Netherlands Organization for Scientific Research (NWO)

December 15, 1997

Resonant detection of gravitational waves is feasible

Physicists at the universities of Amsterdam, Eindhoven, Leiden and Twente and at the NWO's Foundation for Fundamental Research on Matter (FOM), working within the so-called Grail Project, have concluded the feasibility of a resonant antenna with which to detect gravitational waves. These waves have never been observed so far. In order to pick up the waves, a solid sphere made of a copper alloy with a diameter of more than three metres will be required. The sphere will weigh more than a hundred tons and will need to be kept at a temperature which hardly rises above absolute zero. If all goes according to plan, the first measurements will be carried out in 2002.

According to Einstein's theory of relativity, gravitational waves occur when several masses are accelerated with respect to each other. This is the case, for example, when a workman drops a hammer on the earth, but also when a massive star collapses to form a neutron star, or when material is pulled into a black hole. The waves then spread throughout space. Within the massive copper sphere, a few of them can be transformed into mechanical vibration. The spherical form of the antenna means that the waves can reach it from every direction. It will be suspended in such a way that it is not subject to any other vibration. Extremely sensitive microphones will register the signal and amplify it.

In order to detect the very weak gravitational waves, it may be necessary to set up the sphere underground, where there is less interference from charged particles from space which would otherwise cause false signals. The instrument will be a few hundred times more sensitive than existing detectors of a similar nature. In order to measure gravitational energy and distinguish it from thermal interference, an ultra-low temperature is necessary, namely between 10 and 20 thousandths of a degree Kelvin, virtually absolute zero.

This is virtually absolute zero. The instrument will be most effective at registering frequencies of around 700 Hertz, but it will also be able to measure adjacent frequencies. The waveband it will cover will be between 100 and 150 Hertz wide.

The crux of the technology is in converting the mechanical signal into a value which can be displayed on a computer monitor. The sphere is therefore fitted with five amplifiers (SQUIDs) which ensure that its mechanical deformation is converted into a measurable electronic signal. This deformation is extremely slight, less than a hundred billionth of an Angstrom (10-21 metres or 0.000,000,000,000,000,001 millimetres). Casting the sphere will be difficult, but it should be technically possible. The metal used should not show any residual magnetism, and its thermal conductivity must be sufficient so that cooling the sphere down to the necessary low temperature does not take too long.

The researchers have tested the vibration characteristics of various alloys in small spheres and found that an alloy of copper and aluminium had particularly favourable qualities. The price of copper may also influence the choice of material.

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