This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Einstein's theory of general relativity describes gravity as the bending of spacetime, a mathematical model that combines three-dimensional space and time into a single, four-dimensional continuum. The theory also predicts that a rotating object, such as Earth, slightly drags the surrounding spacetime as it rotates, a phenomenon known as frame-dragging.
Researchers at the University of California, Berkeley (UC Berkeley), the Austrian Academy of Sciences and the California Institute of Technology (Caltech) recently introduced the idea of a quantum sensor that could measure this tiny effect in a laboratory setting. Their proposed experiment, described in a paper published in Physical Review Letters, would use helium cooled until it enters a special phase known as the superfluid state inside a gyrometer, an instrument that measures rotation. "The inspiration for this project came when Birgitta Whaley and I were visiting Markus Aspelmeyer at the Institute for Quantum Optics and Quantum Information (IQOQI) in Vienna, Austria, during a sabbatical of Birgitta," Kai-Isaak Ellers, first author of the paper, told Phys.org.
"We were introduced to Marios Christodoulou, who was interested in new ideas for applications of superfluid helium in precision gravity experiments. Since Birgitta has a lot of background working on superfluidity in helium droplets and nanoaperture arrays and since I was working with Birgitta on ideas for tabletop gravity tests, it was a natural connection." When discussing the possibility of using superfluid helium for precision gravity experiments, the researchers realized that it could potentially be used to collect frame-dragging measurements. As Ellers, Whaley and Christodoulou are theorists, they contacted an experimental physicist who leads a lab at Caltech to explore their idea further.
"We reached out to Keith Schwab, who builds superfluid devices," said Ellers. "Keith knew Birgitta from his Ph.D. work at Berkeley in Richard Packard's group and was enthusiastic to work with us to make sure our proposal would be grounded in reality." The gravitational frame-dragging effect predicted by Einstein's general theory of relativity is backed by real-world observations. For instance, Earth's rotation drags the surrounding spacetime, contributing to a slow change in the direction of nearby gyroscopes' spin axes relative to distant stars, a motion called precession.
"Essentially, the Earth drags local gyroscopes along with it very slightly as it spins," explained Ellers. "One can detect this tiny frame-dragging effect in several ways. In our paper, we propose a rotation sensor, or gyrometer, based on superfluid helium." When it is cooled below 2.17 Kelvin, about −271°C (−456°F), helium-4 (the most common form of helium) becomes a superfluid.
This is essentially a liquid that can flow without friction. The researchers propose creating a rotation sensor by filling a doughnut-shaped tube with superfluid helium-4, then detecting changes in its flow via a specially designed barrier with tiny holes or nanopores (i.e., a Josephson junction). "The experimental signature is a quantum mechanical phase difference that appears across the Josephson junction when the device is rotating," said Ellers.
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