In Frequency, a wormhole swallows a communications satellite. Through that satellite, the character John Sullivan finds himself communicating over a ham radio to his father Frank, who is 30 years in the past.
If the movie has a veneer of plausibility, it is thanks to Brian Greene, a Columbia University physicist who served the production both as a consultant and an actor.
Greenes job was to define the rules of space-time travel for the movie, as much as possible within the known laws of physics. He declined to be interviewed for this article, saying he doesnt want to blur the line between the film and his academic work.
The movie draws on several refinements to the notion of time travel that are familiar to physicists.
For one thing, John and Frank exist in parallel universes, not the same one.
This helps avoid a paradox of time travel called the "grandfathers paradox." If a man went back in time and killed his grandfather, he would cancel his own existence.
The parallel universe idea is based on a hypothesis of some scientific repute, the "multi-verse" theory.
This theory says there is a separate universe for every possible event -- countless other universes, some of which also contain you, and which reflect alternate choices you could have made.
The wormhole theory also has scientific imprimatur: it comes from Albert Einstein.
He calculated that space and time are bound together as a single fabric and that a shortcut, or wormhole, could open between two places in space-time. This in principle might allow time travel.
So, it would seem a wormhole plus parallel universes allow time travel -- and bingo!
Not.
There are several problems with using wormholes for time travel, scientists say, though they acknowledge these might just possibly dissipate as our knowledge grows.
One problem: according to Kip Thorne, an eminent physicist from Cal Tech, the type of wormhole that Einsteins equations allow is so short-lived nothing could get through it.
The only way to hold the wormhole open long enough to allow time travel would be using "exotic matter" -- stuff with negative mass and energy.
A few scientists claim to have created minuscule amounts of this material. But by one calculation, human time travel would require using a chunk it comparable in mass to the planet Jupiter.
Victor J. Stenger, a professor of physics and astronomy at the University of Hawaii, adds that wormholes do not allow travel between universes, but only within one.
If the "multi-verse" theory is true, he said, then "you dont get to a parallel world by going through a wormhole. You get to a parallel world just by blinking your eyes, every day."
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