Astrophysicists Detect Gravitational Waves Once Again


The hat-shaped Sombrero galaxy was not involved in the gravitational wave research, but it is lovely. Seen edge-on, it features an unusually large and extended central bulge composed of billions of old stars, while its dust rings harbor many younger and brighter stars. Its center is thought to house a large black hole. Image credit: NASA/Hubble Heritage Team

Just four months after the announcement of the first detection of gravitational waves, physicists say they’ve recorded another burst of these elusive ripples in space-time, again coming from a merging pair of black holes, far beyond our galaxy.

The first gravitation wave detection, announced with great fanfare in February, was sparked by a signal recorded at the twin LIGO detectors on September 14 of last year; this latest signal tripped the detectors on December 26. (The acronym stands for Laser Interferometer Gravitational-wave Observatory.)

“We now know that the first detection wasn’t just luck,” LIGO team member Duncan Brown, of Syracuse University, tells mental_floss. The odds of the earlier signal being a false alarm were on the order of a million to one—but, notes Brown, “people do win the lottery sometimes.” This second detection clinches it, he says. “This tells us that we will be making regular detections of binary black holes” in the coming years.

The LIGO team announced the discovery today at a meeting of the American Astronomical Society in San Diego. Their paper will be published in the journal Physical Review Letters

The paper, which examines data collected by LIGO from September 2015 to January 2016, also hints at a third gravitational wave event, recorded last October, although that event is less certain (and is being described only as a “candidate signal,” and not necessarily a “detection”).

Black holes form when massive stars collapse in the final stage of their evolution. Occasionally black holes end up orbiting other black holes, their orbits gradually shrinking as the system loses energy. Eventually they accelerate and merge, sending a blast of gravitational waves out across the universe.

Until this year, gravitational waves were purely theoretical, a prediction of Einstein’s general theory of relativity, published 100 years ago.

NASA created this visualization of two black holes merging when the discovery of gravitational waves was announced earlier this year.

The black holes that caused the December signal are smaller than those responsible for the earlier event; in this case their masses are believed to have been about 14 and about 17.5 times the mass of the Sun (in the earlier case, they were 29 and 36 times as massive as the Sun). Because of their smaller size, they took longer to execute their final orbits, Brown says. As a result, while the earlier signal was a mere blip, lasting about one-tenth of a second, this event lasted for a relatively leisurely 1.5 seconds. During that time, the two ultra-dense stars, having orbited each other for perhaps 100,000,000 years, performed their final loops. “This time we saw about 30 orbits, before they finally crashed into each other and merged,” Brown says.

The result is an even bigger black hole—though not quite as large as you’d expect by just adding up the masses of the two black holes that gave rise to it. That’s because roughly one solar mass was converted into energy, via Einstein’s famous equation, E = mc2. The magnitude of the explosion boggles the imagination. “When a nuclear bomb explodes, you’re converting about a gram of matter—about the weight of a thumbtack—into energy,” Brown explains. “Here, you’re converting the equivalent of the mass of the Sun into energy, in a tiny fraction of a second.”

As powerful as the blast was—for an instant, it would have produced more energy than all the stars in the universe—the ripples it unleashed were almost vanishingly small by the time they reached the Earth, having traveled across some 1.4 billion light years of space.

For now, scientists can only estimate what direction these signals have come from; however, their ability to “triangulate” locations will greatly improve when another gravitational wave detector, Italy’s Virgo facility, is incorporated into the network of detectors, possibly as early as this autumn. India and Japan are also set to bring gravitational wave detectors online in the years ahead.

LIGO began operation in 2002, but with only a fraction of its current sensitivity. The detectors, located in Louisiana and in Washington state, were upgraded last fall in an effort known as “Advanced LIGO.” The facility is still operating at just one-third of its potential maximum sensitivity, Brown says.

As gravitational wave observations become routine, physicists will be able to tackle some of the outstanding problems in astrophysics and cosmology—many of which involve the puzzling properties of black holes, as University of Florida physicist Clifford Will tells mental_floss: “Where do black holes come from? Were they born small, and then grow? Or are there mechanisms that can produce 30 or 40 stellar mass black holes from the get-go? Did they form within binary systems? Or did one black hole capture another, later in life? These are the questions that astronomers and astrophysicists will be thinking about.”

Adds Brown: “The field of 'gravitational wave astronomy' is now open for business.”