Gravitational Waves Recollection

Written on February 11, 2016

Today's  news  fulfills Einstein's prediction of the existence of gravitational waves and opens up the new field of gravitational wave astrophysics. Interactions in nature are mediated by the exchange of particles, the photon, for instance, in the case of electromagnetism. Only gravity lacked the mediator, the "graviton". LIGO's  announcement is that streams of gravitons, that is gravitational waves, have now been observed directly. 

Because of its weakness , gravitational radiation can be produced only in catastrophic events like the collision of very massive objects like black holes and neutron stars and it is from the distribution and strength of these entirely new signals that a new view of the Universe will emerge. For those who like myself were excited over forty years ago by the audacious prospects that Einstein's theory offered, today's achievement produces profound satisfaction and excitement.

Regina experiment to detect gravitational waves was on a much smaller scale than  LIGO  and our technology was crude in comparison. Our objective was to observe  the short burst of radiation emitted in the sudden collapse of large stars, at a frequency of about 10^5 Hz that could therefore be detected at resonance by a metal bar about 1 meter long and of mass equal to several hundred Kilograms. The resonance factor, about 10^6, was essential in reaching an acceptable sensitivity. The gravitational waves would set up oscillations in the bar. These would then  be converted into electromagnetic signals by a transducer. Several of the antennas produced between 1965 and 1980 in various laboratories were cooled to liquid Helium temperatures to reduce the thermal noise against which the oscillations produced by the gravitational waves had to be observed. The apparatus had also to be isolated from environmental and seismic noise. The Regina antenna was made of quartz and was therefore its own transducer, very much like in a watch the oscillations of  the crystal are transformed into electric signals. This was an advantage, because the antenna was simpler and  the high quality factor of quartz made it almost impervious to noise. Moreover the antenna's mass was smaller and could therefore be more easily cooled to very low temperatures, just a few millidegrees above absolute zero. Because of these characteristics, we  pioneered the use of crystals as antennas and also experimented at length with sapphire. Crystals offered however enormous challenges from the electronic point of view because no suitable, low noise electronic was available  and no funds could be found  to face the extremely difficult task of constructing  a superconducting amplifier of appropriate noise characteristics. The apparatus reached a final sensitivity  of less than a millionth  that of LIGO,  in a totally different frequency range (8846 Hz)  and could have not therefore detected  the coalescing of two black holes which is the phenomenon described in today's announcement.  Because of the rare occurrence of the events that could be detected with the technology of those days, a star explosion in about thirty years in nearby galaxies, all resonating bar antennas  were gradually abandoned in favour of broadband interferometers of the scale of LIGO and VIRGO.  

Giorgio Papini,

Professor Emeritus, Department of Physics, University of Regina