For the past couple of years, I’ve been writing short and medium-length features for U.S. News & World Report. These stories aren’t available online, but you can read them in the special “books” U.S. News puts out called “Best Hospitals”, “Secrets of your Brain,” “Amazing Animals,” and “Mysteries of Space.”

One of the stories I wrote for the “Mysteries of Space” issue didn’t end up running because, I believe, neither I nor my editor could explain wormholes simply enough for a general audience. I liked the piece, though, and thought it should have some sort of life beyond the cutting room floor. So I’ve pasted it below. Enjoy!

Down the Wormhole

Consider a trip to the Alpha Centauri star system, about 4.2 light years away in our Milky Way galaxy. You board a ship capable of the speed of Voyager 1, the United States’ deep space probe, launched in 1977 and fastest craft made by man. Traveling at more than 38,000 miles per hour—a small fraction of the speed of light—your descendants would arrive at the nearest star to our Sun, Proxima Centauri, in about 74,000 years. Even cruising closer to the speed of light, a trip beyond the Milky Way to one of our neighboring galaxies, Andromeda, would take about 2.5-million years.  “We’re trapped in a universe where the speed of light is the speed limit,” says John Cramer, professor of physics at the University of Washington.

That may be the case, but in the 1980’s the late astronomer and science fiction writer Carl Sagan searched for a way to travel through space so that the speed of light didn’t hold astronauts back. He had written a book called Contact (later turned into a movie starring Jodie Foster) in which he proposed a black hole—a dense object with a gravitational grip that nothing can escape—as a portal to another star system. Sagan checked the feasibility of the book’s premise with his friend Kip Thorne, a physicist at the California Institute of Technology. To Thorne, the idea was preposterous. The laws of physics forbid it. However, he happily offered a possible alternative: the wormhole.

The necessary requirements for a wormhole are easy enough to understand. First, one would need to contort space to bring two regions of the universe closer together—similar to curling a sheet of paper so that the top and bottom are nearly touching. Second, one would have to create a shortcut between the two regions that stays open long enough for humans (and their vehicle) to traverse it—an effect similar to punching holes in the top and bottom of the piece of paper and connecting them with a short drinking straw that serves as the portal.

Thorne started with the standard equations of general relativity, which govern the way gravity interacts with space and time. By making assumptions about the type of matter that could constitute a wormhole, he arrived at a solution that described a workable portal. Thorne’s wormhole result not only gave science fiction writers a theoretically plausible shortcut to another star system, but they also prompted other theories, including the possibility of time travel.

The foundations for Thorne’s work were set decades earlier. In 1935 physicists Albert Einstein and Nathan Rosen published a paper describing an “Einstein-Rosen Bridge.” The scientists suggested that fundamental particles such as electrons and positrons could act as tunnels connecting one point in space to another. Not much came of the idea until 1955, when physicist John Wheeler revisited it. He theorized that instead of particles acting as bridges, large objects in space could serve as entrances to tunnels connecting two far-away regions. In 1957, he coined the term “wormhole.” Unfortunately, Wheeler’s wormholes weren’t practical. His calculations suggested they would collapse before even a single photon could pass through.

Kip Thorne saw the problem, but he thought he could find a solution. By the time he turned to wormholes in the 1980s, he was already widely known for his work on black holes and gravity. With his students Mike Morris and Ulvi Yurtsever in 1988, he published a paper that drew wide attention. The three physicists showed that an inter-galactic tunnel could be kept open if it were infused with a sort of exotic matter that could serve as doormen holding open the entrances at each end. This exotic matter would have negative energy, which repels rather than attracts objects gravitationally. As bizarre as negative energy sounds, it has actually been indirectly detected, though in extremely small amounts, in laboratory experiments.

“With enough negative energy, it would be theoretically possible to build both wormholes and warp-drives [the ability to travel faster than light],” says Matthew Visser, professor of mathematics, statistics, and computer science at the Victoria University of Wellington in New Zealand. Of course, no one has figured out how to accomplish this, but that hasn’t stopped some theoretical physicists from continuing to speculate.

There is also still a slim chance that large wormholes exist in nature. “They’re not visible because they’re not illuminated like stars,” Cramer says. But like any massive object in space—a galaxy cluster or a black hole, for instance—wormholes would bend the light of stars behind them. Cramer suggested that astronomers who measure this feature open their search to objects with negative mass, which would bend light in a measurably different direction than objects with positive mass. So far, none have been found.

The study of wormhole physics has also raised the possibility of time travel. Kip Thorne has suggested that if a wormhole tunnel is somehow set in motion and accelerated close to the speed of light it can be used to connect the past and the present. Filmmakers and authors, of course, have jumped on the possibility.

One of these is Cramer who pens science fiction when he’s not writing academic papers on theoretical physics. Wormholes play a starring role in his book Einstein’s Bridge about an aggressive civilization from a parallel universe that is intent on destroying the universe in which Earth exists. These exotic portals, Cramer believes, could set the traveler down in a galaxy cluster billions of light years away or even in some parallel universe which “might have time running in the other direction,” he says. In other words, people would be getting younger not older.

Or it might be just like ours, he suggests, except that everything is made out of antimatter instead of matter.” Unfortunately, when antimatter and matter meet, they annihilate each other. Such a wormhole connection would pose “an explosive problem” for the unwary inter-universe traveler where the first step out of the portal might be his last.

Because of the theoretical work in the 1990s, it’s become clear to most physicists that wormholes and time machines, while mathematically feasible, are not practical to create. Some researchers are hoping that some yet-to-be-devised laws of quantum gravity, which deal with gravity at the scale of subatomic particles, could help solve some of the tricky issues. Until we have those laws, says Thomas Roman, professor of mathematics at Central Connecticut State University, we won’t be able to answer these questions definitively. In the meantime, science fiction writers can feel free to slide their characters through wormholes into another time, another galaxy, or an entirely different universe.