In practice the mechanical oscillator is replaced by a continuous elastic medium. The Grail detector will be a 110 ton copper alloy sphere of three meter diameter. A Gravitational wave would produce a strain h on the sphere equal to DL/L, where L is the diameter of the sphere and DL the change in diameter. Such a strain could be detected by measuring the sphere surface displacement. The required sensitivity corresponds to a displacement of the order of 10^-21 meter (one million less than the nuclear radius!). The measurement of these extremely small displacements is only possible by minimizing all the mechanical disturbances. The first of these are the acoustic and seismic noise, the reduction of which is obtained by suspending the antenna inside a vacuum chamber with some mechanical filters that provide an attenuation of the external vibration of the order of 10^-15. A second source of noise is the disordered thermal motion (Brownian motion) of the antenna atoms which is proportional to the thermodynamic temperature. These noise sources should be reduced to an acceptable level by cooling the ball to temperatures in the order of 10 mK.
The vibrations of the antenna are then converted into an electrical signal through transducers located on the sphere surface and amplified by a low noise device. The amplifier has to operate near the quantum mechanical energy resolution limit. At present the only amplifier with these characteristic is the SQUID (Superconducting Quantum Interference Device), which has to be cooled to below 1 K (possibly 100 mK) in order to decrease the thermal noise contribution.
The possibility to build a detector of the last generation in Holland is being studied by a group of institutes including the Kamerlingh Onnes Laboratory in Leiden (Group Prof. G. Frossati), the Dutch Nuclear and High Energy Physics Institute (NIKHEF) in Amsterdam , the Technical University of Twente, the University of Amsterdam and the Technical University of Eindhoven.