# Low frequency gravitational force sensing

We are working on a controls prototype for a low-frequency gravitational-force sensing. This prototype is currently in air with a static suspension as a proof of concept; to demonstrate our control and measurement method. It will later be placed in vacuum with suspension improvements to lower noise contributions and increase force sensitivity.

Second generation gravitational wave detectors are taking measurements, with strain sensitivities as low as 10-23/Hz in the frequency range from 10 Hz to 10 kHz. At frequencies below 10 Hz, these detectors are potentially limited by seismic and atmospherically induced Newtonian Noise [1]. To understand and characterise Newtonian Noise, a direct measurement of Newtonian Noise is first required.

The Torsion Pendulum Dual Oscillator (TorPeDO) is a low frequency gravitational force sensor, which aims to be limited by Newtonian Noise. The system features two torsion pendulums which are suspended along the same axis of rotation and perpendicular to each other. Local gravity gradient changes cause a differential attraction on the two torsion bars, which causes them to rotate in opposite directions. A differential optical measurement is taken between the two torsion bars to measure this. The mechanical configuration is chosen to maximise common mode rejection between the torsion bars. This is achieved by locating the centres of mass of both bars to the same point in space and tuning the torsion bars to the same frequency for most modes.

Other low frequency gravitational forces can be measured by this system, making it useful for early earthquake warning, giving 10s of seconds advanced warning over seismometers for an earthquake 50 km away [2] Low frequency systems such as the TorPeDO are of interest for testing theories of semi-classical gravity [3], by looking for a semi-classical resonance offset from the main resonance. A smaller version of this sensor is also planned for construction to measure quantum radiation pressure noise.

[1] J. Harms, et al. Phys. Rev. D 88, 122003 (2013)

[2] J. Harms, et al. Geophys J Int, 201(3) (2015).

[3] H. Yang, et. al., Phys. Rev. Lett., 110, 170401 (2013)