In reality, the Earth's axis is not firmly anchored in the sky, as it appears on a globe. Various forces act on it, causing it to wobble to varying degrees. The strongest influence is the Earth's imperfectly round shape; it bulges slightly at the equator compared to the poles. The effect known as precession causes the extension of the Earth’s axis to trace a circle in the sky. Currently, it is aligned precisely with the North Star. But in the future, it will be aligned with other stars before returning to the North Star in a cycle of 26,000 years.
But the gravitational forces of the sun and moon, which sometimes reinforce or weaken each other, also pull on the Earth's axis. This effect, known as nutation, causes small wave movements in the precession circle of the Earth's axis. There is a distinct nutation with a period of 18.6 years, but also many smaller ones with weekly or daily fluctuations. As a result, the axis does not wobble evenly, but with varying degrees of intensity.
Unprecedented precision
The ring laser was able to measure all these effects directly and continuously over 250 days with a level of accuracy previously unheard of for inertial sensors, i.e., sensors that operate independently of external signals. Unlike in the past, this does not require a network of several large radio telescopes (VLBI) on different continents. The ring laser can do all this on its own in a relatively small instrument located in a underground facility in Wettzell. In addition, the temporal resolution of the fluctuations is less than an hour instead of a day – and the results are available immediately, rather than after days or weeks, as is the case with VLBI.
Lead author Prof. K. Ulrich Schreiber from the TUM Institute of Engineering for Astronomical and Physical Geodesy emphasizes: "We have made great progress in measuring the Earth. What our ring laser can do is unique worldwide. We are 100 times more accurate than previously possible with gyroscopes or other ring lasers. The precise measurement of the fluctuations helps us better understand and model the Earth system with high accuracy."
With a further increase in the measurement accuracy and stability of the ring laser by a factor of 10 in the future, it would even be possible to measure the spacetime distortion caused by the Earth's rotation – a direct test of the theory of relativity. This would allow, for example, the Lense-Thirring effect, i.e., the “dragging” of space by the Earth's rotation, to be tested directly at the Earth's surface.
The world's most accurate rotation sensor
“The ring laser in Wettzell is unique and by far the most accurate rotation sensor in the world,” says Prof. Dr. Simon Stellmer, head of the Quantum Metrology working group at the University of Bonn. Together with his team, he is pursuing new approaches to the further development of measurement technology for ring lasers and is operating several setups at the Physics Institute for this purpose. The next campaign is scheduled for November, when a ring laser in New Zealand will be equipped with high-precision optical and electronic components from Bonn: “We are delighted to be able to contribute to these international projects.”