COLLOQUIUM

The large-scale gravitational-wave (GW) detectors Virgo and LIGO have achieved unprecedented sensitivities in relative position measurements between suspended test masses. Technological breakthroughs were necessary concerning, among others, stabilized lasers and the fabrication of optical components of highest mechanical quality and geometric accuracy. While quantum fluctuations of the light and thermal excitations of test masses and their suspensions are relatively easy to predict, the tremendous challenge to identify and suppress all sorts of environmental couplings was hardly anticipated in its full scale when Virgo and LIGO were first conceived. The reduction of environmental noise and noises created by the control systems suppressing the environmental noise is the main challenge of detector commissioning and sensitivity advance today. Among the many environmental couplings, direct gravitational coupling between test masses and their environment stands out as a nearly insurmountable obstacle to extend the observation band of ground-based detectors to frequencies below 10Hz. Hundreds of millions of Euros might be spent for the reduction of terrestrial gravity noise in next-generation GW detectors like the proposed Einstein Telescope. In this colloquium, we will review the various sources of terrestrial gravity noise and outline the mitigation strategies envisioned by GW scientists.