Gravity’s Black Rainbow

The magnificent success of Albert Einstein’s theory of general relativity—the modern theory of gravity, space, and time that supplanted Isaac Newton’s law of universal gravitation—promoted the popular misconception that major advances in fundamental physics and cosmology are the province of theoretical physicists working at blackboards or at their desks. Nothing could be further from the truth. Cosmology, like the rest of physics, is an empirical science. Progress is usually made when new experimental tools become available that produce discoveries that force scientists to adjust to new realities, or when ingenious experimentalists set out to build new devices to probe unexplored aspects of the universe and thereby make discoveries that force scientists to adjust to new realities.

Two new books examine—in very different ways—these two facets of scientific progress by discussing discoveries that have affected our understanding of the universe. Black Hole Blues by Janna Levin provides a case study of how a small group of scientists overcame enormous challenges to build a seemingly impossible experiment that recently made one of the most exciting scientific discoveries of this century. Mapping the Heavens by Priyamvada Natarajan explores how our scientific perspective has changed over millennia as we have learned about the large-scale structure and evolution of our universe, the nature of the Big Bang, and the existence of black holes—the very objects that are central to the discovery described in Levin’s book. The two books, though different in style and content, provide a strong case that science evolves by anything but a straightforward linear progression, and that often it is the scientists involved in making progress who have the hardest time adjusting to new scientific realities. As Max Planck once suggested, science advances one funeral at a time.

Black Hole Blues explores the fascinating, if sometimes tortured, history associated with the building of one of the most ambitious projects ever constructed to advance science—LIGO, the Laser Interferometer Gravitational-Wave Observatory. LIGO consists of two huge sets of vacuum-filled tunnels, each four kilometers long and outfitted with powerful lasers. One set is located in Hanford, Washington, and the other in Livingston, Louisiana. They were designed to detect “gravitational waves,” the elusive ripples in space first predicted by Einstein a hundred years ago. In order to make such a detection LIGO also had to rely on the existence of pairs of huge black holes in distant galaxies, whose collision would create the gravitational waves.

Einstein’s general theory of relativity, which was published in 1915, implied that space is dynamic, responding to mass and energy by curving or expanding or contracting. These distortions in space-time are felt as gravity, which in turn affects the movements of objects within space and time. Shortly after developing his theory of general relativity, Einstein realized that just as tossing a rock into a pond results in wave-like ripples that travel along its surface, so too the motion of mass and energy can create a wave-like disturbance in space itself. This disturbance, in the form of a gravitational wave, would travel outward at the speed of light, and as it passed through a region, the distances between objects in that region would, as a result of gravitational force, literally oscillate back and forth over time.

It seems unreal that gravitational waves of all sorts are traveling through your room as you are reading this. The walls do not appear to be oscillating in and out. But they are. We don’t notice this because gravity is the weakest force in nature, and so the ripples in space caused by even catastrophic events are very faint.

For most of us gravity seems anything but weak. However, the physicist Richard Feynman gave a very simple thought experiment to demonstrate just how weak gravity is. Imagine going to the top of a tall building and throwing something fairly small off it. It takes the gravitational attraction of the entire Earth to accelerate the object as it falls several hundred feet to the ground. However, the force of electromagnetism in the atoms in the sidewalk on which the object lands is much stronger than gravity; the former stops the object within a minuscule fraction of an inch. In fact, solids are mostly empty space; it is the electric forces between the atoms in the ground and the atoms in the object that stop it from passing through the concrete. For the most part, the object would hardly make a dent.

When Einstein first proposed that gravitational waves might be generated, it seemed that trying to detect them would be immensely difficult. (Einstein had his own doubts about whether they even existed and expressed such doubts several times—once in a paper he almost published had it not been for a clever referee.) Still, one can calculate that if two massive black holes—extremely dense objects with a gravitational pull so strong that not even light can escape from them—collide in a nearby galaxy, the effect on Earth of the gravitational waves that pass by as a result would nevertheless be extremely tiny. If one considered, say, two objects separated by four kilometers (the length of the LIGO tunnels), then a passing gravitational wave from such an event would change the distance between these objects by less than one thousandth the size of a single proton.

To measure such a change in length would be like measuring the distance between here and the nearest star to the width of a single human hair. But if such a measurement could be made it would reveal a great deal about both gravity and the universe. It could provide evidence helping to confirm the existence of gravitational waves and also offer strong additional support for the existence and effects of black holes formed by the collapse of massive stars; and this ultimately could help reveal how galaxies formed early in the history of the universe. In short, detecting gravitational waves would open a new window on the universe.

Against all odds, and often in spite of themselves, a group of scientists succeeded in making this measurement. That is the story that Levin tells in Black Hole Blues. Levin is herself a theoretical physicist (as well as an accomplished novelist), but in Black Hole Blues she is more of a journalist, and a good one at that. Most of the book is based on personal interviews she carried out with the principal scientists or on data she retrieved from her visits to archives in various locations, including the sites of the two ligo detectors in Washington and Louisiana.

Levin’s writing is casual and sometimes poetic, and the fortunate existence of an interesting and curious cast of characters makes her book a unique and convincing account of the discovery of gravitational waves. She liberally inserts her own impressions and emotions into the text, and the reader can’t help sharing her surprises, her concerns, and her sympathies.

LIGO, at a cost of $1 billion, is the largest project ever supported by the National Science Foundation. Levin traces its origins to an MIT classroom. In the 1960s, Rainer Weiss, an experimental physicist who had built radios and televisions as a kid, was assigned to teach a class in general relativity at MIT, where he was a professor. Weiss was far from familiar with all the complexities of relativity and struggled with the mathematics, and he focused his class on experimental observations. When it came to teaching about gravitational waves, he assigned his students the task of thinking about how to detect them. He had a good idea: use light bouncing back and forth between three mirrors in a vacuum arranged in a right triangle. One light beam would travel along one leg of the triangle, the other beam along the orthogonal leg. If the beams originated from the same source and if the legs were exactly the same length, then they would reach the central mirror at exactly the same time. But if the distance traveled by one beam changed as the result of the effect of a passing gravitational wave, then the two light beams would become out of synch, and an experimenter could detect this discrepancy at the central mirror.

It seemed as if it might work, but only if scientists could construct a detector a few thousand times bigger than the prototype Weiss and his students built in a makeshift lab at MIT. It took forty-seven years, and a lot of tension among scientists as well as a lot of money, but it eventually happened, using a vastly modified and improved technology that nevertheless grew directly out of the ideas Weiss first tried out in the late 1960s.

Weiss, an unassuming, hands-on scientist (I know almost all the main characters in the book and so my own descriptions are probably biased), comes as close as anyone to being the hero or at least the central character in the book. Having had a fundamental insight, he is driven along by the resulting events and grows into the role that follows. His foil is Ronald Drever, a talented Scottish experimentalist who emerges as a tragic figure, elevated early on to leadership of the experiment but eventually exiled back home. Between Weiss and Drever is the unflappable, brilliant theorist Kip Thorne, promoted at age thirty to a full professorship at Caltech. Thorne saw early on the potential of a grand experiment and decided it had to be done at Caltech. Beginning in the early 1980s, Weiss, Drever, and Thorne led the project together, as part of an arrangement Weiss (and Levin) calls “the Troika.”

There are others who have significant parts in the development of LIGO. Rochus Vogt, the intimidating former provost of Caltech, was hired in 1987 to direct the project after it became clear that the Troika would not be able to lead LIGO effectively, since the three members had difficulty agreeing on almost everything. Vogt helped secure funding for the project from the National Science Foundation and oversaw the construction of laboratories at Caltech.

But Vogt was a flawed leader. While he helped shepherd the project through Congress initially, he alienated many on the team, especially Drever, who was fired in 1992 after he disobeyed Vogt’s orders and presented a paper at a conference in Argentina. In 1994 the particle physicist Barry Barish—the first director with prior experience leading huge research projects—was brought in to turn an operation that looked doomed into a functioning experiment.

By 2000 LIGO had been built, at a cost of about $200 million, requiring the kind of congressional lobbying that caused many outside the LIGO community to turn against the project. One of the two sets of LIGO detectors was built in Louisiana with help from the senator from that state. There had to be two identical sets of detectors located in different parts of the country so that any interference observed in one set of detectors could be compared to that in the second set, in order to get rid of spurious background noise. Besides dealing with the enormous challenges of building two four-kilometer-long tunnels to contain a vacuum more sparse than that of interstellar space, experimentalists had to deal with mold, snakes, spiders, an incursion of bass, and even an alligator.

Barish is, in my opinion, the unsung hero of LIGO, and the only person who I think gets too little attention in Levin’s wonderful tale. While Thorne and Weiss and Drever each helped, in his own way, to create what was to become LIGO, the amazing discovery of gravitational waves made on September 14, 2015, would never have happened without Barish’s leadership. He was most responsible for creating a fully functional laboratory and an experimental and theoretical community to support it.

Nobel Prizes, like all prizes, are in some sense arbitrary, but the reported discovery by LIGO of a gravitational signal from two massive colliding black holes over one billion light years away—confirming the detailed predictions of general relativity and opening a new era for astronomy—will surely win the prize. Only up to three scientists can share the prize but it will be a pity if Barish cannot be honored along with the Troika of Thorne, Weiss, and Drever.

Levin’s story is primarily a history of the people and the circumstances that led to the momentous discovery. But she doesn’t ignore the science, which is interspersed throughout the book in short passages—not too much to overwhelm, but not too little to leave the reader puzzled. It is clear from an epilogue about the discovery itself that Black Hole Blues was written with the expectation of leaving the reader wondering if there would ever be definitive results. It was only in February 2016, after the findings were publicized, that Levin could confirm that LIGO had observed a gravitational wave.

For many years LIGO has been the only laboratory with the capacity to detect gravitational waves. Other remarkable detectors have been built, but none with the sensitivity of LIGO. The tension in Levin’s book results not from competition between LIGO and other research projects, but from doubts about sufficient support for LIGO’s survival. The book isn’t perfect. At a few points Levin’s whimsical style may seem jarring, but that is a small price to pay for the easy grace that generally characterizes her narrative. This short volume will serve as a unique literary resource for those who wish to understand the history of one of the most ambitious science projects of the twentieth century.

By contrast, Priyamvada Natarajan’s historical review of major surprises in cosmology, Mapping the Heavens, presents a more standard, comprehensive overview of the long and winding road, starting from the earliest stages of investigation into the heavens, by which we have stumbled upon our current understanding of the universe.

Natarajan isn’t a fluid storyteller, but fortunately the value of the book doesn’t depend on this aspect of her writing. Hers is a tale focused largely on the science of astrophysics, and also to some degree on her own experience as an astrophysicist. There is much history here, including an enlightening discussion of ancient and medieval astronomy accompanied by beautiful illustrations. But the historical anecdotes sometimes seem like distractions, taking the reader away from the scientific substance. Some of them are nevertheless of strong interest, particularly the story of the unsung American astronomer Henrietta Swan Leavitt. Her discovery of the regular pattern of variable stars called Cepheids, whose fluctuations in brightness are important for measuring cosmic distances, provided the fundamental tool that Edwin Hubble used to make his groundbreaking discovery of the expanding universe. Although Hubble never won the Nobel Prize, which wouldn’t be given to astronomers until the 1970s, one of the committee members wanted to nominate Leavitt for the award in 1925, unaware that she had died several years before.

While much of the material Natarajan presents has been covered in the popular literature, the central premise of her book is both novel and absorbing. Her phrase “Mapping the Heavens” is both literal and metaphorical. Natarajan describes the revolutions that have taken place in our ability to explore the structure of our universe. But the point she makes is deeper. By describing developments in cosmology—including the discovery of other galaxies, the discovery of the expansion of the universe, the existence of dark matter, black holes, and the mysterious dark energy causing the observed expansion of the universe to be speeding up—she succeeds in demonstrating how the progress of fundamental science often challenges the mental “maps” that scientists conceive to represent their ideas. It can take some time for even the scientists involved in the discoveries to accept the implications of their having such mental maps. These may impede progress, but happily the process of science overcomes such biases, even if Planck’s dictum that science progresses one funeral at a time is occasionally proven true.

Many books on cosmology have been written by theorists whose focus is more on the deepest and sometimes most speculative aspects of the field. Natarajan is a theoretical astrophysicist whose work is intimately involved with the analysis of data. While not an observer herself, she works with observers to understand the significance of what is seen and recorded. She describes how gravitational distortions of space-time allow us to map the distribution of dark matter, and how observations of X-rays can reveal information about the gigantic black holes that seem to populate the centers of most galaxies. The discovery of gravitational waves described in Levin’s book here overlaps with Natarajan’s discussions of the astrophysics of black holes.

Readers of Mapping the Heavens will thus get a very good sense of how far cosmology has come, and how often observations have confronted prevailing wisdom. Stylistically, the book falls somewhere between an academic text (although there are no equations) and a popular work of nonfiction. Most chapters begin with some anecdotes to lay the groundwork and then go on to state the scientific conclusion about the subject under discussion. Natarajan then returns to discuss in more detail how that conclusion was arrived at. The chapters usually end with a summary of what has been discussed in the chapter, sometimes quoting other writers to support her thesis, as one might in an academic paper.

This didactic method is useful for presenting the material to readers new to the subject (and as a result the book might also be a valuable supplementary text in an introductory cosmology course), but for some readers the sequence may be difficult to follow. I found this particularly notable in her discussion of Edwin Hubble’s work on the expanding universe, and again in her discussion of the discovery of the Cosmic Microwave Background, a thermal radiation that dates to about 380,000 years after the Big Bang and that was first detected in 1964 with a radio telescope. For those familiar with these discoveries, the sequence of Natarajan’s explanations may not be a problem. But I expect that some readers may have to reread the material a few times to get a sense of the process of discovery.

Because Natarajan’s astronomical work is closely tied to observation, one of the strengths of her book also produces a few weaknesses, at least when she describes some of the more esoteric aspects of theoretical physics. She discusses the Dutch physicist and mathematician Willem de Sitter’s discovery of a solution to Einstein’s equations for general relativity that implied a universe without matter; but today his argument appears to describe our accelerating universe. Here Natarajan fails to emphasize an important point. Both de Sitter and Einstein believed that de Sitter’s solution described a static universe, in which they thought we lived. At the heart of general relativity is the notion that physics is determined by the geometry of space, independent of one’s choice of coordinate system, and as a result some apparent anomalies are merely products of using inappropriate coordinates. Einstein and de Sitter’s interpretation was based on the use of one such coordinate system to describe the solution, in which, as Natarajan writes, de Sitter’s solution had the rate of flow of time appear to vary throughout space.

Also, I disagree with the suggestion that the development of general relativity was completely divorced from observation—an act of pure thought ex nihilo as it is described. For example, the equivalence principle—which states that all objects are affected by gravity in the same way, independent of their nature and composition—had a central part in the development of Einstein’s thinking; and that principle had a strong basis in observation, from Galileo to Newton to the pioneering Eötvös experiment in 1908.

While these are subtle issues that might only bother experts, there are a number of less subtle, if minor, historical and numerical errors that I hope will be corrected in future editions.^* There are also matters where my own recollections differ somewhat from the descriptions in the text—but as the Kurosawa film Rashomon made clear, memory is subjective. Still, in describing two of the discoverers of the accelerating universe, Brian Schmidt and Adam Riess, Natarajan writes that Schmidt, “true to national stereotypes,” is a “relaxed” and “soft-spoken Australian,” while Adam Riess is the “intense” American. Schmidt, however, was born in Montana, grew up in Alaska, and attended college in Arizona and Cambridge, Massachusetts, before moving to Australia to take his current job. He is as American as they come, and while he strikes me as easygoing, he seems anything but relaxed.

These are only quibbles about what is otherwise an insightful treatment of the major recent new discoveries in cosmology. Natarajan’s book is a useful and timely addition to the literature, in part because of its breadth, and in part because it vividly presents several important themes: how the scientific process has evolved over millennia; the complicated interplay between observation and theory; and how traditional views about how science is carried out are often idealized or simplistic.

Science proceeds in anything but a linear and logical fashion. Together Levin’s and Natarajan’s books provide complementary and enlightening reflections on just how remarkable the process of science actually is, and how astonishing is the cosmos that science continues to reveal to us.