History of Resonant Mass Gravitational Wave Antennae

Figure 1: Joseph Weber, inventor of resonant-mass gravitational wave antennas.

For more than 25 years, gravitational waves have eluded confirmed experimental detection. The pioneering proposal to detect gravitational waves was made by Joseph Weber in the early 1960's. He proposed using a large piezoelectric crystal to detect the oscillating strain produced by an oscillating gravitational field.

By 1966 Weber had constructed the first resonant-mass gravitational wave antenna. It was a large, room-temperature aluminum bar that was vibrationally isolated in a vacuum chamber. Quartz strain gauges were used to monitor the bar's fundamental mode of vibration. By 1969 Weber had achieved strain sensitivities of a few parts in 1e16 and had constructed several more gravitational wave detectors. He soon announced that he had observed coincidences between them. These results generated great excitement in the field and other groups began constructing gravitational wave detectors. In the end, however, Weber's findings could not be confirmed by other groups who built similar detectors.

By the early 1970's other groups were involved in building advanced gravitational wave detectors. These groups made a number of significant improvements over Weber's original design. One improvement was to lower the temperature of the bar to liquid helium temperatures (4 Kelvin). The second was a better suspension of the bar with increased vibration isolation. A third was the use of a resonant transducer and low noise amplifier to observe the motion of the bar. The small resonator not only amplified the displacement but attenuated large amplitude vibrations at low frequencies. Today there are three detectors of this type being operated: the LSU ALLEGRO detector, the Rome EXPLORER detector, and the Australian detector.

In 1991 the first tests of an ultra-low-temperature (50 mK) detector were performed. Although the expected improvements have not yet been demonstrated, the techniques look promising.

Figure 2: The LSU ALLEGRO detector with an end cap removed.

The best current antennas, such as the LSU ALLEGRO detector, are sensitive enough to detect a gravitational collapse in our galaxy, if the energy converted to gravitational waves is a few percent of a solar mass. However, the conventional wisdom is that we need to look at least 3 orders of magnitude further in distance, out to the Virgo Cluster, to have an "assured" event rate of several per year. This requires improving the energy resolution of the detector by 6 orders of magnitude.

To acheive this improvement, the LSU group has proposed building a spherical detector. A spherical resonant mass gravitational wave antenna has a number of inherent properties that give it an advantage over other types of detectors. A single sphere is capable of detecting gravitational waves from all directions and polarizations. One would have to construct 5 equivalent bars (bars with the same resonant frequency) to obtain the same amount of information. Therefore, a sphere can be thought of 5 detectors in a single instrument. A sphere is also capable of determining the direction information and tensorial character of an incident gravitational wave. A sphere will have a larger mass than an equivalent bar, which translates into an increased cross section, thus improving the sensitivity of the antenna.

The LSU group proposes using a special arrangement of 6 attached resonators is proposed, which we term a Truncated Icosahedral Gravitational Wave Antenna, or TIGA. They have constructed a small truncated icosahedron to test a model for a spherical resonant mass gravitational wave antenna. This shape was machined from an Al 6063 cylindrical bar and is 84 cm in diameter. The first quadrupole resonances were near 3200 Hz. It was suspended from its center of mass. They observed the motion of the prototype's surface using 6 accelerometers attached to its surface in the symmetric truncated icosahedral arrangement. They have tested a first order direction finding algorithm, which uses fixed linear combinations of six accelerometer responses to first infer the relative amplitudes of the quadrupole modes and from these the location of the impulse.

Figure 3: The truncated icosahedral gravitational wave antenna (TIGA) with secondary resonator locations indicated.

Although a complete investigation of the practicality of a spherical gravitational wave antenna has not been completed, the LSU work has generated great excitement in the field of resonant mass detectors. Several groups have begun exploring the possibility of constructing large spherical antennas. These include GRAVITON in Brazil, GRAIL in the Netherlands, ELSA in Italy, and TIGA in the United States. Two collaborations to build such antennas have also been formed: the US Gravity Wave Co-op and the international OMEGA collaboration.

This page is a modified excerpt of Stephen Merkowitz's Dissertation.