Visser has suggested traversable 3-dimensional wormholes that could plausibly form naturally during Big Bang ination. A wormhole mouth embedded in high mass density might accrete mass, giving the other mouth a net negative mass of unusual gravitational properties. The lensing of such a gravitationally negative anomalous compact halo object (GNACHO) will enhance background stars with a time prole that is observable and qualitatively dierent from that recently observed for massive compact halo objects (MACHOs) of positive mass. We recommend that MACHO search data be analyzed for GNACHOs.

We have carried out a preliminary experimental demonstration of the feasibility of using external electronic circuits to damp mechanical vibrations in optical systems. The significance of the feasibility demonstration is that substantial levels of vibrational amplitude reduction were obtained with very small transducers in nonoptimal positions on noncritical portions of the optical structure. The prototype optical structure used in the experiment consisted of a membrane mirror stretched over a 25-cm diam glass frame with complex cross section. Five small piezoelectric transducers (19.05 x 3.18 x 0.28 mm) were applied with Duco cement at arbitrary positions on the glass frame. Acoustic excitation was then used to excite the resonances in the optical structure. These vibrational responses were measured, and one particular mode was chosen for the feasibility test. The structure was driven by external vibrations at the resonant frequency of the chosen mode until the membrane response was visible. One transducer as used to sense the vibrations in the frame, and this output was used to drive a negative feedback amplifier that drove one of the other transducers on the frame. With the feedback circuit active between two points on the frame, the vibrational response of the membrane to the external excitation was substantially reduced (7:1).

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 kT strain noise in a roomtemperature, 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 singlebar 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. I. WIDEBAND INTERFEROMETER ANTENNA

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We have constructed and tested a laser interferometer transducer for a long, wideband, laser-linked gravitational radiation antenna. Photon-noise-limited performance was achieved using 80 microW from a single mode Spectra-Physics 119 laser in a modified Michelson interferometer on a vibration isolation table in a quiet room. A piezoelectric driver on one of the interferometer mirrors was used to generate subangstrom (3 x 10(-14)-m) vibrations of known amplitude. The measured displacement sensitivity of the system in the kilohertz region was 1.3 x 10(-14)m/Hz(1/2), which compares well with the calculated photon noise limit of 1.06 x 10(-14) m/Hz(1/2). This is the smallest vibrational displacement measured directly with a laser to date.