The ring laser research group is a collaboration between the University of Canterbury Physics and Astronomy Department who operate the and the Technical University of Munich and the Forschungsgruppe Satelliengeodäsie (German Federal Institute for Cartography and Geodesy) who operate the . The goal of this research group is the development of world-leading active ring laser gyroscopes for measuring subtle variations in the rotation rate of the earth. This is important for research in geophysics, geodesy and fundamental physics.
In the 1960s lasers were described at scientific conferences as "a solution in search of a problem". Nowadays lasers are a multibillion dollar business. Every supermarket checkout, every CD player and many builders' toolkits hold one.
In lasers, a light beam bounces along a line between two mirrors. We use the small fraction of light that leaks out at each reflection. Ring lasers, on the other hand, have the mirrors arranged in a triangle or square forming a closed path or 'ring'. This allows two essentially independent laser beams to travel around the ring, one clockwise, the other counterclockwise.
Suppose the whole apparatus is rotated clockwise. Imagine a beam of light beginning at one mirror. Since light travels at the same speed in both directions it will take longer for the clockwise traveling beam to reach its starting position compared to the counterclockwise beam since the starting position has moved since the beam departed.
This effect causes what is known as a 'Doppler shift', and subtly changes the colour of each beam. If the two beams are later mixed together it is possible to measure this change in colour. When expressed as a frequency, this colour change is called the Sagnac frequency after french physicist Georges Sagnac who first demonstrated the principle in 1913.
So a ring laser can measure the absolute rotation of whatever environment it is placed in. Small ring lasers are used in aircraft and submarines for navigation. In our case, it is the much smaller daily rotation of the earth about it's axis that is of most interest to us. It turns out the the larger the ring laser and the better quality the mirrors, the more sensitive the device becomes as a rotation sensor - but it also becomes much harder to construct and operate.
The rotation rate of the Earth makes it a very good clock, but not perfectly so. The rotation axis twists and wobbles since the Earth is not a perfect sphere; atmospheric motion reacts back on the Earth. (Try keeping your body absolutely still as you wave your arms). As a result, one day is a few thousandths of a second longer or shorter than the next.
Improving our knowledge of how the Earth twists and turns in space at the level of parts per billion remains an important scientific goal. This knowledge is important for precise navigation, since it maps the terrestrial reference frame (where you want your position accurately) into the celestial reference frame (where the navigation satellites belong).
Another method of determining extremely precise information on Earth motion comes from the field of intercontinental radio astronomy and satellite laser ranging. After decades of steady development it is now possible to use these techniques to see local motions near earthquake faults and even continental drift after only a few months of observations. It helps to combine results from different technologies. As a sensitive, stand-alone and absolute rotation sensor, a ring laser can give complementary information to astronomical measurements and also short-term information not available using these methods. Short term information is particularly interesting to those who study earthquakes.
One recent and interesting result from the ring laser group is our recent landmark direct measurement cite("polarmotion2004");?> of 'polar wobble', an effect of the moons gravity that causes the Earths rotation axis to be perturbed daily by 60 cm at the poles. Another interesting measurement just on the horizon is the measurement of the subtle periodic movement of mountain ranges caused by wind pushing on them.