Dark energy — a mysterious force pushing the universe apart at an ever-increasing rate — was discovered 26 years ago, and ever since, scientists have been searching for a new and exotic particle causing the expansion.
Pushing the boundaries of this search, University of California, Berkeley physicists have now built the most precise experiment yet to look for minor deviations from the accepted theory of gravity that could be evidence for such a particle, which theorists have dubbed a chameleon or symmetron.
The experiment, which combines an atom interferometer for precise gravity measurements with an optical lattice to hold the atoms in place, allowed the researchers to immobilize free-falling atoms for seconds instead of milliseconds to look for gravitational effects, besting the current most precise measurement by a factor of five.
Though the researchers found no deviation from what is predicted by the theory spelled out by Isaac Newton 400 years ago, expected improvements in the precision of the experiment could eventually turn up evidence that supports or disproves theories of a hypothetical fifth force mediated by chameleons or symmetrons.
The ability of the lattice atom interferometer to hold atoms for up to 70 seconds — and potentially 10 times longer — also opens up the possibility of probing gravity at the quantum level, said Holger Müller, UC Berkeley professor of physics. While physicists have well-tested theories describing the quantum nature of three of the four forces of nature — electromagnetism and the strong and weak forces — the quantum nature of gravity has never been demonstrated.
“Most theorists probably agree that gravity is quantum. But nobody has ever seen an experimental signature of that,” Müller said. “It’s very hard to even know whether gravity is quantum, but if we could hold our atoms 20 or 30 times longer than anyone else, because our sensitivity increases with the second or fourth power of the hold time, we could have a 400 to 800,000 times better chance of finding experimental proof that gravity is indeed quantum mechanical.”
Aside from precision measurements of gravity, other applications of the lattice atom interferometer include quantum sensing.
“Atom interferometry is particularly sensitive to gravity or inertial effects. You can build gyroscopes and accelerometers,” said UC Berkeley postdoctoral fellow Cristian Panda, who is first author of a paper about the gravity measurements set to be published this week in the journal Nature and is co-authored by Müller. “But this gives a new direction in atom interferometry, where quantum sensing of gravity, acceleration and rotation could be done with atoms held in optical lattices in a compact package that is resilient to environmental imperfections or noise.”
Because the optical lattice holds atoms rigidly in place, the lattice atom interferometer could even operate at sea, where sensitive gravity measurements are employed to map the geology of the ocean floor.
Screened forces can hide in plain sight
Dark energy was discovered in 1998 by two teams of scientists: a group of physicists based at Lawrence Berkeley National Laboratory, led by Saul Perlmutter, now a UC Berkeley professor of physics, and a group of astronomers that included UC Berkeley postdoctoral fellow Adam Riess. The two shared the 2011 Nobel Prize in Physics for the discovery.