An Underground Instrument Will Measure How Much Earth Drags Spacetime

The instrument to measure frame-dragging is in a lab underneath Gran Sasso, pictured. Canadastock/shutterstock

From general relativity, we know that mass bends spacetime. However, if the object is rotating, then that also drags spacetime. This phenomenon, known as frame-dragging or the Lense-Thirring effect, is very important in relativity, and it was only proven a few years ago.

Now, a new experiment plans to study frame-dragging with higher precision, but it requires something a bit unusual – the instrument needs to be buried underneath a mountain. The site is located 1,400 meters (almost a mile) deep inside the Gran Sasso, the tallest mountain in central Italy.

Italian physicists from the National Institute for Nuclear Physics (INFN) have tested a prototype for the program GINGER (Gyroscopes in General Relativity) and their results are published in the Review of Scientific Instruments. The system will be composed of several ring laser gyroscopes (RLGs), a series of sophisticated spinning tops that aim to measure the rotational rate of Earth to a precision higher than a part per billion.

When that is achieved, researchers should be able to detect the small Lense-Thirring effect of our planet. With the instrument deep within a mountain, it guarantees that changes in atmospheric pressure and disturbances due to human activities don’t mess with the frame-dragging measurements.

But surprisingly, fundamental physics isn’t the only science that the prototype, known as GINGERino, is capable of performing. The team was able to learn about the region's earthquakes and seismic activity, which has been particularly active in the last few decades.

“One peculiarity of the GINGERino installation is that it’s intentionally located within a high seismicity area of central Italy,” lead author Jacopo Belfi from the INFN said in a statement. “Unlike other large RLG installations, GINGERino can actually explore the seismic rotations induced by nearby earthquakes.”

GINGERino operates next to sophisticated seismic equipment that allows it to both monitor and minimize the effect of earthquakes on the measurement. The prototype is also encased in an isolation chamber to reduce the humidity, which could affect the electronics.

The team is currently investigating whether the gyroscope is coping with other sources of error, with the hope that the mountain is playing its shielding role well.  

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