Scientists have proposed a tabletop device to detect the faint way Earth's daily spin twists the space-time around it, an effect predicted by Einstein's general relativity, Live Science reported. If it were built, the sensor could measure frame dragging with the same precision as the best satellite experiments, but in minutes instead of years, the researchers wrote in a study published Sept. 17 in Physical Review Letters. The work is a proposal, not a completed measurement. "It is impossible to synchronize that collection of clocks when you're in a rotating frame," first author Kai-Isaak Ellers, a physics graduate student at the University of California, Berkeley, told Live Science, describing a classic thought experiment of clocks on a spinning merry-go-round.
Einstein's theory predicts that a massive, spinning object drags space-time with it, like a spoon twirling in honey. The effect is obvious around a supermassive black hole and very faint around Earth. So far, scientists have measured Earth's frame dragging only from orbit. NASA's Gravity Probe B satellite, which carried four gyroscopes, confirmed the prediction in 2011 with a margin of error of about 18 percent. Satellites tracked with lasers have since done better. In July, researchers reported in Nature that the LARES-2 satellite had shrunk that margin to 0.2 percent, after about three years of tracking and after subtracting the much stronger tugs caused by Earth's equatorial bulge and uneven mass. A lab measurement, Ellers said, would also be free of some of the modeling assumptions those satellite experiments depend on.
The proposed device is a loop of superfluid helium-4, helium chilled until it flows without friction and behaves like a single quantum object, interrupted by a thin barrier riddled with tiny holes, a Josephson junction. Superfluid helium can slip through the holes; ordinary liquid helium cannot. "That's where the quantum behavior comes from," said co-author K. Birgitta Whaley, a quantum physicist at UC Berkeley. As Earth spins and drags space-time, it would nudge the helium's quantum state and shift the frequency at which it sloshes in the loop. Superfluid devices, many built at Berkeley, have detected Earth's rotation since the late 1990s, but earlier helium-4 versions had to work close to 2.17 kelvin, where leftover "normal" helium adds noise. The new paper analyzes how the device would behave a few thousandths of a degree above absolute zero. Below 0.7 kelvin, the team found, that noise drops sharply.
The catch is the barrier. Earlier devices used holes about 90 nanometers wide. The colder design would need holes about 1 nanometer across, roughly the size of a helium atom. Co-author Keith Schwab of Caltech and collaborators are developing such materials, but they have not been built into a working sensor yet. If they can be, the researchers calculate the device could reach 0.2 percent precision in about 35 minutes, or about one second if a vibrating membrane were made from the lowest-loss crystals, such as quartz or sapphire. The signal corresponds to a time difference of about 10 to the minus 35 seconds between helium flowing one way around the loop and the other. That is still a billion times larger than the Planck time, Whaley said. The paper does not claim the device already exists in the lab. It claims that, at millikelvin temperatures, the sensitivity could be there if the barrier can be made.