Thought Experiments


Wormholes as Time Machines

You’ve just started reading this post. The decision is done. Seconds have ticked by and you’ve chosen to use them clicking on this article, rather than pursuing hang gliding, water skiing, mountain climbing, chocolate sampling, or countless other options. Sure you could do those things later, but what was “now” is already gone. If only you had a wormhole time machine and could go back in time to undo your choice! But how to make a wormhole time machine? Read on if you’d like some suggestions from the world of theoretical physics.


Step in to my time machine. Credit: NASA/Les Bossinas (Cortez III Service Corp.), via Wikimedia Commons

Flash back to the late 1980s—with your imagination, not a time machine just yet. The extraordinary astronomer and science communicator Carl Sagan, fresh off his award-winning PBS series Cosmos, decided to write a science fiction novel about interstellar travel, “Contact.” Needing a way for his protagonist to travel quickly to another planet, he asked his friend Caltech astrophysicist Kip Thorne for advice.

Thorne is an expert in general relativity, Einstein’s masterful theory of gravity. The equations of general relativity serve as a recipe for how nature kneads the dough of spacetime (space and time combined) into various shapes—from as flat as a pancake to as curvy as a croissant. These shapes determine how other things move. Just as an ant at a picnic would take a more winding route around an apple than across a napkin, objects in the universe (planets, comets, and so forth) veer along curved paths in warped regions. What distorts these sectors of spacetime is the amount and distribution of mass and energy. For example, the gravitational well of the solar system is carved out by the mass of the Sun.

In extreme cases, a glop of mass concentrated in a small enough region will tear the fabric of spacetime, causing what is called a singularity—a point of infinite density where spacetime seems to reach a dead end. Such is the case with what is called the Schwarzschild solution of Einstein’s equations of general relativity, used to describe the ultra-dense, collapsed stellar cores known as black holes. However, as Einstein and his assistant Nathan Rosen showed in 1935, one can mathematically extend the Schwarzschild solution across an “Einstein-Rosen bridge” and link it to another region of spacetime. In the 1960s, the creative Princeton physicist John Wheeler, who was Thorne’s PhD advisor, dubbed these connections “wormholes,” imagining a worm taking a shortcut by crossing an apple’s interior. (Wheeler also coined the term “black hole.”)

When Sagan contacted Thorne he was envisioning something like a Schwarzschild wormhole connecting two otherwise distant parts of space—an interstellar Chunnel, so to speak. But Thorne realized that a Schwarzschild wormhole wouldn’t do. For one thing, it was unstable to matter, meaning that the gravitational effect of even the slightest drop of mass would cause it to collapse. Therefore it would close off if a spaceship tried to enter—that is, if the space voyagers could make it that far. If the wormhole entrance lay in the bowels of a black hole, the travelers would encounter deadly radiation, bone-crushing gravitational forces, and enough stomach-churning acceleration to make even the Dangerous Sports Club give it a miss.

Thorne asked his then-student Michael Morris to help him come up with an alternative. They crafted a novel solution of Einstein’s equations of general relativity that would represent a wormhole that could be traversable by human voyagers, such as the fictional heroine of “Contact.” The solution was custom-designed to eliminate the nasty aspects of navigating into a black hole and allow for a relatively quick, comfortable ride. After passing into the wormhole’s “mouth” (as its entrance was called) and journeying through its “throat” (as its passageway was called), a voyager would find herself emerging from another mouth somewhere in another part of space. Instead of traveling hundreds of years or more to reach another star, if all went well, she’d swiftly arrive in its vicinity.

Morris and Thorne realized that their scheme was extremely hypothetical—requiring a virtually inconceivable engineering feat. For one thing, the amount of mass needed to create the wormhole was comparable to that of a galaxy. Moreover, a new type of negative mass material, called “exotic matter,” would be necessary to prop open the wormhole’s throat and prevent it from collapsing. No known substance has negative mass.

Offering some cause for optimism, physicist Matt Visser of Victoria University of Wellington soon found a way to minimize the amount of exotic matter required. As he and others have pointed out, exotic matter has features in common with the energy of the quantum vacuum, the bedrock state of particle physics, which has a repulsive pressure. Perhaps a future civilization could mine enough of this energy to suffice for wormhole construction. A hypothetical energy called “phantom energy,” a type of dark energy with a considerable amount of negative pressure, used to explain the acceleration of the universe’s expansion, also holds promise as a potential way to stabilize wormholes.

Shortly after Morris and Thorne published their first paper they collaborated with Ulvi Yurtsever, another of Thorne’s PhD students at Caltech, on another remarkable article showing how a wormhole could be used as a time machine. The key would be to speed up one of the mouths of the wormhole to close to the speed of light while leaving the other one fixed. According to the phenomenon of time dilation, an aspect of Einstein’s special theory of relativity, time in the vicinity of a near-light-speed object will slow down significantly relative to a stationary observer. Therefore, while the fixed mouth ages 100 years, the high-speed mouth, if it is fast enough, might experience only one year. If the calendar reads 2112 for the former, it would read 2013 for the latter. Now suppose a space traveler sails into the fixed mouth in 2112. If passage through the throat is quick enough, she would emerge through the moving mouth in 2013.

If you are still thinking about all the things you could have done if you hadn’t clicked on this post, you now know the answer. Assuming you have an advanced spaceship and a CPS device (Cosmic Positioning System), simply find a wormhole, journey through it, go back to the time before you started reading this, and convince yourself to go surfing instead. You are cautioned however that your actions would create a paradox1, because if you never read the article you wouldn’t know how to go back in time (or at least wouldn’t have the need). Proceed to the past at your own risk!

1 To avoid paradoxes such as meeting yourself in the past and convincing yourself never to pursue time travel, or going back in time and accidently eliminating your ancestors, some physicists have asserted that backward time travel is impossible. Stephen Hawking, for example, postulated the Chronology Protection Conjecture to shield the past from tampering. Others such as Igor Novikov of Moscow State University and the Lebedev Physics Institute in Russia have argued, in what he called the Self-Consistency Principle, that past-directed temporal voyages are fine as long as the altered past is consistent with the present—that is, it was really supposed to happen. For example, if you go back in time and convince Carl Sagan that wormholes wouldn’t fit into his novel, maybe that’s just the incentive he needed to contact Kip Thorne and check if they would, leading to what actually happened. Finally, there are some who speculate that backward time travel could lead to a bifurcation of time into parallel realities.

In any case, the work of Thorne, Morris, Yurtsever, Novikov, Hawking, Visser and others has propelled the discussion of time travel and wormholes from fanciful science fiction into serious, peer-reviewed—albeit highly speculative—science. Who knows, perhaps someday our civilization will be advanced enough to test such far-reaching hypotheses and create or find actual wormholes. Only time will tell—and if wormholes exist, we have all the time in the world.

Go Deeper
Editor’s Picks for Further Reading

Daily Mail: Stephen Hawking: How to Build a Time Machine
Stephen Hawking on wormholes and the paradoxes of time travel.

Space Time Travel: Flight Through a Wormhole
Explore computer-generated images of a hypothetical trip through a wormhole.

Wikipedia: Wormholes in Fiction
From “A Wrinkle in Time” to “Fringe,” discover how writers of books, television, and movies have used wormholes in their storytelling.

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Paul Halpern

    Paul Halpern is Professor of Physics at the University of the Sciences in Philadelphia. A prolific author, he has written thirteen science books, including "Einstein's Dice and Schrödinger’s Cat: How Two Great Minds Battled Quantum Randomness to Create a Unified Theory of Physics" (Basic Books). His interests range from space, time and higher dimensions to cultural aspects of science. The recipient of a Guggenheim Fellowship, Fulbright Scholarship, and an Athenaeum Literary Award, he has appeared on the History Channel, the Discovery Channel, the PBS series "Future Quest," and "The Simpsons 20th Anniversary Special." Halpern's books include "Time Journeys," "Cosmic Wormholes," "The Cyclical Serpent," "Faraway Worlds," "The Great Beyond," "Brave New Universe," "What's Science Ever Done for Us?," "Collider," and most recently "Edge of the Universe: A Voyage to the Cosmic Horizon and Beyond" (Wiley 2012). More information about his writings can be found at