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Third Black-Hole Merger Proves (Yet Again) That Einstein Was Right

Astronomers have detected gravitational waves for the third time in less than two years, after two black holes merged to create a single massive void weighing 49 times as much as the sun.

According to data picked up by the Laser Interferometer Gravitational-Wave Observatory (LIGO), the two black holes that smashed together some 3 billion light-years from here were large in themselves, measuring 19 and 32 times the mass of the sun. But when they merged, they created a monster.

Such a dramatic merger may seem alarming, but it appears we should start getting used to it. This is the third black-hole collision scientists have reported in less than two years, suggesting such events might occur quite frequently in the distant reaches of space.

During a news conference yesterday announcing the detection of gravitational waves from the latest collision (which occurred back on January 4), LIGO’s team of scientists said collisions between massive black holes are so common that they expect to start detecting as many as one per day once the observatory begins operating at its full capacity.

Artist's conception shows two merging black holes similar to those detected by LIGO. (Credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet))

Artist’s conception shows two merging black holes similar to those detected by LIGO. (Credit: LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet))

LIGO’s observations come from two detectors—located in Hanford, Washington, and Livingston, Louisiana—and are analyzed by an international collaboration of more than 1,000 scientists. Though the observatory began operating in 2002, it wasn’t sensitive enough to detect much of anything until it underwent a major upgrade, known as Advanced LIGO, which was completed in late 2014.

In September 2015, during a test run several days before the official search began, Advanced LIGO detected gravitational waves for the first time in history. These ripples in the fabric of space and time, caused by a black-hole collision some 1.3 billion light years from Earth, provided the first concrete evidence of a phenomenon first proposed by Albert Einstein in 1916 in his theory of general relativity.

By the time scientists announced that mind-blowing discovery in February 2016, LIGO had already picked up waves from a second black-hole collision in late 2015—on Christmas Day, no less. The third black-hole merger, announced this week, occurred much further away than the previous two. The gravitational waves it caused had to travel some 3 billion light-years to get here, compared with 1.3 billion and 1.4 billion light-years, respectively, for the first two. In addition, the latest merger revealed some new, intriguing clues about how the two black holes were moving in relation to each other, and even how they may have formed in the first place.

Pairs of black holes not only spiral around each other; they also spin individually on their own axes, just like the Earth and most other planets do. A statement from LIGO about the new discovery, Whitney Clavin of the California Institute of Technology (Caltech) describes this dynamic as “like a pair of ice skaters spinning individually while also circling around each other.”

G5AY63 A black hole in space with immense gravity field. Image shot 2016. Exact date unknown.

G5AY63 A black hole in space with immense gravity field. Image shot 2016. Exact date unknown.

When black holes in a pair spin in the same orbital direction as the pair is moving, they’re what’s called “aligned.” But LIGO’s observations of the two black holes that collided in the latest merger suggest that at least one of them was “non-aligned,” meaning it was spinning in the opposite direction from the pair’s orbital motion.

This non-alignment is important because scientists have proposed two theories about how binary pairs of black holes can form: first, they could come into being at the same time, when a pair of stars explodes; second, they could form separately and pair up later in life, in the middle of a dense cluster of stars. In the first scenario, the black holes would likely remain aligned, while in the second they could spin in any direction in relation to their orbital motion.

The latest black-hole collision, then, appears to support the second scenario. “This is the first time that we have evidence that the black holes may not be aligned, giving us just a tiny hint that binary black holes may form in dense stellar clusters,” Bangalore Sathyaprakash of Penn State and Cardiff University, one of the editors of the new paper, noted in the LIGO statement.

As tantalizing as these clues are, scientists hope there will be bigger revelations to come as they collect information about more and more black-hole systems. For example, physicists have long puzzled over the inability to reconcile Einstein’s theory of general relativity, which explains the universe on a macro scale, with quantum mechanics, which gets down to the micro level. To address this, and other lingering questions, LIGO’s scientists are not–just hoping to gain a greater understanding of Einstein’s groundbreaking theory—they’re also hoping to see where it may potentially fall short. As David Reitze, director of the LIGO Laboratory and professor of physics at Caltech, put it the New York Times, “[A]t some point he’s going to be wrong, and we’ll be looking.”

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