General relativity tested on a tabletop
Robin
measurement.
Eric Hand
By measuring a spectacularly small difference in the ticks of two
quantum clocks, physicists have proven a pillar of Albert Einstein's
theory of gravity to be on firmer footing than ever before.
The experiment is the latest in a series of tests in which scientists
have scrutinized one of Einstein's more profound predictions: that
clocks in stronger gravitational fields run more slowly. For decades
they have put clocks at higher elevations, where Earth's gravity is
slightly weaker, and measured the ensuing changes. From a clock in a
tower at Harvard University in Cambridge, Massachusetts, in the 1960s,
to others flown on planes in the 1970s, to a clock that flew thousands
of kilometres into space on a rocket in 1980, physicists have not been
able to show that Einstein was wrong.
Now, a team led by Holger M�ller of the University of California,
Berkeley, has measured the time-shifting effects of gravity 10,000 times
more accurately than ever before. They show that gravity's effect on
time is predictable to 7 parts per billion (H. M�ller, A. Peters and S.
Chu Nature 463, 926�929; 2010). And they did it using two laboratory
clocks with a height difference of just 0.1 millimetres � a set-up that
seems quaintly small in this day of big physics. "Precision experiments
on a tabletop are not something of the past," says M�ller, whose
research team consisted of Achim Peters of the Humboldt University of
Berlin and Steven Chu, the US Secretary of Energy.
Many atomic clocks use the extremely regular pulsations of atoms
shifting between excited energy states. But M�ller's apparatus relied on
the fundamental quantum frequency of a caesium atom associated with the
atom's rest energy. This frequency was so high that physicists never
thought to use it as a clock. But a special interferometer could measure
the difference between two such clocks experiencing gravity's effect.
"What's fascinating about their work is that they were using the entire
atom as a clock," says atomic-clock expert Jun Ye of the Joint Institute
for Laboratory Astrophysics in Boulder, Colorado.
M�ller and his team shot caesium atoms, cooled nearly to absolute zero,
in an arc across a gap. Mid-stream, photons from a laser bumped the
atoms into two, quantum-mechanical alternate realities. In one, an atom
absorbed a photon and arced on a slightly higher path, experiencing a
tiny weakening of gravity and speed-up of time. In the other, the atom
stuck to the lower path, where gravity was stronger and time moved
slightly more slowly. A difference in phase in the atom's fundamental
frequency, measured by the interferometer, indicated a tiny difference
in time.
Laser traps
The experiment takes advantage of the laser atom trap, for which Chu won
a Nobel prize in 1997. The data for the current study were obtained
shortly after that, when Chu was using the set-up to measure a different
constant, the acceleration of gravity (A. Peters, K. Y. Chung and S. Chu
Nature 400, 849�852; 1999).
But M�ller says that in October 2008, he had an epiphany that the same
data could be used to show the constancy of gravity's effect on time. He
e-mailed Chu, then the director of the Lawrence Berkeley National
Laboratory in Berkeley, California, who responded three days later
saying it was a good idea.
Chu says in an e-mail that he found time to work on the current study
during nights, weekends and on planes � after putting in 70�80-hour
weeks as energy secretary. "I like juggling a lot of balls," he says.
The result could one day have practical applications. If gravity's
time-shifting effect were not constant, then researchers might have had
to worry about the accuracy of new atomic clocks as they are flown into
orbit on Global Positioning System (GPS) satellites. But M�ller has
demonstrated the effect to be extraordinarily consistent. "Now we know
that the physics is fine," he says.
The test also puts pressure on the Atomic Clock Ensemble in Space
(ACES), an experiment being run by the European Space Agency that is due
to be attached to the International Space Station in 2013. The current
study already betters ACES's planned measurement of gravity's
time-shifting effect by almost three orders of magnitude. ACES's
principal investigator Christophe Salomon says that the mission will
cost about �100 million (US$136 million), plus the cost of a launch
rocket. By comparison, M�ller says that his tabletop apparatus cost much
less than $1 million. Salomon says that ACES is still justified because
it will perform two other fundamental physics tests, as well as help
researchers to improve the coordination of ground-based atomic clocks.
Physicist Clifford Will of Washington University in St Louis, Missouri,
says that M�ller's result narrows the window for the alternative
theories of gravity that some theorists are exploring. Will was also
impressed that Chu found time to contribute to the study. "When was the
last time that a sitting member of the president's cabinet had a paper
in Nature on fundamental physics?" he asks.
