RIPPLES IN SPACETIME RIPPLES IN SPACETIME Einstein, Gravitational Waves, and the F uture of Astronomy GOVERT SCHILLING The Belknap Press of Harvard University Press Cambridge, Mas sac hu setts | London, England 2017 Copyright © 2017 by Govert Schilling All rights reserved Printed in the United States of Amer i ca First printing Library of Congress Cataloging- in- Publication Data Names: Schilling, Govert, author. Title: Ripples in spacetime : Einstein, gravitational waves, and the future of astronomy / Govert Schilling. Description: Cambridge, Massachusetts : The Belknap Press of Harvard University Press, 2017. | Includes bibliographic references and index. Identifiers: LCCN 2017007571 | ISBN 9780674971660 (alk. paper) Subjects: LCSH: Gravitational waves. | Relativistic astrophysics—Methodology. | Einstein, Albert, 1879–1955. Classification: LCC QC179 .S287 2017 | DDC 539.7/54—dc23 LC record available at https://lccn.loc.gov/2017007571 Jacket art: ©Nicolle R. Fuller / Science Source Jacket design: Annamarie McMahon Why Contents Foreword by Martin Rees vii Introduction 1 1 A Spacetime Appetizer 5 2 Relatively Speaking 22 3 Einstein on Trial 40 4 Wave Talk and Bar Fights 60 5 The Lives of Stars 77 6 Clockwork Precision 94 7 Laser Quest 112 8 The Path to Perfection 129 9 Creation Stories 150 10 Cold Case 167 11 Gotcha 186 12 Black Magic 206 13 Nanoscience 226 14 Follow- Up Questions 246 15 Space Invaders 267 16 Surf’s Up for Einstein Wave Astronomy 288 Notes and Further Reading 307 Acknowl edgments 320 Illustration Credits 321 Index 323 v Foreword MARTIN REES Einstein has a unique place in the pantheon of science—a nd deserv- edly so. His insights into space and time have transformed our under- standing of gravity and the cosmos. Every one recognizes the benign and unkempt sage of poster and T-s hirt. But his best work was done when he was young. He was still in his thirties when he was catapulted to worldwide fame. On May 29, 1919, t here was a solar eclipse. A group led by the astronomer Arthur Eddington observed stars ap- pearing close to the Sun during the eclipse. The meas ure ments showed that these stars were displaced from their normal positions, the light from them being bent by the Sun’s gravity. This confirmed one of Einstein’s key predictions. When these results were reported at the Royal Society in London, the world press spread the news. “Lights All Askew in the Heavens; Einstein Theory Triumphs” was the rather over- the- top headline in the New York Times. Einstein’s theory of general relativity, put forward in 1915, is a tri- umph of pure thought and insight. Its implications for us on Earth are slight. It requires minor adjustments in the clocks used in modern navigation systems, but Newton remains good enough for launching and tracking space probes. In contrast, Einstein’s insight that space and time are linked—t hat “space tells m atter how to move; m atter tells space how to curve”—is vii Foreword crucial to many cosmic phenomena. But it’s hard to test a theory whose consequences are so remote. For almost half a century after it was proposed, general relativity was sidelined from the mainstream of physics. But from the 1960s onward, evidence has grown for a “big bang” that set the universe expanding and for black holes—t wo of Einstein’s key predictions. And in February 2016, nearly a hundred years after the famous Royal Society meeting reporting the eclipse expedition, another announcement— this time at the Press Club in Washington, DC— further vindicated Einstein’s theory. This was the detection of gravi- tational waves by LIGO ( Laser Interferometer Gravitational- Wave Observatory). This is the theme of Govert Schilling’s book. He has a wonderful story to tell, spanning more than a century. Einstein envisaged the force of gravity as a “warping” of space. When gravitating objects change their shapes, they generate r ipples in space itself. When such a r ipple passes the Earth, our local space “jitters”: it is alternately stretched and compressed as gravitational waves pass through it. But the effect is tiny. This is b ecause gravity is such a weak force. The gravitational pull between everyday objects is minuscule. If you wave around two dumbbells, you will emit gravita- tional waves— but with quite infinitesimal power. Even planets orbiting stars, or pairs of stars orbiting each other, don’t emit at a de- tectable level. Astronomers are agreed that the only sources that LIGO might de- tect must involve much stronger gravity than in ordinary stars and planets. The best bet is that the events involve black holes. We’ve known for nearly fifty years that black holes exist: most are the rem- nants of stars twenty or more times more massive than the Sun. These stars burn brightly, and in their explosive death throes (signaled by a supernova), their inner part collapses to a black hole. The material that the star was made of gets cut off from the rest of the universe, leaving a gravitational imprint on the space it’s left. If two black holes w ere to form a binary system, they would grad- ually spiral together. As they get closer, the space around them be- comes more distorted until they coalesce into a single, spinning hole. viii Foreword This hole sloshes and “rings,” generating further waves u ntil it settles down as a single, quiescent hole. It is this “chirp”—a shaking of space that speeds up and strengthens u ntil the merger, and then dies away— that LIGO can detect. These cataclysms happen less than once in a million years in our galaxy. But such an event would give a detectable LIGO signal even if it happened a billion light- years away— and there are millions of galaxies closer than that. To detect even the most pro- pitious events requires amazingly sensitive— and very expensive— instruments. In the LIGO detectors, intense l aser beams are pro- jected along four- kilometer- long vacuum pipes and reflected from mirrors at each end. By analyzing the light beams, it’s pos si ble to de- tect changes in the distance between the mirrors, which alternately increases and decreases as space expands and contracts. The amplitude of this vibration is exceedingly small, about 0.0000000000001 centimeters— millions of times smaller than the size of a single atom. The LIGO proj ect involves two similar detectors about 3,000 kilo- meters apart—o ne in Washington State, the other in Louisiana. A single detector would register microseismic events, passing vehicles, et cetera. To exclude t hese false alarms, experimenters take note only of events that show up in both. For years, LIGO detected nothing. But it went through an up- grade, coming fully on line again in September 2015. A fter literally de cades of frustration, the quest succeeded: a chirp was detected that signaled the collision of two black holes more than a billion light- years away, and it opened up a new field of science—p robing the dynamics of space itself. It is sadly not unknown for hyped-up scientific claims to be mis- taken or exaggerated—a nd the book recounts such claims in this field, too. I count myself a hard-t o- convince skeptic. But what the LIGO researchers claimed— the culmination of literally dec ades of effort by scientists and engineers with high credentials—is compelling, and this time I expect to be fully convinced. This detection is indeed a big deal. It’s one of the great discoveries of the de cade, up there with the detection of the Higgs particle, which caused huge razzmatazz in 2012. The Higgs particle was a capstone ix