A group of researchers have developed a prototype of the ring laser gyroscope (RLG) technology and would be the first to experimentally prove the Lense-Thirring Effect outlined in Einstein’s theory of general relativity.
This effect, also known as frame dragging, is where the orbit of a small body orbiting around a much larger rotating body, is slightly altered by the rotation. The effect was first predicted by Austrian physicists Joseph Lense and Hans Thirring.
The possible detection of frame dragging around neutron stars has been reported by Italian astronomers in 1997. Stronger experimental confirmation of this effect was made using black hole observations by Nasa's Rossi X-Ray Timing Explorer spacecraft.
Einstein’s general relativity predicts the value of this effect near the earth, but the effect has not yet been accurately verified experimentally.
The INFN program, called Gyroscopes in General Relativity (Ginger), would eventually use an array of such highly sensitive RLGs embedded deep underground to experimentally prove this effect. For now, they have successfully demonstrated its prototype, Gingerino, and acquired a host of additional seismic measurements.
Prototype success
The researchers from the Italian National Institute for Nuclear Physics’ (INFN) Laboratori Nazionali del Gran Sasso (LNGS) have installed the prototype single-axis Gingerino instrument inside the INFN's subterranean laboratory LNGS. It has the ability to detect local ground rotational motion, according to the researchers.
Their aim is to measure Earth's rotation rate with a relative accuracy of better than one part per billion to see the miniscule Lense-Thirring effects.
"This effect is detectable as a small difference between the Earth's rotation rate value measured by a ground based observatory, and the value measured in an inertial reference frame," said Jacopo Belfi, lead author working for the Pisa section of INFN.
"This small difference is generated by the Earth's mass and angular momentum and has been foreseen by Einstein's general theory of relativity. From the experimental point of view, one needs to measure the Earth rotation rate vector with a relative accuracy better than one part per billion, corresponding to an absolute rotation rate resolution of 10-14 [radians per second]."
To take these types of sensitive measurements the system must be buried underground and away from external disturbances - from moving water in the ground, temperature or barometric pressure changes.
This prototype is expected to reveal unique information about geophysics, but, according to Belfi, "underground installations of large RLGs, free of surface disturbances, may also provide useful information about geodesy, the branch of science dealing with the shape and area of Earth."
The ultimate goal for Gingerino is to achieve a relative precision of at least one part per billion, to integrate with the less precise information of Earth's changing rotation provided by global positioning system data and the astronomically based measurements of the International Earth Rotation System.
"RLGs are essentially active optical interferometers in ring configuration," Belfi said. "Our interferometers are typically made of three or four mirrors that form a closed loop for two optical beams counter propagating along the loop. Due to the Sagnac effect, a ring interferometer is an extremely accurate angular velocity detector. It's essentially a gyroscope."
The group's approach has enabled the first deep underground installation of an ultrasensitive large-frame RLG capable of measuring the Earth's rotation rate with a maximum resolution of 30 picorads/second.
"One peculiarity of the Gingerino installation is that it's intentionally located within a high seismicity area of central Italy," Belfi said. "Unlike other large RLG installations, Gingerino can actually explore the seismic rotations induced by nearby earthquakes."
Promising research
Professor Ulrich Schreiber, from the satellite geodesy research facility at the Technical University of Munich, Germany, who was not involved in the research, said that if successful, this kind of experiment would be one of the “very few that are in the position to challenge the theory of General Relativity”.
However, he added that while the technology is viable for the purpose, that the Gingerino should be considered a “pathfinder”.
“Since the Lense-Thirring effect is a small DC quantity, a perfect understanding of all involved offsets is required, which is no mean feat,” explained Schreiber. “The challenge ahead is also to improve the sensitivity of the instrument to pick up such a small signal.”
Bias stability and sensitivity are the most important factors to overcome in this research, said Schreiber, and these will not be easy to overcome. “Sensitivity comes with upscaling and upscaling compromises the stability. It really requires a major effort to find the right solution,” Schrieber added.
During Gingerino’s installation the researchers encountered difficulties controlling the natural relative humidity, which was above 90%.
"With this humidity level, long-term operation of Gingerino's electronics wouldn't be viable," Belfi said. To solve this problem they enclose the RLG inside an isolation chamber and increased the internal temperature of the chamber using a set of infrared lamps supplied with constant voltage. This dropped the relative humidity down to 60%.
"It didn't significantly degrade the natural thermal stability of the underground location, which allows us to keep Gingerino’s cavity length stable to within one laser wavelength (633 nanometers) for several days," Belfi said.
Gingerino is now operating, along with seismic equipment provided by the Italian Institute of Geophysics and Volcanology, as a rotational seismic observatory.
"Gingerino and one co-located broadband seismometer makes it possible to retrieve, via a single station, information about the seismic surface wave's phase velocity that in standard seismology requires using large arrays of seismometers," said Belfi.