Wideband laser-interferometer graviational-radiation experiment

Abstract

A wideband laser-interferometer gravitational-radiation antenna was constructed and used to search for gravitational radiation in the frequency band from 1 to 20 kHz. The antenna consisted of a Michelson interferometer with the beamsplitter and retroreflectors attached to masses on soft suspensions that allowed essentially free motion above the suspension frequencies. The strains in the gravitational radiation produce a differential path length change in the two arms of the interferometer which is detected by a pair of balanced photodetectors. The interferometer used a folded-path configuration with an effective length of 8.5 m. The sensitivity of the interferometer was calibrated with signals from a piezoelectric displacement transducer. The strain noise in a 1-Hz bandwidth was less than 0.3 fm/m from 1 to 3 kHz, and less than 0.1 fm/m above 3 kHz, where it was essentially photon-noise limited. (For comparison, the $\mathrm{kT}$ strain noise in a room-temperature, 2-m long, 1000-kg, elastic solid bar antenna is 0.14 fm/m.) The laser interferometer was operated as a detector for gravitational radiation for 150 h during the nights and weekends from the period 4 October through 3 December 1972. During the same period, bar antennas were operated by the Maryland, Glasgow, and Frascati groups, with 18 events reported by the Frascati group in their single bar, 22 single-bar events and no coincidences reported by the Glasgow group in their two bars, and 28 coincidences reported by the Maryland group between the Argonne bar and the Maryland bar and/or disk antennas. The various bar antenna systems were quite different but in general were sensitive to gravitational-radiation strain spectral components with an amplitude of the order of 0.1 fm/m in a narrow band of frequencies about the resonant frequency of the bar. The wideband interferometer data was analyzed by ear, with the detection sensitivity estimated to be of the order of 1-10 fm/m (depending upon the signature of the signal) for the total of the gravitational-radiation strain spectral components in the band from 1-20 kHz. No significant correlations between the Malibu interferometer output and any of the bar events or coincidences were observed.