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.
Accompanying material and information
The Max Planck Institute for Gravitational Physics (Albert Einstein Institute (AEI)) is an institute of the Max Planck Society with sub-institutes in Potsdam-Golm (outside Berlin) and Hannover, where it is closely related to the Leibniz Universität. Since its foundation in 1995, the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) has established itself as a leading international research center. The research program is pursued in five divisions and several independent research groups cover the entire spectrum of gravitational physics: from the giant dimensions of the Universe to the tiny scales of strings. The AEI is the only institute in the world that brings together all of these key research fields. Three of its five divisions are part of the LIGO Scientific Collaboration and played a major role in realizing the first direct detection of gravitational waves.
The Institute for Gravitational Physics of Leibniz Universität Hannover is co-located with the AEI Hannover. Under one roof, scientists from both institutions collaborate closely on all aspects of gravitational wave research. More than 50 PhD students are working towards their doctoral degree at Leibniz Universität Hannover in the joint International Max Planck Research School (IMPRS) on Gravitational Wave Astronomy.
Gravitational wave research in the Max Planck Society has a long history and goes back to the very beginning of the field in the 1960s. It was the Max Planck group that conducted coincidence experiments between resonant mass detectors to disprove the early claims of gravitational wave detection in the 60s. The group then turned to laser interferometry and built the first serious prototypes of laser interferometric gravitational wave detectors, developing and/or demonstrating most of the key concepts that are now an integral part of the large observatories, among them optical mode cleaners, stray light suppression, power recycling, and later in collaboration with Leibniz Universität Hannover, dual recycling, resonant sideband extraction, thermally adaptive optics, multi-stage monolithic suspensions, and high-power stable lasers.
Gravitational waves are an important prediction of Einstein's theory of general relativity. Accelerated motions of large masses create ripples in space-time, which lead to tiny relative distance changes between far-away objects. Even gravitational waves emitted by astrophysical sources, like stellar explosions or merging black holes, change the length of a one-kilometer measurement distance on Earth by only one thousandth of the diameter of a proton (10-18 meters). Only now have the detectors reached a level of sensitivity at which they can measure gravitational waves. The observation of the until now dark “Gravitational Universe” ushers in a new era in astronomy. The interferometric gravitational-wave detectors such as LIGO (in the USA), GEO600 (in Germany), and Virgo (in Italy), as well as planned detectors in Japan and India collaborate closely. A low-frequency gravitational wave detector in space (LISA) is under preparation by ESA and NASA and scientists from Leibniz Universität Hannover and AEI play a leading role.
The signal reported now is referred to as GW150914 since it arrived at Earth on September 14, 2015 at 09:50:45 UTC. It was detected by both LIGO instruments in Hanford and Livingston. It was observed for about 0.2 seconds during which the signal increased both in frequency and amplitude. Over 0.2 seconds its frequency grew from 35 Hz to 250 Hz and it had a peak amplitude (gravitational-wave strain) of 10-21.
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.
The signal matches the predictions of general relativity for those of an inspiral and merger of two black holes with masses of 36 and 29 solar masses, respectively. The black hole resulting from the merger has a mass about 62 times that of our Sun. 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. From the observations a distance of about 410 Megaparsecs (1.3 billion light years) to the black hole system was inferred.
By characterising the random noise fluctuations of the Advanced LIGO detectors the researchers estimate the statistical significance of the signal to be 5.1 standard deviations. This means that such signals in 16 days of observation happen less then once in 200,000 years through coincidence of random detector fluctuations.
Advanced LIGO consists of interferometric gravitational-wave detectors at two sites, one in Hanford (Washington State, USA) and one in Livingston (Louisiana, USA). At each site, lasers beams are bounced down four kilometer long L-shaped vacuum tubes to very precisely monitor the distance between mirrors at each end. According to Einstein’s theory, the distance between the mirrors will change by an infinitesimal amount when gravitational waves pass by the detector. A change in the lengths of the arms smaller than one-ten-thousandth the diameter of a proton (10-19 meter) can be detected.
Independent and widely separated observatories are necessary to verify that the signals come from space and also to determine the direction to the gravitational wave source.
Advanced LIGO concluded its first coordinated three-month data-taking run on January 12, 2016. During that run the sensitivity was 3 to 5 times higher than that of initial LIGO. At design sensitivity, a ten-fold increase in sensitivity over initial LIGO is expected.
GEO600 is an interferometric gravitational-wave detector with 600 meter long laser beam tubes, located near Hannover, Germany. It is designed and operated by scientists from the Max Planck Institute for Gravitational Physics and the Institute for Gravitational Physics at Leibniz Universität Hannover, along with partners in the United Kingdom. GEO600 is part of a worldwide network of gravitational wave detectors and at the moment the only detector taking data almost continuously. GEO600 also is a test bed for advanced detector technologies, such as non-classical (squeezed) light, signal and power recycling, and monolithic suspensions.
Atlas is a large computer cluster at the Albert Einstein Institute in Hannover with enormous computing capacities. Atlas consists of more than 14,000 CPU cores and 250,000 GPU cores, making it the largest computer cluster worldwide dedicated to gravitational-wave data analysis. Atlas is primarily supported (investment and operations) by the Max Planck Society but also receives operational support from Leibniz Universität Hannover.
LIGO operations are funded by the US National Science Foundation (NSF), and the facility is operated by Caltech and MIT. The LIGO upgrade was funded by the NSF with substantial financial and technical contributions from the German Max Planck Society, the UK’s Science and Technology Facilities Council (STFC), and the Australian Research Council (ARC).
GEO600 is funded by the Federal Ministry of Education and Research, the State of Lower Saxony, the Max Planck Society, the Science and Technology Facilities Council (STFC), and the VolkswagenStiftung.
Dr. Hartmut Grote (expert on: experimental aspects of GW detectors, GEO600, Advanced LIGO): “Ein wirklicher Meilenstein der Messtechnik, an dem wir Jahrzehnte gearbeitet haben! Ich bin sehr beeindruckt von der Vorstellung, dass eine Krümmung der Raumzeit als Welle durch das Weltall läuft, und uns, wie in diesem Fall, Signale von Schwarzen Löchern bringt.”
Apl. Prof. Dr. Gerhard Heinzel (expert on: experimental aspects of GW detectors, signal recycling, LISA): “Was wir vor 20 Jahren hier im Labor ausprobiert haben, funktioniert nun reibungslos und vollautomatisch auf vier Kilometer Länge. Und endlich ist ein Ergebnis da, keine technische Rauschkurve mehr sondern ein gigantisches Ereignis aus dem fernen Universum. Ein Traum ist wahr geworden!”
Dr. Harald Lück (expert on: experimental aspects of GW detectors, GEO600, Einstein Telescope): “Nun ist es tatsächlich wahr geworden: zum ersten Mal haben wir den Klang des Weltalls gehört! Unglaublich, dass dieses Signal, das nur klingt wie ein kurzer Schluckauf, von dem gewaltigsten Ereignis produziert wurde, was Menschen je beobachtet haben - für den Bruchteil einer Sekunde kraftvoller als das ganze Universum zusammen.”
Apl. Prof. Dr. Benno Willke (expert on: experimental aspects of GW detectors, aLIGO lasers): “Diese Entdeckung übersteigt die menschliche Vorstellungskraft: Vor mehr als einer Milliarde Jahren sind zwei schwarze Löcher miteinander verschmolzen und haben dabei dreimal die Masse unserer Sonne in Gravitationswellenstrahlung umgewandelt. Diese Strahlung hat im September letzten Jahres auf der Erde den Abstand von zwei 40 Kilogramm schweren Spiegeln um weniger als ein tausendstel eines Atomkerndurchmessers verändert und wir waren in der Lage dies zu messen! Unvorstellbar! Es ist ein erhebendes Gefühl, dass wir am Albert-Einstein-Institut und am Laser Zentrum Hannover e.V. die Laser entwickelt und gebaut haben, die diese Entdeckung ermöglicht haben.”
Dr. Collin Capano (expert on: searches for gravitational waves from coalescing compact objects, GW150914): “My first encounter with GW150914 was my colleague next door banging on my office wall to show me the plot. When it became clear that it was not an injection, I was overwhelmed with excitment. I remember turning to Badri and asking, "What do we do now?"”
Dr. Marco Drago (expert on: search pipeline which discovered the signal (coherent wave burst), GW150914): “I did not think it was real when I saw it. When I went to a colleagues office to talk to him about this, I did not say something like: "I saw an event", I asked: "Do you know if someone is doing hardware injections?"”
Dr. Andrew Lundgren (expert on: aLIGO detectors, detector characterization, searches for gravitational waves from coalescing compact objects, black holes, GW150914): “I never dreamt that one plot could change my view of the universe so much. Not only did we get a beautifully clear first detection, we’ve seen the first glimpse of a universe full of more activity in gravitational waves than we had ever imagined.”
Dr. Reinhard Prix (expert on: searches for continuous gravitational waves): „Gleich dieses erste Signal stellte sich als extrem reichhaltig für die Physik heraus, weit mehr als nur eine erster Nachweis: verschelzende schwarze Löcher, die wunderbare Tests der Allgemeinen Relativitätstheorie erlaubten, mit astrophysikalische interessanten Massen, und vielem mehr. Die Ära der Gravitationswellen-Astronomie ist schon mit dieser ersten Beobachtung voll durchgestartet!”
Dr. Thomas Dent (expert on: searches for compact binary coalescence signals, optimizing search methods to distinguish between signals and detector artefacts, estimating the rate of binary black hole mergers in the Universe based on the detection): “Although it was inevitable that a detection would be made at some point, there was a strong element of luck that it was so early and so clear - it has resulted in a great opportunity for gravitational wave science.”
Dr. Alex Nielsen (expert on: black holes, searches for gravitational waves from coalescing compact objects): “Black holes are mysterious objects. They are right on the frontier of human knowledge. Today's result is the beginning of observing black holes in a way that was not possible before. The discovery indicates that in the coming years we will see many more black holes like this. It marks the beginning of pushing back the frontiers of human knowledge in a major way.”
Dr. Alexander Nitz (expert on: searches for compact binary coalescence signals, software which carried out the most sensitive search for GW150914 presented in the detection paper): “I never expected that we'd find anything this early on, and the signal was so loud that my first reaction was that it could not possibly be real. How glad I am to be wrong.”
Dr. Tito Dal Canton (expert on: searches for compact binary coalescence signals, software which carried out the most sensitive search for GW150914 presented in the detection paper): “Nature really surprised me with such a loud and high-mass signal so early in the advanced detector era. I can't wait to see the data from future scientific runs and I look forward to the detection of binaries containing neutron stars.”
Dr. Roland Haas (expert on: numerical-relativity modeling of compact-object binaries): “I was actually on vacation at the time of the detection, so when I returned to the Albert-Einstein Institute everybody was already hard at work analysing the data. What we had expected to be rare event turned out to have occured quite early on in LIGOs lifetime and we all focused on learning as much as we could from the event, doing work within weeks that we had not expected to attempt for months in the future.”
Dr. Ian Harry (expert on: searches for gravitational waves from coalescing compact objects, GW150914): “I've been working on searches for compact-object mergers since 2007. When I first saw this signal in the data I didn't believe it was real. We never expected the first event to be so loud and clear. As Advanced LIGO rapidly becomes more and more sensitive, we look forward to seeing the field of gravitational-wave astronomy bloom in the coming years.”
Dr. M. Alessandra Papa (expertise: data analysis in the low signal-to-noise ratio regime, gravitational wave searches): “The detection of GW150914 is exciting beyond words: GW150914 represents the first direct detection of gravitational waves, is the first direct messenger of the dynamics of space-time in the strong-field and high-velocity regime, and its signal is strong enough to make this a text-book example. It was an honour to be able to play a role in this historical result. In particular I chaired the committee that reviewed the generic transients search that identified GW150914 within three minutes of data acquisition and was part of the team of six people who functioned as editors of the detection paper.”
Dr. Vivien Raymond (expert on: follow-up analyses of coalescing compact objects, GW150914): “This is such an amazing start to what we call the "advanced detector era"! Before even starting, we made the most impressive discovery we could hope for.”