Rick Robinson

Feb 28th 2020

A Black Hole Collision Really Makes Waves


When two black holes with a combined mass of more than 50 suns crash headlong into each other, you might expect it to shake things up — and you would be right. A black hole collision in a galaxy nearly 2 billion light-years away produced a cosmic jolt so powerful that it was detected in the summer of 2017 by three widely separated observatory instruments on Earth.

Two of the instruments, as the National Science Foundation reports, belong to the Laser Interferometer Gravitational-Wave Observatory, a project funded by the NSF. The two independent LIGO detectors are located in Louisiana and Washington state, while the third detector, called Virgo, is located in Italy.

According to the NSF, the black hole collision, detected Aug. 14, 2017, is the first triple detection of gravitational waves — a key benchmark because three independent observations of an event allow much better pinpointing of the event’s location in the universe.

Things Predicted by Uncle Albert

Black holes and gravitational waves combine two of the most astonishing predictions made by Albert Einstein’s general theory of relativity. One of these predictions is black holes, objects so massive and so compact that not even light can escape their intense gravitational fields.

The other prediction is that gravity, like other forms of energy, is propagated as waves, what the NSF describes as “ripples in space and time.”

Black holes are so called because they cannot be seen directly, since no light can escape them. They can be detected by their effects on nearby space, and have been found by those means for years, according to Until a couple of years ago, gravitational waves had eluded scientists.

The reason, paradoxically, is that gravity is the weakest force in the universe. As PBS notes, gravity is vastly weaker than other forces, such as magnetism — thus a small magnet can lift an iron weight against the gravitational pull of the entire Earth.

Shaking Up the Universe

Gravitational waves are so weak that it took a hundred years for physicists and engineers to come up with instruments, such as LIGO and Virgo, that can detect them. Yet, they are so powerful that they can shake up the universe for billions of light-years in all directions.

The black hole collision detected in 2017 originated with two black holes in orbit of one another. One black hole weighed about 25 times more than the sun; the other about 31 times more. They continued to orbit each other until they collided in an event that took place about 1.8 billion light-years away, resulting in a spinning black hole with a mass of about 53 times the sun, according to the NSF.

That mass was converted into gravitational-wave energy — enough to jiggle our most sensitive instruments here on Earth, 1.8 billion light-years away.

The LIGO and Virgo teams predict that, as they fine-tune their instruments, they will be able to detect gravitational wave signals weekly. And as with every new window into the universe, our wildest imagination will be nothing to the surprises we are sure to discover.