Test masses are placed in vacuum and are either freely falling (e.g., atom clouds ), or suspended and seismically isolated (e.g., high-quality glass or crystal mirrors as used in all of the detectors listed above). Progress in this field is strongly dependent on how well the motion of the test masses can be shielded from the environment. The basic measurement scheme is always the same: the relative displacement of test masses is monitored by using ultra-stable lasers. Second, several groups around the world have already started to adapt the technology to novel interferometer concepts, with potential applications not only in GW science, but also geophysics. First of all, the direct detection of GWs will be a milestone in science opening a new window to our universe, and marking the beginning of a new era in observational astronomy. The impact of these projects onto the field is two-fold. Feasibility of the laser-interferometric detector concept has been demonstrated successfully with the first generation of detectors, which, in addition to the initial LIGO and Virgo detectors, also includes the GEO600 and TAMA300 detectors, and several prototypes around the world. Their goal is to directly observe for the first time GWs that are produced by astrophysical sources such as inspiraling and merging neutron-star or black-hole binaries. The latter will be realized by the advanced generation of the US-based LIGO and Europe-based Virgo gravitational-wave (GW) detectors.
In the coming years, we will see a transition in the field of high-precision gravimetry from observations of slow lasting changes of the gravity field to the experimental study of fast gravity fluctuations. Our understanding of terrestrial gravity fluctuations will have great impact on the future development of GW detectors and high-precision gravimetry in general, and many open questions need to be answered still as emphasized in this article.
The article reviews the current state of the field, and also presents new analyses especially with respect to the impact of seismic scattering on gravity perturbations, active gravity noise cancellation, and time-domain models of gravity perturbations from atmospheric and seismic point sources. The models are then used to evaluate passive and active gravity noise mitigation strategies in GW detectors, or alternatively, to describe their potential use in geophysics. Models of terrestrial gravity perturbations related to seismic fields, atmospheric disturbances, and vibrating, rotating or moving objects, are derived and analyzed. The goal of this article is to provide the analytical framework to describe terrestrial gravity perturbations in these experiments. Alternative designs for GW detectors evolving from traditional gravity gradiometers such as torsion bars, atom interferometers, and superconducting gradiometers are currently being developed to extend the detection band to frequencies below 1 Hz. The technology is pushed to its current limits in the advanced generation of the LIGO and Virgo detectors, targeting gravity strain sensitivities better than 10 −23 Hz −1/2 above a few tens of a Hz. This field is rapidly progressing through the development of GW detectors. Here, we will focus on ground-based gravimetry. Accordingly, terrestrial gravity fluctuations are considered noise and signal depending on the experiment. Gravity changes caused by high-magnitude earthquakes have been detected with the satellite gravity experiment GRACE, and we expect high-frequency terrestrial gravity fluctuations produced by ambient seismic fields to limit the sensitivity of ground-based gravitational-wave (GW) detectors.
For example, atmospheric pressure fluctuations generate a gravity-noise foreground in measurements with super-conducting gravimeters. Different forms of fluctuations of the terrestrial gravity field are observed by gravity experiments.
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