Human Gravity-Gradient Noise in Interferometric Gravitational-Wave Detectors

TL;DR

This paper analyzes human-induced gravity-gradient noise in gravitational-wave detectors, identifying key sources like walking and vehicle deceleration, and suggests distance-based mitigation strategies for advanced LIGO.

Contribution

It quantifies the gravity-gradient noise from human activities and proposes specific distance thresholds to mitigate this noise in LIGO detectors.

Findings

Walking causes measurable gravity-gradient noise in the 2.5-25 Hz band.

Maintaining a 10m distance from test masses reduces human walking noise.

Vehicles should be kept at least 30m away to minimize vehicle-related noise.

Abstract

Among all forms of routine human activity, the one which produces the strongest gravity-gradient noise in interferometric gravitational-wave detectors (e.g. LIGO) is the beginning and end of weight transfer from one foot to the other during walking. The beginning and end of weight transfer entail sharp changes (timescale tau ~ 20msec) in the horizontal jerk (first time derivative of acceleration) of a person's center of mass. These jerk pairs, occuring about twice per second, will produce gravity-gradient noise in LIGO in the frequency band 2.5 Hz <~ f <~ 1/(2 tau) ~= 25 Hz with the form sqrt{S_h(f)} \~0.6 X 10^{-23} Hz^{-1/2} (f/10Hz)^{-6} (sum_i (r_i/10m)^{-6})^{1/2}. Here the sum is over all the walking people, r_i is the distance of the i'th person from the nearest interferometer test mass, and we estimate this formula to be accurate to within a factor 3. To ensure that this noise is neglible in advanced LIGO interferometers, people should be prevented from coming nearer to the test masses than r ~= 10m. A r ~= 10m exclusion zone will also reduce to an acceptable level gravity gradient noise from the slamming of a door and the striking of a fist against a wall. The dominant gravity-gradient noise from automobiles and other vehicles is probably that from decelerating to rest. To keep this below the sensitivity of advanced LIGO interferometers will require keeping vehicles at least 30 meters from all test masses.