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COURTESY OF MICHIO KAKU

A motion picture adaptation of Michael Crichton's time travel adventure story Timeline opens November 26. Crichton cites theoretical physicist Michio Kaku of the City University of New York as one inspiration for the science behind the story. Kaku, a string theorist, is the author of several physics books for a popular audience, including Hyperspace and Visions: How Science Will Revolutionize the 21st Century, and host of a weekly science radio show. He recently spoke with Scientific American.com about the possibility of time travel and his thoughts on science and popular culture. An edited transcript of that conversation follows.

Scientific American.com: How has speculating about time travel changed over the years?

Michio Kaku: About 10 years ago, if you were a serious physicist talking about time travel, you'd be laughed out of the scientific establishment. People would snicker behind your back, your scientific career would be ruined, and you wouldn't get tenure. In the last decade or so, there's been a sea change with regards to the scientific attitude toward time travel, and I think Michael Crichton picked that up. And I tried to convey that in my book Hyperspace. Originally, the burden of proof was on physicists to prove that time travel was possible. Now the burden of proof is on physicists to prove there must be a law forbidding time travel.

SA: When did scientists first start thinking about time travel in a rigorous way?

MK: In 1949 Einstein's colleague at Princeton was Kurt G¿del, one of the greatest logicians of the last thousand years. G¿del found a solution to Einstein's equations [of general relativity] in which the universe rotated. And if the universe rotated, then in a rocket ship, if you went around the universe, you would come back before you left.

Now Einstein was very troubled by this. The river of time, Newton thought, was straight and uniform; it never deviated, it always flowed at the same rate, and it carried everything in its way. Einstein comes along and says, "Not so fast, the river of time meanders, speeds up and slows down around stars and galaxies." The new wrinkle that G¿del showed in 1949 was that the river of time could have whirlpools. These are called "closed timelike curves." And in his memoirs, Einstein says that yes, these are solutions to his equations, but we can dismiss them on physical grounds: the universe expands; it doesn't rotate.

Then scientists looked back at earlier solutions to Einstein's equations and found that there were other solutions which also allow for time travel. In 1937 [W. J.] van Stockum took an infinitely long cylinder that was spinning like a maypole and [it was later found that] if you danced around the maypole you would come back before you left. In 1963 Roy Kerr, a mathematician, found that a spinning black hole collapses into a ring of compressed matter, not a dot. If you fall through the ring, you could wind up backwards in time or perhaps on another universe. The mathematicians call [such spaces] multiply connected spaces. The physicists call them wormholes. In the late 1980s Kip Thorne at Caltech and his colleagues found yet another class of Einstein's equations where these time machines were traversable. Like an elevator connecting parallel universes, these solutions have an up button and a down button. Under certain conditions, you can go through them easily, just like in the movies. You can look through the looking glass and then come back.

SA: Where would the wormhole come from in that case?

MK: We would get the wormhole by grabbing it from the vacuum, because they're everywhere. We think that at very small distances, 10^-33 centimeters, spacetime becomes foamy. The dominant structures at those quantum distances are probably wormholes, little bubbles, universes that pop into existence and then pop right back out of existence. Now if you could manipulate [the so-called] quantum foam, then you could go through one of these bubbles. And in Kip Thorne's original proposal for a time machine, he said that maybe we would obtain a wormhole by grabbing one of these bubbles and expanding it, stabilizing it with negative energy.