The Historic Detection of Gravitational Waves

Monumental Discovery

Long ago and far away, there were two black holes—each 100 miles in diameter, with a mass 30 times that of the sun—that began circling one another in an epic cosmic dance. Gravity drew them closer until, at half the speed of light, they collided. For an instant, the impact radiated more power than all the stars in the universe. It also sent a shudder through the cosmos—invisible ripples in the fabric of space and time, producing a new black hole.

The ripples hurtled through space at the speed of light, fading with distance. Some 1.3 billion years later, in the predawn hours of September 14, 2015, they reached Earth.

That’s where our story begins

Conceptual illustration of gravitational waves
3D visualization of gravitational waves courtesy of Henze, NASA

“Ladies and gentlemen, we have detected gravitational waves. We did it!” exclaims David Reitze, executive director of the Laser Interferometer Gravitational-Wave Observatory (LIGO), during a February 11, 2016 briefing from the National Press Club in Washington, D.C. The announcement, which is simulcast at Syracuse University, draws cheers and applause from an overflowing crowd in Goldstein Auditorium.

Among those at the Syracuse event are Peter Saulson, the Martin A. Pomerantz ’37 Professor of Physics, and Duncan Brown, the Charles Brightman Endowed Professor of Physics. (Their colleague, physics professor Stefan Ballmer, is representing the University at the media briefing in Washington.) At one point during his remarks, Brown pauses to reflect on the amount of time that has passed since LIGO made history by detecting the billion-year-old echo of two black holes colliding. “The past five months have been a rollercoaster,” he tells the enthusiastic audience. “We’ve been doing test after test—nonstop computational analyses—to make sure that what we’ve seen is real, and to understand what the gravitational waves are telling us about the colliding black holes.”

Group photo of the Syracuse Gravitational Wave Group
Gravitational Wave Group, Department of Physics. Photo by Amy Manley.

Saulson, Brown, and Ballmer are part of the Gravitational Wave Group in the Department of Physics, based in the College of Arts and Sciences. They’re also key members of the LIGO Scientific Collaboration, an international community of more than 1,000 scientists, engineers, and students who detect and study gravitational waves. Much of their work takes place at LIGO’s two L-shaped observatories: one in Louisiana called LIGO Livingston, and another, nearly 1,900 miles away, in Richland, Washington, known as LIGO Hanford. Each detector is a giant laser interferometer containing two 2.5-mile-long vacuum arms—tunnels that run perpendicular to one another. A powerful laser beam is split into two and then sent down the tunnels. Mirrors at the end of the tunnels reflect the light back to where the laser beam was split. Since both tunnels are the same length, the light takes exactly the same time to travel to the mirror at the end of each tunnel and back. But if a gravitational wave passes through Earth, it changes the distance between the mirrors, causing the light beams to return at different times. By comparing both beams, LIGO is able to measure the stretching of spacetime caused by gravitational waves. A major, multiyear upgrade begun in 2008, known as Advanced LIGO, fine-tuned the sensitivity of the precise instrumentation even further.

Shortly before 6 a.m. on September 14, 2015, LIGO’s twin observatories picked up the fleeting vibration of a gravitational wave—equal in size to a fraction of the diameter of a subatomic particle. Translated to sound, it was a faint chirp, marking the culmination of more than four decades of hard work and $1.1 billion in taxpayer money.

The reason this is so exciting is that it marks not only the first detection of gravitational waves, but also the first observation of black holes.

—Duncan Brown

The detection also confirms a key prediction of Einstein’s, affording humanity an entirely new understanding of the universe. “I was in LIGO Hanford’s control room, the night before the detection,” says Ballmer, a member of the Advanced LIGO design team, who was the lead commissioner at LIGO Hanford, responsible for making sure the detector worked. “When I returned the next morning, there was a buzz in the air. I’ll never forget staring at the first plots, getting goosebumps.”

According to The New York Times, LIGO’s chirp is destined to become one of science’s great sound bites, on par with Alexander Graham Bell’s “Mr. Watson, come here” and Sputnik’s radio “beeps” from outer space. Comparisons have also been drawn between LIGO’s detectors and Galileo’s first telescope, in terms of modern scientific impact. “The things Galileo saw in his primitive telescope gave rise not only to the science and technology we enjoy today, but also to new and fruitful ways of thinking about everything in human experience,” says Greg Huber, deputy director of the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara. Perhaps the National Science Board got it right when it officially declared LIGO’s detection “one of the coolest discoveries in decades.”

An expert on gravitational-wave astronomy and theoretical astrophysics, Brown agrees that the detection opens a new window onto the universe. “The reason this is so exciting is that it marks not only the first detection of gravitational waves, but also the first observation of black holes,” he says, during a recent meeting in the Physics Building. “We’ll be able to look at the universe in a way that we never have before, getting a better idea of where it has come from and where it’s going.”

Aerial photo of the LIGO Observatory
LIGO Hanford Observatory, Richland, Washington. Photo courtesy of LIGO/Caltech.

Also significant is that LIGO’s detection coincides with the centennial of the publication of Einstein’s general theory of relativity. Saulson considers the theory a “mathematical explanation” of gravity. “Einstein saw gravity not as a force, but as a warping, or curvature, of space and time,” says Saulson, who co-founded the LIGO Scientific Collaboration, and has worked on LIGO for almost 35 years. “Think of the black holes that we’ve seen as two bowling balls, rolling along on a trampoline. They revolve around one another because their mass produces a deep depression in the surface of the trampoline. The balls also jiggle the trampoline’s surface, shooting out energy in the form of ripples, or gravitational waves.”

But that’s where the analogy ends. “In spacetime, the black holes collide with one another to form a sphere, whose energy vaporizes in a flurry of gravitational waves, leaving behind a new, larger black hole,” continues the affable professor, whose job is to assess the authenticity of LIGO signals. “The ripples from this cataclysmic event traveled through the universe for more than a billion years before reaching us last fall. Pretty amazing, really.”

Gabriela González G’95, spokesperson for the LIGO Scientific Collaboration, was Saulson’s first Ph.D. student at Syracuse. “We’re all over the moon and back,” she says of the detection. “Einstein would be very happy, I think.”

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