At the subatomic scale, where the universe is jumpy and discontinuous, physicists don't know how gravity behaves. They theorize that space and time could collapse.
"We may discover some new laws of physics that could change the rules," said Richard Gott, an astrophysicist at Princeton University.
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Relativity theory does not allow for travel into the past. But such travel could possibly be achieved using Einstein-Rosen bridges, better known as wormholes.
The theoretical shortcuts through space and time connect two distant points in space, like a worm tunnel through an apple.
Kip Thorne, a gravitational theorist at the California Institute of Technology in Pasadena, showed in 1988 that these tunnels could be kept open by an exotic form of matter known as Casimir energy.
This energy, which has been measured in a laboratory, is a sort of quantum vacuum. Weighing less than zero, Casimir energy would have an anti-gravitational effect, keeping the wormhole's walls apart.
"It has been conjectured that general relativity prohibits the existence of negative energy densities, [but] quantum mechanics demonstrates that the vacuum may not always have a zero energy density," Lobo said.
Gott, the Princeton scientist, envisions the wormhole effect as being like that of a mirrored garden ball. When looking through the wormhole, however, one would not see a reflection of that same garden, but instead a garden on, say, Alpha Centauri, the star closest to our solar system.
"You can jump through the wormhole and pop out on Alpha Centauri, and you've just gone through a very narrow tunnel that connects these two very distant places," Gott said. "The shortcut of the wormhole allows you to beat a light beam to Alpha Centauri."
If one opening of the wormhole was moved around using the gravitational effect of a spaceship traveling at a speed close to light, a clock at the opening would run slow compared to one on the other end of the wormhole. This could turn the wormhole into a portal between two different times, past and future.
"I should point out that these wormholes are not something you put in your kitchen. Each mouth weighs 100 million solar masses," Gott said, referring to a unit of measurement equal to the mass the sun. "This is a galactic-scale engineering project at best."
But the material needed to construct and support wormholes has been given a boost from the recent discovery that the universe is undergoing an accelerated expansion, says Lobo, the University of Lisbon scientist.
A possible driver of this cosmic expansion, he says, is "phantom energy," a hypothetical matter that may comprise as much as 70 percent of the universe.
Phantom energy may be pushing space apart and is so anti-gravitational that it will eventually rip everything apart, ending everything. However, before then, it could be used to prop open wormholes, Lobo theorizes.
"In a rather speculative scenario, one could imagine an absurdly advanced civilization mining the cosmic fluid for phantom energy necessary to construct and sustain a traversable wormhole," he said.
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