That time-honored standby of science fiction — traveling back in time — has come back into fashion in Hollywood, on the big screen ( "Deja Vu," premiering Wednesday) as well as the small screen ( "Day Break," which made its debut last Wednesday on ABC).
The plot twists have their parallels in classics as old as H.G. Wells' "The Time Machine" (1895) and as recent as Bill Murray in "Groundhog Day" (1993). But is there really anything new under the sun in time travel lore, or are we caught in an infinite time loop?
Despite years of debate, scientists still haven't completely ruled out the possibility of going back in time. "Many physicists have a gut feeling that time travel to the past is not possible," said Columbia University theoretical physicist Brian Greene. "But many of us, including me, are impressed that nobody's been able to prove that."
Over the next few years, some experiments hold out a chance of finally being able to show whether or not time can move backward as well as forward. Theoretically, at least, it might be possible for the future to influence the past, said John Cramer, a physicist at the University of Washington. He and his colleagues plan to try just such an experiment next year.
Cramer acknowledged that the concept of retro-causality doesn't seem to make sense, "but I don't understand why not."
Both Greene and Cramer know the science as well as the fiction side of the time-travel issue: Greene is the author of "The Elegant Universe," a best-selling book on string theory — but he also played a cameo role in "Frequency," a time-travel movie released in 2000, and served as a scientific consultant for "Deja Vu."
"It was a kick to be in the room with [producer] Jerry Bruckheimer and [director] Tony Scott and the writers, talking about special relativity and general relativity and wormholes," he told MSNBC.com.
Cramer, meanwhile, has done research into ultra-relavistic heavy-ion physics at CERN and Brookhaven National Laboratory — but he's also written two science-fiction novels and pens a regular column for Analog magazine called "The Alternate View." If his experiments show that retro-causality is a reality — that one event can determine the outcome of another event taking place 50 microseconds earlier — it could lend support to the ultimate alternate view of quantum physics.
"It opens the door to doing all kinds of really bizarre things," he said.
It's all relative?
Over the past 100 years or so, physicists have come to understand that time travel is all relative: In a sense, we're all traveling through time, and depending on your reference frame, some would seem to be doing it more quickly than others. For example, astronauts returning from a space station mission might find that their watches were a few nanoseconds behind earthly timepieces, thanks to relativity.
But most time-travel plots involve more than just slowing down or speeding up the forward pace of time. What we're talking about here is reversing time's flow, and perhaps influencing the stream of causality to follow another course: one in which, say, Hitler died in childhood, or 9/11 never happened, or Britney Spears stayed happily married.
If you assume that such reversals are possible, Greene said physics would allow for two possibilities:
- Nature would conspire against changing causality, something Cambridge physicist Stephen Hawking has called the "chronology protection conjecture": For example, if you tried to shoot your father before you were born, somehow the gun would fail to go off.
- Causality can be changed, sending the universe down different forks in the road. You could go back and shoot your father, creating a universe where you were never born. But it wouldn't be the same universe you came from. You'd just be an alien visitor from a different reality, living out a scenario that's called the "many-worlds interpretation."
Without giving away the plot, Greene said that the writers of the "Deja Vu" movie "took a very creative approach. ... They said, 'This is an open issue. Let's allow both of these possibilities to have a little bit of play.'"
Reality checks are in the works
Sometime next year, Cramer is hoping to nail down one of those possibilities — or at least find out why those possibilities are actually impossible.
The experiment he's devised to test for backward causality plays off the idea that the states of two photons can become "entangled": Even if the photons are separated by great distances, what happens to one photon is reflected by the other one.
Einstein called this behavior "spooky action at a distance," and held it up as evidence that quantum physics was wrong. However, in recent years physicists have shown that quantum entanglement is indeed an actual phenomenon.
Cramer and his colleagues propose using a special lithium iodate crystal to create two streams of photons. First they would conduct what Cramer called "a rather simple tabletop experiment" to demonstrate quantum entanglement with a two-slit screen and two detectors, much as other researchers have done already.
If they're successful with the first experiment, they would try a more elaborate demonstration: One stream would be sent directly through the screen to a detector — while the other would be split by the slits, then take a detour through two 6-mile (10-kilometer) lengths of optical cable before reaching a second detector.
The researchers would adjust the position of the second detector to find the spot where the stream changes from a fuzzy diffraction pattern to a more defined, wavy interference pattern. Theoretically, because the streams contain entangled photons, the first detector should register the same changes 50 microseconds earlier.
Cramer said some unanticipated factor could spoil the experiment he's planned out. "It's fairly likely that we've missed something someplace that is going to prevent this kind of measurement from being possible," he told MSNBC.com.
But if the entangled photons act as theory dictates, that opens the door to the kind of paradoxes usually found in time-travel fiction. "What happens if you receive the signal that's supposed to be sent 50 microseconds later, and you decide not to send it?" he asked.
In the end, Cramer might find new twists in quantum theory that make retro-causality impossible. "You can't have inconsistent time loops," he said. "It's called a bilking paradox. ... The general consensus is that nature refuses to be bilked."
But sometimes nature turns out to be stranger than fiction.
More fuel for fiction
Over the past few years, advances in quantum theory and string theory have provided plenty of additional fuel for science-fiction speculations. One emerging view holds that time as well as the three spatial dimensions we perceive are all embedded in a higher-dimensional space. If this is so, the universe next door could be on another "brane" just a fraction of an inch away — but in the direction of another dimension we can't sense directly.
Greene says physicists are just beginning to work out the larger implications of brane theory.
"None of it has been parlayed into any stunning revelations about time quite yet," he said. "I think that's the next revolution."
Greene is looking forward to results from Europe's Large Hadron Collider , due to begin operation in 2007, as well as data from the Wilkinson Microwave Anisotropy Probe and the Planck satellite, which are studying the afterglow from the universe's first moments of existence.
Such observations were unlikely to provide direct evidence whether or not time can flow backward as well as forward. However, if they point to the existence of new classes of supersymmetric subatomic particles, or extra dimensions, that would tell physicists that the workings of time — and the universe itself — are more mysterious than scientists thought.
"What they would do more directly is allow us to understand these questions about the origin of the universe," Greene said. "Did time begin with what we consider to be the universe? Or is our universe just one bubble in this big bubble bath?"
An earlier version of this report listed the incorrect network for "Day Break."
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