Gravitational waves are detected for first time

Researchers announced Thursday they have detected gravitational waves for the first time; they observed a signature created by the collision of two black holes more than 1 billion light-years away. The discovery was made on the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) in September 2015.

At a packed news conference in Washington, DC, David Reitze, executive director of the LIGO Laboratory at Caltech, said “Ladies and gentlemen, we have detected gravitational waves. We did it.” He added, “It was exactly the kind of signal that Albert Einstein’s 100-year-old theory of general relativity predicted would occur from the collision.”

France Córdova, David Reitze, Gabriela González, Rainer Weiss, and Kip Thorne at the NSF LIGO announcement press conference.

The signal was picked up nearly simultaneously by LIGO’s two arms, at the Hanford Site in Washington State and at Livingston, Louisiana. After the LIGO collaboration checked and analyzed the data over several months to ensure it was not an artifact, they calculated the false alarm chance at less than 1 event per 203 000 years. Their results were published in the 12 February issue of Physical Review Letters.

Scientists estimated that in a fraction of a second, the collision of the black holes—each about 30 times the mass of the Sun, roughly 150 km in diameter, and traveling at half the speed of light—converted three solar masses into gravitational waves, producing a peak power output equivalent to 50 times that of the entire visible universe. Because the wave arrived at the Louisiana observatory 7 milliseconds earlier than the one in Washington, the researchers concluded that the source was located in the Southern Hemisphere, in the direction of the Large Magellanic Cloud.

The detection of the wave came just days after LIGO began its first observational run following a seven-year upgrade to increase its sensitivity. Italy’s Virgo observatory was not operating at the time, and Germany’s GEO 600 was not sufficiently sensitive to detect the event, according to the article.

The discovery is a triumph for NSF, which has funded LIGO beginning in 1992. “It was a big risk,” said NSF director France Córdova. “But NSF is the agency that takes these kinds of risks.”

Slides from the press conference display the signals at the two sites and the calculated origin of the signal.

Rush Holt, executive officer of the American Association for the Advancement of Science, said “NSF deserves great credit.” He noted that the agency was the lone US funder for the project. Germany’s Max Planck Society, the UK’s Science and Technology Facilities Council, and the Australian Research Council also contributed to Advanced LIGO.

“What’s really exciting is what comes next,” said Reitze, who compared the discovery to Galileo’s observations 400 years ago. “I think we’re doing something equally important here. I think we’re opening a window on the universe with gravitational astronomy.”

Reitze said that the event was proof of the existence of binary black holes, which have never before been observed. He said the size of their signal as seen from Earth is 10^-18 m, 1/1000th the diameter of a proton. LIGO is the most sensitive measuring instrument in the world, he said, capable of determining the distance between the Sun and the nearest star, 3 1/4 light years away, to the width of a human hair.

Caltech physicist Kip Thorne, one of the three original proposers of LIGO, said statistical analysis indicates that other gravitational wave events should be observed in the coming year. He noted that Advanced LIGO is currently operating at just one-third of its ultimate level of sensitivity.

Each L-shaped LIGO observatory consists of two 4-km-long legs, in which laser interferometers measure the infinitesimal difference in the distances of the legs caused by the distortion in spacetime from the passing gravitational wave.

Gabriela González, spokesperson for the 1004-member LIGO Scientific Collaboration, said the frequencies of the waveform indicated the masses of the two black holes and showed that the combined black hole was three solar masses smaller than the pair were. The distance of the event, 1.3 billion light-years, could also be deduced from the waveform, she said.