Working in concert with the two LIGO detectors, the Virgo detector (above) can pinpoint the sources of gravitational waves on the sky.

N. Baldocchi/The Virgo Collaboration

Trio of detectors homes in on black hole sources of gravitational waves

For a fourth time, physicists have spotted gravitational waves—ripples in space itself—set off by the merger of two massive black holes. But this time, they detected the waves not only with two detectors in the United States, but also with a third detector in Europe: the Virgo detector near Pisa, Italy. The three-way detection enabled researchers to home in on the location of the black holes on the sky with 10 times greater precision than before, and to probe the polarization of gravitational waves in new ways. The result also independently confirms the blockbuster discovery of gravitational waves made 2 years ago.

"Virgo is in the game and that's very important," says Clifford Will, a gravitational theorist at the University of Florida in Gainesville who was not involved in the work.

Gravitational waves are a spectacular prediction of Albert Einstein's theory of gravity, general relativity. Einstein explained that gravity arises because massive objects warp space and time. When these objects spin around each other like twirling barbells, he predicted, they should produce ripples in spacetime, or gravitational waves, that spread at light-speed.

That prediction came to fruition in September 2015. Physicists working with the Laser Interferometer Gravitational-Wave Observatory (LIGO), which has twin instruments in Livingston, Louisiana, and Hanford, Washington, spotted a burst of gravitational waves from black holes 29 and 36 times as massive as the sun that spiraled into each other 1.3 billion light-years away. Since then, the 1000-member LIGO team has spotted two other black hole mergers, using its exquisitely sensitive L-shaped optical instruments called interferometers, which use lasers and mirrors to compare the stretching of space in one direction to that in the perpendicular direction. LIGO completed its two interferometers, with 4-kilometer-long arms, in 1999.

But LIGO hasn't been alone in the hunt for gravitational waves. In 2003, European physicists completed construction of Virgo, a €300 million interferometer with 3-kilometer-long arms, funded by French national research agency CNRS and the Italian National Institute of Nuclear Physics (INFN). In 2007, LIGO and Virgo researchers signed a data-sharing agreement, and on 1 August, after a 5-year, €24 million upgrade, Virgo rejoined LIGO in the search for gravitational waves.

The Virgo and LIGO detectors found that the new black-hole merger occurred in a patch of sky measuring 60 square degrees.

Virgo-European Gravitational Observatory

It didn't take Virgo long to strike scientific gold. On 14 August at 12:30:43 p.m. in Italy, the detector’s automated triggering system sensed a potentially exciting tremor, as did the systems of the two LIGO detectors. “This is really a great surprise, having an event just 2 weeks after the start of the run,” says Benoit Mours, a Virgo team member and a physicist from the Annecy Laboratory of Particle Physics in France. Subsequent analysis showed that the signal came from a black hole merger, Virgo researchers announced at a press briefing today in Turin, Italy.

The observation should reassure the roughly 280 Virgo scientists, who just a few months ago were dealing with technical difficulties with their machine. “For the experimenters it's tremendous because you have to see the light at the end of the tunnel,” says Ettore Majorana, a physicist and Virgo team member with INFN in Rome, who worked on the specific technical problems. During the observing run, which ended 25 August, Virgo ran with between a quarter to a half the sensitivity of LIGO, Mours says.

The new black hole merger is similar to the first one seen by LIGO. In it, black holes 25 and 31 times as massive as the sun spiraled together in a galaxy 1.8 billion light-years away. By timing the arrivals of the signals at all three detectors, which differ by milliseconds, researchers were able to determine that the black hole merger took place somewhere within a 60-square-degree patch of sky in the Southern Hemisphere. That's a big chunk of sky—the full moon covers only 0.2 square degrees—but it's an area 10 times smaller than what could have been determined with the LIGO detectors alone.

Such pointing capability could prove crucial for finding flashes of light that accompany the pulses of gravitational waves. Although no such flash is expected from the merger of black holes, it would be expected in the merger of two neutron stars. Rumors have been swirling that such a case has occurred after astronomers last month trained several different telescopes on a particular galaxy.

The new observation also tests a key property of the gravitational waves themselves, their polarization. Just as light waves can be polarized horizontally or vertically depending on which way the electromagnetic field in them jiggles, gravitational waves can be polarized in two ways, according to general relativity, Will says. In one way, an oncoming gravitational wave can squeeze space vertically and stretch it horizontally, and then vice versa, in a repeating cycle. The second way is for that pattern to be tilted by 45°.

However, if Einstein was wrong and general relativity is incorrect, then, in principle, gravitational waves could come with four other polarization patterns, Will says. “If you see any of the other four it kills general relativity,” he says. With the data from the three detectors, physicists found no evidence for polarization in three of the four unacceptable ways, Will says. So general relativity lives to fight another day.

Perhaps most important, the latest result shows that the infant field of gravitational waves continues to live up to scientists’ sky-high expectation, Will says. “Nature has just blown us away.”