MILWAUKEE _ For the first time, scientists have observed ripples in the fabric of space-time 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 (09:51 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.
“This measurement is incredibly exciting,” said Patrick Brady, a professor of physics and director of the Leonard E. Parker Center for Gravitation, Cosmology and Astrophysics at the University of Wisconsin-Milwaukee. “It confirms Einstein’s prediction of gravitational waves and, at the same time, is proof positive of the existence of black holes.”
Based on the observed signals, LIGO scientists estimate that the black holes for this event were about 29 and 36 times the mass of the sun, and the event took place 1.3 billion years ago. About 3 times the mass of the sun was converted into gravitational waves in a fraction of a second—with a peak power output about 50 times that of the whole visible universe. By looking at the time of arrival of the signals—the detector in Livingston recorded the event 7 milliseconds before the detector in Hanford—scientists can say that the source was located in the Southern Hemisphere.
According to general relativity, a pair of black holes orbiting around each other lose energy through the emission of gravitational waves, causing them to gradually approach each other over billions of years, and then much more quickly in the final minutes. During the final fraction of a second, the two black holes collide into each other at nearly one-half the speed of light and form a single more massive black hole, converting a portion of the combined black holes’ mass to energy, according to Einstein’s formula E=mc2. This energy is emitted as a final strong burst of gravitational waves. It is these gravitational waves that LIGO has observed.
UW-Milwaukee scientists were instrumental in developing the analytical tools that identified the wave as the product of colliding black holes. Along with Brady, UWM LIGO team is led by faculty members Jolien Creighton, Xavier Siemens and Alan Wiseman. The team led the development and operations of the LIGO Data Grid, a network of supercomputers used to analyze copious amounts of data coming from the LIGO instruments.
With partners at the Albert Einstein Institute in Germany, Caltech, Cal State Fullerton, Louisiana State University, MIT, Penn State, and Syracuse University, the team created a system to analyze the data within minutes of its acquisition. Physicists at UWM also played a key role in the calibration of the data – the conversion of electrical signals from the LIGO detectors into data that could be analyzed by the LIGO team. Using these systems, scientists first identified the Sept. 14 signal about three minutes after the waves passed the detectors.
“This discovery inaugurates the field of gravitational wave astronomy,” Creighton said. “It opens a completely new window. Until now, we’ve explored the cosmos using photons and particles. With gravitational waves, we can now see the final third of our observable Universe.”
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 US 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 the City of New York, and Louisiana State University.
“For many years, the LIGO group at UWM has been a leader in bringing computer resources – both software and hardware – to bear on the problem of analyzing gravitational wave data,” Wiseman said.
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.
UWM physicists have been part of LIGO since the early 1990s and trained scientists who are now faculty at universities around the world.
“We’ve trained a generation of scientists to analyze LIGO data,” Siemens said. “Many key members of the LIGO Scientific Collaboration trained at UWM as students or postdoctoral researchers.”
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.
To listen to a gravitational wave, watch this video.
To learn more about UWM’s role in the search, read this story.
About UWM
As Wisconsin’s only public urban research university, UW-Milwaukee has established an international reputation for excellence in research, community engagement, teaching and entrepreneurism. On a budget of $667 million, UW-Milwaukee educates more than 27,000 students and is an engine for innovation in southeastern Wisconsin. Its economic impact is more than $1.5 billion per year in Wisconsin alone. The Princeton Review named UWM a “2016 Best Midwestern” university based on overall academic excellence and student reviews. The Carnegie Classification of Institutions of Higher Education elevated UWM to “R1” status in 2016, recognizing it as one of 115 institutions with the “highest research activity.”