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 September 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.
University of Maryland Physics Professor Joseph Weber (1919-2000) with one of the world's first gravitational wave detectors. Credit: Special Collections and University Archives, University of Maryland Libraries
Researchers in the University of Maryland Department of Physics contributed to the international effort that led to the discovery of these gravitational waves, building on the university’s long history in this field. In the early 1960s, the late UMD Physics Professor Joseph Weber built the world’s first gravitational wave detectors on the university’s College Park, Md. campus, inspiring a new field of research.
“Sharing the detection of this binary black hole merger event is a sheer joy for all of us who have been working in this field,” said Peter Shawhan, an associate professor of physics at UMD and a LIGO Scientific Collaboration (LSC) principal investigator. “It’s a dream finally realized, but it is just the beginning of the science that we can do with LIGO and other gravitational wave detectors.”
Shawhan helped to validate the analysis software that identified the black-hole merger signal a few minutes after the LIGO detectors recorded it. He also acted as a liaison with astronomers before and during the LIGO observing run. UMD physics graduate student Min-A Cho developed software to communicate the properties of promising signals to astronomers for follow-up observations with their telescopes and other instruments.
Cregg Yancey, also a UMD physics graduate student, helped to check that the detectors operated properly when the signal was detected. The black-hole merger signal stood up to all scrutiny during months of painstaking analysis and cross checks and was ultimately named GW150914, indicating the date of its arrival at Earth.
Alessandra Buonanno, a UMD Research Professor of Physics who also has an appointment as Director at the Max Planck Institute for Gravitational Physics in Potsdam, Germany, together with many students and postdoctoral researchers at both institutions, have developed highly accurate models of gravitational waves that black holes would generate in the final process of orbiting and colliding with each other.
UMD alumnus Andrea Taracchini (Ph.D. '14, physics), who is now a postdoctoral researcher in Buonanno's division at the Max Planck Institute in Germany; Buonanno; Yi Pan, a former assistant research scientist in physics at UMD; and Enrico Barausse, a former postdoctoral researcher in physics at UMD, developed waveform models that were employed in the search that observed the black-hole merger with high-enough significance to be confident in its detection.
"We spent years modeling the gravitational-wave emission from one of the most extreme events in the universe: pairs of massive black holes orbiting each other and then merging. And that’s exactly the kind of signal we detected!” said Buonanno, who is also an LSC principal investigator.
Buonanno also co-led an effort to determine whether the signal detected by LIGO matches exactly the predictions of Einstein’s theory of general relativity. So far, all tests find the signal to be consistent with this theory.
“It is overwhelming to see how exactly Einstein’s theory of relativity describes reality,” said Buonanno. “GW150914 gives us a remarkable opportunity to see how gravity operates under some of the most extreme conditions possible.”
UMD physicists are continuing a long tradition of gravitational wave research that began over 50 years ago. Weber’s early detectors used “resonant bars”, which were designed to ring when a gravitational wave passed through them. UMD Physics Professors Emeriti Ho Jung Paik and Jean-Paul Richard improved on Weber’s technique to develop more sensitive resonant detectors. Later technology improvements enabled the more-sensitive laser interferometer technique used by LIGO.
Over the years, the gravity theory research group at UMD has also made many key contributions to the theory of black hole dynamics, gravitational wave emission and possible alternative theories of gravity, through the work of UMD Physics Professors Emeriti Dieter Brill and Charles Misner and UMD Physics Professor Ted Jacobson.
“As we continue to improve our detectors and collect and analyze more data, we expect many more discoveries that will give us a fuller picture of the gravitational dynamics that have shaped our universe, with all its galaxies and stars, along with weird, wonderful things like neutron stars and black holes,” said Shawhan.
LIGO research is carried out by the 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: six from Centre National de la Recherche Scientifique (CNRS) in France; eight from the Istituto Nazionale di Fisica Nucleare (INFN) in Italy; two 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 with 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 of New York and Louisiana State University.
Press conference by the LIGO Observatories:
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