A Tabletop Laser Test Improves Accuracy of Einstein’s Gravitational Redshift

17. February 2010

By Patricia Daukantas

A trio of scientists – including the current U.S. secretary of energy, OSA Honorary Member Steven Chu – has verified one prediction of Albert Einstein’s general theory of relativity with 10,000 times more accuracy than previous experiments yielded.

The scientists used a cesium atom interferometer to test the gravitational redshift predicted by Einstein. The results appear in the Feb. 18, 2010, issue of Nature.

The scientists obtained the data for this week’s article more than 10 years ago, when Chu was still at Stanford University (U.S.A.). At that time, Chu, who shared the 1997 Nobel Prize in physics for developing laser-cooled atom traps, and his colleagues were measuring the acceleration of gravity, according to a news article in Nature (subscription required). One of Chu’s then-graduate students, Achim Peters, now heads the optical metrology group at Humboldt University in Berlin (Germany).

Much later, a former student of Peters who also worked in Chu’s Stanford group -- Holger Müller, now an assistant physics professor at the University of California, Berkeley (U.S.A.) – realized that the data from the late-1990s experiment might yield a high-precision test of gravitational redshift. He contacted Chu, who has been juggling his work on the study with his leadership of the Cabinet-level department with a $24.1-billion budget.

The part of Einstein’s theory that Müller, Peters and Chu tested involves the slowing down of clocks in a gravitational potential U by a factor of 1 + U/c2, where c is the speed of light. (That “slowing down” is relative to a clock outside the gravitational potential, of course – everything’s relative!) The previous experiment, which involved taking a hydrogen maser clock on a rocket ride in 1976, found that Einstein’s prediction held true to a precision of 7 × 105. By studying the rise and fall of cesium atoms in a laser trap and measuring the slight phase differences among the frequencies of the atoms, the researchers improved the accuracy of the redshift prediction to 7 × 10–9. 

For more information, read the Berkeley press release here.

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