Wormhole Wonders: Hunting Down Spacetime Shortcuts

Science fiction literature is full of stories in which tunnels in space-time - known as wormholes - are used for time travel. How much fact lies within the fiction? The answer is, more than you might think. Scientists are looking at ways to use traversable wormholes (if they exist) to travel faster than the speed of light - and even to travel through time itself.

"A traversable wormhole is a hyperspace tunnel, also called a throat, that connects together two remotely distant regions within our universe, or two different universes - if other universes exist - or two different periods in time, as in time travel, or different dimensions of space," physicist Eric Davis told Space.com by email.

Top 5 Sci-Fi Time Travel Methods

Davis specializes in the field of space-time as a member of the Tau Zero Foundation, where he uses equations from Einstein's general theory of relativity to think about possible (or impossible) designs for traversable wormholes, warp drives and time machines.

Building a Wormhole Wormholes were first proposed in 1916 by mathematician Ludwig Flamm, who was toying around with equations from Einstein's theory of general relativity that describe how gravity can curve space-time, which refers to the fabric of physical reality. While these tunnels through space-time are a fascinating theoretical possibility, according to physicist Kip Thorne, a professor emeritus at the California Institute of Technology, scientists have not yet come up with an agreed-upon way that wormholes could form in nature, and no wormholes have ever been detected.

Thorne and some of his colleagues also showed that even if a wormhole appeared, it would likely collapse before an object (or person) could pass through it. To keep the wormhole open long enough to traverse it would require some kind of scaffolding, but normal matter wouldn't stand up to the job - it would require an "exotic material."

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"Dark energy is one form of naturally occurring exotic matter whose negative pressure produces the gravitationally repulsive force that pushes the space inside our universe outward, thus producing the inflationary expansion of the universe," Davis said.

Along with dark energy, scientists also know of an exotic material called dark matter, which is five times more prevalent in the universe than regular matter. To date, scientists have been unable to directly detect either dark matter or dark energy, so much about them is still unknown. Scientists can learn about these materials, though, by examining the effect they have on the space around them.

According to Ali Övgün of Eastern Mediterranean University in Cyprus, it's possible that wormholes could form where dark matter is present, and thus that they could exist in the outer regions of the Milky Way, where dark matter lies, as well as within other galaxies. Övgün is working to prove that wormholes could exist in regions dense with dark matter. He and his colleagues have run simulations that show that wormholes in dense regions of dark matter found in galactic halos would satisfy the physical requirements scientists think the tunnels require.

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"But it is only mathematical proof," Övgün said. "I hope one day it will be possible to also find direct experimental evidence."

So, what happens to a person or instrument traveling through a wormhole?

"Nothing! The space-time geometry of traversable wormholes requires that there be no nasty, intolerable gravitational tidal forces acting upon the spacecraft or its passengers while they move through the wormhole tunnel," Davis said. "They go into the throat at their departure location near Earth and get shunted through the tunnel to emerge out the other side near the destination star."

Because these theoretical tunnels cut through space-time, they would allow travelers to achieve speeds that appear to an outside observer to be faster than light (FTL). However, from the travelers' points of view, they would never actually outpace the speed of light - it would just seem that way to outside observers because the travelers would be taking a route that's shorter than they would have taken through ordinary space.

Before scientists could use wormholes, they would first have to find them. To date, wormholes have not been discovered. However, if they exist, locating a tunnel through space-time may not be as difficult as it sounds.

"As it is visualized in the movie 'Interstellar,' in the future, there will be some experiments to observe indirectly," Övgün said.

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Based on certain wormhole theories, he compared peering through a wormhole to Alice's glimpse through the looking glass, in Lewis Carroll's novel of the same name. The region of space at the far end of the tunnel should stand out from the area around the entrance thanks to distortions that would be similar to the reflection in curved mirrors. Another indication may be the way light is concentrated as it moves through the wormhole tunnel, much as the wind blows through a physical tunnel.

Davis refers to what is seen at the near end of a wormhole as a "rainbow caustic effect." Such effects could be seen from a distance.

"Astronomers were planning to use telescopes to hunt for these rainbow caustics as a sign of a naturally occurring, or even an alien-made, traversable wormhole," Davis said. "I never heard if that project got off the ground."

Traveling Through Time As part of his study of wormholes, Thorne also proposed a thought experiment in which a wormhole could be used as a time machine. Thought experiments about time travel often run into paradoxes. Perhaps the most famous of these is the grandfather paradox: If an explorer went back in time and killed his or her grandfather, that person could not be born, and would never have gone back in time in the first place. This seems to suggest that backward time travel is impossible, but according to Davis, Thorne's work opened up a new avenue for scientists to explore.

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"An entire cottage industry of theoretical physics was born after that, which led to the development of other space-time techniques that can produce causal, nonparadox time machines," Davis said.

But although using wormholes for time travel may appeal to fans of fiction (and those who'd like to change their past), Davis said current theories show that to make a wormhole time machine, one or both ends of the tunnel would need to be accelerated to velocities approaching the speed of light.

"It would be extremely difficult to construct a wormhole time machine," Davis said. "It's relatively much simpler to use wormholes for FTL interstellar travel between the stars."

Other physicists have suggested that using a wormhole to travel through time would cause a massive buildup of energy that would destroy the tunnel just before it could be utilized as a time machine - a process known as quantum back reaction. Nonetheless, it is still fun to dream about the potential.

"Think of all the possibilities of what people could do and the discoveries they could make if they could travel through time," Davis said. "Their adventures would be very interesting, to say the least."

More from SPACE.com:

'Interstellar' Science: Physicist Kip Thorne Writes the Book Inside 'Interstellar': Q&A with Jonathan Nolan and Kip Thorne What 'Black Holes and Time Warps' Means to Me Originally published on Space.com. Copyright 2015 SPACE.com, a Purch company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

Artist's impression of a spacecraft passing from one part of the universe to another through a traversable wormhole.

Top 5 Time Travel Methods from the Movies

Oh, how we often long to travel back in time and change our pasts, to stop some horrible event, to rewrite history. Movies often indulge and inspire us with their time travel adventures, but how many of these have any basis in real science? Let's, for this purpose, ignore how much the movies tug at the heartstrings, entertain us, or tickle our funny bone. We can never forget great ones like "Back to the Future," "Bill and Ted's Excellent Adventure," "Time Cop" and many others. But this is a focus on how the heck we are going to get these movies to come true. Ron Mallett, professor of physics at the University of Connecticut and author of Time Traveler: A Scientist's Personal Mission to Make Time Travel a Reality, has spent his whole career studying the possibilities of time travel and he weighed in on what aspects of the movies are on the edge of possibility and which ones are not. Derived from the expertise of Mallett, here are the top five time travel movies ranked on their basis in science and their true feasibility. Sorry "Hot Tub Time Machine", you didn't make the cut.

Frequency (2000)

Frank Sullivan, played by Dennis Quaid, communicates with his son John (Jim Caviezel) 30 years into the future through a radio. They work together to save Frank's life and to find John's mother's would-be-killer. Frequency's time travel is largely based around an unusual environmental phenomenon in the aurora borealis. This solar disturbance causes Frank's radio to send its signal to the same radio in the future, where John now has it in the same home. The idea that the energy from a solar disturbance could alter spacetime in some way that sends radio waves through time is very much outside the realms of possibility. The aurora borealis could never produce that much energy and if it were to do so, there would likely be some disastrous side effects. But being outside the realms of possibility isn't a problem when it comes to sci-fi time travel. However, Brian Greene, physicist and author of several books including The Elegant Universe, was a consultant for the film, so his input certainly gave the movie some extra weight as far as feasibility goes.

Déjà Vu (2006)

After a ferry bombing, Agent Doug Carlin (Denzel Washington) joins forces with a special team that has technology to see four days into the past. This technology turns out to be a "time window" which Carlin convinces them to use as a time machine and send him into the past. He ends up saving the ferry from the bomber, played by Jim Caviezel. This is a stretch and would involve technology and an understanding of wormholes that we just don't have right now. The "Snow White" project, as it's called in Déjà Vu, somehow controls wormholes or what they call a "time window" to travel through time and space. Wormholes, also called Einstein-Rosen bridges, are a valid theoretical method for time travel, but solely theoretical. Their existence has never been proven and requires something called "exotic matter" to keep them stable. Yet, Mallett says that any movie involving theories of Einstein for time travel is more feasible than those that do not. The movie takes the theory to the extreme in a way that is very entertaining and smart. Brian Greene, who consulted in "Frequency," was also a consultant for "Déjà Vu". You definitely need a theoretical physicist to keep all the timelines straight in this one.

The Time Machine (1960)

H.G. Wells wrote the original book and the movie's lead character is named H. George Wells (Rod Taylor) after him. He uses his machine to travel from the year 1900 through the future seeing two world wars and accidentally ending up in the year 802,701, where he finds a future people who he tries to help. Mallett has this high on his list (of course he says the book is even better) because it's the only movie that accurately states time as a fourth dimension. Although in modern relativistic physics, space and time are combined into one metric called "spacetime." The machine itself; not so feasible. A chair with spinning parts and a lever that controls time travel is something today's scientists have yet to find viable. It's quite miraculous that the fictional H. George Wells was able to invent it in the early 1900s. Another positive note for this version: it does not have him traveling to the past beyond the invention of his machine. The movie doesn't explain this rule, but it does follow the true physics of time travel. Mallett says, "Since it is the device that creates the effect, then is not possible to go back before the device was created."

Star Trek (2009)

Time travel is featured in the "Star Trek" series many times, but the way the most recent movie dealt with time travel makes it very easy to understand without having to be a physics expert. "They actually brought in a number of current ideas," says Mallett. The movie's use of parallel universes is done very well and explained very well. There is a valid scientific theory in quantum mechanics that says there are many parallel universes. Plus, when you go back into the past you actually arrive in the past of a parallel universe, where you can change the future of that universe, but in your time those things have already happened. It's also important to know that once you are in this alternate reality you cannot, as another time travel movie suggests, go back to the future. Black holes are a very popular method for sci-fi time travel and one that could actually be possible. Einstein's general theory of relativity basically says that gravity effects time and since black holes have gravity so strong that light can't even escape, they create the possibility for time travel. If you were able to get close to a black hole, time would slow down. When you escaped, time outside would have passed a lot more quickly. Making things more complicated, when the black hole is rotating, it can cause time to be twisted into a loop that allows you to go into the past. That still leaves some scientific flaws in the movie, but nobody's perfect. It was still one of the most (if not the most) entertaining "Star Trek" movies.

Planet of the Apes (1968)

Put briefly, astronaut Taylor (Charlton Heston) unknowingly goes on a journey to a future Earth where apes have guns, don't believe in the possibility of flight, and rule the planet. Humans are still around in this future, but are mute and quite unintelligent. The best thing about this movie is that they talk about real scientific theories in explaining how they traveled through time. "According to Dr. Hasslein's theory of a vehicle traveling near the speed of light, the earth has aged nearly 700 years since we left it, while we've aged hardly at all," says Taylor in the opening scene of the movie. Actually, it's based more on theories of a man named Albert Einstein. His special theory of relativity says that time for a moving clock slows down. In 1971, the Hafele-Keating experiment proved this with very accurate atomic clocks. One was on Earth and another was flown around the world on a passenger jet. When the jet landed, the clock on the jet was about 50 nanoseconds behind the one on Earth, just like Einstein predicted in 1905. "This means, for a space traveler traveling close to the speed of light, that this effect will happen dramatically. A few years will pass for those on board, but when the rocket lands, decades will have passed on Earth," explains Mallett. So as Heston and his crew travel near the speed of light, they would in fact be traveling through time relative to those of us on Earth. Getting a ship close to the speed of light is the practical challenge in this case, but very plausible in theory. The portion of the movie in which apes control the planet is a little more questionable.