Gravitational Waves From Black Holes Are Detected for Third Time

The ripples in spacetime reached Earth from a cosmic collision about 3 billion light-years away.

Artist's conception of two merging black holes
Artist's conception of two merging black holes (LIGO / Caltech / MIT / Sonoma State / Aurore Simonnet)

Scientists have detected for the third time gravitational waves coming from the merging of two massive black holes somewhere in the universe, the wrinkles in the fabric of space and time created by a powerful cosmic collision.

About 3-billion light-years away from Earth, the two black holes, far more massive than our sun, whirled around each other and eventually collided, generating waves like ripples in a pond. The waves spread out into the universe, expanding and contracting spacetime as they went. They reached Earth in January, where they were detected as tiny vibrations by sensitive instruments in twin observatories in Louisiana and Washington state. The collision created a single, bigger black hole, with a mass about 49 times that of the sun, the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced Thursday.

LIGO first detected gravitational waves in September 2015 and publicly announced the discovery in February 2016, a century after Albert Einstein predicted their existence. The observatory announced a second detection last June, made in December 2015.

The first and second detections were located about 1.3 billion and 1.4 billion light-years away, making the third detection the farthest and earliest yet. The collision in the first detection produced a new black hole of 62 solar masses, and the second 21 solar masses. The measurements shocked scientists. Before LIGO’s first detection of gravitational waves, “all the black holes we knew about had masses less than six or seven solar masses,” said David Spergel, an astrophysicist at Princeton University and one of cosmology’s top experts. “People thought stellar evolution would have a hard time producing black holes of masses of 20 or 30 solar masses.” Suddenly, they were seeing black holes measuring 20 to 30 times the mass of the sun are colliding to make even more massive black holes—which meant massive stars exist to make them.

“Am I surprised that black holes collided a little bit earlier in the universe’s history? No, it’s an ongoing process,” Spergel said. “Am I surprised that black holes this massive exist? Yes.”

The latest detection, of a newfound black hole of 49 solar masses, suggests there’s indeed a class of stellar-mass of black holes out there, gobbling up matter unlucky enough to cross their paths.

The LIGO observatories in Louisiana and Washington state, run by Caltech and MIT, are laser interferometers, experiments designed to pick up interference in light waves. Laser light is beamed into the arms of L-shaped tubes, each arm measuring two and a half miles, or four kilometers, and kept under near-vacuum conditions to protect the experiment from outside noise. Mirrors are positioned at the ends of each arm. The laser beams travel back and forth down the arms, bouncing off the mirrors.

When there’s no interference, the distance the light travels between the mirrors remain the same. But when gravitational waves reach the observatories, that changes. As gravitational waves wash over Earth and distort spacetime, they stretch and shrink the lengths of the interferometers’ arms, which alters the distance the laser beams travel between the mirrors. The change is incredibly small, about one thousandths of the width of a proton, the particle inside the nucleus of an atom.

The collision of two black holes releases more energy than all the stars and galaxies in the universe radiate at any time. To us, that moment is practically nothing.

Here’s that moment in GIF form:

S. Ossokine / A. Buonanno /T. Dietrich / R. Haas / SXS project
SXS Collaboration

LIGO can’t tell where exactly in the universe the gravitational waves are coming from. But using mathematical models, scientists can estimate the sizes of the black holes, and get some clues about the formation of binary black holes, in which the objects orbit each other closely. When black holes are barreling toward each other, they’re also spinning on their own axis. If their spins are aligned, traveling in the same direction as the moving pair, the black holes may have been born together, formed when a pair of stars exploded. If their spins don’t match, the black holes may have come together later in their lifespans, perhaps inside a stellar cluster. The latest LIGO data found that the spin of at least one of the black holes did not line up with the motion of the pair, providing some evidence for the second theory for binary black holes.

LIGO is currently in the middle of an observational run that began last November and will continue through the summer. Its next run, scheduled to begin in late 2018, will feature improvements to the interferometers’ sensitivity. Scientists hope the next detection could bring something new, like evidence of a collision between a black hole and a neutron star, the core left over from the explosion of dying star and another puzzling object in the universe. But technology can’t go and look for these collisions and the gravitational waves like telescopes might search inside specific chunks of sky. The observatories just have to sit and wait, bouncing laser beams between mirrors, ready to detect a tiny shift that signals a seismic event billions of light-years away.