Einstein Unruffled: Relativity passes stringent new test

By Davide Castelvecchi
From Science News, November 24th, 2007; Vol.172 #21 (p. 324)

By tracking the moon’s location to within 1 centimeter, astronomers have put general relativity, Albert Einstein’s theory of gravity, to a stringent new test. The theory stood up. In a separate experiment, physicists reconfirmed Einstein’s older predictions on the stretching of time.

While both general relativity and quantum theory so far fit experimental data very well, their incompatibility makes physicists believe that at small scales either one of them or both must be wrong. Scientists constantly work to improve the sensitivity of their experiments to violations that might point to a new “theory of everything.”

Astronomers at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., have now performed a new analysis of 35 years’ worth of data on the moon’s distance from Earth, including data they recently collected themselves with centimeter precision. The data tracked the time it took for a laser beam to reach a mirror on the lunar surface—left behind by the Apollo 11 astronauts—and bounce back to Earth. The results, described in an upcoming Physical Review Letters, confirmed one of general relativity’s cornerstones: The laws of gravity are the same in all frames of reference.

The results place some strict requirements on any conceivable theory of everything, says team member James Battat.

Last year theorists Alan Kostelecký and Quentin Bailey of Indiana University in Bloomington, calculated the implications for gravity of a broader theory developed by Kostelecký. Called the standard-model extension, it represents the most general form that a theory of everything must have to fit the best known data. It involves more than 200 parameters—like so many knobs physicists can turn to change the equations underlying general relativity and quantum theory.

For example, some of the knobs would change Einstein’s equations, Kostelecký says. “You don’t have E = mc2 anymore,” he says as an example. “You have E = mc2 plus a little bit”—a large number of tiny, hypothetical correction terms. The Harvard-Smithsonian results found no deviation from relativity for six of those knobs, with precisions ranging from one part in a million to 1 part in 100 billion.

Separately, in an upcoming Nature Physics, physicists confirm Einstein’s prediction that an object moving at a high speed appears to have its own flow of time slowed down. At the Max Planck Institute in Heidelberg, Germany, the researchers shot a beam of lithium ions at up to 6 percent of the speed of light and hit them with two laser beams. The laser frequency was tuned to excite some of the ions’ electrons. To do so, the physicists had to make a small correction in the laser tuning, since the speeding ions’ dilated sense of time affected their reaction to the frequency of the lasers’ waves.

The precise measurement of this correction nailed down another knob with a three-fold improvement in sensitivity over the team’s own 2003 results, says team member Gerald Gwinner, now at the University of Manitoba in Winnipeg.

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