Miércoles 15 de Junio de 2005, Ip nº 113

Wormhole wanderers face a deadly dilemma
Por Maggie McKee

Would-be wormhole travellers may have to choose between danger and unpredictability for their journeys through space-time, a new study suggests. The research may spell doom for time machines, but it suggests the universe will survive to a ripe old age instead of being ripped apart by a particularly repulsive form of dark energy.

As fans of science fiction know, wormholes provide short-cuts through space and time, sucking in objects at one end and spitting them out at the other. The distance from one point to the other would be much shorter than conventional travel across the universe.

A useful analogy in helping to visualise the wormhole phenomenon is to imagine a sheet of paper - which represents the universe - which is then folded neatly in half. Next, near the edge furthest from the fold, a pin is pushed through the paper. This creates a “wormhole” connecting two distant points in the universe.

But for this trick to work in space-time, the hypothetical tunnels are suggested to be coated with an unknown form of matter. This "exotic" matter exerts negative pressure - if a balloon were being filled with the stuff, it would deflate.

Now, physicists Roman Buniy and Stephen Hsu at the University of Oregon in Eugene, US, have studied the properties of such matter in two theoretical wormhole types. The first type mainly obeys the laws of classical physics and does not fluctuate in time, while the second follows quantum mechanical rules and therefore carries with it inherent uncertainties.

These uncertainties mean someone is not guaranteed to come out at a given location in space or time after every use of the quantum wormhole. "The danger is the endpoint of the wormhole which, if it is fluctuating around unpredictably, might be in a wall or under the Pacific Ocean," Hsu says. "Alternatively, you might exit a year before or after you thought you would."

Rocks and hard places
So researchers had believed classical wormholes could serve as more practical portals through space-time. But in a paper recently published on an online preprint server, Buniy and Hsu show these classical objects are inherently unstable.

"If someone nudged the apparatus a little, it would cause the system to fall apart, the way a bridge would collapse," Hsu told New Scientist. "It would probably not last long enough for you to get through to the other side."

That puts hopeful wormhole travellers between a rock and a hard place, forcing them to choose between reliability and (relative) safety for their mode of transportation, says Hsu. But the newly discovered instability actually has positive implications for the fate of the universe.

Their work suggests that the exotic matter hypothesised to fill the throats of wormholes shares similar properties with the dark energy that appears to be speeding up the expansion of the universe. Observations offer a range of possible values for this dark energy, but Hsu says the new research shows most of these values - which produce the highest rates of acceleration - are unstable.

That suggests the universe will not end in a "big rip" in which quickening cosmic acceleration will eventually tear apart galaxies, stars, and even atoms, says Hsu. "If the dark energy has properties that would lead to a big rip, it is likely to fall apart due to instability long before we get anywhere near the rip," he says.

  24/05/2005. New Scientist Magazine.