Peripheral, William Gibson calls his time-branches “stubs.” Parallel universes drive the 1998 movie Sliding Doors, in which a Londoner played by Gwyneth Paltrow slips back and forth between two timelines, one happy and one sad. (The portal between universes seems to be a train on the Tube.) Star Trek and Doctor Who have explored parallel universes; Donnie Darko is another parallel-universe time-travel movie; and so is, when you think about it, the James Stewart classic It’s a Wonderful Life.

So sure, it could be science. Or it could be mumbo-jumbo. Remember what young Murph says: Science is about admitting what you don’t know.

Read more of the Quartz Ideas series on The Nostalgia Economy.