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Three billion years ago, in a third of a second, two black holes crashed into each other and merged into a single entity, converting two solar masses into energy that shook the fabric of spacetime, sending gravitational ripples across the universe that were detected on Earth last January, researchers announced Thursday.
It was the third confirmed detection of coalescing black holes detected so far by the U.S.-led Laser Interferometer Gravitational-Wave Observatory, or LIGO, a project made up of two observing stations, one near Hanford, Washington, and the other 1,800 miles away near Livingston, Louisiana, and hundreds of scientists around the world. The observations confirmed a prediction first made by Albert Einstein a century earlier.
As the gravitational waves passed by, they caused space to lengthen in one direction and compress in the other, squeezing and stretching the LIGO detectors ever so slightly and causing laser beams to cover slightly different distances as they bounced back and forth between massive mirrors.
Exhaustive tests and analyses confirmed the reality of the signal in another milestone for the growing field of gravitational wave astronomy.
"We have observed, on the fourth of January, 2017, another massive black hole-to-black hole binary coalescence, the merging of black holes roughly 20 and 30 times the mass of our sun," David Shoemaker, the spokesperson for the LIGO Scientific Collaboration, told reporters.
"The key thing to take away from this third event is we're really moving from novelty to new observational science, a new astronomy of gravitational waves."
The discovery was detailed in a paper accepted by the journal Physical Review Letters.
The ripples detected by LIGO indicate the single black hole formed by the merger has a mass of about 49 times that of the sun, midway between the black holes detected by LIGO in September and December 2015. Two times the mass of Earth's sun was converted directly into energy in a fraction of a second.
Black holes are among the most bizarre objects in the known universe. They are believed to form when massive stars run out of nuclear fuel at the end of their lives. Without the outward pressure generated by nuclear fusion to offset the inward pull of gravity, the core suddenly collapses as the star is blown apart.
LIGO's latest discovery "likely favors the theory that these two black holes formed separately in a dense stellar cluster, sank to the core of the cluster and then paired up rather than being formed together from the collapse of two already paired stars," said Laura Cadonati, a LIGO researcher at the Georgia Institute of Technology.
"This is an important clue in understanding how black holes form," she said. "We have found a new tile to put in the puzzle of understanding the formation mechanism."
Gravitational waves were predicted in 1916 by Einstein's general theory of relativity. The equations showed that massive bodies under acceleration, like binary black holes or the collapsing cores of huge stars in supernova explosions, would radiate gravitational energy in the form of waves distorting the fabric of space.
The waves would spread out in all directions, traveling at or near the speed of light. But detecting them is a major challenge. By the time a wave from an event many light years away reaches Earth, its effects are vastly reduced, becoming hard-to-detect ripples rather than powerful waves.
To detect those ripples, the LIGO observatories were designed to measure changes in distance that are vastly smaller than the width of an atomic nucleus.
"Gravitational waves are distortions in the metric of space, in the medium that we live in," said Michael Landry, director of the LIGO observatory near Hanford. "Normally, we don't think of the nothing of space as having any properties at all, so it's quite counter intuitive that it could expand or contract or vibrate.
"But that's what Einstein's relativity tells us. When a gravitational wave passes, the medium that we live in is distorted, and that causes what looks to us like length changes."
By way of analogy, Landry likened spacetime to the canvas of a painting.
"If I stretch the medium of a painting, I can see the painting get distorted," he said. "It's the medium that's vibrating, that's really what a gravitational wave is, and so we register the passage of those gravitational waves by comparing the length of the two long arms of our L-shaped detector."
(CBS)
