The 2016 Oskar Klein Memorial Lecture Gravitational Waves: The Physics and Astrophysics of LIGO

Gravitational waves are so radically different from electromagnetic waves that they are likely to revolutionize our understanding of the universe. LIGO, the Laser Interferometer Gravitational Wave Observatory, has recently opened up the first of four gravitational-wave windows onto the universe (the high-frequency window); and over the coming two decades, three more gravitational-wave windows will be opened. The astrophysical phenomena that LIGO is likely to explore are remarkable: Already it is exploring the collision and merger of spinning black holes and the resulting nonlinear dynamics of curved, empty spacetime. LIGO is likely also to detect and explore spinning neutron stars, collisions of neutron stars, black holes tearing neutron stars apart, the central engines of gamma ray bursts, perhaps the cores of supernova explosions, and perhaps vibrating cosmic strings (thought to have been produced by inflation of fundamental strings in the earliest moments of our universe). But most wonderful of all will be completely unexpected phenomena: big surprises. The physics of LIGO is also remarkable: Gravitational waves stretch and squeeze the separations of mirrors that hang from overhead supports at the ends of 4 kilometer “arms”; and those mirrors’ motions are monitored using light beams and interferometry. The wavelength of each light beam in LIGO gets stretched and squeezed by the same fractional amount as the mirror separations, so how can LIGO possibly see the waves? And the stretch and squeeze is 1/100 the diameter of a proton, or less — which means 10-11 of the wavelength of the light that is used to measure the stretch and squeeze. How can light possibly reveal such tiny motions? The atoms of which the mirrors are made have sizes 10 million times greater than the stretch and squeeze, and they vibrate, thermally, with amplitudes that are a million times larger than the stretch and squeeze; why doesn’t this produce noise that masks the waves’ signals? Ambient seismic motions in the ground beneath LIGO are ten million times greater than the waves’ stretch and squeeze; why don’t these seismic motions mask the signals? When LIGO reaches its design sensitivity, several years from now, the Heisenberg uncertainty principle, applied to LIGO’s 40 kilogram mirrors, says that the very act of measuring the mirror motions will perturb the mirrors so much that it may mask the signal. How can this quantum behavior of human sized mirrors be controlled, so as to preserve the signals?