Gravitational Waves Strike Twice

Scientists announce the second detection of gravitational waves

black holes gravitational waves
An artist's rendition of merging black holes that produce gravitational waves. LIGO/A. Simonnet

For the second time this year—and the second time in history—scientists confirmed the detection of ripples in the fabric of space-time known as gravitational waves.

Since Albert Einstein predicted these elusive events over a century ago in his General Theory of Relativity, physicists have studied the skies hoping to catch the waves he described. With this second detection, researchers have not only confirmed their ability to detect gravitational waves, but illustrated that perhaps these space-time ripples aren’t as rare as they once thought.

Physicists at the Advanced Laser Interferometer Gravitational Waves Observatory (LIGO) made history in February of this year when they announced the first confirmed gravitational waves. But just a few months earlier, on December 26, 2015, the LIGO instrumentation logged logged a second space-time ripple. 

“We did it again,” LIGO researcher Salvatore Vitale tells Jennifer Chu for MIT News. “The first event was so beautiful that we almost couldn’t believe it.” With the confirmation of the second ripple, scientists are increasingly hopeful that these events could provide a new way to study the mysteries of the cosmos.

The faint but distinctive “chirp” that characterizes a gravitational wave is produced when two supermassive objects collide. While the fabric of space-time is stiff, immensely heavy objects like black holes can warp it, Geoff Brumfiel reports for NPR. When that happens, the distances between objects actually changes as the ripples pass by—much like the effect of dropping a stone into a pond.

"It will get longer and shorter and longer and shorter without us doing anything, without us feeling anything," Gabriela González, the head of LIGO's scientific collaboration tells Brumfiel.

In order to detect the waves, scientists have developed a way to sense these incredibly tiny shifts. As Liz Kruesi reported for in Feburary:

Inside each L-shaped LIGO observatory, a laser sits at the meeting point of two perpendicular tubes. The laser passes through an instrument that splits the light, so that two beams travel the roughly 2.5 miles down each tube. Mirrors at the ends of the tubes reflect the light back towards its source, where a detector waits.

Typically no light lands on the detector. But when a gravitational wave passes though, it should stretch and squish space-time in a predictable pattern, effectively changing the lengths of the tubes by a tiny amount—on the order of one-thousandth the diameter of a proton. Then, some light will land on the detector.

Once researchers detect the changes, they can trace the origins back into space to determine the cause. The latest waves emanated from the collision of two giant black holes about 1.4 billion light years away, Maddie Stone reports for Gizmodo.

“The objects are about as far away but because they are lighter, it’s a much weaker signal,” MIT researcher and LIGO leader David Shoemaker tells Stone. “We had to be more careful to look for airplanes, lighting strikes, seismic noises, people dropping hammers—all the things that could go wrong.”

Now that those possible interference have been eliminated, the researchers are confident that this second chirp is truly a gravitational wave.

"This is like Galileo turning his telescope to the sky 400 years ago," David Reitze, LIGO’s executive director, tells Brumfiel. "We're now looking at the universe in an entirely new way, and we're going to learn new things that we can't learn any other way."

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