Gravitational Wave Detectors

Radio wave detectors are calibrated by sending out radio waves from a transmitter several wavelengths away and more. Why can’t builders of gravitational wave detectors do the same thing? After all, one could put two large masses at opposite sides of a rotating platform and spin them around to have a gravitational wave source for detector calibration.

Answer

For all kinds of waves, for locations beyond several wavelengths, the solutions of the wave equation correspond to the radiation field transporting energy and momentum from the source into the surrounding space. When considering possible sources of gravitational waves in the Galaxy and beyond, the wavelengths are typically several kilometers or more. One could place the rotating laboratory gravitational wave source several kilometers away or more from the gravitational wave detector, but the decrease in the radiation field intensity with distance squared combined with the low sensitivity of the detectors make this arrangement unlikely to work with present detectors. Therefore, as far as we know, there has never been a true test of the gravitational wave response of any detector to gravitational radiation using laboratory sources of gravity waves.

There have been two fundamental types of gravitational wave detectors: the Weber bar antenna, named after pioneering physicist Joseph Weber, who began this research field in the 1950s with his one meter diameter suspended aluminum bar; and the interferometer type such as LIGO, first analyzed by the same Joseph Weber and his students. The classical calculation of the resonant response of the Weber bar reveals just how limited is its sensitivity to gravitational waves originating in our Solar System and Galaxy. However, if the Weber bar antenna actually behaves differently than originally expected, as a collective quantum oscillator responding coherently, say, then it will respond well to all frequencies of the incident gravitational waves. Hundreds to thousands of vibrational modes could be excited in a large range of frequencies with an increase in sensitivity of many powers of ten.

At this time there has been no substantiated detection of gravitational waves by either type of detector. Weber reported a twice-daily response of his two side-by-side almost-identical bar antennas for orientations pointing toward the center of the Galaxy during a period of almost two decades, but no other researcher has verified this behavior with an independent detector. So we must wait for the first detection of gravitational waves by LIGO or other detectors. Unfortunately, interferometer types of detectors such as LIGO and VIRGO cannot operate as a collective quantum oscillator.