Cosmic breakthrough: Physicists detect gravitational waves from violent black-hole merger


Scientists announced Thursday that, after decades of effort, they have succeeded in detecting gravitational waves from the violent merging of two black holes in deep space. The detection was hailed as a triumph for a controversial, exquisitely crafted, billion-dollar physics experiment and as confirmation of a key prediction of Albert Einstein's General Theory of Relativity.It may inaugurate a new era of astronomy in which gravitational waves are tools for studying the most mysterious and exotic objects in the universe. "Ladies and gentlemen, we have detected gravitational waves. We did it!" declared David Reitze, the executive director of the Laser Interferometer Gravitational-wave Observatory (LIGO), drawing applause from a packed audience at the National Press Club that included many of the luminaries of the physics world. Some of the scientists gathered for the announcement had spent decades conceiving and constructing LIGO. “For me, this was really my dream. It’s the golden signal for me," said Alessandra Buonanno, who started working on the problem of gravitational waves as a postdoctoral student in 2000 and is now a theoretical physicist at Germany's Max Planck Institute for Gravitational Physics. The observatory, described as "the most precise measuring device ever built," is actually two facilities in Livingston, La., and Hanford, Wash. They were built and operated with funding from the National Science Foundation, which has spent $1.1 billion on LIGO over the course of several decades. The project is led by scientists from the California Institute of Technology and the Massachusetts Institute of Technology, and is supported by an international consortium of scientists and institutions. LIGO survived years of management and funding turmoil, and then finally began operations in 2002. Throughout the first observational run, lasting until 2010, the universe declined to cooperate. LIGO detected nothing. Then came a major upgrade of the detectors. LIGO became more sensitive. On Sept. 14, the signal arrived. Though only a "chirp," it was a clear, compelling signal of two black holes coalescing, LIGO scientists said. It lasted only half a second, but it captured, for the very first time, the endgame of two black holes spiraling together. "This was truly a scientific moonshot," Reitze said during the announcement. "I really believe that. And we did it. We landed on the moon." These black holes were each about the diameter of a major metropolis. They orbited one another at a furious pace at the very end, speeding up to about 75 orbits per second — warping the space around them like a blender cranked to infinity — until finally the two black holes became one. The pattern of the resulting gravitational waves contained information about the nature of the black holes. Most significantly, the signal closely matched what scientists expected based on Einstein's relativity equations. The physicists knew, in advance, what gravitational waves from merging black holes ought to look like — with a rising frequency, culminating in that chirp, followed by a "ring-down" as the waves settle. Gabriela Gonzalez, a physics professor at Louisiana State University who is the spokesperson for LIGO, revealed images of the waves picked up by the two detectors and then played an audio version of the same signal. "Did you hear the chirp? There's a rumbling noise, and then there's a chirp," she told the Press Club audience. "That's the chirp we've been looking for. And that's what they saw and heard, both in Louisiana and Washington state. It was such a strong signal that everyone knew it was either a real detection of a black hole merger, or "somebody had injected a signal into the interferometers and not properly flagged it into the data set. It turned out that fortunately that wasn’t the case,” as Reitze put it in advance of the news conference. He said the team, knowing the checkered history of gravitational wave detections that were later discredited, took special care to have the results verified and peer-reviewed prior to the big announcement. The scientists even looked for the possible handiwork of a computer hacker, Reitze said. All reviews held up. The LIGO success has been a poorly kept secret in the physics world, but the scientists kept their historic paper detailing the exact results secret until Thursday morning.

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