Revealing Gravity's Secrets with Gravitational Waves

Everhart Lecture by Maximiliano Isi
On September 14, 2015, the two LIGO instruments directly detected, for the very first time, a gravitational wave produced by the collision of two black holes—an achievement that crowned decades of effort by hundreds of scientists, and inaugurated a new era of observational astronomy. These elusive spacetime ripples, which alternatively stretch and squeeze space as they whiz by at the speed of light, have since been observed at least five other times. Most remarkably, last August, when LIGO and its European counterpart, Virgo, made the first observation of the merger of two neutron stars. This gravitational signal was also accompanied by exuberant emission across the electromagnetic spectrum, leading to groundbreaking astrophysical insights.
Besides providing an invaluable wealth of astrophysical data, gravitational waves also carry important information about the nature of gravity itself. Although Einstein's general theory of relativity encapsulates our best understanding of this fundamental force, there are strong reasons to believe it is not its final description. In this Lecture, I will review how gravitational waves provide a vital window into the most interesting regimes of highly-dynamical and strong-field gravity. I will explain how observations with LIGO and Virgo have already provided some of the most stringent constraints on deviations from general relativity, focusing on work done at Caltech to measure fundamental properties of gravitational waves themselves. Finally, I will provide an overview of future prospects as we enter the new and exciting era of multi-messenger gravitational wave astronomy.