When two black holes merged 1.8 billion light-years away, their violent union sent shock waves through space and time. On Aug. 14, three precisely tuned machines sensed the cosmic fallout, a ripple known as a gravitational wave. August's event marked the fourth time that astronomers have observed black hole collisions.
An international team of scientists announced the discovery on Wednesday from Turin, Italy, at a meeting of the G7 science ministers.
The science of hunting gravitational waves is old on paper and young in practice. Albert Einstein, through his General Theory of Relativity, predicted in 1916 that the waves should exist. It would remain a prediction for 98 years, until the LIGO Scientific Collaboration detected the first gravitational wave in September 2015.
The two L-shaped detectors that make up LIGO, located in Washington state and Louisiana, recently partnered with a third: the Virgo detector near Pisa, Italy.
"LIGO and Virgo are the most sensitive instruments ever built by mankind," said Jo van den Brand, a physicist and Virgo Collaboration spokesman, during the announcement. The detectors hear waves as a spike in frequency sometimes called a cosmic chirp. August's chirp was the first signal detected by all three observatories.
[Earlier this summer: Cosmic waves felt from an ancient black hole collision]
Virgo was officially online for just two weeks when it detected the gravitational wave. Now that Earth's three-detector network for sensing gravitational waves is operational, astronomers hope to zoom in on the source of the waves.
Massachusetts Institute of Technology research scientist David Shoemaker, spokesman for the LIGO Scientific Collaboration, used the analogy of a camera tripod at Wednesday's conference. Work your way up the three legs of a tripod, and you'll find the location of the camera. In this sense, the LIGO detectors and the Virgo detector form the tripod's feet. Where the legs met, 1.8 billion light-years away, was a ball of space so compact light cannot escape.
The two black holes that merged were massive: one hole was 31 times the mass of the sun, and the other was 25 solar masses. They twisted together to form a single spinning hole 53 times more massive than our star. The missing three suns' worth of mass became energy, expelled as gravitational waves.
Gabriela González, a professor of physics and astronomy at Louisiana State University and former spokeswoman for the LIGO team, said this is a milestone detection for the collaboration.
The Virgo detector – a nearly 2-mile-long, V-shaped instrument dug into the Italian countryside – is less sensitive than the two American detectors that first found gravitational waves two years ago. But its inclusion in the collaboration allows scientists to triangulate the origin of these space-time ripples with greater precision.
"We go from hundreds of square degrees, almost thousands, to only 30 square degrees," González said. ("Square degrees" are the units with which astronomers measure the celestial sphere of the night sky; the full moon takes up about 0.2 square degrees, and the large constellation Hydra covers 1,303.)
Astronomers said they were able to trace the gravitational wave, GW170814, to a region of sky of 60 square degrees. Virgo's addition shrank the space by a factor of 10.
What's more, the detector in Italy added "a new dimension to the fundamental tests we can carry out," said Frédérique Marion, a Virgo Collaboration senior scientist, during Wednesday's announcement. The LIGO detectors are essentially aligned. Virgo is not. With the third detector, researchers can analyze the space-time distortion in three dimensions. They found, just as Einstein predicted, that wherever space stretched, it contracted in the dimension at a right angle – as what happens when you tug on a wad of putty.
Such precision becomes even more important as gravitational wave detectors begin to detect signals from events involving objects other than black holes. Whereas black holes emit no radiation and are impossible to directly observe, other potential sources of gravitational waves – colliding neutron stars, supernovas, binary star mergers – can be seen through conventional telescopes.
Knowing exactly where a gravitational wave came from, González said, will help LIGO scientists correlate their detection to optical counterparts – heralding a new era of astronomy in which celestial events are "heard" as well as "seen."
"This proves we are now ready to do that kind of astronomy," González said.
In fact, since mid-August, rumors have circulated that the detectors sensed gravitational waves from colliding neutron stars on Aug. 17. If so, it is an event that might also have been seen on the electromagnetic spectrum by telescopes like Hubble and the Chandra X-ray Observatory. In mid-August, these telescopes were all abruptly aimed toward the origin of a gamma-ray burst, an explosion predicted to come from the impact of two neutron stars.
(In a news release for Tuesday's announcement of the gravitational wave, the LIGO and Virgo researchers hoped to tamp down some of the speculation: "LIGO and VIRGO's partner electromagnetic facilities around the world didn't identify a counterpart for GW170814, [the gravitational wave from colliding black holes] which was similar to the three prior LIGO observations of black hole mergers. Black holes produce gravitational waves but not light.")
González said that the LIGO team is still seeking to confirm several potential detections from the latest observing run, which started in January and ended Aug. 25. She would not say whether the Aug. 17 signal was among them. But González explained that gravitational wave events with optical counterparts can take longer to confirm because of the consistency tests involved.
And there's always next year. The next observational run will begin in late 2018.