Cosmic Wormholes: Where Science Meets Science Fiction

In a book to be published this October, physicist Paul Halpern will explain how to build and use a cosmic wormhole as an interstellar shortcut to distant parts of the universe, or to travel backward in time. As fanciful as this sounds, many physicists say Halpern's book is not necessarily science fiction. There is indeed serious science at work here—at least on a theoretical level—they say. Halpern, who doesn't think such devices could be found or constructed anytime soon, nonetheless would like to see them created someday. "I really think there is some life out there. But the current space program is very restricted. There are very few options for us if the speed-of-light barrier is not broken," says Halpern, an associate professor at the Philadelphia College of Pharmacy and Science and author of the forthcoming book Cosmic Wormholes: The Search for Interstellar Shortcuts (New York, E. P. Dutton). "If we can't take these shortcuts, the more distant stars Would be barred from us forever." Despite the recent surge in the number of wormhole-related papers, job prospects for physicists focusing on this subject and other aspects of general relativity are a bit grim. David Hochberg, who has been working on wormholes in the early universe, is now on his third postdoctoral position, this time at Vanderbilt University. He says there's a big supply of people involved in theoretical physics, but there are very few permanent faculty positions opening up each year. Another complication is that most of these postdoc positions are funded by the National Science Foundation, which allows a maximum two-year stay at a university, resulting in a game of musical chairs that keeps many wormhole theorists constantly on the move. Matt Visser, who is also often on the move, having recently arrived at Washington University in St. Louis from Los Alamos National Laboratory, agrees the going is tough, and doesn't recommend a career in theoretical physics. "I got into it [physics] a long time ago. And in all honesty, I would not take on a graduate student at this stage," he says. But despite the survival problems, Hochenberg says, it is a passion for physics—one that began in his junior year of high school—that keeps him going. And he can't see doing anything but physics. "I enjoy the field, and it's very satisfying. But there's one small glitch—you can't get a job," says Hochenberg. As for Halpern, survival is assured through his associate professor position. Although Halpern's professional publications are in cosmology, he has not worked on wormhole theory himself. So, the book represents a compilation of the work of a number of theoretical physicists who have been making inroads in the growing field of wormhole theory. According to Halpern, wormholes represent severe distortions in the fabric of space caused by extremely powerful gravitational fields. Einstein's field equations, as part of the general theory of relativity, relate the curvature of space to the amount of matter and energy in a particular region: Paraphrasing physicist John Wheeler, he says, "Space tells matter how to move, matter tells space how to curve." Space can be compared to the surface of a trampoline, and matter is like a heavy rock. Place that rock in the center of the trampoline and it bends. But put a boulder in the center, and it will tear a hole in the trampoline. A wormhole represents just such a hole in the surface of space, connecting distant parts of the universe. "With wormholes, what we're doing is constructing a gravitational field to have space curve in a way we'd like it to curve," says Halpern. While researchers concede that the use of wormholes as shortcuts to distant parts of the universe is a long way off, they are still useful today as "thought experiments," according to physicist Kip Thorne at the California Institute of Technology, noted for his coauthorship of the widely used textbook Gravitation. As Thorne wrote in a 1991 article in the Annals of the New York Academy of Sciences (631:182-93): "Much of the forefront of theoretical physics deals with situations so extreme that there is no hope to probe them experimentally. Such, largely, was the case nearly a century ago for Einstein's formulation of general relativity, and such is the case today for the attempt to quantize gravity. In these situations, however, thought experiments can be helpful; and of all thought experiments, perhaps the most helpful are those that push the laws of physics in the most extreme ways." The question that Thorne asks is: Do the laws of physics allow an advanced civilization to construct devices to bridge vast distances of space and to cross backward and forward in time? The answer is yes, according to Thorne and other physicists, and the device—at least on the theoretical level—is the traversible wormhole. Many who are now looking at wormholes trace the burst of interest in the subject to a 1988 paper from Thorne and coworker Michael Morris that appeared in the American Journal of Physics (56:395-412). Washington University physicist Visser says this paper was seminal because once somebody "relatively serious," such as Thorne, puts his imprimatur on a subject as speculative as wormholes, it gives other, lesser-known physicists, such as himself, license to jump aboard the topic without looking silly. Thorne, in turn, gives Cornell University astronomer Carl Sagan credit for stimulating him to work on the wormhole problem. In 1985, Sagan was in the process of writing his novel Contact, and asked Thorne to develop a means of rapid interstellar transportation that was within the bounds of known physical laws. Sagan wanted to avoid the common science fiction error of breaking the speed-of-light barrier. And that's when Thorne developed the traversible wormhole—in which a ship could bridge large distances without exceeding the speed of light. But the idea of wormholes is hardly new. As Thorne pointed out in his 1988 paper, Wormholes as the means of studying mathematical relativity predate black holes. In 1916, the Viennese physicist Ludwig Flamm found that one of the solutions to Einstein's field equations was a type of wormhole—the Schwarzschild wormhole, named after German astronomer Karl Schwarzschild. The trouble with this type of wormhole is that it expands and contracts so rapidly that a spacecraft, even one traveling at nearly the speed of light, could not duck through it fast enough to avoid being crushed by gravitational tidal forces. And besides, such a wormhole has a tremendous gravitational pull—not unlike a black hole—and the spaceship would be torn apart if it approached the entrance, called the mouth. Although such a wormhole might exist, it would not be useful for flitting around the universe. To hold the throat of the wormhole open would require tremendous amounts of a material having enormous tensile strength. A steel rod, for instance, has a breaking tension of about 100,000 pounds per square inch, Halpern says. But the material needed to thread the throat of a wormhole must have a tensile strength of a million trillion pounds per square inch. And the only conceivable substance capable of such strength would be "exotic matter," with a negative mass. Exotic matter is a neat solution on paper, but can it exist? Thorne and others say yes. As Halpern points out, the Heisenberg principle of uncertainty provides that the properties of atomic particles are always fluctuating. The position, energy, and momentum of particles vary over time in an unpredictable manner. The averages of these values can be predicted, but never the exact values all at once. During such fluctuations, particles of negative mass may be created. And Halpern says that in the vicinity of black holes it appears that these particles, during their brief lifetimes, could be captured and collected to form enough negative mass matter to build a wormhole. Halpern says the quantity of exotic matter would be enormous, equivalent to the amount of matter in a very large star. He concedes this is not a task that will soon be undertaken. His seat-of-the-pants guess is that it could be done by 2500 A.D. But the concept of exotic matter is not generally accepted among physicists, says Visser, and "if exotic matter is impossible, wormholes are impossible." He says exotic matter has been seen in the lab but only in tiny amounts, and an ongoing discussion within the community of general relativity theorists is whether it will ever be possible to put enough exotic matter together in one place to build a wormhole. If wormholes exist or could be constructed, their use would not be limited to space travel, says Visser. "A rather embarrassing side effect is that it would be easy to turn a wormhole into a time machine," he notes. "And that makes me a little nervous." With one mouth left stationary and the other in motion, a spacecraft entering a wormhole could go into the past, which potentially violates the laws of causality. Time travel results in the "grandmother paradox," in which you could kill your grandmother before your mother is born, which in turn means you could never have taken the trip into the past, as Halpern points out. One notion is that if you attempted to turn a wormhole into a time machine by moving the mouth, the wormhole would collapse, says Visser. Physicist Stephen Hawking has proposed that there must be a blockage against using wormholes to go backward in time; otherwise, hordes of tourists would be here today. And Visser is steering clear of the time machine aspect. "Let's just keep the time machines out of the wormhole theories until there's no other solution possible," he says. Vanderbilt physicist Hochberg says he got interested in wormholes not as a means of transportation, but rather to solve a cosmological problem: If remote parts of the universe never communicated with each other in the past, the average temperature of space would be different in different directions—but it's the same. Wormholes could provide the solution, Hochberg says, because back when the universe was extremely young—on the order of 10-43 seconds old—all the forces of the nature, including gravity, were quantum in nature. And this is the time when wormholes may have existed in abundance and connected all parts of the cosmos, resulting in the equalization of temperatures observed today, 5 x 1017 seconds later. "If you understand something like a wormhole, what that's doing is giving you a handle on a small aspect of quantum gravity," Hochberg says. Although wormholes may have existed in the early universe, today, he says, "it's very unlikely that wormholes exist." Visser believes that wormhole work is many decades away from any kind of experimental test. If wormholes have formed naturally and still exist in the cosmos, they would be difficult to observe. Black holes can be detected by the halo of radiation emitted as matter is sucked in. Wormholes do not emit such a radiation signature. "Are we going to find one of these things? Right now, the answer is almost certainly no," says Visser. Without such direct observation, proving the existence of wormholes is linked to discovering the weakly interacting massive particles possibly involved in gravity. To find these particles, Visser says, "we would need to manipulate energies that would have to be, at a minimum, several billion times greater than the superconducting supercollider—assuming it ever gets built."

Cosmic Wormholes: Where Science Meets Science Fiction

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Scott Veggeberg

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July 1992

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