By Eric Hand of Nature magazine

Can the time-warping ways of Einstein's theory of general relativity be measured by the quantum 'ticking' of an atom? In 2010, researchers at the University of California, Berkeley, claimed in Nature that they had used an inexpensive table-top apparatus to show how gravity had altered a fundamental oscillation of two atoms.

But a group of French researchers now say that these atomic oscillations don't work like clocks at all. "We found that these claims cannot be supported," says Luc Blanchet, a theorist at the Astrophysical Institute of Paris. Blanchet and his colleagues publish a critique today in Classical and Quantum Gravity, alongside another paper that independently criticizes the Berkeley researchers.

For decades, researchers have been putting general relativity to the test. The theory has held up so far, but any deviation from expectations, however small, could point to an overhaul of physics.

One much-tested aspect of relativity, 'gravitational redshift', explains how clocks run faster at higher elevations where gravity is weaker--an effect that has to be accounted for in the operation of global positioning system (GPS) satellites. The tests for this effect, using clocks in towers, airplanes and rockets, have become increasingly precise.

But the Berkeley researchers garnered acclaim by doing the experiment on a table-top, with their 'clocks' separated by a height of just 0.1 millimeters (see 'General relativity tested on a tabletop'). Nobel laureate Steven Chu, the US Secretary of Energy, was a co-author, which also helped to raise the paper's profile. The French team, however, is hitting back with a Nobel prize-winner of it own: Claude Cohen-Tannoudji, who shared the 1997 physics prize with Chu.

Clocking off

The debate comes down to whether a fundamental atomic oscillation, based on the rest mass of a cesium atom, can be used as a clock. The table-top setup relied on an atom interferometer, which tracked the offset in oscillations, or phase difference, of the cesium atoms as they flew on paths of marginally different heights. But Blanchet's team argue that the phase difference between any two atoms due to the fundamental oscillation will always be zero, and therefore could never be used to detect a gravitational redshift.

They say that the Berkeley researchers were instead using their interferometer as an accelerometer to measure a different aspect of general relativity: the universality of free fall. That is no less interesting in its own right, but it has already been tested to greater levels of precision.

Berkeley physicist Holger Müller, lead author of the Nature paper, stands by his claim, and, in April, published a paper further explaining his team's approach. He says that the teams are engaged in semantic disagreements over what defines a clock. He is already at work on a new table-top setup that will eliminate the accelerating effect of gravity. If his cesium atoms still accumulate a phase difference, then it would be due to the gravitational redshift, not the universality of free fall.

Clifford Will, who studies general relativity at Washington University in St Louis, Missouri, says he thinks Blanchet and his team currently have the upper hand. "Their argument to me seems completely convincing," says Will, who was not one of the reviewers of the critiques, but was familiar with them as editor of Classical and Quantum Gravity.

If the critiques of the French team's findings hold up, they could bolster the mission goals of the Atomic Clock Ensemble in Space (ACES), a European Space Agency experiment that is slated to join the International Space Station in 2014. The experiment, which will perform important synchronizations of atomic clocks around the world, is also supposed to test the gravitational redshift effect--but is capable of far less precision than the table-top experiment. If the Berkeley results end up being as ghostly as the quantum oscillations on which they rely, the ACES team could take the lead as being Einstein's best clock-keepers.

This article is reproduced with permission from the magazine Nature. The article was first published on June 9, 2011.