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Astronomy

Gravitational waves detected!

A hundred years after Einstein predicted them, gravitational waves from a cataclysmic event a billion years ago have been observed.

11 February 2016, SPIE Newsroom. DOI: 10.1117/2.2201602.01
LIGO detects gravitational waves

For the first time, scientists have observed gravitational waves, ripples in the fabric of spacetime arriving at 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 to the cosmos.

The discovery was announced on 11 February at a press conference in Washington, DC, hosted by the National Science Foundation, the primary funder of the Laser Interferometer Gravitational Wave Observatory (LIGO).

The 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 event took place on 14 September 2015 at 5:51 a.m. EDT (09:51 UTC) by both of the twin (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington. The LIGO observatories are funded by the National Science Foundation (NSF), and were conceived, built and are operated by the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology (MIT).

SPIE video and related article: LIGO Hanford scientists celebrate while looking ahead to future discoveries

This discovery comes at the culmination of decades of instrument research and development, through a world-wide effort of thousands of researchers, and made possible by dedicated support for LIGO from the NSF. It marks the beginning of a new era of gravitational-wave astronomy - the possibilities for discovery are as rich and boundless as they have been with light-based astronomy.

Gravitational waves carry information about their dramatic origins and about the nature of gravity that cannot otherwise be obtained. 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, in the approximate direction of the Magellanic Clouds.

How our sun and Earth warp space and time, or spacetime, is represented here with a green grid. As Albert Einstein demonstrated in his theory of general relativity, the gravity of massive bodies warps the fabric of space and time-and those bodies move along paths determined by this geometry. His theory also predicted the existence of gravitational waves, which are ripples in space and time. These waves, which move at the speed of light, are created when massive bodies accelerate through space and time. (LIGO)

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.

"Our observation of gravitational waves accomplishes an ambitious goal set out over 5 decades ago to directly detect this elusive phenomenon and better understand the universe, and, fittingly, fulfills Einstein's legacy on the 100th anniversary of his general theory of relativity," says Caltech's David H. Reitze, executive director of the LIGO Laboratory. 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.

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.

"In 1992, when LIGO's initial funding was approved, it represented the biggest investment the NSF had ever made," says France Córdova, NSF director. "It was a big risk. But the National Science Foundation is the agency that takes these kinds of risks. We support fundamental science and engineering at a point in the road to discovery where that path is anything but clear. We fund trailblazers. It's why the U.S. continues to be a global leader in advancing knowledge."

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.

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.

"The description of this observation is beautifully described in the Einstein theory of general relativity formulated 100 years ago and comprises the first test of the theory in strong gravitation. It would have been wonderful to watch Einstein's face had we been able to tell him," says Weiss.

"With this discovery, we humans are embarking on a marvelous new quest: the quest to explore the warped side of the universe-objects and phenomena that are made from warped spacetime. Colliding black holes and gravitational waves are our first beautiful examples," says Thorne.

In a 1987 interview with SPIE, Thorne talked about his vision for the LIGO detectors.

"The kinds of things one will see with gravitational waves will be very different from what one sees with electromagnetic waves," Thorne said. "The region from which the gravitational waves would come would be the region of very strong gravity, deep in the interior of the imploding star...Gravitational waves are the only things that can get out unimpeded, with no loss of information."

The SPIE video below is from a visit in September 2015 to the Hanford LIGO observatory. The first gravitational wave had been detected a week earlier, but was not announced until February 2016.