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As the key measurement load of Taiji-1 satellite, inertial sensor detects the acceleration disturbance of test mass (TM) under nonconservative force in line with the basic principle of capacitive sensing, while keeping the TM in equilibrium position through electrostatic drive. In order to ensure the smooth progress of the mission, it is necessary to test and evaluate the performance of inertial sensor on the ground. In this paper, a torsion pendulum system is designed to eliminate the influence of the Earth’s gravity so as to meet the requirements of ground test. The experimental results show that the inertial sensor in closed-loop control mode can stably keep the TM at equilibrium position. At the same time, the ground detection of acceleration resolution of inertial sensor is greatly affected by ground vibration noise. If the inertial sensor operates normally in space, its acceleration resolution can reach$3.96\times {10}^{-9}\mathrm{\text{m}}/{\mathrm{\text{s}}}^2/\sqrt{\mathrm{\text{Hz}}}$, thus meeting the requirement of Taiji-1.

Many theories which unify gravity with the other known forces of nature predict the existence of an intermediate-range "fifth force" similar to gravity. Such a force could be manifest as a deviation from the gravitational inverse-square law. Currently, at distances near 10^-1 m, the inverse-square law is known to be correct to about one part per thousand. I present the design of an experiment that will improve this limit by two orders of magnitude. This is accomplished by constructing a torsion pendulum and source mass apparatus that are particularly insensitive to Newtonian gravity and, simultaneously, maximally sensitive to violations of the same.

Theoretical speculations about the quantum nature of the gravitational interaction have motivated many recent experiments. But perhaps the most profound and puzzling questions that these investigations address surround the observed cosmic acceleration, or Dark Energy. This mysterious substance comprises roughly 2/3 of the energy density of the universe. Current gravitational experiments may soon have the sensitivity to detect subtle clues that will reveal the mechanism behind the cosmic acceleration. On the laboratory scale, short-range tests of the Newtonian inverse-square law (ISL) provide the most sensitive measurements of gravity at the Dark Energy length scale, λ_d = (ħc/ρ_d)^1/4 ≈ 85 μm, where ρ_d ≈ 3.8 keV/cm³ is the observed Dark Energy density. This length scale may also have fundamental significance that could be related to the "size" of the graviton. At the University of Washington, we are conducting the world's most sensitive, short-range test of the Newtonian ISL.

An electrostatically controlled torsion pendulum has been constructed to investigate the performance of the ASTROD I inertial sensor on the ground. The twist motion of the pendulum is monitored by a capacitive transducer and controlled by an electrostatic actuator. The preliminary experimental results show that the torque resolution of the pendulum is 2 × 10^-12 N m Hz^-1/2 at 3 mHz. Further improvements under consideration for the inertial sensor are discussed.

Expanding the observational frequency of gravitational waves is important for the future of astronomy. Torsion-Bar Antenna (TOBA) is a mid-frequency and low-frequency gravitational wave detector using a torsion pendulum. The low resonant frequency of the rotational mode of the torsion pendulum enables ground-based observations. The overview of TOBA, including the past and present status of the prototype development, is summarized in this paper.

We describe a new actuation technique for gravity experiments based on a liquid field mass. The characterizing idea is to modulate the gravity force acting on a test mass by controlling the level of a liquid in a suitable container. This allows to obtain a periodical gravity force without moving parts (except the liquid level) close to the TM. We describe in detail the most relevant aspects of the liquid actuator and discuss how it can be used in gravity experiments. In particular, we analyze an application to test the inverse square law in the mm to cm distance region.

A new determination of Newtonian gravitational constant G is presented by using a torsion pendulum with time-of-swing method. Features of the design include: (1) two SS316 stainless steel spheres with more homogeneous density were used as the source masses; (2) a rectangular glass block was used as the main body of the pendulum; (3) both the pendulum and source masses were set in a vacuum chamber to reduce the error of measuring the relative positions conveniently; (4) exchanging of the configurations between "near" and "far" positions was remotely operated by using a stepper motor; (5) the anelastic effect of the torsion fiber was first measured directly by using two disk pendulums with the help of a high-Q factor quartz fiber. We repeated the G measurements twice independently, and two G values have only a difference of 9 ppm. The combined final value of G was found to be (6.67349±0.00018) × 10^-11 m³kg^-1s^-2 with a relative uncertainty of 26.33 ppm.