The rotating Earth very weakly drags the surrounding spacetime along with it. This prediction of Einstein's general theory of relativity is called the frame-dragging effect. Scientists from the University of California, Berkeley, the Austrian Academy of Sciences and the California Institute of Technology have proposed measuring this tiny effect with a quantum sensor based on superfluid helium. The work was published in the journal Physical Review Letters.
Superfluidity is a state of a liquid in which it flows without friction. Helium-4 becomes superfluid when cooled below 2.17 kelvin, that is, to approximately minus 271 degrees Celsius. The authors propose filling a ring-shaped tube with such helium and monitoring changes in the flow through a partition with nanopores, a so-called Josephson junction. When the device rotates, a quantum phase difference arises at the junction, which is registered by the motion of a mechanical membrane coupled to the superfluid circuit.
Similar sensors have already been created in Professor Packard's laboratory at Berkeley and have measured the Earth's rotation. However, the weak signal from spacetime dragging has so far been lost in thermal noise - random fluctuations associated with temperature. To assess the feasibility of the experiment, the researchers analyzed the thermal noise of the proposed device.
The calculations showed that if the sensor is cooled well below the superfluid transition temperature, for example to 10 millikelvin, it could match or surpass the best existing measurements of the frame-dragging effect in sensitivity, and in less time. Unlike previous experiments, such a test could be conducted in an Earth-based laboratory rather than in space.
In addition, according to the authors, the sensor would make it possible to record extremely small differences in the passage of time along different paths around the circuit - another way of testing general relativity. The researchers also suggest that the experiment could make it possible to observe a general-gravitational effect in a macroscopic quantum system, since Newton's theory of gravity does not predict frame dragging.
For now this is only a proposal. To implement it, Josephson junctions for superfluid helium-4 operating at millikelvin temperatures must be created in Keith Schwab's laboratory at Caltech. Theorists, meanwhile, are considering whether effects of quantum gravity or interactions beyond Einstein's theory might leave traces in such a device.