Syracuse University Magazine

Monumental Discovery

Monumental Discovery

Syracuse physicists play major role in historic detection of gravitational waves

By Rob Enslin

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.

“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 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.”

Saulson_Brown_Ballmer.jpg3D visualization of gravitational waves (top) courtesy of Henze, NASA; faculty photos 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 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.”

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.”


Peter Bergmann (pictured) was a research assistant to Einstein and a leader in advancing Einstein’s theories; he taught at SU from 1947 to 1982, when he retired from the faculty.


Joshua Goldberg G’50, G’52 (right), now a professor emeritus at SU, was instrumental in the physics department’s research on general relativity.

Bergmann and Goldberg photos courtesy of the College of Arts and Sciences

Deep-Rooted Research History

The University has deep roots in gravitational-wave research. After World War II, Einstein assistant Peter Bergmann, who taught at Syracuse until 1982, founded the nation’s first general relativity research group. Intent on unifying general relativity with quantum theory, the group attracted many stellar faculty members, including Professor Emeritus Joshua Goldberg G’50, G’52, who supported the first international conference on general relativity; Roy Kerr, a research scientist who eventually cracked “Einstein’s code” (six interlocking equations at the core of general relativity); and Abhay Ashtekar, Lee Smolin, and Professor Emeritus Rafael Sorkin, all of whom pioneered the study of quantum gravity in the 1980s and ’90s.

Kerr, who figured out what spacetime looks like in the presence of a large spinning black hole, regards LIGO as “one of the most outstanding contributions of science and technology ever…. [LIGO] required not only unbelievable technological advances to be able to measure incredibly small gravitational waves, but also several decades of theoretical work to calculate the signals that have been observed,” emails Kerr, professor emeritus of physics and astronomy at the University of Canterbury in Christchurch, New Zealand. “From the frequency of the signal, it is clear that this is not two neutron stars colliding, but a pair of heavy black holes. Spinning black holes do exist.”

Gravitational waves were mostly the stuff of theory until the 1960s, when researchers began figuring out how to detect them. Early proponents included Joseph Weber, a University of Maryland physicist who claimed to have detected gravitational waves using a six-foot aluminum cylinder as an antenna, and Rainer “Rai” Weiss, who worked out the basic ideas of LIGO as part of a physics course he taught at MIT.

Weiss has mentored several generations of gravitational physicists, including Saulson, Ballmer, and González. “For the life of me, I couldn’t figure out what Weber did,” says the retired physics professor from his office in Cambridge, Massachusetts. “So I came up with my own way of measuring gravitational waves, taking freely floating masses in space and then measuring the time it took to travel between them. The presence of a gravitational wave would change that.”

With Saulson in tow, Weiss teamed up with Kip Thorne (Brown’s research mentor) and Ronald Drever, gravitational physicists at Caltech, to form LIGO. Some thought their plan of using two large identical detectors to detect gravitational waves was more science fiction than hard science, but persistence won the day.

Saulson revels in the fact that LIGO has always been on time and on budget. Although initial data runs in 2005-07 and 2008-10 did not detect any gravitational waves, they were promising enough for the National Science Foundation (NSF) to approve the Advanced LIGO upgrade. “Currently, Stefan, Duncan, and others are knee-deep into the next phase of the project, going far beyond Advanced LIGO,” Saulson says proudly.

Although LIGO involves approximately 50 colleges and universities, Syracuse is home to one of the largest, most diverse cohorts in the country. It encompasses more than two-dozen professors, research scientists, postdocs, graduate students, and undergraduates, all with a broad range of backgrounds and experiences. Members include Laura Nuttall, a postdoc who studies how noise in LIGO detectors affects the sensitivity of the project’s search for black holes and other astrophysical objects; and Samantha Usman ’16, a double major in physics and mathematics, who is using an Astronaut Scholarship to develop algorithms to search for binary black holes. “At first, I thought [the chirp] was some freak noise or a fake signal that was put in the data to test us,” Usman says. “Only after we spent months looking at the results of my analysis were we convinced that the gravitational wave was real.”



The graph (below) shows the “chirp” heard round the world, the long-awaited signal detected at LIGO’s Hanford and Livingston (left) observatories that confirmed the existence of a gravitational wave produced by two colliding black holes.

Chirp_frequency.jpgGraph and Livingston photo courtesy of LIGO/Caltech

Future Explorations

While LIGO’s detection is only months old, researchers are already plotting the future. Saulson sees tremendous potential for the booming field of gravitational-wave astronomy. He is among the first to propose that neutron star binaries—extremely small, dense stars, born from the explosive deaths of larger stars—are a dominant source of gravitational waves. “Their signals are so weak that we still haven’t achieved the level of sensitivity needed to detect them, but we should be able to find them soon,” he says.

LIGO detectors are so sensitive that even the slightest trace of background noise—the hum of an air compressor, the rumbling of traffic, the crashing of ocean waves hundreds of miles away—can drown out a gravitational signal. “Even quantum noise from photons in laser beams, during interferometry, is problematic,” says Ballmer, who makes sure LIGO’s lasers and high-precision mirrors are “perfectly tuned” for desired interferometer sensitivity. “That’s why I do most of my work at night, when there’s less chance of human interference with the detector.”

Meanwhile, a wide range of fields, including nuclear astrophysics, is likely to benefit from LIGO’s detection. “I combine the experimental aspects of Peter’s and Stefan’s work with my own research in theoretical astrophysics,” Brown says. “Detecting gravitational waves lets us perform experiments with energies, masses, and speeds that are inaccessible in a lab on Earth. Future observatories may even be able take ‘baby pictures’ of the universe, capturing the first moments of creation.”

Saulson says LIGO’s detection is a testament to the vision and patience of campus leadership. “What we do takes a long time to produce results, and not many universities of our size are willing to play the long game,” he says. “I am extremely proud of and grateful for the leaders of Syracuse University and the College of Arts and Sciences. They’ve stood by us the whole way.”  «


Gravitational Wave Group, Department of Physics

Photo by Amy Manley

Computing Firepower

LIGO researchers need massive computing firepower, as well as sensitive detectors to study gravitational waves. At Syracuse, this is made possible by two cutting-edge computational environments overseen by Information Technology Services (ITS): Orange Grid, a research cloud made from University desktop computers; and Crush, a supercomputer housed in the Green Data Center on South Campus.

Eric SedoreEric Sedore, associate CIO at ITS, works closely with LIGO to provide the computing infrastructure needed to achieve the project’s ambitious goals. “Finding gravitational waves in the detector’s noise and then understanding where they come from requires a huge amount of computing,” he says.

Professor Duncan Brown, whose work lies at the intersection of physics, astronomy, and computing, has created many of the algorithms for detecting and studying gravitational waves. He also collaborated with Sedore to marshal the computing hardware necessary for LIGO’s detection. “In addition to traditional supercomputers, we relied on unused CPU cycles from desktop computers all over campus,” Brown says. “Those PCs worked overnight on LIGO data analysis, helping our students make LIGO’s groundbreaking detection.”

Brown and Sedore have also worked with the Open Science Grid (OSG), a consortium that facilitates access to distributed high throughput computing. “OSG has enabled LIGO computing to flow from Syracuse to clusters all over the country, some as far away as San Diego,” Sedore says.

Student Physicist with a Punch

If anyone eschews stereotypes, it’s Samantha Usman ’16, a double major in physics and mathematics. The College of Arts and Sciences senior has been integral to the success of the University’s Gravitational Wave Group, parlaying her interest in the astrophysics of gravitational waves into NSF fellowships at Caltech and the Laboratoire de l’Accélérateur Linéaire in Paris. She also has been awarded a prestigious Astronaut Scholarship, supporting her research in gravitational-wave astrophysics. “I never thought I’d be involved with anything as cool as LIGO,” Usman says. “I ran the analysis that first determined the significance of the binary black hole signal that we saw. When we realized that the detection was real, I was ecstatic. We can learn so much about the universe from gravitational waves.” 

Samantha UsmanWhen she’s not participating in the Renée Crown University Honors Program or serving as a campus tour guide for University 100, the Pittsburgh native may be found in the gym. A formidable boxer, she captains the University’s co-ed boxing team, and placed second in her weight class at the 2015 United States Intercollegiate Boxing Championship. “Being at Syracuse has changed my life,” says Usman, who, after graduation, is using a fellowship to continue LIGO work at Cardiff University in Wales (U.K.). “I want to keep pushing the project in bold, new directions.”

Photo by Amy Manley

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