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Time-travel physics seems stranger than fiction

Physicists say future tests could tell whether we can change the past

By Alan Boyle
Science editor
updated 3:18 a.m. ET Nov. 21, 2006

Alan Boyle
Science editor

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

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.'"

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