Time travelogue

Time travel has been a standard feature of science fiction, but never more so than today: The latest "Star Trek" and "Terminator" movies, as well as the TV series "Lost" and "Heroes," play off the classic paradoxes, and still more shows are on the way. In fiction, all it takes to travel back in time is a black hole or a flash of energy, with nothing more than a hand-waving explanation. If only real-life experiments in time and causality were that simple ...

It's easy to get your mind tied in a knot if you think too much about moving through loops of time - the sort of thing that inspires Heinlein short stories, "Twilight Zone" episodes, "Back to the Future" sequels and "Simpsons" parodies. To make things simple, let's just accept the first item in Cosmic Variance's Rules for Time Travelers: that there are no paradoxes.

This leads to three main conceptual avenues for time-travel plots:

• "Whatever happened, happened": The past can't be changed, even if you could try. Something would keep you from preventing Abraham Lincoln's assassination, and you might even end up helping John Wilkes Booth if you interfere. Famed physicist Stephen Hawking sides with this view, calling it the "chronology protection conjecture." And I suppose that, by implication, the future must take its course even if you could manage to foresee it. This is the approach suggested by "Lost," although there are hints that things may change next season.
• Many-worlds interpretation: This is the favorite way for Hollywood to do time travel. Why travel at all unless it makes a difference? But what happens to the chain of causality if you make a change - for example, killing your father before you were born? The many-worlds interpretation of quantum physics comes to the rescue, by suggesting there are a nearly infinite number of possible universes branching out from each moment. You can either stay in the changed past, or go back to the original or a different future. (But how do you choose? Maybe "The Simpsons" had it right after all.)
• "Change these shadows": The third possibility applies to visions of the future or the past, rather than actual travel back and forth in time. One of the practitioners of this concept was Charles Dickens, who provided a loophole for Scrooge when the Ghost of Christmas Yet to Come showed the old humbug his possible future. "Assure me that I may yet change these shadows," Scrooge pleaded. Such visions may turn out to be alterable or just plain wrong. This is the sort of thing you see in TV shows like "Life on Mars" and "Medium" - and "Dallas," for that matter.

We have a limited ability to anticipate and change the future, of course, but is it at all possible to change the past? In a sense, that's what University of Washington physicist John Cramer has been looking into.

For more than two years, he's been trying to set up an experiment that would test a phenomenon suggested by quantum mechanics: If you change the quantum state for one of two entangled photons, it might be possible to have that change reflected in photon No. 2 before you make the change in photon No. 1. The implications of the experiment are so intriguing that Cramer's fans contributed more than $35,000 to keep it going.

I've checked in with Cramer in 2007 and 2008 (twice!) to find out about his progress, and the latest is that the lab apparatus is still not right to do the experiment. The crystals that he originally planned to use have a "huge signal-to-noise problem," he told me today. The few entangled photons produced by the crystals are overwhelmed by stray photons that muck up the detection effort.

"Phase 1 and phase 2 [of the experiment] hit the wall, and we're about to start phase 3," Cramer said.

To get around the problem, Cramer is planning to switch to periodically poled crystals, which he said fellow quantum researcher Anton Zeilinger has used to produce millions of entangled photon pairs per second. Cramer still thinks his experiment is a "long shot," however. He suspects some factor will always prevent him from observing retrocausality in action.

For example, there appears to be a complementarity between quantum coherence and entanglement, he said. The more certain you are that the photons are really entangled, the less certain you are about the photons' quantum coherence. Both factors need to be nailed down in order to verify that retrocausality really, really works. "That could be the showstopper," Cramer said.

Until Cramer actually sets up the experiment with the new, improved crystals, he'll never know. But even if the experiment fails, it won't be a total loss. Because Cramer is a columnist and novelist as well as a physicist, that will just give him something to write about in his next book. Who knows? Retrocausality may soon be coming to a bookstore near you - or at least an alternate universe near you.

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