Seismic and Newtonian Noise in the GW Detectors

Lucia Trozzo,Francesca Badaracco

doi:10.3390/galaxies10010020

Abstract

Gravitational wave detectors aim to measure relative length variations of the order of ΔL/L≃10−21, or less. Thus, any mechanism that is able to reproduce such a tiny variation can, in principle, threaten the sensitivity of these instruments, representing a source of noise. There are many examples of such noise, and seismic and Newtonian noise are among these and will be the subject of this review. Seismic noise is generated by the incessant ground vibration that characterizes Earth. Newtonian noise is instead produced by the tiny fluctuations of the Earth’s gravitational field. These fluctuations are generated by variations of air and soil density near the detector test masses. Soil density variations are produced by the same seismic waves comprising seismic noise. Thus, it makes sense to address these two sources of noise in the same review. An overview of seismic and Newtonian noise is presented, together with a review of the strategies adopted to mitigate them.

Highlights

Current gravitational wave (GW) detectors are sensitive to signals in a frequency band that ranges between 10 Hz and 10 kHz
Seismic noise can directly couple with the detector through what is known in GW physics as Newtonian noise (NN)
To isolate and suppress the seismic noise transmitted from the ground to the test masses, a GW detector requires complex mechanical suspensions that are essentially lowpass filters and that allow one to consider the test masses as free-falling bodies in the horizontal direction

Summary

Introduction

Current gravitational wave (GW) detectors are sensitive to signals in a frequency band that ranges between 10 Hz and 10 kHz. Their sensitivity can be affected by several dominant sources of noise, such as ground motion [1], local terrestrial gravity [2], magnetic noise [3], thermal [4] and quantum noise [5] These noises cause a phase variation of the laser light detected at the interferometer output port in the same way that a passing GW would. Seismic noise can directly couple with the detector through what is known in GW physics as Newtonian noise (NN). It is well known from Newtonian physics that a variation in mass density leads to fluctuations in the surrounding gravitational field.

An Introduction to Seismic Noise in GW Detectors

Seismic Noise

Seismic Noise in Underground Sites

Mechanical Attenuators

Seismic Isolation Systems

LIGO Seismic Isolation System

Newtonian Noise

Atmospheric Newtonian Noise

Seismic Newtonian Noise

Other Sources of Newtonian Noise

Newtonian Noise Cancellation

Findings