In this time-exposure shot of one of LIGO’s interferometer arms in Livingston, Louisiana, the red lights symbolize gravitational waves. 

Joe McNally

Ripples in space: U.S. trio wins physics Nobel for discovery of gravitational waves

Two years ago, physicists detected for the first time the infinitesimal ripples in space itself set off when two black holes whirled into each other. The observation of such gravitational waves fulfilled a century-old prediction from Albert Einstein and opened up a whole new way to explore the heavens. Today, three leaders of the massive experiment that made the discovery received the Nobel Prize in Physics.

Rainer Weiss, 85, of the Massachusetts Institute of Technology (MIT) in Cambridge and Kip Thorne, 77, of the California Institute of Technology (Caltech) in Pasadena hatched plans for the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 1984. Barry Barish, 81, a Caltech particle physicist, later guided the construction of the twin LIGO observatories in Hanford, Washington, and Livingston, Louisiana. Weiss will receive one half of the $1.1 million prize, and Thorne and Barish the other half. LIGO's third founder, Ronald Drever, died in Edinburgh on 7 March at age 85. (Nobel Prizes are not awarded posthumously.)

Other physicists rate the discovery of gravitational waves among the most important ever in physics. “It's revolutionary,” says Abraham Loeb, a theorist at Harvard University. It's very rare that we open a completely new window on the universe.”

Weiss, however, says that he finds the prize somewhat embarrassing. “Receiving money for something that was a pleasure to begin with is a little outrageous,” he says. "The best way I can think of it is we're symbols for the much bigger group of people who made [LIGO] happen.” Weiss says he has arranged to donate the prize money to MIT to help support students.

The entire notion of gravitational waves is mind-bending. In 1915, Albert Einstein explained in his general theory of relativity that gravity comes about when mass and energy warp spacetime, causing freely falling objects to follow curving trajectories. A year later he predicted that a twirling barbell-shaped arrangement of mass—such as two spiraling black holes—should radiate ripples in space that would zip through the universe at light-speed.

Detecting the incredibly feeble waves is a challenge. Each of LIGO's L-shaped interferometers acts like a pair of perpendicular rulers. A passing gravitational wave will generally stretch the two 4-kilometer-long arms by different amounts, and by comparing laser light bouncing back and forth in the arms, physicists can detect that slight differential stretching. LIGO’s interferometers can detect a difference in length as small as 1/10,000 the width of a proton.

Rainer Weiss of the Massachusetts Institute of Technology in Cambridge.

Ken Richardson

Kip Thorne of the California Institute of Technology in Pasadena.

Caltech Alumni Association

Weiss, a consummate tinkerer who once flunked out of college, wasn't the first person to think of using an interferometer to try to detect gravitational waves. In the 1960s, U.S. physicist Robert Forward built a small interferometer for the task. However, Weiss analyzed the problem far more thoroughly and recognized the need for kilometers-long interferometers. He also identified the main sources of extraneous noise, and explained how to deal with them in an unpublished report in 1972 that became the basis for LIGO.

After overcoming his initial skepticism, Thorne championed the project and pressed Caltech to pursue gravitational wave research by hiring Drever in 1979. Thorne also shaped LIGO’s scientific goals, says Saul Teukolsky, a theorist at Cornell University. For example, early on, many physicists thought the most likely sources of gravitational waves would be supernova explosions. Thorne realized that pairs of spiraling neutron stars or black holes would be more powerful sources and encouraged experimenters to tailor LIGO to spot them, Teukolsky says. Thorne also pushed physicists to assemble a vast catalog of numerical simulations to help them spot potential signals in their data.

Barry Barish of the California Institute of Technology in Pasadena.

Caltech

If Weiss and Thorne conceived of LIGO, Barish made it a reality. He took over leadership of the project in 1994, when it was stalled and the National Science Foundation was thinking of canceling it. Barish expanded the LIGO collaboration, made a key design change, and saw the project through construction before stepping down in 2005. “LIGO wouldn't have happened without his leadership,” says Stanley Whitcomb, a physicist at Caltech and an original member of LIGO. “It was something that Rai[ner] and Kip couldn't do.”

Barish, too, says he's uncertain what to think of the prize. “I have somewhat ambivalent feelings about the recognition of individuals when so much of this was a team effort,” he says.

Had he lived, Drever would have shared in the prize, many physicists think, even though his difficult personality got him drummed out of LIGO in 1990. Drever invented several key elements of the LIGO design, Whitcomb says, but he was not as self-effacing as Weiss and Thorne. “It will be a cruel thing to Kip and Rai[ner] because they have to live with this [attention] for the rest of their lives,” Whitcomb says. “And it's cruel to Ron because he would have enjoyed it.”

LIGO appears to be delivering on its promise. The first discovery revealed the merger of star-sized black holes that were more massive than theorists had thought possible. Last month, researchers announced the discovery of a fourth black-hole merger that had been spotted not only by the LIGO detectors, but also by Europe’s premiere detector—the freshly upgraded Virgo detector near Pisa, Italy—which enabled scientists to better pinpoint the merger’s location on the sky. And rumors are swirling that LIGO has spotted the merger of two neutron stars in a violent explosion that was also seen by conventional telescopes. Such “multimessenger” astronomy could provide unprecedented insight into numerous astrophysical phenomena.

The one thing LIGO has yet to deliver is a surprise, Weiss notes. “We haven’t found anything that we can’t explain at all,” he says. “I hope that will happen.”