Gravitational waves detected 100 years after Einstein's prediction
LIGO opens new window on the Universe with observation of gravitational waves from colliding black holes – key contributions from Max Planck Society and Leibniz Universität Hannover researchers
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 a.m. 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.
Researchers at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute; AEI) in Hannover and Potsdam, Germany, and from the Institute for Gravitational Physics at Leibniz Universität Hannover (LUH) have made crucial contributions to the discovery in several key areas: development and operation of extremely sensitive detectors pushed to the limits of physics, efficient data analysis methods running on powerful computer clusters, and highly accurate waveform models to detect the signal and infer astrophysical information from it.
From its foundation the AEI played a leading role in gravitational-wave research thanks to Bernard F. Schutz, one of its founding directors, who retired in 2014. Bernard Schutz was - and still is - a vital part of the effort to foster productive cooperation between theoretical and experimental astrophysicists worldwide. He played a key role in establishing the field of gravitational wave astronomy, in particular as one of the three initiators (with Karsten Danzmann and the British physicist Jim Hough) of the GEO600 detector in Hannover; and he prepared the ground for the detection with numerical simulations and theoretical studies of gravitational wave sources, and with the development of methods for analyzing gravitational wave data.
“Very soon after turning on the Advanced LIGO detectors, what we observed was a strong signal from a kind of system that we expected to have to wait a much longer time to see, two very heavy black holes merging together”, said Schutz. “This encourages me to believe that our new field of gravitational wave astronomy will be rich with further surprises and discoveries from the dark side of our Universe. This is a success story that rewards the vision and the efforts of hundreds of scientists over the last thirty years, as well as the long-term support for this work provided by scientific funding bodies in many countries, including of course the Max Planck Society.”
Advanced detector techniques from GEO600
The GEO collaboration includes Max Planck and Leibniz Universität researchers together with UK colleagues. They designed and operate the GEO600 gravitational-wave detector near Hannover, Germany. It is used as a think tank and testbed for advanced detector techniques. Most of the key technologies that contributed to the unprecedented sensitivity of Advanced LIGO (aLIGO) and enabled the discovery have been developed and tested within the GEO collaboration. Examples of these are signal recycling, resonant sideband extraction, and monolithic mirror suspensions. AEI researchers together with the Laser Zentrum Hannover also developed and installed the aLIGO high-power laser systems, which are crucial for the high-precision measurements.
“Scientists have been looking for gravitational waves for decades, but we’ve only now been able to achieve the incredibly precise technologies needed to pick up these very, very faint echoes from across the Universe,” says Prof. Karsten Danzmann, director at the Max Planck Institute for Gravitational Physics in Hannover and director of the Institute for Gravitational Physics at Leibniz Universität Hannover. “This discovery would not have been possible without the efforts and the technologies developed by the Max Planck, Leibniz Universität, and UK scientists working in the GEO collaboration.”
Computing power and analysis methods for the discovery
Max Planck scientists developed and implemented advanced and efficient data analysis methods to search for weak gravitational-wave signals in the aLIGO detector data streams and carried out most of the production analysis. In addition, the majority of the computational resources for the discovery and analysis of the Advanced LIGO data were provided by Atlas, the most powerful computer cluster in the world designed for gravitational-wave data analysis, operated by the AEI. Atlas has provided more than 24 million CPU core hours for the analysis of Advanced LIGO data.
“I am proud that the first two scientists to look at the signal were at the Max Planck Institute for Gravitational Physics and that our institute played a leading role in this exciting discovery,” says Prof. Bruce Allen, director at the Max Planck Institute for Gravitational Physics in Hannover. “Einstein himself thought gravitational waves were too weak to detect, and didn’t believe in black holes. But I don’t think he’d have minded being wrong!“
Accurate models of gravitational waves pave the way for observing merging black holes
Max Planck researchers developed highly accurate models of gravitational waves that black holes would generate in the final process of orbiting and colliding with each other. These waveform models were implemented and employed in the continuing search for binary coalescences in LIGO data. It is this search that observed the black-hole merger known as GW150914 with greater than 5-sigma confidence. Max Planck scientists also used the same waveform models to infer the astrophysical parameters of the source, such as the masses and spins of the two black holes, the binary’s orientation and distance from Earth, and the mass and spin of the enormous black hole that the merger produced. The waveform models were also employed to test whether GW150914 is consistent with predictions from general relativity.
“We spent years modeling the gravitational-wave emission from one of the most extreme events in the Universe: pairs of massive black holes orbiting with each other and then merging. And that’s exactly the kind of signal we detected!” says Prof. Alessandra Buonanno, director at the Max Planck Institute for Gravitational Physics in Potsdam. “It is overwhelming to see how exactly Einstein’s theory of relativity describes reality. GW150914 gives us a remarkable opportunity to see how gravity operates under some of the most extreme conditions possible.”
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.
The Deutsche Forschungsgemeinschaft has provided significant financial support for gravitational-wave research through the QUEST cluster of excellence. Five different projects within QUEST were aimed at improving and developing gravitational-wave measurement instruments and applied gravitational-wave astronomy.
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 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 New York, and Louisiana State University.