By Alexis Soloski
By Molly Grogan
By R. C. Baker
By Christian Viveros-Fauné
By Alexis Soloski
By Alexis Soloski
By Lilly Lampe
Time dilation has been proven by placing atomic clocks aboard long-haul flights, but, Davies says, "people are often surprised, indeed disbelieving, when you tell them that simply by flying in an airplane you can jump ahead in time." Not by much, admittedly. Our fastest spacecraft travels at a dismal 0.01 percent of the speed of light, and the resulting time warp is measured in imperceptible microseconds. J. Richard Gott, professor of astrophysics at Princeton, notes in his book Time Travel in Einstein's Universe (Houghton Mifflin) that the most successful time traveler we know of is the cosmonaut Sergei Avdeyev, who was in orbit aboard the Mir space station for a total of 748 days. Allowing for Einstein's principle of equivalence, which asserts that gravity slows time (i.e., a clock at a high altitude would tick slightly faster than on the surface of the Earth), Gott calculated that Avdeyev, whizzing around the planet at more than 17,000 miles per hour for more than two years, traveled into the future by approximately 1/50th of a second.
These numbers don't bode well for aspiring century-straddlers. But even if engineering capabilities were to dramatically improve, facilitating high-speed propulsions through space and into the future, these would be strictly one-way trips: You could never return to the moment of your departure. Moving at a speed faster than light would allow travel to the past (it's precisely what Superman does to save Lois Lane in the 1977 movie). But one of Einstein's most famous results is that the light barrier is insurmountablethe ultimate speed limit. Physicists eventually realized that, rather than striving to attain superliminal speed, theories of backward time travel might be developed by considering the curvature of space-time (another Einstein conclusion) and contriving shortcuts where it would be possible to beat a light beam in a race.
In 1949, the Austrian mathematician Kurt Gödel observed that in a rotating universe, trajectories of light would loop in such a way that a time traveler could outpace them without having to surpass the light barrier. Available evidence suggests that the universe is notrotating, but Gödel's model was a turning point all the same, in demonstrating that travel to the past is possible in principle. The next major landmark came, once again, from science fiction. If Wells can take credit for being the first to articulate this whole crazy idea, Carl Sagan's 1985 novel, Contact, was the most important, albeit unintentional, catalyst of the last two decades. The book (later turned into a Jodie Foster movie) sends the heroine on a journey to the star Vega, 26 light-years away; Sagan's first impulse was to shorten the trip by having her pass through a black hole.
His friend Caltech physicist Kip Thorne objectedwith good reason. A black hole is a region of infinite density and intensely high gravity (generally thought to be left over from an inward-collapsing star), from which, by definition, nothingescapes. Thorne suggested that Sagan use a wormhole, essentially a black hole with mouths on either end, which could conceivably form a passageway between two distant swaths of space-time. He and his colleagues also investigated ways to counteract the lethal gravitational effects and make the tunnel safely traversablethe solution involved propping it open with "negative energy" or anti-gravitating matter. Contactposits a wormhole as a shortcut for space travel, but physicists soon realized that such a structure could also serve as a time machine; Thorne's workdetailed in Black Holes and Time Warps: Einstein's Outrageous Legacyforms the basis for much current research. Another popular theory for reverse time travel, proposed by Gott in 1991, is predicated on cosmic strings: infinitely long strands of residual high-density matter that date from the Big Bang. Gott hypothesizes that if two strings moving in opposite directions zip by each other, a spaceship whose path traces a loop around them could arrive back where it took off before it left.
One might well wonder what tangible use any of this is to us. These are highly conjectural theories entailing extreme circumstances, and the jury is still out on whether space oddities like wormholes and cosmic strings exist in the first place. (They're theoretically plausible, most astrophysicists agree, but there's still no hard evidence.) Even if they do, the project of turning them into time machines, as Thorne and Gott have pointed out, would require the resources of a supercivilization. Davies's book, in keeping with its mock-utilitarian title (which fittingly, though perhaps unwittingly, derives from pataphysics, echoing French absurdist Alfred Jarry's 1899 Wells-inspired essay "How to Construct a Time Machine"), devotes a full chapter to the engineering of a wormhole time machine. His four-step guide for manufacturing a wormhole in a quantum vacuum and blowing it up to everyday dimensions is, to say the least, not your average science fair project.
Cynics have been known to scoff, "There's speculation, there's pure speculation, and there's cosmology." Cosmologists would respond that an elucidation of the nature of time is crucial to a complete theory of physicsthe elusive "theory of everything" that reconciles Einstein's general relativity (which explains gravity and the curvature of space-time) and quantum theory (which is focused on the subatomic world). Physicists describe the quest for this holy grail in grandly mystical terms"reading the mind of God" is a favorite phrase. Michio Kaku, theoretical physicist at CUNY and author of Hyperspace, says the leading contender is the still incomplete solution known as superstring theory, which involves nine spatial dimensions. "String theory would settle once and for all the question of whether time travel is possible. If it is, it would allow us to make calculations about the stability of a wormhole."