Laser gyroscope measures the variations in Earth’s rotation rate

The length of Earth’s day is not constant—it can vary by fractions of a millisecond. The planet’s rotation rate depends on gravitational forces from the Sun and the Moon and on the flows from the oceans, moving ice sheets, air currents, and more. Traditionally, the combined effects have been measured using the signals from satellite networks or radio telescope arrays. But the interferometry performed with those systems provides only a single measurement averaged over a day.

Now K. Ulrich Schreiber and his colleagues in Germany and New Zealand have built a large ring-laser gyroscope that can observe tiny variations in Earth’s rotation at the level of five parts per billion over three months of continuous operation. What’s more, the gyroscope integrates more than three hours for each data point.

The gyroscope is rigidly grounded to bedrock in an underground laboratory at the Geodetic Observatory Wettzell in southern Germany. Two laser beams propagate in opposite directions inside a 16 m square-shaped ring, with 4 m laser cavities on each side, seen in the photo with Schreiber next to one of them.

As Earth fluctuates in space, so does the gyroscope. The wavelength of the beam aligned in the direction of Earth’s rotation elongates, while the one traveling against it shrinks. The effects of those fluctuations show up in the interference pattern produced when the two beams recombine. The researchers track the resulting beat signal to measure variations in the length of day (LoD), accurate to a few milliseconds, as seen in the plot below. It shows those variations over four months as measured by the ring-laser gyroscope (black) and superimposes them atop data from the International Earth Rotation and Reference Systems Service (red).

The new measurements are relevant to understanding the global water cycle and atmospheric circulation and may provide an important constraint on the effect of all those processes together. But they may also help researchers better understand the dynamics in Earth’s interior. By increasing the gyroscope’s stability, Schreiber expects to extend the window over which he and his colleagues can see mass transport on slow time scales. That may provide insight on the interaction between the solid core and the mantle. (K. U. Schreiber et al., Nat. Photon., 2023, doi:10.1038/s41566-023-01286-x .)