William Joseph Weber, Professor, Dipartimento di Fisica Università di Trento and TIFPA/INFN, Italy
A space gravitational wave observatory will measure the time-varying gravitational tidal deformation on a constellation of distant free-falling test particles. LISA Pathfinder, in orbit at the first Earth-Sun Lagrange point since early 2016, tests a miniature version of one link in such a constellation, with an interferometric measurement of the relative acceleration of two test masses inside a single spacecraft. Pathfinder has demonstrated a typical resolution of 15~atto-g on 1000~s time scales, a several order-of-magnitude improvement upon current gradiometers for geodesy and sufficient, if achieved in a LISA-like observatory configuration, for high precision observation of merging super-massive black holes and other low frequency gravitational wave sources. Demonstrating sub-femto-g free-fall has required deep space, drag-free control, and innovations in interferometry and inertial sensing, but also a growing experimentally-anchored physical model of femto-Newton force sources ranging from cosmic ray charging to Brownian motion to spacecraft self-gravity.
This talk presents the LISA Pathfinder measurement results and discusses the legacy for gravitational wave astronomy in space and experimental gravitation in general.