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As long-period ground motion holds the key to understanding the interior of the Earth’s Moon, reducing long-period noise sources will be an essential area of focus in the design of future lunar seismometers. For the proposed Lunar Geophysical Network (LGN), the International Lunar Network (ILN) Science Definition Team specifies that an LGN enabling seismometer will need to be more sensitive than any previous seismometer at frequencies below 1 Hz. In an effort toward lowering the seismometer noise floor for lunar geophysical missions, we evaluate the 1/f Brownian noise and the temperature sensitivity of a seismometer. Temperature sensitivity of a seismometer is related to an important component of the seismometer output noise that is proportional to the temperature noise in the environment. The implications of the ILN requirement are presented in the context of the state-of-the-art InSight Seismic Experiment for Interior Structure (SEIS) Very Broad Band (VBB) planetary seismometer. Brownian noise due to internal friction was estimated for future lunar operation after accounting for the rebalance of the product of mass and distance to the center of gravity of the pendulum for the SEIS-VBB sensor. We find that Brownian noise could be a limiting factor in meeting the ILN requirement for lunar seismometers. Further, we have developed a formalism to understand the temperature sensitivity of a seismometer, relating it quantitatively to the local gravity, the thermoelastic coefficient of the spring, change in center of gravity, and the coefficient of thermal expansion of the mechanical structures. We found that in general the temperature sensitivity of a seismometer is proportional to the local gravity, and so the temperature sensitivity can be reduced when operating on a planetary body with lower gravity. Our Brownian noise and temperature sensitivity models will be useful in the design of the next generation of planetary seismometers.

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 targeted signal frequencies were in the kHz range. Weber also pointed out that monitoring of vibrations of Earth or Moon could reveal gravitational waves in the mHz band. His Lunar Surface Gravimeter experiment deployed on the Moon by the Apollo 17 crew had a technical failure rendering the data useless. In this article, we revisit the idea and propose a Lunar Gravitational-Wave Antenna (LGWA). We find that LGWA could become an important partner observatory for joint observations with the space-borne, laser-interferometric detector LISA, and at the same time contribute an independent science case due to LGWA's unique features. Technical challenges need to be overcome for the deployment of the experiment, and development of inertial vibration sensor technology lays out a future path for this exciting detector concept., 29 pages, 17 figures

Broadband seismometers and gravitational wave detectors make use of mechanical resonators with a high quality factor to reduce Brownian noise. At low frequency, Brownian noise is ultimately dominated by internal friction in the suspension, which has a 1/f noise compared with the white noise arising from viscous dissipation. Internal friction is typically modeled as a frequency-dependent loss and can be challenging to measure reliably through experiment. In this work, we present the physics and experimental implementation of electrostatic frequency reduction (EFR) in a mechanical oscillator-a method to measure dissipation as a function of frequency. By applying a high voltage to two parallel capacitor plates, with the center plate being a suspended mass, an electrostatic force is created that acts as a negative stiffness mechanism to reduce the system's resonance frequency. Through EFR, the loss angle can be measured as a function of frequency by measuring amplitude decay response curves for a range of applied voltages. We present experimental measurements of the loss angle for three metal helical extension springs in the nominal frequency range 0.7-2.9 Hz at 0.2 Hz intervals, demonstrating the possibility for fine adjustment of the resonance frequency for loss angle measurements. A quality factor proportional to the resonance frequency squared was measured, an indication that internal friction and other non-viscous dissipation elements, such as electrostatic damping, were the prominent loss mechanisms in our experiments. Finally, we consider the implications of Brownian noise arising from internal friction on a low 1/f noise seismometer.

Detection of gravitational waves (GWs) from merging binary black holes (BHs) by Advanced LIGO has ushered in the new era of GW astronomy. Many conceivable sources such as intermediate-mass BH binaries and white dwarf binaries, as well as stellar-mass BH inspirals, would emit GWs below 10 Hz. It is highly desirable to open a new window for GW astronomy in the infrasound frequency band. A low-frequency tensor detector could be constructed by combining six magnetically levitated superconducting test masses. Such a detector would be equally sensitive to GWs coming from anywhere in the sky, and would be capable of resolving the source direction and wave polarization. I will present the design concept of a new terrestrial GW detector, named SOGRO, which could reach a strain sensitivity of 10−19-10−21 Hz−1/2 at 0.1-10 Hz. Seismic and Newtonian gravity noises are serious obstacles in constructing terrestrial GW detectors at frequencies below 10 Hz. I will explain how these noises are rejected in SOGRO. I will also report the progress made in designing the platform and modelling its thermal noise.

Laser interferometer gravitational-wave (GW) detectors are observing signals from merging black hole and neutron star binaries with a frequency window from 10[Formula: see text]Hz to several kHz. Future space-based laser interferometers will open a new window of 0.1[Formula: see text]mHz to 0.1[Formula: see text]Hz. In this paper, we discuss the possibility of constructing a terrestrial GW detector named Superconducting Omni-directional Gravitational Radiation Observatory (SOGRO), which can fill the missing frequency window, 0.1 to 10[Formula: see text]Hz, with astronomically interesting sensitivity. SOGRO measures all five tensor components of the spacetime metric, which results in uniform sensitivity for all-sky directions and enables identification of the source direction and wave polarization with a single detector. Seismic and Newtonian gravity noise pose the greatest challenges for constructing ground-based detectors below 10[Formula: see text]Hz. SOGRO utilizes enhanced mechanical and electrical stabilities of materials at cryogenic temperatures to reject common-mode seismic noise to a very high degree. Further, its full-tensor characteristic gives an advantage in the rejection of the Newtonian noise over conventional detectors.

With continuous advances in technology, future satellite gradiometry missions will be capable of performing precision relativistic experiments and imposing constraints on modern gravity theories. To this end, the full first-order post-Newtonian tidal tensor under inertially guided and Earth-pointing local frames along post-Newtonian orbits is worked out. The physical picture behind the "Mashhoon-Theiss anomaly" is explained at the post-Newtonian level. The relativistic precession of the local frame with respect to the sidereal frame will produce modulations of Newtonian tidal forces along certain bases, which gives rise to two different kinds of secular tidal tensors. The measurements of the secular tidal force from the frame-dragging effect is also discussed., Comment: Typos in Eq. (11) and Eq. (22b) are corrected. 10 pages, 3 figures

This paper focuses on the next detectors for gravitational wave astronomy which will be required after the current ground based detectors have completed their initial observations, and probably achieved the first direct detection of gravitational waves. The next detectors will need to have greater sensitivity, while also enabling the world array of detectors to have improved angular resolution to allow localisation of signal sources. Sect. 1 of this paper begins by reviewing proposals for the next ground based detectors, and presents an analysis of the sensitivity of an 8 km armlength detector, which is proposed as a safe and cost-effective means to attain a 4-fold improvement in sensitivity. The scientific benefits of creating a pair of such detectors in China and Australia is emphasised. Sect. 2 of this paper discusses the high performance suspension systems for test masses that will be an essential component for future detectors, while sect. 3 discusses solutions to the problem of Newtonian noise which arise from fluctuations in gravity gradient forces acting on test masses. Such gravitational perturbations cannot be shielded, and set limits to low frequency sensitivity unless measured and suppressed. Sects. 4 and 5 address critical operational technologies that will be ongoing issues in future detectors. Sect. 4 addresses the design of thermal compensation systems needed in all high optical power interferometers operating at room temperature. Parametric instability control is addressed in sect. 5. Only recently proven to occur in Advanced LIGO, parametric instability phenomenon brings both risks and opportunities for future detectors. The path to future enhancements of detectors will come from quantum measurement technologies. Sect. 6 focuses on the use of optomechanical devices for obtaining enhanced sensitivity, while sect. 7 reviews a range of quantum measurement options.

The Moon and the moons of Mars should be extremely quiet seismically and could therefore become sensitive gravitational wave detectors, if instrumented properly. Highly sensitive displacement sensors could be deployed on these planetary bodies to monitor the motion induced by gravitational waves. A superconducting displacement sensor with a 10-kg test mass cooled to 2 K will have an intrinsic instrument noise of 10 −16 m Hz −1/2 . These sensors could be tuned to the lowest two quadrupole modes of the body or operated as a wideband detector below its fundamental mode. An interesting frequency range is 0.1–1 Hz, which will be missed by both the ground detectors on the Earth and LISA and would be the best window for searching for stochastic background gravitational waves. Phobos and Deimos have their lowest quadrupole modes at 0.2–0.3 Hz and could offer a sensitivity h min ⩽ 10 −22 Hz −1/2 within their resonance peaks, which is within two orders of magnitude from the goal of the Big Bang Observer (BBO). The lunar and Martian moon detectors would detect many interesting foreground sources in a new frequency window and could serve as a valuable precursor for BBO.

The angular momentum of the Earth produces gravitomagnetic components of the Riemann curvature tensor, which are of the order of 10−10 of the Newtonian terms arising from the mass of the Earth. Due to the dragging of the local inertial frame by the spinning Earth, there are also secular terms, which grow in time. These fields can be detected in principle by a set of orbiting superconducting gravity gradiometers. The Riemann tensor components for various spacecraft orientations have been computed and the principle of detecting the gravitomagnetic tidal force has been published. In this paper, we review the conclusions of the earlier analyses and discuss the feasibility of the gravity gradiometer experiment.

The objective of ISLES (Inverse-Square Law Experiment in Space) is to perform a null test of Newton's law in space with a resolution of 10 ppm or better at a 100 μm distance. ISLES will be sensitive enough to detect the axion, a dark matter candidate, with the strongest allowed coupling and probe large extra dimensions of string theory down to a few micrometers. The experiment will be cooled to < 2 K , which permits superconducting magnetic levitation of the test masses. This soft, low-loss suspension, combined with a low-noise SQUID, leads to extremely low intrinsic noise in the detector. To minimize Newtonian errors, ISLES employs a near-null source, a circular disk of large diameter-to-thickness ratio. Two test masses, also disk-shaped, are suspended on the two sides of the source mass at a nominal distance of 100 μm. The signal is detected by a superconducting differential accelerometer.

Space-based experiments today can uniquely address important questions related to the fundamental laws of Nature. In particular, high-accuracy physics experiments in space can test relativistic gravity and probe the physics beyond the Standard Model; they can perform direct detection of gravitational waves and are naturally suited for precision investigations in cosmology and astroparticle physics. In addition, atomic physics has recently shown substantial progress in the development of optical clocks and atom interferometers. If placed in space, these instruments could turn into powerful high-resolution quantum sensors greatly benefiting fundamental physics. We discuss the current status of space-based research in fundamental physics, its discovery potential, and its importance for modern science. We offer a set of recommendations to be considered by the upcoming National Academy of Sciences' Decadal Survey in Astronomy and Astrophysics. In our opinion, the Decadal Survey should include space-based research in fundamental physics as one of its focus areas. We recommend establishing an Astronomy and Astrophysics Advisory Committee's interagency ``Fundamental Physics Task Force'' to assess the status of both ground- and space-based efforts in the field, to identify the most important objectives, and to suggest the best ways to organize the work of several federal agencies involved. We also recommend establishing a new NASA-led interagency program in fundamental physics that will consolidate new technologies, prepare key instruments for future space missions, and build a strong scientific and engineering community. Our goal is to expand NASA's science objectives in space by including ``laboratory research in fundamental physics'' as an element in agency's ongoing space research efforts., a white paper, revtex, 27 pages, updated bibliography

Strange quark matter made of up, down and strange quarks has been postulated by Witten [E. Witten, Phys. Rev D 30 (1984) 279]. Strange quark matter would be nearly charge neutral and would have density of nuclear matter (1014 gm/cm3). Witten also suggested that nuggets of strange quark matter, or strange quark nuggets (SQNs), could have formed shortly after the Big Bang, and that they would be viable candidates for cold dark matter. As suggested by de Rujula and Glashow [A. de Rujula and S. Glashow, Nature 312 (1984) 734], an SQN may pass through a celestial body releasing detectable seismic energy along a straight line. The Moon, being much quieter seismically than the Earth, would be a favorable place to search for such events. We review previous searches for SQNs to illustrate the parameter space explored by using the Moon as a low-noise detector of SQNs. We also discuss possible detection schemes using a single seismometer, and using an International Lunar Seismic Network.

Because the Moon is much quieter seismically than the Earth, science to be performed on the Moon using seismology is currently limited by the best available seismometer. We describe the development of a lunar seismometer with sensitivity at least 100 times higher than current state of the art. Analysis of the fundamental noise of this seismometer shows that at frequency below ~1 Hz the noise will be dominated by Brownian motion of the test mass. Above ~1 Hz, it will be dominated by thermal noise in the electronic readout. The electronic noise corresponds to ~10 −13 m Hz −1/2 . A way to reduce the Brownian motion noise of the test mass using electrostatic force is also discussed.

As part of the refocusing of NASA from Space Station research to exploration research, we are pursuing a number of proposed and funded projects for lunar exploration using cryogenic techniques. This paper gives a summary of these projects which include: (1) Using passive radiative cooling for separation and storage of volatiles from lunar regolith; (2) Studies of boiling and two-phase flow under lunar gravity; (3) SQUID-based MRI for monitoring astronaut health; (4) Studies of volatiles and water ice in permanently shadowed craters at the lunar poles; and (5) Seismic search for strange quark matter using the Moon as a large and seismically quiet detector.

The superconducting differential accelerometers both for the Equivalence Principle experiment and geodesy within the European STEP mission have common design principles. The test masses are suspended by stable superconducting magnetic levitation. The suspension design makes all the degrees of freedom of the masses stiff, except for the axial differential mode which is made compliant in order to obtain a high intrinsic sensitivity of the differential accelerometer, ⪅ 10 −14 m s −2 Hz −1 2 . Trimming of persistent currents circulating in the levitation system allows to achieve rejection ratios for the unwanted common and radial accelerations ⪆ 106. Two separate superconducting circuits couple the axial displacements of the test masses to two SQUIDs. Persistent currents are stored in the two circuits such that one SQUID is only coupled to the differential displacement while the other only senses the common one. By differencing the signal before its detection, one highly reduces the dynamic range needs of the SQUIDs, of the following amplifiers and of the final A/D converters.

A sensitive angular accelerometer has been developed. The accelerometer contains a superconducting test mass suspended by a weak flexure pivot and has a bandwidth exceeding 10 Hz. A superconducting quantum interference device amplifier and superconducting circuit are used to detect angular displacement of the test mass. Three nearly identical units have been constructed and tested. An analysis of the instrument is presented along with experimental results. At 4.2 K, performance better than 3×10−9 rad s−2 Hz−1/2 has been demonstrated. The measured noise floor has been traced to seismic motion and temperature fluctuations. The thermal noise contributes less than 10−10 rad s−2 Hz−1/2. Application of the superconducting angular accelerometer to gravity gradiometry is discussed.

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 demons...

The rotation of the Earth produces gravitomagnetic components of the Riemann curvature tensor, which are of the order of $$10^{-10}$$ of the Newtonian terms arising from the mass of the Earth. Detection of the gravitomagnetic field is very important, since it tests one of the most fundamental predictions of general relativity. We review the experimental scheme for detecting the gravitomagnetic effect with an orbiting gravity gradiometer in an inertial frame. Two factors which can affect the experiment, satellite positioning errors and Earth’s multipole moments, are analyzed. We derive the satellite positioning accuracy required for construction of matched templates with sufficient precision. We find that the satellite orbit must be carefully selected to avoid some dangerous altitudes at which Earth’s multipole moments can mask gravitomagnetic signals. Allowable orbit altitudes are computed. We also examine an alternative scheme, in which the gradiometer is fixed to a local inertial frame defined by gyroscopes. In this experiment, the gradient signal grows linearly with time due to the Lense–Thirring effect, and the satellite positioning errors and Earth’s multipole moments become negligible.

The superconducting differential accelerometers for the equivalence principle and geodesy experiments on the European STEP mission have been designed under common guiding principles. Both accelerometers rely on stable superconducting magnetic levitation. The suspension is stiff against all unwanted degrees of freedom while the axial differential mode is kept compliant to obtain a high intrinsic sensitivity of the differential accelerometer, . The sensitive axis of each component accelerometer is aligned to by adjusting persistent currents in the alignment coils and a restoring force is provided by persistent currents to prevent unwanted rotation about the sensitive axis. The dynamic alignment of the sensitive axes improves the common-mode rejection ratio to . The axial displacements of the two test masses in each differential accelerometer are coupled through two separate superconducting circuits to two DC SQUIDs. Persistent currents are stored in the two circuits such that the acceleration signals are differenced in one circuit and summed in the other before detection by the respective SQUIDs. The signal differencing before detection reduces the dynamic range requirement of the following amplifiers and analogue-to-digital (A/D) converters by orders of magnitude.

A really inertial platform for highest sensitivity instruments which need to be absolutely free from any disturbances like drag and other forces in space is proposed for the case of an Equivalence Principle test with differential accelerometers. A spin-stable platform, inside a co-rotating spacecraft shielding it, constitutes the reference proof mass for a simple position servo system. Position measurements, as well as signal transmissions, are made optically. Power is supplied by microwave beams. The proposed ambient-temperature EP-mission EOTVOS in a circular, equatorial orbit at 800 km altitude would constitute a simple, very cost-effective experiment. Limitations to the potentially very high precision (goal: one part in 1017) are expected to be set by thermal disturbances.

The superconducting gravity gradiometers for the European STEP and GEM missions have common design features. Both gradiometers have their test masses magnetically levitated, stiff against all unwanted degrees of freedom. The sensitive axes of the component accelerometers are aligned to with respect to each other, by adjusting persistent currents in the alignment coils, to improve the common-mode rejection ratio to . The axial displacements of the two test masses in each gradiometer are coupled through two superconducting circuits to two DC SQUIDs. Persistent currents are stored in the circuits such that the acceleration signals are summed and differenced at the respective SQUID inputs. This signal differencing before detection reduces the linearity and dynamic range requirements of the electronics by several orders of magnitude. The STEP gradiometer will be a single-axis device with a baseline of about 60 cm and with its sensitive axis oriented along the orbit normal. Its intrinsic noise is expected to be above . Below this frequency 1/f power noise should appear. A compact three-axis superconducting gravity gradiometer with a baseline of 12 cm is proposed for GEM. This gradiometer will have an intrinsic noise of above . Below this frequency 1/f noise will dominate.

The Superconducting Gravity Gradiometer (SGG), which can measure gravitational field fluctuations to unprecedented precisions, is currently being evaluated for use in an Earth-orbiting platform. Two missions are planned: a Shuttle-based demonstration flight and a dedicated gravity surveying mission. The Shuttle mission will feature a fully functional, self-contained instrument with an intrinsic noise level of 10 −3 E Hz −1 2 , where E is defined at 10 −9 m s −2 m −1 . Calibration hardware, residual error correction techniques, and surface tension devices which control liquid helium motion in the dewar will be tested. For a dedicated free-flier gravity mission, two concepts have been studied. Both have been formulated to achieve low cost while maintaining an instrument sensitivity of 10 −3 E Hz −1 2 . One concept uses the spacecraft developed for the Sub-millimeter Wave Astronomical Satellite (SWAS), and would be in a polar orbit, decaying from an initial altitude of 380 km. A mission lifetime of 6–9 months is planned. The other concept is based on a re-usable SPARTAN 400 carrier which would be deployed from, and retrieved by, the Shuttle. The carrier can be periodically reboosted and would maintain an altitude between 275 and 350 km. The mission lifetime is restricted to about 3 months, and the orbit inclination is limited to 57 °, the maximum the Shuttle can reach. Since the carrier is retrieved, the instrument could be periodially reflown. The free-flier missions, including experiment and spacecraft design, are described.

A high-resolution global gravity map would provide tremendous benefits to the Earth sciences. Mapping the lateral fluctuations of a planet's gravity field is one of the few means of probing its interior. For the Earth, a high-resolution gravity map would also reveal dynamic ocean current information presently hidden in existing sea surface height data. The use of gradiometers for gravity mapping has inherent advantages over other techniques and has the potential of providing the highest spatial resolution for global surveys from orbit. Extremely sensitive superconducting gravity gradiometers (SGGs) have been developed for space applications under NASA support. The present laboratory model has demonstrated a sensitivity almost two orders of magnitude better than any other gradiometer ( Hz −1 2 , where 1 mE ≡ 10−12 m s−2 m−1 ≈ 10−13 gE m−1). In preparation for a dedicated gravity mapping mission, we have proposed a flight test of a spaceworthy version of the instrument in the SHOOT (Superfluid Helium On Orbit Transfer) dewars, which flew on the Shuttle in 1993. We have examined potential error sources for the instrument operating in the Shuttle environment. Of the largest error sources, most are motion-related. We obtained the motion of the Orbiter in six degrees of freedom (d.o.f.) by examining the SAMS (shuttle acceleration measurement system) and the CAS (calibrated ancillary system) data sets. Using this information and the measured error coupling coefficients of the laboratory SGG, we can predict the resolution with which the Earth's gravity gradient signal can be recovered on this Shuttle test flight. Although the Orbiter provides a much worse environment for the SGG than a dedicated satellite, our analysis shows that during quiet periods on the Orbiter, the ability to recover gravity data will approach the limit imposed by the instrument thermal noise.

We have calculated signal-to-noise ratios for eight spherical resonant-mass antennas interacting with gravitational radiation from inspiralling and coalescing binary neutron stars and from the dynamical and secular bar-mode instability of a rapidly rotating star. We find that by using technology that could be available in the next several years, spherical antennas can detect neutron star inspiral and coalescence at a distance of 15 Mpc and the dynamical bar-mode instability at a distance of 2 Mpc., Comment: 39 pages, 4 EPS Figures, some additional SNRs for secular instabilities, some changes to LIGO SNRs, Appendix added on the asymptotic expansion of energy sensitivity, corrected supernova rates. Results available at http://www.physics.umd.edu/rgroups/gen_rel_exp/snr.html Submitted to Phys. Rev. D

Terrestrial gravity noise produced by ambient seismic and infrasound fields poses one of the main sensitivity limitations in low-frequency ground-based gravitational-wave (GW) detectors. This noise needs to be suppressed by 3-5 orders of magnitude in the frequency band 10 mHz to 1 Hz, which is extremely challenging. We present a new approach that greatly facilitates cancellation of gravity noise in full-tensor GW detectors. It makes explicit use of the direction of propagation of a GW, and can therefore either be implemented in directional searches for GWs or in observations of known sources. We show that suppression of the Newtonian-noise foreground is greatly facilitated using the extra strain channels in full-tensor GW detectors. Only a modest number of auxiliary, high-sensitivity environmental sensors is required to achieve noise suppression by a few orders of magnitude.

A differential equivalent of the inverse-square force law is Gauss's law for the field, , where . Thus, by summing the outputs of an in-line gravity gradiometer rotated in three orthogonal directions, one can perform a near-null test of the inverse-square law. The primary advantage of this type of experiment is its reduced sensitivity to the density and metrology errors of the source. We have developed a three-axis superconducting gravity gradiometer and carried out such a test using a lead (Pb) pendulum to produce a time-varying field. This experiment places a new () limit of at , where and are parameters of the generalized potential . This result represents an improvement of an order of magnitude over the best existing limit at . To achieve further improvement in the resolution of with the present instrument, a near-null source is needed. We plan to carry out an improved test of the inverse-square law using a long cylindrical shell as the source. Looking farther into the future, we hope to be able to conduct a geological-scale null experiment by moving the gradiometer up and down a tower above a plain.

After giving a brief historical review of their technological development, we discuss the principle of a superconducting accelerometer. We then expand into the design and optimization of various specialized accelerometers: superconducting gravity gradiometers and a superconducting six-axis accelerometer. We report the performance of these devices. Finally, we discuss how the superconducting accelerometer techniques are applied to various differential accelerometers in proposed space missions.

Over the past 25 years, a spaced based test has been been proposed to measure nearly every aspect of General Relativity from gravitational waves to tests of Einstein’s Equivalence Principle. To date, very few experiments speci cally designed to explore General Relativity have been launched. The notable exceptions have been Gravity Probe-A, testing the gravitational redshift, and Gravity Probe-B, testing the geodetic and the frame-dragging e ects. The scarcity of realized experiments speaks to the di culty of making exquisitely sensitive experiments in the space environment. Despite the di culties, the advantages of the space environment provide great promise in understanding many open questions in General Relativity.

The objective of ISLES (Inverse-Square Law Experiment in Space) is to perform a null test of Newton's law in space with a resolution of 10 ppm or better at a 100 μm distance. ISLES will be sensitive enough to detect the axion, a dark matter candidate, with the strongest allowed coupling and probe large extra dimensions of string theory down to a few micrometers. The experiment will be cooled to < 2 K, which permits superconducting magnetic levitation of the test masses. This soft, low-loss suspension, combined with a low-noise SQUID, leads to extremely low intrinsic noise in the detector. To minimize Newtonian errors, ISLES employs a near-null source, a circular disk of large diameter-to-thickness ratio. Two test masses, also disk-shaped, are suspended on the two sides of the source mass at a nominal distance of 100 μm. The signal is detected by a superconducting differential accelerometer.

Space offers unique experimental conditions and a wide range of opportunities to explore the foundations of modern physics with an accuracy far beyond that of ground-based experiments. Space-based experiments today can uniquely address important questions related to the fundamental laws of Nature. In particular, high-accuracy physics experiments in space can test relativistic gravity and probe the physics beyond the Standard Model; they can perform direct detection of gravitational waves and are naturally suited for investigations in precision cosmology and astroparticle physics. In addition, atomic physics has recently shown substantial progress in the development of optical clocks and atom interferometers. If placed in space, these instruments could turn into powerful high-resolution quantum sensors greatly benefiting fundamental physics. We discuss the current status of space-based research in fundamental physics, its discovery potential, and its importance for modern science. We offer a set of recommendations to be considered by the upcoming National Academy of Sciences' Decadal Survey in Astronomy and Astrophysics. In our opinion, the Decadal Survey should include space-based research in fundamental physics as one of its focus areas. We recommend establishing an Astronomy and Astrophysics Advisory Committee's interagency "Fundamental Physics Task Force" to assess the status of both ground- and space-based efforts in the field, to identify the most important objectives, and to suggest the best ways to organize the work of several federal agencies involved. We also recommend establishing a new NASA-led interagency program in fundamental physics that will consolidate new technologies, prepare key instruments for future space missions, and build a strong scientific and engineering community. Our goal is to expand NASA's science objectives in space by including "laboratory research in fundamental physics" as an element in the agency's ongoing space research efforts.

A superconducting device that is capable of measuring acceleration in all six degrees of freedom with great sensitivity is discussed. The device makes use of the fact that a superconducting mass levitated over a set of superconducting coils by a persistent current behaves as if it were supported by spring with a very stable spring constant. The displacement of the levitated mass modulates the inductance of spiral coils in close proximity to it. By forming inductance bridges from sets of such coils and reading the bridge misbalance signal with a SQUID, it is possible to sense very small displacements and rotations of the proof mass. In the first full tests of the device, its scale factor for acceleration (angular acceleration) was found to be approximately 2*10/sup 5/ Phi /sub 0//g (10/sup 3/ Phi /sub 0//rad/s/sup 2/) at the SQUID, in good agreement with values obtained from an analytical model. The inherent acceleration (angular acceleration) noise was found to be approximately 5*10/sup -10/ g/ square root Hz (10/sup -7/ rad/s/sup 2// square root Hz) which corresponds to the expected input noise for the commercial RF SQUID used. A substantial augmentation of the scale factors is possible by increasing the area of the sensing coils and improving the resolution with which the proof mass can be positioned. Such improvements, along with the use of lower noise DC SQUIDs, should allow the inherent acceleration (angular acceleration) noise to be reduced to a value close to the Brownian noise limit, approximately 10/sup -13/ g/ square root Hz (7*10/sup -11/ rad/s/sup 2// square root Hz).

A sensitive gravity gradiometer suitable for operation on an aircraft or ship is being constructed. The design is based on superconducting accelerometer technology developed at the University of Maryland over the past two decades. A cryogenic instrument not only benefits from reduced thermal noise, but more importantly from the extraordinary stability of superconducting circuits and material properties at near-zero temperatures. The latter permits precise matching of scale factors and accurate rejection of dynamic errors. One of the main perceived drawbacks of the Superconducting Gravity Gradiometer (SGG) has been the necessity for a liquid helium cryostat. To avoid this constraint, the SGG is being integrated with a closed-cycle refrigerator for operation at 3 K.

We report on the sensitivity of a transducer which uses a low noise SQUID amplifier. The transducer is made of niobium and was electropolished and heat treated to improve the Qs. The mechanical Q was found to be 3.15×106 at 4 K and the electrical Q was 2.52×105. With a low noise SQUID designed for this transducer, the noise temperature of the transducer was found to be 1.1 mK, of which 0.7 mK was due to the SQUID. This is about 200× larger than the expected noise from the SQUID. Noise from flux trapped on the chip is the most likely cause.

We report on the total noise from an inductive motion transducer for a gravitational-wave antenna. The transducer uses a two-stage SQUID amplifier and has a noise temperature of 1.1 mK, of which 0.70 mK is due to back-action noise from the SQUID chip. The total noise includes thermal noise from the transducer mass, which has a measured Q of 2.60 X 10^6. The noise temperature exceeds the expected value of 3.5 \mu K by a factor of 200, primarily due to voltage noise at the input of the SQUID. Noise from flux trapped on the chip is found to be the most likely cause., Comment: Accepted by Applied Physics Letters tentatively scheduled for March 13, 2000

A test of the University of Maryland’s Superconducting Gravity Gradiometer1 (SGG) on the Space Shutde is currendy being studied. The SGG can measure the spatial gradient of Earth’s gravity to very high precision, having a sensitivity of 10-3 E Hz-1/2 (1 E≡10-9 m/s2 per m) in each axis. For successful application of this instrument in an orbital platform, a number of difficulties must be overcome. One is that the measured gravity signal is strongly dependent on the orientation of the instrument with respect to Earth. Another is that centrifugal acceleration and gravity gradients are indistinguishable in a rotating reference frame. Also, angular accelerations can couple to the gradient output through imperfections in the instrument. It is therefore necessary to measure the angular position, velocity and acceleration of the gradiometer in order to recover the true gravity signal. Superconducting angular accelerometers are described which can meet the sensitivity requirements for all three quantities. The acceleration signal will be integrated once to obtain angular velocity and then again for position. Angular position measurement is most critical, and sets a requirement of measuring accelerations to 10-10 rad/s2 Hz-1/2. The sensors must be compatible with a cryogenic environment since they will be mounted onto the SGG. Two designs are being pursued, differing significantly only in how the proof mass is confined to rotations about a single axis. Both use niobium proof masses, superconducting position sensing circuits, and SQUIDs for signal readout as does the SGG. Design parameters and anticipated performance are described.

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-sigma) limit of alpha = (0.9 + or - 4.6) x 10 exp -4 at lambda of 1.5 m, where alpha and lambda are parameters for the generalized potential phi = -(GM/r)(l + alpha e exp -r/lambda).

Significant improvements of the present gravity field models of the Earth are required to advance various branches of solid-Earth geophysics and oceanography (McNutt and Flinn, 1987). A powerful method of acquiring high-resolution global gravity data is remote sensing by means of an orbiting gravity gradiometer (Paik et al, 1988). In order to recover the gravity field to the required precision of 2 to 3 mGals with a resolution of 50 km from an altitude of 200 km, a gradiometer sensitivity of 3 × 10−4 E Hz−1/2 (1 E ≡ 10−9 s−2) is needed. Such a sensitivity represents improvement by five orders of magnitude from the sensitivity achieved in a room-temperature gravity gradiometer (Wells, 1983), and requires an unconventional approach to the sensor design and platform control.