Generation and Detection of Dynamic Gravitational-Gradient Fields

Abstract

We have constructed a generator of dynamic Newtonian gravitational-force-gradient fields and used it as a signal generator to calibrate the response of the gravitational-gradient detectors being developed in our research work on gravitational-mass sensors. The gravitational-gradient-field generator is a flat aluminum cylinder 14 cm in diameter, with four holes than can be filled with slugs of different density to create a rotating mass-quadrupole moment. The generator is mounted on an air-bearing-supported motor and rotated at a nominal speed of 44 rps (2640 rpm). Because of the bisymmetric mass distribution, the dynamic gravitational-gradient fields generated have a frequency of 88 Hz, or twice the rotation frequency. The detector is a 12-cm-diam cruciform-shaped structure which responds to 88 Hz gravitational-gradient forces. The small (10−11 cm) motions induced in the detector arms are sensed by piezoelectric strain transducers attached to the arms near the point of maximum strain. A simple vacuum system, an iron shield plate, and spring mounts suffice for acoustic and magnetic isolation, since most of the nongravitational noises were generated at 44 Hz, the rotation frequency, rather than at 88 Hz, the gravitational-gradient frequency. Data taken with four different mass distributions varying from 0 to 1000 g and separation distances varying from 4.8 to 12 cm agree well with the theory, indicating that only gravitational energy was being transmitted from the generator to the detector. The minimum dynamic gravitational-gradient field observed during this test was 6×10−9 sec−2 or 0.002 of the earth's gradient. The equivalent differential acceleration exerted on the sensor arms by this field was 3×10−11 g's.