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Black hole merger
A computer simulation shows two black holes shortly before they merge into one. (Credit: SXS)

WASHINGTON, D.C. – After more than a decade of looking, scientists say they’ve detected the gravitational waves given off when two black holes merged into one bigger black hole.

“Ladies and gentlemen, we have detected gravitational waves. We did it!” Caltech physicist David Reitze, executive director of the Laser Interferometer Gravitational-Wave Observatory, declared here today at the National Press Club.

Reitze compared the LIGO project to a “scientific moonshot,” and then added, “We landed on the moon.”

The news was greeted with applause at the Washington briefing – and at a gathering of scientists and journalists in Hanford, Wash., the home of one of LIGO’s miles-long, L-shaped detectors.

The detection represents what’s likely to be a Nobel Prize-worthy discovery. It provides the best confirmation yet for a claim made a century ago in Albert Einstein’s general theory of relativity: that gravitational interactions should give off energy in the form of ripples in the fabric of spacetime.

For more than a decade, LIGO’s detectors in Hanford and in Livingston, La., have been watching for the signature of those ripples, using sets of cross-interfering laser beams. The laser detectors are so finely tuned they can register distortions in spatial dimensions that amount to less than a thousandth of the width of a proton.

Last year, scientists made changes in the LIGO equipment to improve its sensitivity dramatically. The “Advanced LIGO” observational campaign began last September, and it didn’t take long for the physics community to start buzzing about rumors that the upgraded experiment was picking up gravitational waves. Today, LIGO spokesperson Gabriela Gonzalez could joke about all those months of buzzing.

“The facts are so beautiful, how can you talk about rumors now?” she said.

The detection was made on Sept. 14, just a few days before the campaign’s official start. “We actually had not declared that we were ready to take data,” Caltech physicist Stan Whitcomb, the LIGO project’s chief scientist, told GeekWire.

Caltech theoretical physicist Kip Thorne, who was one of LIGO’s early advocates, said the signal was “just barely not strong enough” to have been detected years ago during the initial LIGO campaign. But with Advanced LIGO on the case, the signal stuck out like a sore thumb.

“My reaction was, ‘Wow, I couldn’t believe it,'” Reitze said.

False signals are sometimes inserted into the LIGO data to test the science team’s verification process, but in this case the signal came in before that “blind insertion” system was in effect. After weeks of analysis, scientists at LIGO and other gravitational-wave research centers verified the gravitational-wave signal so definitively that only the lower bound of the statistical confidence level could be estimated. (For the stat geeks out there, that value is 5.1 sigma.)

Scientists say the gravitational waves came from the merger of two black holes about 1.3 billion light-years from Earth, lying somewhere in the southern celestial hemisphere. The black holes were about 29 and 36 times the mass of the sun. In the course of merging, a share of the mass amounting to about three times the mass of the sun was converted into gravitational-wave energy, in accordance with Einstein’s theory.

“This is the first time that this phenomenon has ever been seen,” Reitze said.

The research is being published in Physical Review Letters.

The detection is a big deal, and not only because it confirms one of Einstein’s big claims.

Gravitational-wave observatories will provide a whole new way to “see” the universe – to study invisible black holes, to probe the dynamics of a supernova in the midst of an explosion, and to get a better grip on how gravity works.

Kip Thorne, who designed the fictional black hole in the movie “Interstellar,” said a detailed analysis of LIGO’s data could help physicists narrow down their search for the graviton, an as-yet-unobserved particle that’s thought to play a role in mediating the force of gravity.

Scientists expect still more gravitational-wave observatories, such as Italy’s upgraded EGO-Virgo detector and Japan’s KAGRA detector, to bring further discoveries to light. And last year, the European Space Agency launched a mission called LISA Pathfinder that could eventually set the stage for a space-based gravitational-wave observatory.

Thorne wrapped up today’s briefing by providing a big-picture perspective on LIGO’s quest. Just as modern-day historians hail the Renaissance for its impact on art, architecture and music, future historians are likely to hail the current age for its contribution to humanity’s understanding of the universe, he said.

“LIGO is a big part of that,” Thorne said. “I think we should be proud of what we give to our descendants culturally.”

More than 1,000 researchers from the LIGO Scientific Collaboration and the Virgo Collaboration are listed as co-authors of the paper in Physical Review Letters, titled “Observation of Gravitational Waves From a Binary Black Hole Merger.” The first listed author is B.P. Abbott, by virtue of alphabetical order.

This report includes information from GeekWire contributor John Stang.

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