Scientists Detect Fourth Gravitational Wave, Homing in on an Ancient Black Hole Collision

For for the fourth time since early last year, astronomers announced the detection of gravitational waves—ripples in the fabric of space-time created by the powerful collision of two black holes.

Just over two years ago, detectors in the United States picked up these ripples, a century after Albert Einstein predicted their existence. The discovery of gravitational waves confirmed a crucial tenant in the theory of relativity: The movement of objects can create tiny ripples in the space-time continuum. The find was met with much excitement, shaking up the astronomical world and winning its discoverers awards and acclaim.

Now, as methods are tweaked and instrumentation improves, detecting gravitational waves could soon become more routine. "With the next observing run planned for fall 2018, we can expect such detections weekly or even more often," astrophysicist David Shoemaker says in a statement.

The gravitational waves in question, which were spotted late last month, emanated from the ancient collision of two black holes with masses 31 and 25 times that of our Sun, reports Hannah Devlin for the Guardian. The impact took place roughly 1.8 billion years ago, and converted a relatively small portion of their masses into energy that began rippling through the underlying fabric of the galaxy as gravitational waves. It's a little like the radiating waves that form from tossing a pebble in a pond.

These ripples are absurdly small—less than the width of an atom, reports Pallab Ghosh for BBC News.​Their minute size is why scientists have only just sucessfully fine tuned their instruments enough to spot them. 

But this latest find is particularly notable: Scientists were able to determine the origin of the ripples with unprecedented precision. Pinpointing where these gravitational waves came from is a challenging task, notes Adrian Cho for Science. To tackle the problem, researchers tried some international teamwork.

The newest detection was a combination of efforts between two U.S.-based detectors of the Laser Interferometer Gravitational-Wave Observatory, or LIGO, and the Italian Virgo observatory. By teaming up with these distant detectors, astronomers can triangulate the gravitational waves in the same way GPS satellites pinpoint a device's location on Earth, reports Loren Grush for The Verge.

By coordinating the measurements, astronomers were able to narrow the source to an area ten times smaller with Virgo's data than LIGO could pinpoint alone. They were also able to observe the waves in what is essentially 3D, notes Elizabeth Gibney for Nature, meaning that the orientation of the waves in relation to Earth's orbit could be calculated, giving scientists more data to estimate how much energy was originally released by the black holes.

"This increased precision will allow the entire astrophysical community to eventually make even more exciting discoveries," astrophysicist Laura Cadonati says in a statement from the LIGO team, referring to the possibility of observing other sources of gravitational waves such as colliding neutron stars.

With the power of combined forces, scientists are optimistic for the future of space-time ripple research. As Shoemaker says in the statement: "This is just the beginning of observations with the network enabled by Virgo and LIGO working together."

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Ben Panko is a staff writer for Smithsonian.com