In H.G. Wells' 1895 novel, The Time Machine, a radical scientist, weary from his travels to the future and back, warns his colleagues that his story will be difficult to believe.
"I don't mind telling you the story," the Time Traveler says to his friends. "But I can't argue…Most of it will sound like lying. So be it!"
The Time Traveler's story may have sounded outrageous to his colleagues, but today physicists think Wells was onto something. In fact, according to Albert Einstein's famous equation, E = mc² , time travel is possible, at least in one direction. Going the other way — back to the past — presents a trickier challenge.
All About Einstein
This Friday, the stories of the Time Traveler, updated to suit a 21st century world, will once again be presented in a Dream Works and Warner Bros. Picture film, starring Guy Pearce. And, once again, the concept of time travel will seem far-fetched and improbable.
But physicists warn just because the feat may seem impossible, doesn't mean it is.
"We have a hard time perceiving how time can bend just like other dimensions, so Einstein's predictions seem strange," said J. Richard Gott, author of the book Time Travel in Einstein's Universe and a professor of astrophysics at Princeton University. "But this appears to be the world we live in."
Part of the "strange" world that Einstein explained in 1905 in his theory of relativity is that time and space are joined in our universe as a four-dimensional fabric known as space-time. Stranger yet is the concept that both space and time warp as mass or speed is increased.
Travel fast and time moves more slowly. Increase the mass around you to near collapsible levels and you get the same effect.
The phenomenon has already been proven, albeit at minute scales.
Slowed Clocks, Younger Particles
In 1975 Carol Allie of the University of Maryland synchronized two atomic clocks and placed one on a plane and flew it around for several hours and left the other on Earth. When the airborne clock was returned to Earth, she compared its time with the one that hadn't moved and found that time had moved a fraction of a second more slowly for the clock on board the plane.
In other experiments, scientists used particle accelerators to speed elementary particles to nearly the speed of light. They found the accelerated particles decayed slightly more slowly than ones that remained sitting in the lab.
As for the effect of mass on time, scientists have measured the ticking of atomic clocks at the top and base of skyscrapers. They found the clocks at the base — closer to the mass of Earth — ticked ever more slowly than those perched high.
Why does all this mean time travel is possible? Gott explains, the same principles that make the clocks tick slower on planes and low on Earth, should also work at extremes.
Building a Fast Ship
In The Time Machine, the traveler catapults through time by activating his hand crafted crystal and polished brass time traveling unit. Physicists believe true time travel might require something more along the lines of a very fast space ship.
By riding on a spacecraft that can travel at speeds of two hundred million meters per second, or about four hundred and fifty million miles per hour, a passenger would experience significantly slowed time.
The slowed time would not be noticeable to the traveler — the same way riding an airplane doesn't feel any different than sitting on Earth. But once the traveler returned, he'd find that those who remained on Earth had aged at a faster rate.
Time passed at its regular cadence on Earth, while to the spaceship traveler, it crawled. So the traveler's return to Earth is, in effect, a trip to the future.
Gott estimates that our best current time traveler is Russian cosmonaut Sergei Avdeyev who was in orbit for a total of 748 days during three space flights. Avdeyev's prolonged travel made him a younger man — by about one-50th of a second — than those of us who have remained on Earth.
That may not seem like much, but as Gott points out in his book, "the journey of a thousand years must begin with a fraction of a second."
There are, of course, significant obstacles — namely building a spaceship that can travel at speeds close to the speed of light. The task would require intense levels of energy that we currently can't achieve.
Still, Paul Halpern, a physicist at the University of the Sciences in Philadelphia and author of the book Time Journeys: A Search for Cosmic Destiny and Meaning, believes the prospect isn't so far off.
"There's no reason to think that in the next few hundred years, we won't be able to reach these speeds," he said. "There's no reason to discount the idea."
Then again, even if we manage to bolt into the future, there remains the tricky issue of how to return by traveling to the past.
Shortcuts in Time
According to Einstein's theory, approaching the speed of light would theoretically slow time, traveling at the speed of light would make it stand still and traveling faster than the speed of light would reverse time.
But Einstein also showed that traveling at or faster than the speed of light is impossible because mass at these speeds becomes infinite. Does that mean traveling back in time is impossible? Some, like British theoretical physicist Stephen Hawking, have said so. But others think there may be a way to find "shortcuts" to the past.
In the late 1980's Kip Thorne of the University of California at Berkeley suggested that objects known as wormholes exist in space. These objects would essentially be two connecting black holes whose mouths make up a tear in the fabric of space-time.
By finding a wormhole and stretching it so one mouth extends light years away from the other, the wormhole could provide a passageway to a past or future point on the undulating river of time.
Thorne developed the theory after Carl Sagan had asked him whether there could be a way he could send the heroine of his novel, Contact, billions of miles to meet an alien and return home the same day. The book and the movie by the same name, starring Jodie Foster, featured Thorne's wormhole phenomenon.
The idea may have worked nicely for Sagan's novel, but the actual theory still has significant problems.
Besides locating a real wormhole, scientists would also need to find a way to keep the wormhole's entrances open long enough for a person to pass through. Due to quantum mechanics — the field of physics that governs the mechanics of the inner world of atoms — forces would cause the time machine to instantly squeeze shut.
Some have proposed solutions to this problem, including filling the wormhole with large amounts of exotic, or negative matter, but the solutions would require enormous energy and ingenuity. It would also require that scientists find a way of merging Einstein's law of relativity with those laws governing quantum mechanics in a so-called Theory of Everything.
"Time machines to the past are projects only a super civilization could attempt," said Gott. "It would require a civilization that has the resources of the galaxy at its command."
The Grandmother Paradox
Finally, there is the stubborn issue of the time travel paradox. For example, what happens if a person travels back in time and kills his grandmother before the traveler's mother is conceived. Does that mean the time traveler suddenly no longer exists?
Hawking has suggested that time protects itself from such scenarios by preventing time travel to the past. Others suggest that the time traveler would simply enter a parallel universe that evolves along its own separate route in space. And others, like Halpern, say that past, present and future, might all exist and influence each other simultaneously in our universe.
"Perhaps all of space and time exist at once and our travels through time are simply something that our conscious minds undertake," said Halpern. "If we could break this force that is propelling us forward, maybe we could travel back in time."
Confused yet? Physicists explain part of the reason why time travel may be difficult to grasp is our perception is limited to a fairly moderate world where the laws of physics are not pushed to their limit.
H.G. Wells, and later, Einstein were able to take a leap of imagination and envision what could happen when these laws are pushed to extremes. The results may seem bizarre, but, as the Time Traveler says, "So be it!"