Superconducting differential accelerometers have been used to test Newton’s inverse square law and have been proposed for other sensitive experiments. These include searches for spin-mass coupling, detecting Earth’s gravitomagnetic field, and testing the Equivalence Principle. This article discusses the principle and performance of a sensitive three-axis gravity gradiometer. This device utilizes quantized flux and the Meissner effect to provide stable test mass levitation and signal coupling, and superconducting quantum interference devices to provide very low-noise amplification of the signals. The instrument comprises a total of nine superconducting accelerometers, six linear and three angular. This configuration permits simultaneous measurement of the diagonal components of the gravity gradient tensor as well as platform acceleration in all six degrees of freedom. An analysis of this instrument is presented along with experimental results. Methods to correct for various motion-induced errors are demonstrated. Other error sources are also discussed. The resulting performance of the superconducting gravity gradiometer is 2×10−11 s−2 Hz−1/2.

The angular momentum of the Earth produces gravitomagnetic components of the Riemann curvature tensor, which are of the order of 10 ' of the Newtonian tidal terms arising from the mass of the Earth. These components could be detected in principle by sensitive superconducting gravity gradiometers currently under development. We lay out the theoretical principles of such an experiment by using the parametrized post-Newtonian formalism to derive the locally measured Riernann tensor in an orbiting proper reference frame, in a class of metric theories of gravity that includes general relativity. A gradiometer assembly consisting of three gradiometers with axes at mutually right angles measures three diagonal components of a 3 X 3 "tidal tensor, " related to the Riemann tensor. We find that, by choosing a particular assembly orientation relative to the orbit and taking a sum and difference of two of the three gradiometer outputs, one can isolate the gravitomagnetic relativistic effect from the large Newtonian background.

A very sensitive resonant superconducting accelerometer has been developed as a component of a cryogenic gravitational-radiation detector. The device consists of a superconducting test mass and superconducting coils carrying a persistent current. The displacement of the test mass modulates the inductances of the coils and generates an ac magnetic field which is detected by a Josephson-junction magnetometer. The restoring force provided by the magnetic field is used to tune the resonant frequency of the transducer. The expected sensitivity of the system is better than 10−12gE/Hz1/2 (gE=9.8 m/s2) when used to detect accelerations at frequencies lower than 50 Hz. The system has been thoroughly tested and is being used to detect small accelerations of a gravitational-wave antenna caused by the Brownian motion and other external disturbances. When used as a resonant displacement sensor in a gravitational-wave detector cooled to 3 mK, this transducer is capable of converting a displacement of 4×10−20 m at 1 kHz into an electrical signal detectable with unity signal-to-noise ratio for 1-Hz bandwidth. The gravitational-radiation-flux sensitivity implied by this is 0.1 erg/cm2 Hz. This will make not only the observation of expected galactic events possible, but will allow one to extend the scope of observation beyond the Milky Way. The system can be modified to make a sensitive gravity gradiometer. When two accelerometers are coupled to the same Josephson-junction magnetometer with their transformer coils wound in the opposite sense, direct subtraction of acceleration signals can be accomplished. The system will be easy to build and mechanically rugged. The device in various applications is discussed and the theory of transducer energy coupling, frequency tuning, and parameter optimization is presented. Some experimental results confirming the theory are reported. Included are data showing the temperature dependence of the Q of a niobium diaphragm and the measurement of the low-frequency background acceleration of a magnetically levitated gravitational-wave antenna.

A null test of the gravitational inverse-square law can be performed by testing Gauss's law for the field. We have constructed a three-axis superconducting gravity gradiometer and carried out such a test. A lead pendulum weighing 1500 kg was used to produce a time-varying field. This experiment places a new (2= -(GM/r)(\+ae ~r /x ).

Monitoring of vibrational eigenmodes of an elastic body excited by gravitational waves was one of the first concepts proposed for the detection of gravitational waves. At laboratory scale, these experiments became known as resonant bar detectors first developed by Joseph Weber in the 1960s. Due to the dimensions of these bars, the