The Perfect Theory

by Pedro G. Ferreira

  A fully functioning interferometer that could truly detect gravitational waves coming in from outer space was an almost impossible beast to build. The laser beam would have to travel for kilometers at a time without deviating from its path by more than the width of an atom. The equipment would have to be set up as if it were floating on air, protected from all the ambient noises of everyday life, with perfect mirrors and state-of-the-art signal processing to be able to tease out the imperceptible deflections. It would have to be able to separate out the effect of the Earth’s tides which could shift things around by a fraction of a millimeter, the rumbling of trucks on distant highways, and the vibrations from the electrical grid.

  It would have to be perfect in every way, and it would have to be big. As interferometers began slowly to take over the field of gravitational waves, it became clear that their size and expense would limit the number that could be built. In Europe, the British and the Germans joined forces to build an interferometer with arm lengths of about 600 meters. Based near Sarstedt in Germany, it was named GEO600. A far bigger one named after the Virgo cluster of over a thousand galaxies, with arms of 3 kilometers, was conceived by the French and Italians and built in Cascina, Italy. In Japan, a smaller interferometer, TAMA, was built with arm lengths of 300 meters.

  The poster child for gravitational wave interferometry was to be LIGO, the Laser Interferometer Gravitational Wave Observatory. It was originally led by two experimentalists, Rainer Weiss from MIT and Ronald Drever from Caltech, and the theorist Kip Thorne. First conceived in the early 1970s, LIGO had a difficult, fractious birth.

  It was to be, by far, the grandest of all the interferometers. In fact, it wasn’t one but two interferometers, one based in Hanford, Washington, and the other in Livingston, Louisiana. With two detectors located far apart, it would be possible to rule out results due to local noise, earthquakes, or traffic. And if it joined forces with one of the other detectors, like GEO600, it just might be able to pinpoint the direction of the gravitational wave sources and so would be a true observatory, a proper telescope. No one was yet sure exactly what they should expect to detect or whether the instrument would be sensitive enough. LIGO would have to be built in two steps. First they’d need to build a “proof of concept,” a gigantic prototype that would work the way the relativists and experimenters wanted it to, a process that was expected to take more than a decade. Only after that could LIGO be upgraded and start looking for the interesting stuff. The projects would take a long time, but the payoff if LIGO actually saw gravitational waves would be staggering. Their detection would allow us to observe the universe in a completely new way, not using light or radio waves or any of the other conventional approaches. It would also be a completely new window on Einstein’s general theory of relativity for, although most people believed gravitational waves were out there, no one had actually seen them directly. LIGO’s discovery of gravity waves would be on a par with the discovery of the electron, proton, and neutron at the beginning of the twentieth century. It would be a Nobel Prize–winning experiment for sure.

  The excitement over LIGO was not universal. The project was expected to cost hundreds of millions of dollars to build and run, draining funding from other research projects. LIGO inevitably took money away from the other gravitational wave experiments, but its impact on funding would also encroach on other fields. And by calling itself an observatory, LIGO was also stepping on the astronomers’ toes. They could see LIGO sucking away precious cash from their own research. In a 1991 article in the New York Times, Tony Tyson from Bell Labs, who had worked on gravitational waves in the early days, wrote, “Most of the astrophysical community seems to feel it would be very difficult to get any important information from a gravitational-wave signal even if one should be detected.” As Jeremiah Ostriker, a leading Princeton astrophysicist, said to the New York Times, the world “should wait for someone to come up with a cheaper more reliable approach to gravity waves.” The astrophysicists were vocal, almost rabid in their opposition to LIGO. When asked to rank what astronomical projects should be given priority by the US funding agencies in the beginning of the 1990s, a panel of astronomers led by John Bahcall of the Institute for Advanced Study at Princeton didn’t even bother to include LIGO in their rankings.

  The American National Science Foundation turned down the first two proposals for LIGO and took five years from when the first proposal was submitted to finally approve a third proposal with a budget of $250 million, a seemingly exorbitant amount of money for an instrument that would quite probably see nothing and was, on the face of it, technologically impossible to build. Yet finally, in 1992, after almost twenty years of scheming, designing, and dreaming, the perfect experiment could go ahead.

  Kip Thorne and his collaborators were already discussing their plans for LIGO when Frans Pretorius was born in South Africa. Pretorius grew up in the United States and Canada and completed his PhD at the University of British Columbia in Vancouver, learning the trade at one of the nerve centers of numerical relativity. He was offered a fellowship at Caltech, Kip Thorne’s stomping ground, that let him do whatever he wanted. Pretorius decided to tackle the problem of inspiraling black holes on his own terms. In contrast to the big teams of computer programmers, working on the insurmountable problem of simulating the inspiral, chirp, and ringdown, Pretorius worked alone, “under the radar” as he recalls, not taking part in any of the big collaborations that were designing computer programs to solve the problem. Pretorius stepped back and looked at all the failed attempts of the past decades and picked out bits of different ideas that could be promising. He then set about writing a numerical program from scratch, in his own way, incorporating all of these ideas. He had an incredible instinct for what might and might not work. In his resulting code, Einstein’s equations became much simpler, so simple that they looked almost like those of electromagnetism. And electromagnetic waves were easy to solve and evolve.

  Then he ran it. It took several months for the program to run, a period Pretorius recalls as “pure agony.” But to his growing surprise and elation, Pretorius was able to run his program all the way through, from the moment the black holes started inspiraling until they coalesced, sent out a burst of waves, and then settled down into one fast-spinning black hole. There was the precise, accurate description of the gravitational waves that everyone had been so desperately looking for. Pretorius had finally solved Einstein’s field equations on a computer. He had built on a battery of ideas that emerged before him, but it had taken his new, fresh look at the problem to put them together in exactly the right way.

  Pretorius announced his results at a conference on general relativity in Banff, Alberta, in January 2005. Einstein’s field equations had finally been cracked open, and it was possible for the first time to simulate two black holes orbiting one another, each sucking the other into its inexorable pull until the two coalesced into one, spitting out a barrage of gravitational waves that would gradually disappear with time. “There was quite a bit of excitement,” Pretorius recalls. “People were interested enough to go outside of the talk to organize a session where people could ask all the detailed questions.” Half a year later, two other groups announced that they had also been able to crack the problem using completely different methods of evolving the black hole binaries. Just like Pretorius, they were able to follow the catastrophic collapse of a pair of black holes all the way through. It was as if Pretorius’s discovery had mentally unblocked all the work being done by other teams, and the results started to pour in, confirming Pretorius’s calculation.

  There was now a palpable sense of euphoria and relief. Finally, finally, it would be possible to describe the elusive waveforms. The observers would now know how to pick out the ghostly signals buried in the mayhem of noise measured by the interferometers.

  Toward the end of his life, Joseph Weber came across as a bitter man. He bristled with anger at any discussion of gravitational waves. At the few conferences or workshops he at
tended, the audience would be subjected to decades of pent-up fury. He would rage at the mildest attempt to question him. He had seen gravitational radiation before everyone else and no one would take that away from him. Freeman Dyson, one of his early supporters, had in Weber’s later life written to him pleading that he back down. Dyson had written, “A great man is not afraid to admit publicly that he has made a mistake and has changed his mind. I know you are a man of integrity. You are strong enough to admit that you are wrong. If you do this, your enemies will rejoice but your friends will rejoice even more. You will save yourself as a scientist.”

  Weber did no such thing. On the contrary, he had become the drag anchor of gravitational wave research, actively campaigning against LIGO. Weber had been in the press enough to have made a name for himself in the wider world as the expert on gravitational waves. When he spoke out, the powers that be would sometimes listen. In the early 1990s, when LIGO was making its third, desperate bid for funding, Weber wrote to Congress, stating that funding such a hugely expensive instrument would be a waste of money. His bars, he claimed, had seen gravitational waves and cost a fraction of a million dollars. There was no need to spend hundreds of millions. His ranting had little impact; throughout his career, Weber had made so many ludicrous claims that, as Bernard Schutz recalls, “by the time he was opposing LIGO, no one really wanted him on their side.” If Weber felt ignored, he was making things worse for himself. He was now the enemy of the field he had created.

  Weber died in 2000, before LIGO started operations. It had taken decades of devotion to get the most perfectly tuned instrument to work. Along the way, there had been delay after delay. Kip Thorne had made a number of bets with colleagues in the 1980s and 1990s that gravitational waves would be discovered before the turn of the millennium, and he lost them all. Even in the beginning of the twenty-first century, LIGO faced setbacks, from the loggers with their circular saws in the Louisiana forest who set off the detectors at Livingston, to mysterious whirrings in the nuclear reactors around the Hanford site in Washington. But when it was finally turned on in 2002 and run for a few years, LIGO was able to achieve the sensitivity everyone had been gunning for. It was the first stage in the experimental journey laid out in the proposal in the early 1990s. Its detectors could pick up vibrations of less than a proton’s width, as had been envisioned decades before. In fact, the LIGO team announced, the instrument was even more sensitive than they had predicted. LIGO was, by all means, a resounding success, even though it didn’t see anything. As expected in its first incarnation, LIGO was not yet sensitive enough to actually detect gravitational waves, but it did show the way forward. The LIGO team can now improve the existing instrument so that at some point it will see the ripples in spacetime that Einstein had first predicted.

  It is a long game. Unlike Weber’s results, which came fast and steady the moment he turned on his instrument, LIGO will have used up thousands of technicians over many decades before it can actually detect gravitational waves. The founding trio, Ron Drever, Kip Thorne, and Rainer Weiss, now in their seventies and eighties, might not all be around when that moment comes, and they may have devoted their lives to something they will never see. But there is unwavering confidence that waves are out there; Einstein’s theory predicts them, and they have been seen, albeit indirectly, through the gentle but steady orbital decay of the millisecond pulsars. It is just a matter of time before gravitational waves are seen, and then a field of research that started with Weber’s bang will end with a whimper: the whimper of spacetime shimmering as it passes through Earth.

  11

  The Dark Universe

  AT THE 1996 Critical Dialogues in Cosmology meeting in Princeton, the stars of the field engaged in one-on-one combat over the state of the universe. The organizers had picked a series of contentious open issues for public debate, clearly asking for a fight. Pairs of invited speakers—leading astronomers, physicists, and mathematicians—abandoned the usual ceremony of conference protocol when they took the stage. They went on the attack, trying to tear each other’s cases apart. It was an odd yet riveting way to discuss science.

  Martin Rees, who had by then contributed so much to the understanding of black holes and the theory of the Big Bang and become one of the big beasts of relativistic astrophysics, opened the hostilities. He argued that cosmology is “a fundamental science” and “the grandest of environmental sciences.” It provides the ultimate application of the beautiful mathematics and physics developed in the twentieth century by Einstein, Dirac, and many others. Furthermore, it grapples with the plethora of observations of galaxies, quasars, and stars, seeking to explain how their seemingly messy mechanisms fit together into the grand picture of the universe. Cosmology’s task is difficult, controversial, and unfinished, but, as Rees argued, it is also of the utmost importance.

  The picture of the universe that cosmology was revealing by the time of the Princeton conference was truly bizarre. It seemed that we understood far less of the universe than we had originally thought. In fact, a large fraction of the universe appeared to be in the form of exotic substances we had never seen in a laboratory. Dubbed “dark matter” and “dark energy,” they were out there, affecting spacetime, yet strangely elusive and undetectable. The case for a dark universe emerged forcefully one afternoon when the large-scale structure of the universe was discussed. It was that one topic that had drawn me into cosmology in the first place.

  When we look out at the universe, we see an elaborate tapestry of light, with galaxies clumped into clusters, filaments, and walls, leaving large voids of emptiness. It is rich, full of information and complexity. Where does this large-scale structure of the universe come from? This was the most pressing question for the conference attendees, for the answer was still completely up for grabs, and the conference organizers dedicated a full afternoon to the topic. J. Richard Gott, a tall, gangly astronomer from Princeton with a deep and slow southern drawl, stood up and defended common sense. At a first glance the universe looks very empty, so Gott proposed a universe almost completely devoid of matter that slowly evolved to form a tapestry of galaxies and clusters of galaxies that would populate the night sky. Another young and energetic astronomer from Princeton named David Spergel proposed that the universe is not at all empty, but rather full of an invisible, dark form of matter. Spergel’s dark matter would be made up of some fundamental particle unaccounted for in the standard model of particle physics that had not yet been observed in any experiment. But it was the final speaker, Michael Turner, a sharp-witted theoretical cosmologist from Chicago, who made the most outlandish proposal of the afternoon: Why not assume that the universe is permeated by the energy of a cosmological constant? In Turner’s universe, about two-thirds of the overall energy would be accounted for by the constant Einstein had so firmly rejected almost seventy years before. The crowd was not impressed with Turner’s proposal. Anything but a cosmological constant—it was Einstein’s biggest blunder.

  Chairing the gladiatorial combat between the universes was Phillip James (Jim) Peebles, then the Albert Einstein Professor of Science at Princeton University. A tall, slim man with a thoughtful face lifted from a portrait by Modigliani, Peebles was the consummate gentleman, courteously moderating the debate. While he was careful to the keep the conversation on track, he would sometimes chuckle with almost childish glee at the jibes and comments being thrown across the stage. The Critical Dialogues meeting was partly organized to celebrate Peebles’s sixtieth birthday, a fitting tribute. For the previous three decades, Peebles had been the prime architect of the theory of large-scale structure of the universe at the heart of modern cosmology.

  In the early 1970s, Jim Peebles published a slim volume, Physical Cosmology, a summary of a set of graduate lectures he gave at Princeton in 1969. John Wheeler had attended, taken notes, and, according to Peebles, bullied him into publishing the lectures. In the introduction to Physical Cosmology, Peebles briefly mentioned the cosmological constant, saying that �
��the cosmological constant Λ [the Greek capital letter “lambda,” which is the mathematical symbol for the cosmological constant] is seldom mentioned in these notes.” For Peebles, the constant was an unnecessary complication, “the dirty little secret” of cosmology. Everyone knew that the mathematics allowed for it, but because it made the physics too bizarre and troublesome, everyone pretended it wasn’t there. Now, a quarter of a century later, despite being reviled by the majority of Peebles’s colleagues, the cosmological constant was about to make a comeback. It would do so with a vengeance.

  When Jim Peebles arrived in Princeton in 1958, fresh out of engineering school at the University of Manitoba, he found John Wheeler and his crew chipping away at black holes and the final state. Wheeler was not the only acolyte of general relativity at Princeton; there was also Robert Dicke. Like Wheeler, in the mid-1950s, Dicke realized what dire straits Einstein’s theory was in, with little or no progress being made in testing it. He created his own gravity group at Princeton, where general relativity could be discussed and, most important, measured and tested. “Rather quickly in my career I got into orbit around Bob and into doing things that were exciting,” Peebles says. He joined Dicke’s team as a PhD student and, after graduating, focused his research on testing gravity physics. He would stay in Princeton for the next fifty years.