MANY physicists are convinced that we are surrounded by extra dimensions as big as a few tenths of a millimetre wide. But so far, their most ingenious experiments have failed to either confirm their existence or rule them out.
But now an Icelandic physicist, Steinn Sigurdsson, says he knows how to detect them. His idea is to take advantage of the strange state of matter known as a Bose-Einstein condensate, in which clumps of a million ultra-cold atoms behave as a single entity.
Extra space dimensions are a prediction of leading theories that attempt to unify all the known forces into one overall “theory of everything”, such as superstring theory. In some theories these extra dimensions are curled up very small – fantastically smaller than the width of a proton. But in some versions of superstring theory they are relatively large: perhaps as big as a fraction of a millimetre across.
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Gravity is thought to be the only force that pervades these extra dimensions, which is why they are so hard to detect. If large extra dimensions do exist, then the gravitational pull between two tiny objects less than about a millimetre apart would be hugely stronger than Newton’s theory predicts. But the gravitational pull is still tiny, making it very hard to measure over such small distances.
The best attempt so far measured the pull between two masses that were just over 0.2 millimetres apart, and found that it was no stronger than conventional theory predicts.
Now Sigurdsson, who works in the US at Pennsylvania State University, says the experiment could be run on even smaller scales by using Bose-Einstein condensates (). He suggests letting two BECs in free fall pass on either side of a thin horizontal glass cylinder 20 micrometres in diameter and filled with a dense substance such as mercury, lead or gold.
“If there exists an extra dimension between a hundredth and a tenth of a millimetre in extent, then as the condensates pass the cylinder they will be pulled towards it with a gravitational force a hundred times bigger than expected,” says Sigurdsson.
The key thing that makes this measurable is that each BEC behaves like a single atom with a single wavelength. So if the two are later combined, they form an interference pattern. When the BECs fall past the mass, the peaks and troughs of the interference pattern will be shifted, with the size of the shift depending on the strength of the gravitational pull. “The shift should be measurable, making it possible to deduce the strength of gravity,” says Sigurdsson.
The effect will be stronger and easier to measure if the free-falling BECs spend as long as possible close to the cylinder. So Sigurdsson proposes doing the experiment on the International Space Station, where the BECs would fall past the cylinder extremely slowly. “In space, the effect should be 10,000 times greater than on Earth, and correspondingly easier to measure,” he says.
“It’s well worth doing this kind of experiment,” says England’s Astronomer Royal, Martin Rees of Cambridge University. “It’s excellent that this fundamental work can be done without vast teams.”