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Time after time travel

Moviegoers received a double dose of time-travel fiction this week, with the present-day(s) thriller "Deja Vu" on one hand and "The Fountain," a time-trippy love story, on the other. As we discussed earlier this week, "Deja Vu" reflects a bit more of the current scientific thinking about what time travel into the past, a.k.a. retro-causality, might look like if it were possible. But if you ask the physicists to list their favorite time-travel tales, the ones they mention are golden oldies going back to the days of classic "Star Trek."

Of course, the Starship Enterprise's crew was ready to travel back in time at the drop of a hat (preferably one that covered Spock's pointy ears). But for string theorist Brian Greene, a Columbia University physicist who was a consultant for "Deja Vu," the episode that stands out is "City on the Edge of Forever," in which Dr. McCoy changes mid-20th-century history and it's up to Kirk and the gang to change it back.

As history is rewritten and re-rewritten, items (such as the Enterprise) disappear and reappear to reflect the universe's changing timeline. That follows the same time-travel conventions put in place for the "Back to the Future" time-travel films (and "Star Trek IV: The Voyage Home," for that matter), but does not follow the two main scientific possibilities that Greene and others have laid out:

  • The flow of past events is unchangeable, because any effect time travelers had is already reflected in the universe (Stephen Hawking's "chronology protection conjecture").
  • The flow of past events can be changed - for example, by sending a signal or a person through a wormhole. But that merely creates a new branching in what is a multitude of universes. The universe from which the time traveler was sent back is unchanged (the "many-worlds" interpretation of quantum mechanics).

Thankfully, as a work of fiction, "City by the Edge of Forever" doesn't have to pass strictly scientific muster, and that's why Greene felt confident giving the episode an endorsement. "Put me down for that one," he told me.

Other physicists who have written about time travel (such as the University of New Hampshire's Paul Nahin and Princeton's J. Richard Gott III) have given thumbs-up to an even more unlikely entrant in the time-travel oeuvre: "Bill and Ted's Excellent Adventure," with a pre-"Matrix" Keanu Reeves as one of the slacker stars.

Of course, Greene is partial to the time-travel films in which he's been personally involved, including "Deja Vu" as well as "Frequency," where he played a time-warped cameo role. And Greene said he's involved in yet another time-travel movie project titled "Mimzy," based on the short story "Mimsy Were the Borogoves" and slated for release next year.

The physicist said he doesn't want to give the impression that time travel is a major focus of research. "Many of us see it as a lingering question in the back of our minds," Greene said. He and his colleagues are even more intrigued by the trippy concepts spawned by string theory - indicating that the universe could follow any of 10500 possible courses, and that our course seems to be going down just the right path to allow for the development of stars, galaxies and life.

For some physicists, this multiplicity of potential universes - a seemingly unprovable claim - is a scientific scandal. The criticism is voiced loud and clear in books such as "Not Even Wrong" and "The Trouble With Physics." But for Greene, the idea poses a challenge well worth accepting.

"If this is a feature of the theory, then it's one we have to better understand," he said. "If it is true, it's telling us something mind-bogglingly remarkable about the universe."

The deeper implications of physics will surely set the tone for the start-up of the Large Hadron Collider, scheduled for next year - as well as for the World Science Festival, a science-plus-art celebration scheduled to make its debut in New York in 2008. "I'm one of the festival's co-founders," Greene said.

Here are some of your own thoughts about time travel:

Stuart Greene: "I have always wondered myself about time travel. I was fascinated as a kid. My take on time travel is very simple.

"Let's assume in the future that there is a way to travel back in time. This would imply that time travel would have been infinitely possible, given that someone from the future has already traveled back to the past. And this person who traveled from the future would have transferred this and other technologies and knowledge to the past. From the fact that we know very little about the future, we could conclude that either the traveler from the future is either very selfish (giving his past self scores of the World Series), or he thinks that our knowledge of the future would be detrimental, or there is no future, or time travel is impossible. ...

"My opinion is that time is linear, and the only time travel possible would be to see images of  the past by traveling faster than the speed of light. Kind of sad, but way more interesting not to know the future."

Darrell A. Jones: "If you get a chance, go back and research the Philadelphia Experiment - not the movie, but the actual military research. ... It revealed that the time-travel experiment started out as a failure, then did succeed, yet was still a failure for they had to [go] back because of the hole that had developed and was left open. The scientist involved left a message and documents behind, so that the government did not get the complete files. ..."

Harold Estep, Clarksville, Tenn.: "I once read a novel which dealt with time travel.  It was titled 'Time and Again,' by Jack Finney.  Over the years I have read it three times because the author's premise of how to travel back in time, seemed so believable.  If you like this genre of fiction, I highly recommend you obtain a copy."

That sounds like this month's selection for the Cosmic Log Used Book Club, which highlights books with cosmic themes that you should be able to find at your local library or used-book shop. Before he passed away, Finney wrote a sequel to the novel titled "From Time to Time."

Do you have additional recommendations for time-travel tales - or observations on the facts and fictions surrounding the flow of time and causality? Feel free to add your comments below.