February 17, 2010 | 14
Albert Einstein’s theory of general relativity makes a number of counterintuitive predictions about the workings of gravity, and experimentalists nearly 100 years after the theory was developed continue to confirm those predictions with increasing accuracy. A new paper co-authored by U.S. Energy Secretary Steven Chu measures the gravitational redshift, illustrated by the gravity-induced slowing of a clock and sometimes referred to as gravitational time dilation (though users of that term often conflate two separate phenomena), a measurement that jibes with Einstein and that is 10,000 times more precise than its predecessor.
To confirm the nature of the gravitational redshift, researchers in 1971 flew atomic clocks in commercial airliners; in 1980 another group launched a device to an altitude of 10,000 kilometers on a rocket. The new research, a repurposing of a decade-old experiment, takes a more down-to-Earth approach, tracking cesium atoms, whose oscillations act as a sort of atomic clock, launched on vertical trajectories differing by only 0.12 millimeter.
As reported in the February 18 issue of Nature by Holger Müller of the University of California, Berkeley, and Lawrence Berkeley National Laboratory, Achim Peters of Humboldt University of Berlin and Chu, cesium atoms, which oscillate with a characteristic frequency, are shot through a gauntlet of laser beams. (Scientific American is part of Nature Publishing Group.) The first beam diverts the atom onto one of two paths, one being slightly higher than the other, with equal probability. A second and third laser act to reunite the paths [see diagram above], producing what is known as an atom interferometer.
In quantum-mechanical terms, the cesium atoms can be thought of as a matter wave. The first laser splits the wave into two waves propagating simultaneously along both paths that the atom could take, while the third and final laser splices them back together into a single wave. If the wave components remain unchanged along their respective paths, the wave will emerge whole once it is recombined. But if one of the waves is waylaid on its path, it will be out of phase with its counterpart, yielding destructive interference when the two are recombined. By monitoring the interference after recombination, the researchers can trace the phase difference induced by the waves traveling along different paths.
In the case of the cesium atoms, the researchers found that the waves traveling along the two trajectories oscillated a different number of times. That is just what is predicted by general relativity, which holds that clocks close to Earth tick more slowly than those at higher elevations—even a fraction of a millimeter higher in elevation.
Image credit: Nature