Scientists have directly observed gravitational waves, ripples in the fabric of spacetime, for the second time—confirming the start of a new era of astronomy.
Much like the historic first observation announced in February, the second event marked the cataclysmic embrace of two black holes more than a billion years ago. This fatal attraction sent shockwaves rippling across the universe, which astronomers recorded as they washed through Earth.
Produced by some of the universe’s most violent events—such as mergers of black holes and collisions of ultra-dense neutron stars—gravitational waves give scientists an entirely new way of observing the universe, potentially allowing them to measure and study things that are invisible in any wavelength of light.
The latest event means that the first observation, made by the Laser Interferometer Gravitational-wave Observatory (LIGO), wasn’t a mere fluke but was the first of what are assuredly hundreds, if not thousands, more detections to come.
“If you just detect one magnificent event, that’s terrific, but it almost has a magical quality about it,” says France Córdova, an astrophysicist and the director of the National Science Foundation, the U.S. government agency that funds LIGO.
“When you detect a second event, then you know you have a real observatory. It’s shown us that we really do have a new way of looking at the universe.”
The first detection of gravitational waves, made on September 14, 2015, marked the end of a century-long search that had been frustrating physicists since Albert Einstein predicted the ripples in 1916.
Though Einstein later had misgivings about their existence, his theory of general relativity all but requires them. Then, pulsar observations made in the 1970s offered tantalizing evidence they should be out there, garnering the scientists involved the 1993 Nobel prize in physics.
The first LIGO detection involved two whopping black holes each about 30 times more massive than the sun. But the second signal came from a lighter set—one black hole with 14 times the mass of the sun, and the other with about 7.5 times the sun’s mass.
When these black holes spiraled into each other and merged, they created a new black hole roughly 21 times the mass of the sun and released as much energy as the sun will produce over its entire 10-billion-year lifespan.
The echoes of the collision careened through space for 1.4 billion years, passing through Earth early on December 26, 2015.
With the help of lasers and mirrors placed kilometers apart, the two L-shaped detectors in Louisiana and Washington measured the fantastically tiny ripples, which stretched Earth less than a ten-thousandth the width of a proton, one of the particles that makes up an atom’s nucleus.
An aerial view of the LIGO gravitational wave detector in Livingston, Louisiana. [PHOTOGRAPH COURTESY LIGO]
Some 70 seconds later, Pennsylvania State University physicist Chad Hanna got a phone call while celebrating the holidays with his family.
“It was pretty wild,” says Hanna, who co-leads the LIGO group in charge of analyzing its detections of black hole mergers. “I leapt out of my chair, recognized it was some sort of event, grabbed my laptop and phone, and ran upstairs.”
“My family was concerned,” he adds.
Hanna and more than a thousand scientists and engineers were still in the throes of verifying the September 2015 detection, and they ended up working double-time over the winter of 2015 to juggle both analyses. Ultimately, the scientists were able to tease out the comparatively quiet December detection, which is described in an analysis published today in Physical Review Letters.
Bagging that second detection before even announcing the first emboldened LIGO scientists to move ahead with their February unveiling, which is now among the most significant physics discoveries in decades.
“It was clearly a bit of a tease for us,” says David Shoemaker, director of the MIT LIGO Laboratory and leader of many of the observatory’s recent upgrades. “We felt more comfortable about it because we already knew what we had in the can.”
The announcement comes at a time when prospects for gravitational-wave astronomy has never been brighter—or, rather, louder.
Researchers point out that LIGO is operating at only a third of its designed sensitivity. If upgrades go as planned, the observatory will be able to scan 27 times more of the cosmos by 2019, all but guaranteeing a surge of detections.
What’s more, detecting gravitational waves will soon become a global affair. Last month, the National Science Foundation signed an agreement with India that may bring a LIGO-like observatory to the country as soon as 2023. Japanese researchers are also working on an underground detector of their own, which may open as soon as 2018.
And the pending opening of VIRGO, a gravitational-wave observatory in Italy, will help researchers triangulate where in the sky individual ripples originated, allowing astronomers to do follow-up measurements with optical telescopes—a tag-team Córdova describes as a “holy grail.”
“The era of gravitational-wave astronomy is upon us,” says astronomer Scott Ransom of the National Radio Astronomy Observatory. “I think it’s spectacular.”