Rainer Weiss, Barry Barish and Kip Thorne. [Images: MIT/M. Scott Brauer (left); Caltech (center and right)]
In a widely anticipated decision, the Swedish Academy of Sciences has awarded the 2017 Nobel Prize in Physics to three key figures in the history of the Laser Interferometer Gravitational-wave Observatory (LIGO)—Rainer Weiss of the Massachusetts Institute of Technology (MIT), USA, and Barry C. Barish and Kip S. Thorne of the California Institute of Technology (Caltech), USA. The award citation recognized the trio “for decisive contributions to the LIGO detector and the observation of gravitational waves.”
In announcing the prize, Göran Hansson, the secretary general of the Swedish Academy, called that observation “a discovery that shook the world.” And that discovery rested on an infrastructure of incredibly precise, cutting-edge optical and photonic technology—as well as on the staying power and commitment of scientists, engineers and funders to a massive project spanning more than four decades.
In its selection of Weiss, who will receive half of the prize’s 9.0 million Swedish kronor (~US$1.1 million), and Barish and Thorne, who will split the other half, the Nobel physics committee seemed consciously to be viewing them in part as avatars for a much larger group of scientists, engineers and others. “This year’s Nobel laureates represent in an excellent way,” said Nils Mårtensson, the acting chairman of the committee, “the diverse competencies needed for LIGO’s success.”
Weiss, reached by telephone by the committee, sounded a similar note. “I view this,” he said, “as a thing that recognizes the work of about a thousand people … Many of us were in this thing.”
Weiss’s early prototype
For Weiss himself, the LIGO journey started 50 years ago, in 1967. At that time, as an assistant professor at MIT, he hit upon the notion that an L-shaped laser interferometer could detect the faint ripples in spacetime, gravitational waves, that Einstein had predicted decades earlier as an outcome of general relativity. Since a passing gravitational wave would be expected to lengthen one arm of the interferometer and shorten the other—albeit by strains of less than the width of an atomic nucleus—the interference pattern of the combined lasers in the two arms could in principle detect the event.
While several other groups had sketched out similar ideas, Weiss followed them up with a shot-noise-limited tabletop prototype. A few years later, in 1972, he produced a detailed internal MIT report, in which he laid out the requirements of an interferometer capable of picking up gravitational waves from pulsars in the Crab Nebula.
The requirements were daunting indeed. The interferometer would somehow need to find a way to screen out or adjust for a dizzying array of noise sources, including seismic noise, gravitational-field gradients, thermal noise in the precision-shaped mirrors, or “test masses,” suspended at the end of the interferometer arms, laser instabilities and more. And, Weiss noted, to search for gravitational waves emanating from pulsars, the interferometer arms would need to be at least a kilometer long.
Kip Thorne, at Caltech, was also working on gravitational waves—both with a prototype detector of his own, and, more important, on the theoretical side. In particular, Thorne calculated the expected, detailed gravitational-wave signal and shape for specific astrophysical events—calculations that would prove remarkably accurate, and practically useful, when the waves were finally detected decades later. Thorne also assembled a gravitational-wave research group at Caltech that included future OSA Fellow Stanley Whitcomb and the late Ronald Drever, who had been lured to Caltech from the University of Glasgow, Scotland, where he had been working separately on gravitational waves.
By 1984, Weiss, Thorne and Drever had joined forces under the aegis of a demonstration project endorsed by the U.S. National Science Foundation (NSF). Six years later, in 1990, NSF upped the ante, approving US$300 million in funding for the construction of two facilities with 4-km-long laser interferometer arms, one in Hanford, Washington, and one in Livingston, Louisiana. The LIGO project was born.
Barish: From iLIGO to aLIGO
The LIGO facilities in Hanford, Washington (top), and Livingston, Louisiana (bottom).
In selecting Barry Barish as the third 2017 Nobel Prize recipient, the Nobel physics committee explicitly recognized the importance not only of basic science and engineering, but also of management and leadership in the success of “big science” undertakings such as LIGO.
Barish took the reins of the fledgling LIGO project in 1994, and, according to the committee, “transformed LIGO from a limited MIT/Caltech endeavor to a major international, gravitational-wave project.” In particular, Barish mapped out a strategic plan in which LIGO would proceed in two phases: An Initial LIGO (iLIGO) that would prove out the technology, and an Advanced LIGO (aLIGO) sufficiently sensitive to make detection of gravitational waves highly likely.
Barish guided the project from iLIGO’s construction through its initial data run in 2005, during which the facility met its design specifications—a crucial hurdle in getting NSF funding for the more ambitious aLIGO project. At that point Barish stepped down as project lead to accept a position with the International Linear Collider project, but remained part of the LIGO Scientific Collaboration.
Construction of the exquisitely sensitive aLIGO, the optical setup of which blazed new trails in sensitivity and error tolerances, was finished in mid-2015. The first detection of a gravitational wave, from the collision of two black holes, followed swiftly thereafter, in September 2015—while the machine was still in test mode, before its official observation campaign had even begun.
More to come
Since that initial, landmark detection, the Advanced LIGO detectors have sensed gravitational waves from three additional black-hole collisions. The most recent of those detections, announced only days ago, also included data from a third detector, the European Advanced Virgo project in Italy. The LIGO-Virgo collaboration now includes more than 1,000 scientists, engineers and others from roughly 100 institutions on five continents. The LIGO project has indeed come a long way.
And there is likely more to come. In a recent talk at OSA’s Laser Congress, Robert Byer of Stanford University, USA, noted that the LIGO-Virgo collaboration’s next press conference would take place in mid-October—and, in Byer’s words, would announce “one of the most important events in the history of astrophysics.”
Rainer Weiss, reflecting on the potential of the new gravitational-wave astronomy that he had helped pioneer, sees virtually unlimited prospects for what LIGO, Virgo, and other, similar detectors may someday tell us.
“The thing is, we hope there will be all sorts of phenomena that you can see only by the gravitational waves they emit,” he said. “That will open up a new science.” He speculated that the gravitational radiation patterns from supernovae, for example, might help resolve open questions about how those massive events operate, and that we may someday even be able to look at the background radiation from the Big Bang through a gravitational-wave lens.
“There is now a way of looking at part of the universe that’s never been seen before,” Weiss said. “We fully expect that we’ll learn things we don’t know about.”