REVIEWS OF MODERN PHYSICS, VOLUME 90, OCTOBER–DECEMBER 2018 Nobel Lecture: LIGO and gravitational waves III* Kip S. Thorne California Institute of Technology, Pasadena, California 91125, USA (published 18 December 2018) DOI: 10.1103/RevModPhys.90.040503 CONTENTS in accepting the Nobel Prize, I regard myself as an icon for the Collaboration. I. Introduction and Overview 1 1 Because this was a collaborative achievement, Rai, Barry and II. Some Early Personal History: 1962–1976 1 III. Sources of Gravitational Waves 3 I have chosen to present a single, unified Nobel Lecture, in IV. Information Carried by Gravitational Waves, and three parts. Although my third part may be somewhat com- Computation of Gravitational Waveforms 5 prehensible without the other two, readers can only fully A. Observables from a compact binary’s inspiral waves 5 understand our Collaboration’s achievement, how it came to B. Post-Newtonian approximation for computing inspiral be, and where it is leading, by reading all three parts. Our three- waveforms 5 part written lecture is a detailed expansion of the lecture we C. Numerical relativity for computing merger waveforms 6 actually delivered in Stockholm on December 8, 2017. D. Geometrodynamics in BBH mergers 9 In Part 1 of this written lecture, Rai describes Einstein’s V. Theorists’ Contributions to Understanding and Controlling prediction of gravitational waves, and the experimental effort, Noise in the LIGO Interferometers 11 from the 1960s to 1994, that underpins our discovery of A. Scattered-light noise 11 gravitational waves. In Part 2, Barry describes the experi- B. Gravitational noise 11 mental effort from 1994 up to the present (including our first C. Thermal noise 11 observation of the waves), and describes what we may expect D. Quantum noise and the standard quantum limit for a as the current LIGO detectors reach their design sensitivity in gravitational interferometer 11 about 2020 and then are improved beyond that. In my Part 3, E. Quantum fluctuations, quantum nondemolition, and I describe the role of theorists and theory in LIGO’s success, squeezed vacuum 13 VI. The Future: Four Gravitational Frequency Bands 14 and where I expect gravitational-wave astronomy, in four A. LISA: The Laser Interferometer Space Antenna 14 different frequency bands, to take us over the next several B. PTAs: Pulsar timing arrays 15 decades. But first, I will make some personal remarks about C. CMB polarization 15 the early history of our joint experimental/theoretical quest to VII. The Future: Probing the Universe with Gravitational open the first gravitational-wave window onto the universe. Waves 16 A. Multi-messenger astronomy 16 II. SOME EARLY PERSONAL HISTORY: 1962–19761 B. Exploring black holes and geometrodynamics with gravitational waves 16 I fell in love with relativity when I was a teenage boy C. Exploring the first one second of our Universe’s life 17 growing up in Logan, Utah, so it was inevitable that I would VIII. Conclusion 18 go to Princeton University for graduate school and study Acknowledgments 18 under the great guru of relativity, John Archibald Wheeler. References 18 I arrived at Princeton in autumn 1962, completed my Ph.D. in spring 1965, and stayed on for one postdoctoral year. At I. INTRODUCTION AND OVERVIEW Princeton, Wheeler inspired me about black holes, neutron stars, and gravitational waves: relativistic concepts for which The first observation of gravitational waves, by LIGO on there was not yet any observational evidence; and Robert September 14, 2015, was the culmination of a near half Dicke inspired and educated me about experimental physics, ∼1200 century effort by scientists and engineers of the LIGO/ and especially experiments to test Einstein’s relativity theory. Virgo Collaboration. It was also the remarkable beginning In the summer of 1963 I attended an eight-week summer of a whole new way to observe the universe: gravitational school on general relativity at the École d’Ét´e de Physique astronomy. Theorique in Les Houches, France. There I was exposed to the “ ” The Nobel Prize for decisive contributions to this triumph elegant mathematical theory of gravitational waves in lectures was awarded to only three members of the Collaboration: by Ray Sachs, and to gravitational-wave experiment in Rainer Weiss, Barry Barish, and me. But, in fact, it is the entire lectures by Joe Weber. Those lectures and Wheeler’s influ- collaboration that deserves the primary credit. For this reason, ence, together with conversations I had with Weber while hiking in the surrounding Alpine mountains, got me hooked *The 2017 Nobel Prize for Physics was shared by Rainer Weiss, Barry C. Barish, and Kip S. Thorne. These papers are the text of the 1For greater personal detail that feeds into this lecture, see my address given in conjunction with the award. Nobel Biography. 0034-6861=2018=90(4)=040503(20) 040503-1 © 2018 Nobel Foundation, Published by the American Physical Society Kip S. Thorne: Nobel Lecture: LIGO and gravitational waves III FIG. 1. John Wheeler, Robert Dicke, and Joseph Weber. Credit: Wheeler: AIP Emilio Segr`e Visual Archives, Wheeler Collection. Dicke: Department of Physics, Princeton University. Weber: AIP Emilio Segr`e Visual Archives. on gravitational waves as a potential research direction. So it • Astrophysical electromagnetic waves are all too easily was inevitable that in 1966, when I moved from Princeton to absorbed and scattered by matter between their source Caltech and began building a research group of six graduate and Earth. Gravitational waves are never significantly students and three postdocs, I focused my group on black absorbed or scattered by matter, even when emitted in the holes, neutron stars, and gravitational waves. earliest moments of the Universe’s life. My group’s gravitational-wave research initially was quite These huge differences implied, it seemed to me, that theoretical. We focused on gravitational radiation reaction • Many gravitational-wave sources will not be seen (whether and how gravitational waves kick back at their source, electromagnetically. like a gun kicks back when firing a bullet). More importantly, • Just as each new electromagnetic frequency band (or we developed new ways of computing, accurately, the details of “window”) when opened—radio waves, X-rays, gamma the gravitational waves emitted by astrophysical sources such rays, …—had brought great surprises due to the differ- as spinning, deformed neutron stars, pulsating neutron stars, ence between that band and others, so gravitational and pulsating black holes. Most importantly (relying not only waves with their far greater difference from electromag- on our own group’s work but also on the work of colleagues netic waves are likely to bring even greater surprises. elsewhere) we began to develop a vision for the future of • Indeed, gravitational astronomy has the potential to gravitational-wave astronomy: What would be the frequency revolutionize our understanding of the universe. bands in which observations could be made, what might be the In 1972, while Bill Press and I were writing our first vision strongest sources of gravitational waves in each band, and what paper, Rai Weiss at MITwas writing one of the most remarkable information might be extractable from the sources’ waves. We and prescient papers I have ever read (Weiss, 1972). It proposed described this evolving vision in a series of review articles, an L-shaped laser interferometer gravitational-wave detector beginning with one by my student Bill Press and me in 1972 (gravitational interferometer) with free swinging mirrors, whose (Press and Thorne, 1972) and continuing onward every few oscillating separations would be measured via laser interferom- years until 2001 (Cutler and Thorne, 2002) when, with etry. The bare-bones idea for such a device had been proposed colleagues, I wrote the scientific case for the Advanced- earlier and independently by Michael Gertsenshtein and LIGO gravitational-wave interferometers (Thorne et al., 2001). Vladislav Pustovoit in Moscow (Gertsenshtein and Pustovoit, Particularly important to our evolving vision was the 1963), but Weiss and only Weiss identified the most serious extreme difference between the electromagnetic waves with noise sources that it would have to face, described ways to which astronomers then studied the universe, and the expected deal with each one, and estimated the resulting sensitivity to astrophysical gravitational waves: gravitational waves. Comparing with estimated wave strengths • Electromagnetic waves (light, radio waves, X-rays, from astrophysical sources, Rai concluded that such an inter- gamma rays, …) are oscillating electric and magnetic ferometer with kilometer-scale arm lengths had a real possibility fields that propagate through spacetime. Gravitational to discover gravitational waves. (This is why I regard Rai as the waves, by contrast, are oscillations of the “fabric” or primary inventor of gravitational interferometers.) shape of spacetime itself. The physical character of the Rai, being Rai, did not publish his remarkable paper in a waves could not be more different! normal physics journal. He thought one should not publish • Electromagnetic waves from astrophysical sources are until after building the interferometer and finding gravitational almost always incoherent superpositions of emission waves, so instead he put his paper in an internal MIT report produced by individual charged particles, atoms, or series, but provided copies to colleagues. molecules.