Atom experiment tests Einstein's redshift

While aircraft and rocket experiments have proved that gravity makes clocks tick more slowly—a central prediction of Albert Einstein's general theory of relativity—a new experiment has measured this slowdown 10,000 times more accurately than before, and finds it to be exactly what Einstein predicted.

The result shows once again how well Einstein's theory describes the real world, said Holger Muller, an assistant professor of physics at the University of California, Berkeley.

"This experiment demonstrates that gravity changes the flow of time, a concept fundamental to the theory of general relativity," Muller said.

The phenomenon is often called the gravitational redshift because the oscillations of light waves slow down or become redder when tugged by gravity.

A report describing the experiment appears in the February 18 issue of the journal Nature.

Pushing atoms with lasers

Muller tested Einstein's theory by taking advantage of a tenet of quantum mechanics … that matter is both a particle and a wave. The caesium atoms used in the experiment can be represented by matter waves that oscillate nearly a million billion billion times per second.
When the caesium atom matter wave enters the experiment, it encounters a carefully tuned flash of laser light. The laws of quantum mechanics step in, and each caesium atom enters two alternate realities, Muller said. In one, the laser has pushed the atom up one-tenth of a millimetre, giving it a tiny boost out of Earth's gravitational field. In the other, the atom remains unmoved inside Earth's gravitational well, where time flies by less quickly.

While the frequency of caesium matter waves is too high to measure, Muller and his colleagues used the interference between the caesium matter waves in the alternate realities to measure the resulting difference between their oscillations, and thus the redshift.

The equations of general relativity predicted precisely the measured slowing of time, to an accuracy of about one part in 100 million—10,000 times more accurate than the measurements made 30 years ago using two hydrogen maser clocks, one on Earth and the other launched via rocket to a height of 10,000 kilometres.

Practical benefits

Far from merely theoretical, the results have implications for Earth's global positioning satellite system, for precision timekeeping and for gravitational wave detectors, Muller said.

"If we used our best clocks, with 17-digit precision, in global positioning satellites, we could determine position to the millimetre," he said. "But lifting a clock by one metre creates a change in the 16th digit. So, as we use better and better clocks, we need to know the influence of gravity better."

Muller also noted that the experiment demonstrates very clearly "Einstein's profound insight, that gravity is a manifestation of curved space and time, which is among the greatest discoveries of humankind."

This insight means that what we think of as the influence of gravity—planets orbiting stars, for example, or an apple falling to Earth—is really matter following the quickest path through spacetime. In a flat geometry, the quickest route is a straight line. But in Einstein's theory, the flow of time becomes a function of location, so the quickest path could now be an elliptical orbit or a plumb line to the ground.