Historic first detection of gravitational waves
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.
The gravitational waves were detected by the Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, based in the USA. Glasgow researchers have been working for decades to support the worldwide effort to detect gravitational waves, and co-led the group inside the collaboration which first detected the gravitational wave signal.
The gravitational wave event GW150914 observed by the LIGO Hanford (H1, left panel) and LIGO Livingston (L1, right panel) detectors. (Ligo.org)
The collaboration has concluded that the detected gravitational waves were produced during the final fraction of a second of the merger of two black holes to form a single, more massive spinning black hole.
Professor Sheila Rowan, Director of the Institute for Gravitational Research, said: “This is a monumental leap forward for physics and astrophysics – taking Einstein’s predictions and turning them into an entirely new way to sense some of the most fascinating objects in our Universe.
“In the past, we’ve relied on the information we collected from the electromagnetic spectrum to help learn more about the cosmos, from the other planets in our solar system to star systems millions of light years away.
“Now gravitational wave astronomy will give us the ability to make many exciting new discoveries. This first detection, in addition to confirming Einstein’s prediction, also gives us the first direct evidence of the existence of black holes, and the first observation of black holes merging, which is a fantastic result. We’re very much looking forward to new data from LIGO in the coming months and years, and to making our detectors even more sensitive.”
Glasgow scientists describe the significance of detecting gravitational waves
Scientists from the University’s Institute for Gravitational Research led on the conception, development, construction and installation of sensitive mirror suspensions in the heart of the LIGO detectors, which were crucial to the first detection.
That technology, developed in partnership with the University of Birmingham, the University of Strathclyde, and the STFC Rutherford Appleton Laboratory, was based on Glasgow’s pioneering work for the UK/German GEO600 detector.
Those suspensions rely on delicate 400-micron-wide fibres made from silica. Despite their fragility, each suspension fibre is very strong, capable of holding up to 70 kilograms. In the LIGO detectors, the mirror suspensions hold 40kg mirrors and keep them from being interfered with any outside force or vibration except for gravitational waves.
The mirrors are held so still by the suspensions that the LIGO detectors can detect movements, caused by gravitational waves, close to one-ten-thousandth the diameter of a proton.
Silicate suspension being prepared at the Institute of Gravitational Research
The longest-serving member of the University’s gravitational research community is Professor James Hough, who has worked in the field at the University since 1971.
Professor Hough said: “Alongside Professor Ronald Drever, I was involved in building early gravitational wave detectors here in Glasgow, which monitored outputs from piezoelectric transducers attached to aluminium bars.
“We thought it would take us about a year to make an initial detection, and in 1972, we found what looked very much like evidence of gravitational waves. However, since no other detectors we operating at the same time, we weren’t able to verify our observation. Nonetheless, that finding convinced me that we would one day find the evidence we were looking for.
“This discovery, 43 years later, is the culmination of my career in science. I’m immensely proud to have been involved in the project and I’m very excited to see the fascinating new discoveries gravitational wave astronomy will bring us in the future.”
Detecting the gravitational waves
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.
At each observatory, the two-and-a-half-mile (4 km) long L-shaped LIGO interferometer uses laser light split into two beams that travel back and forth down the arms (four-foot diameter tubes kept under a near-perfect vacuum). The beams are used to monitor the distance between mirrors precisely positioned at the ends of the arms. According to Einstein’s theory, the distance between the mirrors will change by an infinitesimal amount when a gravitational wave passes by the detector.
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.
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.
Aerial view of the LIGO Hanford Observatory (Ligo.org)
More about the LIGO Scientific Collaboration
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.
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 analyse 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, and formerly of the University of Glasgow.
Glasgow works closely with a number of UK partners within Advanced LIGO UK Operations. These include Cardiff University, University of Birmingham and the University of Sheffield.
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 in the City of New York, and Louisiana State University.
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