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Submitted on 1 Jan 1985
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ATTEMPTS TO DETECT GRAVITATIONAL WAVES
WITH RESONANT BARS
W. Robinson
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C10, supplbment au n012, Tome 46, dbcembre 1985 page C10-773
ATTEMPTS TO DETECT GRAVITATIONAL WAVES WITH RESONANT BARS
W.H. ROBINSON
Physics & Engineering Laboratory, Department of Scientific &
Industrial Research, Lower Hutt, New Zealand,
T h e radiative solution of Einstein's theory of General Relativity indicates t h a t gravitational waves should exist, travel at t h e speed of light and produce ripples in t h e curvature of space-time. These waves a r e predicted to b e a
"dimensionless strain ,of space" which, for a wave travelling in direction '3', consist of t w o orthogonal polarizations h l l and h l 2 with h l l = -h22 and
h i 2 = hZI. In 1960 Weber published a paper suggesting experimental techniques for measuring gravitational radiation. The "resonant bar" detector h e
suggested is now being used by a t least ten groups throughout t h e world. W e have made a gravitational wave d e t e c t o r using a pair of PZT-8 ceramic transducers 100 mm in diameter and 20 mm thick. These transducers have a rms strain amplitude in the thickness mode of -2 x 10-14 a t their resonant
frequencies of 110 kHz due t o 'kT1 of noise. T h e frequencies of t h e two transducers have been adjusted, by capacitive loading, t o within -10 Hz of e a c h other. The output signals from t h e t w o transducers (A, B) a r e amplified by tuned preamplifiers before entering summing (A
+
B) and difference (A-
8 ) amplifiers. An electronic switch samples t h e sum then t h e difference a t a r a t e of 5Hz before t h e signal is amplified, demodulated and s e n t t o alock-in-amplifier operating a t t h e sampling frequency. This process, which enables t h e d e t e c t o r t o b e switched on (A
+
B) and then off (A-
B), improves t h e strain sensitivity from t h e initial value of -2 x 10-14 t o -3 x 10-16. T h e next improvement of sensitivity is obtained by sweeping t h e transducers slowly around an axis parallel t o t h e earth's polar axis with measurements being t a k e n every If0. These readings a r e stored on magnetic tape before they a r e processed t o determine t h e variation in strain amplitude with d e t e c t o r position, universal time and siderial time. For an integration time of 6 months the sensitivity is b e t t e r than 3 xA brief summary of t h e theoretical reasons for t h e existance of gravitational radiation will be presented together with a survey of the various d e t e c t o r s based on t h e resonant bar. Finally, a f t e r describing our piezoelectric d e t e c t o r and t h e d a t a obtained over t h e last 18 months, possible improvements t o t h e sensitivity of resonant bar d e t e c t o r s will b e discussed.
