Time Travel? Don't Forget to Pack Your Wormhole
Though wormholes have never been proven to exist, these theoretical passageways through space-time are predicted by Einstein's general theory of relativity.
Top 5 Sci-Fi Time Travel Methods
There is no shortage of time machines in the world of science fiction. You could probably name a bunch of them off the top of your head, from H.G. Wells' iconic creation to such mainstays as Dr. Who's Tardis and Dr. Brown's flux-capacitated DeLorean. But just how many fictional time machines can you explain? In many works of fantasy and science fiction, the time machine is just a magical plot device. No actual science is thrown at the audience. Most of the time, no one asks for any. After all, you're probably not watching Life on Mars or Terminator Salvation for a lesson in theoretical physics. Plus, if you're writing time-traveling fiction, then skipping the science spares you the embarrassment of getting something wrong. Isn't it enough that you described 1997 as being a world full of flying cars and busty android life partners? Let's take a look at five examples of the plausible and ridiculous ways fictional TV and film characters have traveled through time.
5. Superman Spin Control
If we learned anything about the physics of time and space from Richard Donner's 1978 film Superman, it's that if you fly around the Earth really fast, you can reverse its rotation and roll back time. Although physicists agree that space and time are interconnected, you'd be hard-pressed to find anyone who would back the "science" behind reversing planetary rotation to turn back time. Far from saving Lois Lane's life, the feat likely would have caused global chaos. Slam on the brakes in a moving car and everything inside it continues moving forward. Now imagine this scenario on a global scale, only with oceans, mountains and weather systems continuing to surge forward at up to 1,000 miles per hour, depending on your latitude. Way to go, Superman.
Credit: AP Photo
4. The Voyage Home to 1986
The Star Trek universe is full of fantastic ideas: aliens with rippled foreheads, holodecks and more time travel than you can shake a stick at. According to the Star Trek Wiki, 50 episodes of the six TV series featured time travel, as did four of the 11 films. You'd think the space-time continuum would just be circling the drain after all that tinkering. Time paradoxes aside, Star Trek always flirted with real science. Take 1986's Star Trek IV: The Voyage Home, for example. In this film, the crew of the Starship Enterprise send a Klingon Bird-of-Prey vehicle back to the 1980s by sling shotting it around the sun. The Star Trek slingshot method involves using the sun's gravitational pull as an accelerator to reach speeds necessary to travel through time. The premise falls in line with some theories about time travel and Einstein's theory of special relativity. The theory says if time slows the closer you get to the speed of light, then travel into the future -- or the past -- may be possible. One slight problem: faster-than-light travel is physically impossible. Plus, as Lawrence M. Krauss points out in The Physics of Star Trek, the gravitational field near the surface of the sun doesn't produce anywhere near the boost you'd need to go talk to whales in the past.
3. Trekking into a Black Hole
Paradoxical time travel isn't a thing of the past for the Star Trek legacy. The plot of the new film concerns two starships that are sucked into an artificial black hole, sending them 154 years into the past. While the time-travel method employed in Star Trek IV: The Voyage Home depended on a far too weak gravitational slingshot, many physicists believe that a black hole might indeed provide the necessary portal to the past. Anything that crosses a black hole's event horizon heads toward an incredibly tiny point of infinitely compressed matter called a singularity. That's also one of the huge problems with the new Star Trek film's plot: What's to keep the two starships from winding up as one with the singularity? Physicists point to Kerr black holes as a less destructive alternative. These theoretical cosmic phenomena first described by Roy Kerr in the 1960s lack the matter-smashing singularity at the center, potentially making it possible to pass the event horizon and come out the other side -- in another time.
2. Donnie Darko, Creepy Rabbits and Wormholes
The 2001 cult favorite Donnie Darko spends most of its time exploring the possible effects of time-travel paradoxes and tangent universes on its characters, but it also features a portal through time: a wormhole. Also called Einstein-Rosen bridges, these hypothetical cosmic structures might offer a traveler the necessary means of not just taking a shortcut through space, but also through time itself. Einstein's theory of relativity states that mass curves in spacetime. The most common visual example of this concept is that of space depicted as a curved, two-dimensional plane. Think of a racetrack: If you're speeding around a curve, you're bound to that curve, but what if you could forge a new line of track between its two parallel sides? That's the idea behind a wormhole. If a mass on one side of the spacetime curve applies enough force and a mass on the other side of the spacetime curve applies enough force, then the two could meet, creating a tunnel.
Credit: AP Photo
1. Lost on a Time-Traveling Island
If you've watched ABC's "Lost," then you're probably used to things not making a lot of sense. A big reason for this is that the show's mysterious island bounces the characters around through time seamlessly. Seriously, by the end of the series, everyone will be lucky to make it off the island without becoming their own grandparent. Yet "Lost" at least makes an effort to prop up the fiction with a little science. According to blog analysis at Popular Mechanics, the science behind the show's time travel seems to depend on quantum mechanics, a mysterious substance in the ground called "exotic material" and possibly a wormhole. Might this buried, volatile substance produce the necessary energy to manipulate a breach in spacetime? To varying degrees, you could argue that this is all any writer can achieve when crafting a piece of time-travel fiction -- not counting writers who are actually from the future, of course.
The concept of a time machine typically conjures up images of an implausible plot device used in a few too many science-fiction storylines. But according to Albert Einstein's general theory of relativity, which explains how gravity operates in the universe, real-life time travel isn't just a vague fantasy.
Traveling forward in time is an uncontroversial possibility, according to Einstein's theory. In fact, physicists have been able to send tiny particles called muons, which are similar to electrons, forward in time by manipulating the gravity around them. That's not to say the technology for sending humans 100 years into the future will be available anytime soon, though.
Time travel to the past, however, is even less understood. Still, astrophysicist Eric W. Davis, of the EarthTech International Institute for Advanced Studies at Austin, argues that it's possible. All you need, he says, is a wormhole, which is a theoretical passageway through space-time that is predicted by relativity.
"You can go into the future or into the past using traversable wormholes," Davis told LiveScience.
Where's My Wormhole?
Wormholes have never been proven to exist, and if they are ever found, they are likely to be so tiny that a person couldn't fit inside, never mind a spaceship.
Even so, Davis' paper, published in July in the American Institute of Aeronautics and Astronautics' journal, addresses time machines and the possibility that a wormhole could become, or be used as, a means for traveling backward in time.
Both general relativity theory and quantum theory appear to offer several possibilities for traveling along what physicists call a "closed, timelike curve," or a path that cuts through time and space -- essentially, a time machine.
In fact, Davis said, scientists' current understanding of the laws of physics "are infested with time machines whereby there are numerous space-time geometry solutions that exhibit time travel and/or have the properties of time machines."
A wormhole would allow a ship, for instance, to travel from one point to another faster than the speed of light -- sort of. That's because the ship would arrive at its destination sooner than a beam of light would, by taking a shortcut through space-time via the wormhole. That way, the vehicle doesn't actually break the rule of the so-called universal speed limit -- the speed of light -- because the ship never actually travels at a speed faster than light.
Theoretically, a wormhole could be used to cut not just through space, but through time as well.
"Time machines are unavoidable in our physical dimensional space-time," David wrote in his paper. "Traversable wormholes imply time machines, and (the prediction of wormholes) spawned a number of follow-on research efforts on time machines."
However, Davis added, turning a wormhole into a time machine won't be easy. "It would take a Herculean effort to turn a wormhole into a time machine. It's going to be tough enough to pull off a wormhole," he told LiveScience.
That's because once a wormhole is created, one or both ends of it would need to be accelerated through time to the desired position, according to general relativity theory.
Artist's impression of the mouth of a wormhole.DCL
There are several theories for how the laws of physics might work to prevent time travel through wormholes.
"Not only do we assume (time travel into the past) will not be possible in our lifetime, but we assume that the laws of physics, when fully understood, will rule it out entirely," said Robert Owen, an astrophysicist at Oberlin College in Ohio who specializes in black holes and gravitation theory.
According to scientists' current understanding, keeping a wormhole stable enough to traverse requires large amounts of exotic matter, a substance that is still very poorly understood.
General relativity can't account for exotic matter -- according to general relativity, exotic matter can't exist. But exotic matter does exist. That's where quantum theory comes in. Like general relativity, quantum theory is a system for explaining the universe, kind of like a lens through which scientists observe the universe.
However, exotic matter has only been observed in very small amounts -- not nearly enough to hold open a wormhole. Physicists would have to find a way to generate and harness large amounts of exotic matter if they hope to achieve this quasi-faster-than-light travel and, by extension, time travel.
Furthermore, other physicists have used quantum mechanics to posit that trying to travel through a wormhole would create something called a quantum back reaction.
In a quantum back reaction, the act of turning a wormhole into a time machine would cause a massive buildup of energy, ultimately destroying the wormhole just before it could be used as a time machine.
However, the mathematical model used to calculate quantum back reaction only takes into account one dimension of space-time.
"I am confident that, since (general relativity) theory has not failed yet, that its predictions for time machines, warp drives and wormholes remain valid and testable, regardless of what quantum theory has to say about those subjects," Davis added.
This illustrates one of the key problems in theories of time travel: physicists have to ground their arguments in either general relativity or quantum theory, both of which are incomplete and unable to encompass the entirety of our complex, mysterious universe.
Before they can figure out time travel, physicists need to find a way to reconcile general relativity and quantum theory into a quantum theory of gravity. That theory will then serve as the basis for further study of time travel.
Therefore, Owen argues that it's impossible to be certain of whether time travel is possible yet. "The wormhole-based time-machine idea takes into account general relativity, but it leaves out quantum mechanics," Owen added. "But including quantum mechanics in the calculations seems to show us that the time machine couldn't actually work the way we hope."
Davis, however, believes scientists have discovered all they can about time machines from theory alone, and calls on physicists to focus first on faster-than-light travel.
"Until someone makes a wormhole or a warp drive, there's no use getting hyped up about a time machine," Davis told LiveScience.
Accomplishing this will require a universally accepted quantum gravity theory -- an immense challenge -- so don't go booking those time-travel plans just yet.