Absolute gravity measurements in California

We have constructed an absolute gravity meter that determines the local gravitational acceleration by timing a freely falling mass with a laser interferometer. The instrument has made measurements at 11 sites in California, four in Nevada, and one in France. The uncertainty in the results is typically 10μGal (1 Gal ≡ 1 cm/s2; 1 μGal = 10−6 Gal = 10−8 m/s2). Repeated measurements have been made at several of the sites; only one shows a substantial change in gravity.