Multiple mode gravitational wave detection with a spherical antenna

Abstract

Apart from omnidirectional, a solid elastic sphere is a natural multimode and multifrequency device for the detection of Gravitational Waves (GW). Motion sensing in a spherical GW detector thus requires a multiple set of transducers attached to it at suitable locations. Resonant transducers exert a significant back action on the larger sphere, and as a consequence the joint dynamics of the entire system has to be properly understood before reliable conclusions can be drawn from its readout. In this paper, we present and develop a mathematical formalism to analyse such dynamics, which generalises and enhances currently existing ones, and which clarifies their actual range of validity, thereby shedding light into the physics of the detector. In addition, the new formalism has enabled us to discover a new resonator layout (we call it PHC) which only requires five resonators per quadrupole mode sensed, and has mode channels, i.e., linear combinations of the transducers' readouts which are directly proportional to the GW amplitudes. The perturbative nature of our proposed approach makes it also very well adapted to systematically assess the consequences of small mistunings in the device parameters by robust analytic methods. These prove to be very powerful, not only theoretically but also practically: confrontation of our model's predictions with the experimental data of the LSU prototype detector TIGA reveals an agreement between both consistently reaching the fourth decimal place.