On Sept. 14, 2015, LIGO detected gravitational waves for the first time. Northwestern's Vicky Kalogera and Shane Larson, both LIGO collaborators, describe the event. Produced by Erin Meyer and Kristin Samuelson


Northwestern University’s Vicky Kalogera absolutely could not believe it. The data revealed that gravitational waves -- ripples in the fabric of spacetime first predicted by Albert Einstein in 1916 -- had been detected for the first time by an international scientific team to which she belongs. Even Einstein didn’t think such a discovery would ever happen.


The extremely difficult-to-detect gravitational waves arrived at the earth from a cataclysmic event in the distant universe: the merger of two black holes to produce a single, more massive spinning black hole. This event emitted more energy than anything directly observed before in the universe. Such a collision of two black holes had been predicted but never observed.


The gravitational waves were detected at 5:51 a.m. Eastern Daylight Time Sept. 14, 2015, by both of the twin Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors. The surprising detection came only three days after the detectors were turned on again after a five-year renovation and upgrade.


The LIGO Scientific Collaboration (LSC), which carries out LIGO-related research, and the Virgo Collaboration in Europe made the discovery, confirming a major prediction of Einstein’s 1915 general theory of relativity.


Kalogera, an expert in black-hole formation in binary systems and in LIGO data analysis, had worked for nearly two decades for this moment. Soon after realizing there was a binary black hole represented by gravitational wave signals in the LIGO data, she emailed her Northwestern colleague Shane Larson -- both are members of the LIGO Scientific Collaboration -- to tell him the stunning news.


Since that day, Kalogera feels like she has been riding a roller coaster. The ride -- with 1,000 other scientists and engineers – has included thorough checking of data, multiple simulations, countless teleconferences and the writing of a flurry of scientific papers. Though the whirlwind permits little sleep, she and Larson are thrilled to be part of the discovery.


“To detect something in the first few days after turning on our new detectors and to have a detection of an unexpected source – ‘heavy’ binary black holes -- is just amazing,” Kalogera said.


An LSC member for more than 15 years, Kalogera is one of LIGO’s most senior astrophysicists and led the LSC’s astrophysics effort as the LIGO co-editor of the paper about the discovery’s implications. At Northwestern, she is director of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). Kalogera also is the Erastus O. Haven Professor of Physics and Astronomy and associate chair of the physics and astronomy department in the Weinberg College of Arts and Sciences.


Larson is a research associate professor of physics and astronomy at Northwestern, a CIERA member and an astronomer at the Adler Planetarium in Chicago. He has been involved with LIGO for five years and with the gravitational-wave community for more than a decade.


Kalogera leads the LIGO research team at Northwestern, which currently includes Larson, two postdoctoral fellows, three graduate students and several undergraduate students. The team’s contributions to the Sept. 14 discovery include making predictions for anticipated detections, interpreting the astrophysics, analyzing the data and characterizing the detectors.

Northwestern alumnus David Reitze, executive director of the LIGO Laboratory at California Institute of Technology, knows Kalogera’s contributions well.


“Professor Kalogera’s group has had a very substantial and outsized impact within the LIGO Scientific Collaboration for the past 15 years, ranging from making astrophysical predictions for what LIGO will discover and how often detections will be made to the development of advanced statistical methods for extracting information from LIGO signals, particularly in the determination of black hole masses and spins,” Reitze said.


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