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Human gravity-gradient noise in interferometric gravitational-wave detectors

Thorne, Kip S. and Winstein, Carolee J. (1999) Human gravity-gradient noise in interferometric gravitational-wave detectors. Physical Review D, 60 (8). Art. No. 082001. ISSN 0556-2821.

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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 (time scale τ∼20 msec) in the horizontal jerk (first time derivative of acceleration) of a person’s center of mass. These jerk pairs, occurring about twice per second, will produce gravity-gradient noise in LIGO in the frequency band 2.5 Hz≲f≲1/(2τ)≃25 Hz with the form sqrt[Sh(f)]∼0.6×10-23 Hz-1/2(f/10 Hz)-6[∑i(ri/10 m)-6]1/2. Here the sum is over all the walking people, ri 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 negligible in advanced LIGO interferometers, people should be prevented from coming nearer to the test masses than r≃10 m. A r≃10 m 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 m from all test masses.

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Additional Information:©1999 The American Physical Society Received 5 October 1998; published 24 September 1999 We are grateful to Robert Spero for triggering our interest in this problem and for helpful discussions early in this work. We thank Ernest L. Bontrager (Associate Director of Engineering Research at the Pathokinesiology Service of Rancho Los Amigos Medical Center, Downey, California) for providing us with the force-plate data that underlie Fig. 1 and the analysis in Sec. II A, and Ge Wu (Assistant Professor of Physical Therapy, University of Vermont) for helpful discussions of her accelerometer data that underlie Table II. We thank James Ipser and Lee Lindblom for information about automobile motion, and Albert Lazzarini for providing us with details of the LIGO site design. This research was supported in part by NSF Grant PHY-9424337.
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