We describe the design improvements incorporated in a new generation of absolute gravimeters, the FG5. A vertically oriented (in-line) interferometer design is used to remove the influence of floor vibration and tilt on the optical path length. The interferometer uses an iodine-stabilized laser as a primary length standard, with circuitry for automatic peak detection and locking. The seismic isolation system is an active long-period seismometer (Super Spring). The new design has improved passive isolation and thermal drift characteristics over previous systems. Programming flexibility and control of the test mass trajectory have been improved. The computer system has also improved real-time analysis and system capability. The FG5 instrument has a higher level of robustness, reliability and ease of use. These design advances have led to an instrumental uncertainty estimate of 1,1 × 10-8 m s-2 (1,0 μGal). Instrument agreement among nine similar devices is 1,8 μGal and observations under optimal conditions exhibit standard deviations of 5 μGal to 8 μGal.

We have carried out a direct free-fall experiment to measure the differential acceleration between two different materials (copper and uranium) falling in the Earth's gravitational field. The differential acceleration was measured to be less than 5 parts in 10 10 of the normal gravitational acceleration. This null result puts new limits on the strength and range of the proposed fifth force.PACS numbers: 04.90.+e, 04.80,+z Our experiment is a modern-day counterpart to the experiment Galileo is alleged to have performed from the leaning tower of Pisa. We dropped two objects of differing composition (copper and depleted uranium) in a vacuum, and monitored their motion interferometrically in order to measure any differential acceleration. This experiment was designed to test for a possible fifth force coupled to hypercharge, as proposed by Fischbach et al. l to explain apparent anomalies in three different types of experiments. The proposed form of the anomalous hypercharge potential between two point masses iswhere / and X are the effective charge and range of the interaction, respectively, and B is the total baryon number. The magnitude of the charge would be about 10 ~1 9 of the elementary electron charge. The range may be as large as 10 4 m. The effect of this hypothesized fifth force would be to decrease the normal gravitational acceleration of a freely falling body. The anomalous acceleration would depend on the baryon-number-to-mass ratio which depends on the nuclear structure of the substance. Thus, two objects of differing composition would fall toward the Earth at different rates.The differential acceleration of two bodies toward the Earth is given by the following equation for ranges (X) smaller than the radius of the Earth (R):The parameter a is the anomaly in G which would arise from the hypothesized fifth force (GQ=GOO [\ +a]). B/ji is the ratio of the baryon number to the mass given in units of atomic hydrogen. Fischbach et al. presented Eq. (2) in their reanalysis of the Eotvos data, except that the density factor shown in brackets was not included. The ratio of the surface density to the mean density (p s /p m = 1/2) of the Earth is necessary because of the short range of the hyperphoton. The function £ expresses the interaction of the hypercharge force with the more dense inner layers of the Earth. £ can be computed with use of a linear radial density which overestimates the magnitude for ranges less than 2000 km. This model yields ^4X/R.r Dropping chambers were used from two of the absolute gravimeters recently developed in our laboratory. 2These gravimeters measure g by dropping an object in a vacuum. The vertical position of the dropped object is monitored interferometrically and its acceleration is then calculated. In this Galilean experiment, two dropping chambers are used. They are placed together over a single interferometer base in order to directly measure the differential acceleration of the objects. The apparatus and a block diagram of the optical measurement system is shown in Fi...

Recent determinations of the Newtonian constant of gravity have produced values that differ by nearly 40 times their individual error estimates (more than 0.5%). In an attempt to help resolve this situation, an experiment that uses the gravity field of a one-half metric ton source mass to perturb the trajectory of a free-falling mass and laser interferometry to track the falling object was performed. This experiment does not suspend the test mass from a support system. It is therefore free of many systematic errors associated with supports. The measured value was G = (6.6873 ± 0.0094) × 10 −11 m 3 kg −1 sec −2 .