Is Einstein Still Right?

by Clifford M. Will

  Is Einstein Still Right?

  Is Einstein Still Right?

  Clifford M. Will and Nicolás Yunes

  Great Clarendon Street, Oxford, OX2 6DP, United Kingdom

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  © Clifford M. Will, Nicolás Yunes 2020

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  For Leslie

  For Penny, Jessica, Roberto and Lita

  Contents

  Preface

  1. A Very Good Summer

  2. Wrinkles in Time

  3. How Light Sheds Light on Gravity

  4. Does Gravity Do the Twist?

  5. Celestial Lighthouses for Testing Relativity

  6. How to Use a Black Hole to Test General Relativity 141

  7. Gravitational Waves Detected At Last!

  8. What Do Gravitational Waves Tell Us?

  9. A Loud Future for Gravitational Wave Science

  10. A Dialogue

  Suggestions for Further Reading

  Index

  Preface

  A little over a century ago, over four consecutive Wednesdays in November 1915, Albert Einstein described to the Prussian Academy of Sciences a theory of gravity that he had been working on for eight years. Among the audience of German scientists, a few were excited and impressed, many were mystified, and some were openly hostile. Outside the world of German science, the lectures had almost no impact. This was the middle of World War I, and, except for a few neutral countries such as Switzerland and the Netherlands, Germany was effectively cut off from the rest of the world. Einstein, seriously ill from the grueling days and nights of calculating, and from the food rationing and other privations of wartime Berlin, returned to his office to continue toiling on his new theory in relative obscurity.

  Just four years later, after British astronomers declared that Einstein was right about the Sun’s gravity bending light, international headlines proclaimed Einstein to be the successor to Isaac Newton, the herald of a strange new universe governed by rubbery time, warped space, and mathematics so abstruse that only a handful of people could possibly comprehend it. Einstein became an overnight science superstar, a status that he thoroughly enjoyed and occasionally disliked. But his brainchild, called general relativity, soon languished, burdened by a shortage of relevance, a lack of experimental support and a reputation for being just too complicated. General relativity soon became little more than an afterthought in the world of physics.

  But by 2015, the hundredth anniversary of general relativity, Einstein’s theory had assumed its rightful place in the pantheon of physics. Its predictions had been tested and retested countless times, sometimes with mind-boggling precision. College bookshelves displayed textbooks on general relativity alongside conventional tomes on quantum mechanics, solid-state physics and astronomy, and physics departments routinely taught general relativity to graduate and undergraduate students. The theory’s relevance was being touted in fields ranging from high-energy physics to astronomy to cosmology. And modern-day science superstars, such as Stephen Hawking, could be seen or heard expounding on warped spacetime on YouTube or in television shows such as The Big Bang Theory. It was even said that general relativity helps you to navigate your car or to find your misplaced smartphone, through the manner in which its rubbery time must be accounted for in global navigation systems such as GPS.

  The crowning event of that centennial year was the September 14 2015 detection of gravitational waves emitted by a pair of colliding black holes a billion light years away from Earth. Einstein first predicted these waves in 1916, doubted their reality for a while in the 1930s, and believed that it would never be feasible to detect them. That detection, announced at a press conference in February 2016, made similar world-wide headlines proclaiming that Einstein was right. More importantly, it initiated a new way of doing astronomy, by “listening” to the universe rather than by looking at it. It also opened up new ways of putting Einstein’s theory to the test, using black holes, neutron stars and gravitational waves.

  This book is about Einstein’s creation of over a hundred years ago, the general theory of relativity, with a definite slant toward experiment and observation. General relativity is a very beautiful theory. Einstein was guided toward its final form by aesthetic criteria of beauty, simplicity and elegance. In the end, while he appreciated the role of experimental tests, deep down he believed that the theory was so beautiful that it had to be correct. But, as the great American physicist Richard Feynman once said, “It doesn’t matter how beautiful your theory is, it doesn’t matter how smart you are. If it doesn’t agree with experiment, it’s wrong.”

  In this book we will describe how general relativity has passed every experimental test to which it has been subjected, an almost unbelievable perfect score. Yet the 1687 gravitation theory of Newton had a similar perfect score until general relativity took over. There is no reason to assume that general relativity is the last word on gravity. The observation of some anomalous effect or of a disagreement with Einstein’s theory could tell us that it’s time for a new theory. The 1998 discovery that the expansion of the universe is speeding up rather than slowing down is an example of an anomaly that has many people scratching their heads. Some of them are working hard on devising alternatives to general relativity to account for this. Therefore, we must keep testing general relativity, especially in new and unfamiliar arenas, such as near black holes, or using gravitational waves, in order to discover where or how, or even if, it might be superseded.

  The authors are theoretical general relativists, but we have both spent a substantial fraction of our research careers investigating how to verify (or disprove) general relativity by experiment or observation. We don’t actually do experiments or make observations; our experimental colleagues get nervous when we get too close to their equipment. Yet, we have spent enough time talking to them and collaborating with them that we think we have a good feeling for what they do and how observations and experiments can test Einsteins theory. In this book you will learn about some of the absolutely brilliant people who design the experiments, build the apparatus and instrumen
ts, and analyze the data. Some of them work alone or in small groups, some belong to enormous collaborations of thousands of scientists, engineers and technicians. These are the people who are doing the real work of finding out if Einstein is still right.

  Acknowledgments

  We are very grateful to the many friends and colleagues who read parts of this book and sent us criticisms, corrections and suggestions: Bruce Allen, Imre Bartos, Peter Bender, Donald Bruns, Alejandro Cárdenas-Avendaño, Katerina Chatziioannou, Ignazio Ciufolini, Karsten Danz-mann, Sheperd Doeleman, Philip Eaton, Jim Hough, Cole Miller, Jenny Meyer, Paolo Freire, Reinhard Genzel, Ramesh Narayan, Jorge Pullin, Jessica Raley, David Reitze, Bernard Schutz and Norbert Wex. Ultimately, we are responsible for any remaining errors or omissions.

  Clifford Will is grateful to the University of Florida for its support and to the Institut d’Astrophysique de Paris for its hospitality during extended stays in 2018 and 2019 while this book was being written. He is also grateful to the US National Science Foundation for support under various grants. Nicolás Yunes is grateful to Montana State University and the University of Illinois at Urbana-Champaign for their support, as well as to the Kavli Institute for Theoretical Physics for its hospitality during extended workshops in 2019 while this book was being written. He is also grateful to the US National Science Foundation and the US National Aeronautics and Space Administration for support under various grants.

  In this book we will present “thought experiments” or situations where an observer does something. Rather than referring to the observer as “he or she” or “they”, we will go back and forth, sometimes using “he” and sometimes using “she”. This is by no means meant to exclude the possibility of observers or scientists who might be in gender transition or gender non-binary. Indeed, as physicists, we are well aware that our profession still needs to work hard to become more diverse in terms of gender, gender identification, race, ethnicity and disability status, and we are committed to doing our part. We hope that our usage of pronouns in this way will keep our examples readable and enjoyable, while still fostering a spirit of inclusiveness.

  CHAPTER 1

  A Very Good Summer

  The summer of 2017 was a smashing success for Albert Einstein. On Thursday 25 May, in the waning weeks of spring that year, the journal Physical Review Letters posted a paper on its website. It described a nineteeen-year campaign to watch two stars revolving around the gargantuan black hole at the precise center of our own Milky Way. These two stars are special because their orbits are very close to the black hole, so they whirl around it at speeds as high as a few percent of the speed of light, or 20 to 30 million kilometers per hour. These orbits are ideal to test Einstein’s theory and to search for any possible deviations from its predictions. No deviations were found by the team, headed by Andrea Ghez of the University of California in Los Angeles. Einstein passed yet another test, the first one to involve orbits around a black hole.

  On Tuesday 18 July, at 8 p.m., standing in front of a forest of computer monitors in the German town of Darmstadt, a scientist at the mission operation center gave the kill signal. Five seconds later, 1.5 million kilometers from Earth, the LISA Pathfinder satellite shut down. A sigh of relief mixed with sadness was heard through the room. For sixteen months, two identical cubes of a gold and platinum alloy, 1.8 inches on a side, floated freely inside evacuated chambers in the satellite, maintaining almost exactly the same separation. The satellite had to periodically adjust slightly to account for shifts in its position caused by the bombardment of protons and radiation from the Sun. If either cube made contact with the walls of its chamber, it would be a disaster. Specially made spacecraft thrusters and delicate sensors were essential if this mission was to succeed. For sixteen months, the inside of this satellite was the quietest place in the universe. The success of the mission moved scientists one step closer to fulfilling a dream: the observation of gravitational waves with a space detector, known as LISA.

  The month of August that summer was even better. On Monday 14 August the LIGO gravitational wave detectors in the USA and the Virgo detector in Italy picked up the signal from two black holes that merged 1.4 billion years ago. This wasn’t the first gravitational wave signal detected—that momentous discovery had happened almost two years earlier—but it was the first to be detected simultaneously by the LIGO instruments in Washington state, near the Hanford nuclear reservation, and in Louisiana, near Baton Rouge, and by the newly operational Virgo detector near Pisa, Italy. The triple detection enabled the scientists to do a much better job of pinpointing the location of the source on the sky.

  Three days later, another burst of gravitational waves jiggled the sensitive mirrors of the LIGO and Virgo detectors. A few seconds after that, the Fermi Gamma-Ray Space Telescope, orbiting 534 kilometers above the Earth, sensed a burst of gamma rays coming from the same part of the sky. Some rapid detective work located the galaxy where both signals originated, and during the subsequent hours and days astronomers around the world observed light in all its forms, from X-rays to radio waves, arriving at Earth from that same location. This time, the source was two neutron stars about 140 million light years from Earth producing waves of gravity as the stars spiraled toward each other and merged. This was followed by a nuclear fireball of unimaginable power.

  This single observation revealed wonders about the universe that not even Einstein would have imagined. If you are wearing a gold necklace or a platinum ring, then there is a good chance that those precious (and expensive) elements were produced in a nuclear cataclysm just like the one observed on that day. In fact, most of the gold and platinum in the universe is now thought to have been produced in the explosions that result when neutron stars collide.

  If that wasn’t enough, the mere fact that the gravitational waves, emitted right before the neutron stars merged, and the first gamma rays, emitted right after the merger, arrived within 2 seconds of each other after traveling 140 million light years revealed that the speed of gravitational waves and the speed of light are the same to fifteen decimal places. Amazingly, this is precisely what Einstein had predicted in 1916.

  The following week, on Monday 21 August, a lone amateur astro- nomer named Don Bruns settled into a folding patio chair near the top of Casper Mountain in Wyoming. With the press of a key, his laptop began instructing a TeleVue Optics NP101is telescope to take a series of photographs of the Sun during the total solar eclipse that crossed the United States that day. His goal was to replicate a famous 1919 experiment carried out by a British team of professional astronomers headed by Arthur Stanley Eddington. Eddington’s measurements showed that gravity bends light exactly as Einstein had predicted, thereby overturning Newton’s theory and making Einstein an international celebrity. Bruns wanted to see what could be done by a non-professional astronomer armed only with a modern commercial telescope, a CCD (charge-coupled device) camera, and computer-controlled instruments. After analyzing his data Bruns also verified that light is bent by the Sun just as Einstein predicted, and the accuracy of his measurements beat Eddington’s by a factor of three.

  Many of these events were covered by the press, using headlines like “Einstein was right, again.” They reinforced an almost fairy-tale version of the story of general relativity that goes something like this: in 1905, Einstein, working as a lowly clerk at the Patent Office in Bern, Switzerland, created the Special Theory of Relativity. He then turned his attention to gravity, and after ten years of hard work created the General Theory of Relativity. In 1919 Eddington verified the theory by measuring the bending of starlight. Einstein became famous, his theory was triumphant, and everybody lived happily ever after.

  The actual story of general relativity is more complicated. Back in the 1920s there was considerable skepticism about Eddington’s results, particularly among American astronomers. Attempts in 1917 to measure the shift in the wavelength of sunlight toward the red end of the spectrum, an effect that Einstein considered another crucial tes
t of his theory, failed to detect the effect. This apparently hurt Einstein’s chances for the Nobel Prize until 1921, when it was finally awarded for his work on the photoelectric effect, not for general relativity.

  The theory was considered to be extremely complicated, with exotic new concepts like curved spacetime that baffled most physicists and astronomers at the time, to say nothing of the general public. The headline of an article on relativity in the 9 November 1919 issue of the New York Times stated, “A book for 12 wise men / No more in all the world could comprehend it, said Einstein when his daring publishers accepted it.” Einstein himself may have used some such phrase as early as 1916 in reference to a popular book on relativity that he had written. Another story has this idea originating with Eddington. Soon after the publication of the final form of general relativity in 1916, Eddington was one of the first to appreciate its importance, and set out to master the theory and then to organize a team to measure the bending of light. At the close of the November 1919 joint meeting of the Royal Astronomical Society and the Royal Society of London at which Eddington reported the successful measurements, a colleague purportedly said, “Professor Eddington, you must be one of three people in the world who understand general relativity!” to which Eddington demurred. The colleague persisted, saying, “Don’t be modest, Eddington.” Eddington replied, “On the contrary, I am trying to think who the third person could be.”

  Perhaps only a handful of people understood it, but millions were fascinated by it and wanted to read about it and about Einstein. In the popular press, the scientific revolution engendered by general relativity was placed on a par with the insights of Copernicus, Kepler and Newton. Editorial after editorial marveled at what was called one of the greatest achievements in the history of human thought, but at the same time complained about the difficulty of understanding it. Einstein himself wrote a long article for The Times of London in late 1919, attempting to explain the theory to a general audience. His picture graced the cover of the 14 December 1919 issue of the German news magazine Berliner Illustrirte Zeitung, with the caption “A new great figure in world history.”