Ring-laser tests of fundamental physics and geophysics

HeNe ring-laser gyros are standard sensors in inertial guidance; minor reflectances now reach 99.9999%. Present research instruments have an area of similar to 1 m(2), a passive quality factor of greater than or equal to 10(11), and a resolution of the frequency difference of counter-rotating optical beams approaching microhertz. In the Sagnac effect, this difference is proportional to the angular velocity. Present resolution is limited by thermal drifts in frequency pulling, itself reflecting mirror backscatter. The capability of ring lasers for measurements of geodesic interest, including seismometry and earth tides, and for detection of other sources of nonreciprocal refractive indices, including axions and CP violation, are discussed. In standard polarization geometries the observable is necessarily rime-reversal odd. Scaling rules for dimensions, finesse etc summarizing past progress and suggesting future potential are given.