Published online 17 June 2010 | Nature | doi:10.1038/news.2010.303

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Delicate super-cold experiment falls for science.

The canister containing the experiment is dropped down a 146-metre-high tower in Bremen, Germany. Science

It is any researcher's nightmare: you spend months setting up a delicate experiment and then, just as it is about to take data, you drop it.

That's exactly what a team of German scientists did again and again in an effort to understand gravity better. Their nearly indestructible experiment, the results of which are published in this week's Science1, could soon lead to ultra-sensitive tests of gravity or better sensors for spacecraft.

Gravity is mercilessly impartial — on Earth, it accelerates heavy and light objects alike with a tug of 9.8 metres per second squared. That property is the cornerstone of Albert Einstein's theory of general relativity, which states that gravity is indistinguishable from any other type of acceleration. But some physicists wonder whether gravity's tug might be fractionally different on objects of different mass, or whether it might change its behaviour at short distances — such as those at which the rules of quantum mechanics come into play.

The problem is that any deviation would be miniscule. Current experiments, which use twisting pendulums or precise laser measurements of the changing distance between Earth and the Moon, can detect incredibly small variations in gravity's expected tug. So far, no changes have been seen.

Physicist Ernst Rasel of the Leibniz University of Hanover in Germany and his colleagues want to detect even smaller gravitational variations. The idea is to chill a cluster of atoms to a temperature that is within a fraction of absolute zero. At that extreme, the atoms all assume the same quantum-mechanical state and begin to behave collectively as a sort of super-atom, known as a Bose-Einstein condensate (BEC).

Researchers would like to split the super-atom cloud, send each half along a separate path and then recombine them. When the halves join, they generate a characteristic interference pattern of light and dark fringes, just like the pattern that a laser beam generates when it is split and recombined. Researchers might be able to use that interference pattern to detect tiny differences in gravity's behaviour, and also to see how it acts on a quantum-mechanical object.

Research in free fall

It's an ingenious plan with two major problems: first, the super-cold atom clouds are extraordinarily hard to make. Second, the best way to test gravity is to make sure that no other forces are acting on an experiment. Short of launching it into orbit, the best way to do that is to drop the whole experimental apparatus so that it goes into free fall.

Incredibly, Rasel and his team have now licked both problems. They devised a special self-contained canister that can automatically generate a BEC. They then dropped the canister from the 146-metre-high drop tower at the Center of Applied Space Technology and Microgravity in Bremen, Germany.

"I was very worried," Rasel says of the moments before his team first dropped their experiment. "It was coming towards the end of a PhD thesis of a student," he adds, explaining that it would have caused serious problems if anything went wrong.

But the canister worked perfectly. Rasel and his team carried out 180 drops and were able to measure multiple atom clouds over about a second of free fall, a considerable time for such a tiny object.

Redefining gravity

"It's a marvellous technical accomplishment," says Eric Adelberger, a physicist at the University of Washington in Seattle, who conducts gravitational measurements with twisting pendulums known as torsion balances. Rasel's team has not yet split the cloud in two to make gravitational measurements, but assuming that it can be done, it will rival other techniques, he says. "They can measure extremely tiny acceleration differences, ones that are much smaller than a torsion balance is likely to do."

Rasel says he would eventually like to put a version of his experiment into continuous freefall by launching it into orbit around the earth. Such an experiment would be able to conduct numerous gravity measurements with very high-precision, he says. But first, the team will have to overcome some lingering but serious difficulties. Most immediately, Rasel says that the team will have to get rid of stray magnetic fields that are disturbing the atom cloud.

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BECs could be useful for more than just science experiments, adds Dana Anderson, a physicist at JILA on the campus of University of Colorado at Boulder. Because ultra-cold matter is so sensitive to jolts and jiggles, atom clouds could make supersensitive sensors for inertial guidance systems. Although the Global Positioning System is the main way aircraft navigate day-to-day, inertial guidance is still an important back-up for when GPS is not available because of jamming or an outage. "Nearly every vehicle that's in the air has an inertial guidance system," Anderson says.