LIGO’s Latest Black-Hole Merger Confirms Einstein, Challenges Astrophysics

Jun 6, 2017

By Lee Billings

Some three billion years ago, when Earth was a sprightly ocean world dotted with protocontinents and inhabited solely by single-celled organisms, a pair of black holes spiraled together and collided in a far-off region of the universe, leaving behind a single black hole some 50 times heavier than our sun. Emitting no light, the entire affair should have remained forever lost to the void.

Instead, the invisible violence of the pair’s final moments and ultimate merging was so great that it shook the fabric of reality itself, sending gravitational waves—ripples in spacetime—propagating outward at the speed of light. In the early morning hours of January 4, 2017, those waves washed over our modern Earth and into the most precise scientific instrument ever built, the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO). There the waves shifted the positions of vacuum-insulated, laser-bathed mirrors by less than the radius of a single subatomic particle. Traveling at light-speed, the waves first perturbed LIGO mirrors set up in Hanford, Wash., before passing through a second set of mirrors in Livingston, La., some three milliseconds later. Synced together from each station’s moving mirrors and converted to audible frequencies, the cosmos-quaking gravitational waves sounded like a single, soft “chirp.” Analyzing it, researchers are teasing out remarkable and otherwise-inaccessible details about the hidden lives of black holes. Announced Thursday by members of the LIGO team, the findings are described in Physical Review Letters.

As inconceivable as it may seem, tuning in to such chirps is now becoming routine. First predicted by Einstein more than a century ago as a consequence of his theory of general relativity, gravitational waves were long thought to be beyond observational reach—if not entirely nonexistent. But the chirp from January 4, dubbed “GW170104,” is actually LIGO’s third and farthest-reaching detection of gravitational waves, coming from somewhere about 3 billion light-years away. It follows earlier chirps from two other events detected separately in late 2015 that each occurred closer by, yet still more than a billion light-years distant.

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