Gravitational Waves Detected 100 Years After Einstein's Prediction

• Two Northwestern astrophysicists contributed to historic discovery by LIGO-Virgo collaborations

EVANSTON, Ill. --- For the first time, scientists have observed ripples in the fabric of spacetime called gravitational waves, arriving at the earth from a cataclysmic event in the distant universe. This confirms a major prediction of Albert Einstein’s 1915 general theory of relativity and opens an unprecedented new window onto the cosmos.

Gravitational waves carry information about their dramatic origins and about the nature of gravity that cannot otherwise be obtained. Physicists have concluded that the detected gravitational waves were produced during the final fraction of a second of the merger of two black holes to produce a single, more massive spinning black hole. This collision of two black holes had been predicted but never observed.

The gravitational waves were detected on Sept. 14, 2015, at 5:51 a.m. Eastern Daylight Time (9:51 UTC) by both of the twin Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington, USA. The LIGO Observatories are funded by the National Science Foundation (NSF) and were conceived, built and are operated by Caltech and MIT. The discovery, accepted for publication in the journal Physical Review Letters, was made by the LIGO Scientific Collaboration (which includes the GEO Collaboration and the Australian Consortium for Interferometric Gravitational Astronomy) and the Virgo Collaboration using data from the two LIGO detectors.

The news of the first definitive detection of gravitational waves and the first observation of colliding black holes is being announced today (Feb. 11) at a news conference starting at 10:30 a.m. (EST) at the National Press Club in Washington, D.C. The event, hosted by the National Science Foundation, is being simulcast live online.

Two Northwestern University astrophysicists, Vicky Kalogera and Shane L. Larson, are members of the LIGO Scientific Collaboration (LSC), which carries out LIGO-related research. Their contributions to the discovery include making predictions for anticipated detections, interpreting the astrophysics, analyzing the data and characterizing the detectors.

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 astrophysical implications. This companion paper is accepted for publication in The Astrophysical Journal Letters.

Kalogera is attending today’s news conference and can answer questions about the discovery’s implications for astrophysics; she also is available for comment after the event.

Larson has been involved with LIGO for five years and with the gravitational-wave community for more than a decade. Larson can comment on the importance of gravitational-wave exploration and research.

Almost everything we currently know about the universe has been discovered with light of some kind, said Kalogera, director of Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). “Gravitational waves carry completely new information about black holes and other cosmic objects and will unlock a new window onto the universe. These waves are very weak and challenging to detect, but now we have detected our first burst of gravitational waves.”

An expert in black-hole formation in binary systems and in LIGO data analysis, Kalogera also is the Erastus O. Haven Professor of physics and astronomy and associate chair of the department of physics and astronomy in the Weinberg College of Arts and Sciences.

A black hole’s gravity is so strong, not even light can escape it. But black holes do radiate gravitational waves, produced by accelerating masses. The gravitational-wave event detected Sept. 14 was two black holes, one orbiting around the other, moving eventually at half the speed of light, colliding and merging to form a new, bigger black hole.

“This is a new kind of astronomy -- observing the universe using gravity itself,” said Larson, research associate professor of physics and astronomy at Northwestern, a CIERA member and an astronomer at the Adler Planetarium in Chicago.

“We can’t ‘see’ black holes with telescopes. This is the first time black holes have been directly detected by measuring them, through their gravity, as opposed to measuring the effect they have on other matter in the universe,” Larson said.

Kalogera leads the LIGO research team at Northwestern that currently includes Larson, two postdoctoral fellows, three graduate students and several undergraduate students.

“The long-term goal for the LIGO detectors and its observations is to do astrophysics,” Kalogera said. “We want to use the gravitational-wave observations to learn about our universe for decades and centuries to come.”

In this context, Northwestern has played a major role within the LIGO collaboration, with Kalogera being the first “card-carrying” astrophysicist to join the collaboration, back in 2000.

Northwestern’s Selim Shahriar also is a LIGO member, working on the laser physics of the enterprise. He is looking for ways to improve the sensitivity of the LIGO detectors and broaden the spectrum that the detectors are sensitive to. Shahriar is a professor of electrical engineering and computer science at the McCormick School of Engineering and Applied Science.

LIGO research is carried out by the LIGO Scientific Collaboration (LSC), a group of more than 1000 scientists from universities around the United States and in 14 other countries. More than 90 universities and research institutes in the LSC develop detector technology and analyze data; approximately 250 students are strong contributing members of the collaboration.

The LSC detector network includes the LIGO interferometers and the GEO600 detector. The GEO team includes scientists at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI), Leibniz Universität Hannover, along with partners at the University of Glasgow, Cardiff University, the University of Birmingham, other universities in the United Kingdom, and the University of the Balearic Islands in Spain.

LIGO was originally proposed as a means of detecting these gravitational waves in the 1980s by Rainer Weiss, professor of physics, emeritus, from MIT; Kip Thorne, Caltech’s Richard P. Feynman Professor of Theoretical Physics, emeritus; and Ronald Drever, professor of physics, emeritus, also from Caltech.

Virgo research is carried out by the Virgo Collaboration, consisting of more than 250 physicists and engineers belonging to 19 different European research groups: 6 from Centre National de la Recherche Scientifique (CNRS) in France; 8 from the Istituto Nazionale di Fisica Nucleare (INFN) in Italy; 2 in The Netherlands with Nikhef; the Wigner RCP in Hungary; the POLGRAW group in Poland and the European Gravitational Observatory (EGO), the laboratory hosting the Virgo detector near Pisa in Italy.

The discovery was made possible by the enhanced capabilities of Advanced LIGO, a major upgrade that increases the sensitivity of the instruments compared to the first generation LIGO detectors, enabling a large increase in the volume of the universe probed -- and the discovery of gravitational waves during its first observation run.

The U.S. National Science Foundation leads in financial support for Advanced LIGO. Funding organizations in Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council, STFC) and Australia (Australian Research Council) also have made significant commitments to the project. Several of the key technologies that made Advanced LIGO so much more sensitive have been developed and tested by the German UK GEO collaboration.

Significant computer resources have been contributed by the AEI Hannover Atlas Cluster, the LIGO Laboratory, Syracuse University and the University of Wisconsin-Milwaukee. Several universities designed, built, and tested key components for Advanced LIGO: The Australian National University, the University of Adelaide, the University of Florida, Stanford University, Columbia University in the City of New York, and Louisiana State University.