16 Mar 2017
Italian scientists dip deep for laser experiments to measure Earth’s rotational effects at greatest ever sensitivity.
The ring laser gyroscope (RLG) technology enabling these Earth-based measurements provide, unlike those made by referencing celestial objects, inertial rotation information, revealing fluctuations in the rotation rate from the grounded reference frame.
A group from the Italian National Institute for Nuclear Physics’ Laboratori Nazionali del Gran Sasso (LNGS) are researching the measurement of the gyroscopic precession Earth undergoes due to a relativistic effect called the Lense-Thirring effect.
This research program, called Gyroscopes in General Relativity (GINGER), would eventually use an array of such highly sensitive RLGs. For now, the team say they have successfully demonstrated its prototype, called “GINGERino”, and acquired many additional seismic measurements necessary to help achieve their aims.
The work has just been published in Review of Scientific Instruments, in which the group reports their successful installation of the single-axis GINGERino instrument inside the INFN's subterranean laboratory LNGS, and its ability to detect local ground rotational motion. Ultimately, GINGER aims to measure Earth’s rotation rate vector with a relative accuracy of better than one part per billion to observe the miniscule Lense-Thirring effects.
"The 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 and a researcher 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."
This pilot system is expected to reveal new information about geophysics. Belfi added, “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, within a few hours’ investigation time, to integrate with the less precise information of Earth’s changing rotation provided by global positioning system data and the astronomically-based measurements provided by the International Earth Rotation System, based in Germany.
“RLGs are essentially active optical interferometers in a ring configuration,” Belfi explained. “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 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 curiosity of the GINGERino installation is that we have intentionally located it within a high-seismicity area of central Italy,” Belfi said.
“With this humidity level, long-term operation of GINGERino’s electronics wouldn’t be viable,” Belfi said. “So we enclosed the RLG inside an isolation chamber and increased the internal temperature of the chamber via a set of infrared lamps supplied with a constant voltage.”
By doing so, the group was able to reduce the relative humidity to 60 percent. “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,” he said.