Scientists observe 4th black hole collision using US, European detectors

The collision of two black holes. Caltech/MIT/LIGO Laboratory/Handout via Reuters

Scientists detected gravitational waves that indicate two black holes have collided for the fourth time in the past two years, the Laser Interferometer Gravitational-Wave Observatory announced Wednesday.

The latest detection was picked up Aug. 14 by a combination of US LIGO gravitational wave detectors and a European detector known as Virgo located near Pisa, Italy, the observatory said in a statement. This was the Virgo’s first detection.

“It is wonderful to see a first gravitational-wave signal in our brand new Advanced Virgo detector only two weeks after it officially started taking data,” Jo van den Brand, a spokesman for the Virgo collaboration, said in the statement.

Gravitational waves allow scientists to observe black holes, said Nergis Mavalvala, a physics professor at the Massachusetts Institute of Technology and a member of the university’s LIGO lab.

“Black holes are stars, and ordinary stars give off light,” she said. “Black holes don’t shine because they have too much gravity.”

Instead, they emit gravitational waves, ripples in space and time that scientists use to track a black hole’s approximate location and size, Mavalvala said.

The waves detected in August collided 1.8 billion light years away from Earth, the observatory said. One was 31 times the mass of the sun, the other 25 times the mass of the sun.

They combined to create a black hole 53 times the mass of the sun, spinning in space at about the speed of a blender’s blades, Mavalvala said. That’s pretty fast for something as large as a black hole.

The remaining three solar masses from the collision escaped in the form of gravitational-wave energy, the observatory said.

In addition to the fact that August’s collision was recorded by a combination of US and European detectors for the first time, it’s also unique because it gave scientists a clear look at the geometry of gravitational waves, Mavalvala said.

Scientists can detect the location, shape, and size of the waves, but with limited precision.

“Imagine someone drops a rock in a pond, and you’re far away from where the rock was dropped, but you want to know where the wave came from,” Mavalvala said.

With the help of the third observatory in Italy, the detectors were able to record more precise locations for the black holes and estimate the shape and size of the gravitational waves more accurately.

The LIGO detectors, located in Livingston, La., and Hanford, Wash., are aligned the same way, Mavalvala said. Since the Virgo detector is aligned differently, it allows scientists to better measure the geometry of the waves.

“This is just the beginning of observations with the network enabled by Virgo and LIGO working together,” said David Shoemaker, an MIT research scientist and a spokesman for the LIGO Scientific Collaboration. “With the next observing run planned for Fall 2018, we can expect such detections weekly or even more often.”