Characterization of rock vibrations at the Homestake mine - Gravitational-wave detection

By: Jan Harms, Angelo Sajeva, Riccardo Desalvo and Vuk Mandic 

Physicists describe a gravitational wave as an oscillatory disturbance of spacetime. For most purposes, it can be understood as a force field traveling at the speed of light which changes distances between free masses. A gravitational-wave detector has to monitor these changes. The LIGO scientific collaboration has built kilometer scale antennas which use laser light to measure the distance between suspended mirrors.

Many additional forces acting on the mirrors may mimic the influence of a gravitational wave: vibrations of the ground, thermally excited vibrations of the mirror’s surface, fluctuations inherent to the laser light and fluctuations of the gravitational field due to density fluctuations of the environment (i.e. Newtonian noise). Of all mentioned forces, it is the latter one which is considered to be the most problematic in the long run. Our goal at Homestake is to better understand the Newtonian noise and to develop methods which attenuate its detrimental effects on the gravitational-wave detector.

Gravitational waves are very difficult to measure. That is why we have to aim for the most energetic processes in the universe, like inspirals of binary black holes or neutron stars, the explosion of stars (supernovae) or the Big Bang itself which marks the birth of everything we observe today. Even then, distances between two mirrors inside a gravitational-wave antenna would not change more than a tiny fraction of the radius of atomic nuclei due to one of these cosmic events. Once we measure these waves, antennas around this world will open a completely new window to our universe. This time we would not look out of the window, but hold our ears to the glass pane and listen to the death of stars and the birth of our observable universe.