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An historic and simple experiment has been revitalised through the availability of supercavity mirrors and also through a heightened interest in interferometry as a test of physical theory. We describe our helium-neon ring laser, and present results demonstrating a fractional frequency resolution of 2·1x10-18 (1·0 mHz in 474 THz). The rotation of the earth unlocks the counterrotating beams. A new field of spectroscopy becomes possible, with possible applications to geophysical measurements such as seismic events and earth tides, improved measurements of Fresnel drag, detection of ultraweak nonlinear optical propert~es of matter, and also searches for preferred frame effects in gravitation and for pseudoscalar particles.
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Stedman et al. (1993) studied this question.
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