Feature

As the World Turns...

Modern physics and military technology meet in the depths of a WWII bunker in Christchurch.

Geoff Stedman

The Canterbury Ring Laser project recently celebrated its 10th year, and continues to provide new insights and develop new facilities. Buried in a WWII bunker built in Christchurch to repel a possible Japanese invasion, the project has also demonstrated how military advances can be ploughed back into pure science research. The supermirrors used in the ring lasers have unprecedented quality, once available only for US military hardware.

Ring lasers can perform the function of gyroscopes, where the absolute rotation of the light travelling around the ring from two different directions shows up as a colour difference between the two laser beams (clockwise and anticlockwise). These are now used routinely in aircraft inertial navigation systems, and there are plans to produce inexpensive units for cars to provide drivers with local guidance maps.

Big rings, such as the Canterbury rings, use the same process to measure the Earth's rotation. In our laser systems, we are talking very small fractions of the laser frequency to see the Earth rotate at all [A Ring of Light and Time, March 1994].

Mirror and gyro advances are proceeding still. Even today a curtain of secrecy surrounds some topics, and the commercial leaders continue to wrangle in lawsuits worth over billions of dollars. Last September, I attended the International Symposium on Gyro Technology in Germany, where representatives of 17 countries, including Russia, China, Japan and the US, presented their latest advances in gyro technology. The chairman pointed out that 10 years ago such a meeting would have been impossible. I was delighted to be able to tell the symposium that we had found a pacifist use for a war bunker.

The Cashmere facility is internationally unique. The war bunker environment reduces thermal drifts and mechanical vibrations, allowing greater sensitivity and unusual applications such as using the facility as a novel seismograph. We are also interested in detecting lunar and solar earth tides, and have performed a preliminary experimental test of relativity -- looking in the ring laser signal for evidence of the cosmic microwave background. The theory suggesting this possibility was developed at the University of Canterbury.

The Earth is a very good clock; but it is not perfect. There are many causes of fluctuations in the Earth's rate of rotation, each with its own time scale, from seconds to centuries and beyond. C-II is the harbinger of an instrument which will measure this in a comparatively rapid time scale.

New Ring Laser

The project's first decade has been celebrated with a major advance in the international programme. A year ago the original large helium-neon ring laser, now dubbed C-I (C for Canterbury), was joined by a greatly superior device, C-II. This new ring laser is the outcome of an international collaboration between German, US and Cantabrian researchers.

C-I was built locally from raw materials on a very limited budget. No such gross economies were made on C-II, which is a superbly engineered device. It is based on a monolithic block of high-grade Zerodur, a very stable form of the ceramic commonly used to make oventops. The C-II slab is 1.2 metres square and 18 centimetres thick; the initial blank alone cost $400,000, but is now worth much much more after years of careful precision machining. Its stability is such that it makes stainless steel look like jelly in comparison.

Funding for C-II was provided by by Bundesamt für Kartographie und Geodaesie and Technische Universitaet München. Many local expenses such as the adaptation of the Cashmere cavern environment were funded by University of Canterbury and the Marsden fund of the Royal Society of New Zealand.

Once the Zerodur blank was delivered to the builders, Carl Zeiss in Germany, the real work started. Several holes had been drilled, some to make the square path for the lasing gas and the light beam. Then the four corners of the block were cut, finished and hand-polished -- this last task alone taking on order of a man-year of work -- to attain a surface which was optically flat and angled to an absolute accuracy of 30 arc seconds; and so on. Some of the technology embodied is new, for example the use of ultra-high vacuum seals between stainless steel flanges and the Zerodur body.

Nor has the rest of physics stood still. Nowadays the best interferometer-type (as opposed to C-I -- or laser-type) gyros use the quantum waves of whole atoms. A little rotation of such a heavy mass as an atom goes a long way in the sensitivity stakes. This glamorous topic in physics invokes some horrendous technical obstacles, which have been overcome only very recently. It speaks volumes for the vigour of research in the antipodes that New Zealand experimental scientists at the Universities of Auckland and Otago are working energetically in the general field of atom interferometry. It is good to report that Canterbury measurements last year on C-II show that as a gyro it performs better than the best atomic gyros to date.

The interest in Earth rotation goes well beyond earthquake measurement, covering a large range of scales. The Earth is a very good clock; but it is not perfect -- every now and then extra seconds have to be slipped into our time base as the Earth slows down. There are many other causes of fluctuations in the rate of rotation, each with its own time scale, from seconds to centuries and beyond.

C-II is the harbinger of an instrument, called G, which will measure this in a comparatively rapid time scale. These effects are in the region of a few billionths of the rotation rate -- a few thousandths of a second per day. These are beyond the reach of C-II, but should be within the reach of G.

Still, extrapolation is risky. So controversial, indeed, that it was widely considered in the gyro industry that devices like C-I would never work: they would be too noisy and unstable. C-I dramatically proved that wrong. But is there actually an impassable limit? Is bigger really better?

At a public seminar given as part of the celebrations marking C-II's formal opening, Professor Hermann Seeger of Bundesamt fuer Kartographie und Geodaesie, Frankfurt, announced formally that a committee of the German Bundestag had approved the funding of G.

Since then, in preparation for the new machine, a very much simplified form (in the style of C-I, scaled up) called G0 is being built at Cashmere. G0 is the third world-record laser to be built there. It is expected to test the optics design of G at full scale (4 metres square), and to detect earthquakes much more sensitively than C-II.

Professor Seeger's dramatic announcement set a splendid seal of achievement on this day of ceremony, and on a decade of adventure underground in Cashmere. This seems just the end of the beginning. It is heartening news for New Zealand when such projects attract major international interest and investment, and trigger the creation of a totally new generation of unique devices.

Professor Geoffrey Stedman is in the Department of Physics and Astronomy at Canterbury University.