Capturing the Sound of Colliding Black Holes

Julian Smith Writer

Colliding black holes produce a cosmic sound that can now be detected on Earth through gravitational wave astronomy, giving scientists a new window into the cosmos and the nature of gravity.

The Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) was online less than two days before the first signal arrived at Earth from a cataclysmic black hole collision more than a billion light years away.

Sweeping up from 35 to 250 Hz like a bass tone turning into a low bird whistle, gravitational wave signal GW150914, as it was called, occurred on September 14, 2015.

It peaked in about a fifth of a second, but its significance echoes on: the cosmic “chirp” was the first direct detection of gravitational waves, or ripples in space and time generated as the black holes spiraled in toward each other, collided and merged.

These infinitesimal vibrations in spacetime were predicted by Albert Einstein’s theory of general relativity exactly 100 years earlier.

Based on the observed signal, LIGO scientists estimate that two black holes, which were about 29 and 36 times the mass of the sun, collided 1.3 billion years ago and merged to form one massive black hole.

At the point of impact, about three times the mass of the sun was converted into gravitational waves in a fraction of a second, releasing 50 times more energy than all the stars in the observable universe.

“This is the dawn of gravitational wave astronomy, a completely new way of observing our universe,” said Erik Katsavounidis, a senior research scientist at MIT and a member of the LIGO team. “This new tool gives us a picture that cannot be obtained with any other means.”

Miles of Detection

Supported by the National Science Foundation (NSF) and operated by Caltech and MIT, LIGO has two ground-based detector sites, one in Hanford, Washington and another in Livingston, Louisiana.

The L-shaped detector has two arms — each 2.5 miles long — that use laser light split into two beams that travel back and forth on each arm. The beams monitor the distance between mirrors precisely positioned at the ends of the arms.

According to Einstein’s theory, the distance between the mirrors will change by an infinitesimal amount when a gravitational wave passes by the detector.

Gravitational waves are like ripples through water, except on a mind-bendingly smaller scale and at the speed of light. When a wave passes through the detectors, it makes each arm alternately shrink and expand, changing the distance each laser travels.

LIGO detector site
The LIGO detector site, near Hanford in eastern Washington state, has arms 2.5 miles long. Photo credit: Caltech/MIT/LIGO Lab.

It’s hard to detect something this faint and fleeting, said Haisheng Rong, a principal engineer at Intel Labs who worked on the project.

These displacements are smaller than one-ten-thousandth the diameter of a proton. The length of the arms makes these miniscule changes easier to detect, he said.

Blocking Noise

“Not only do you need a stable and powerful laser, but you have to isolate it from all vibrations, like earthquakes or even cars driving past,” said Rong.

Contained within 4-foot-diameter tubes, the lasers are suspended by fused silica fibers to keep motion to a minimum. Seven sequential stages of active and passive vibration isolation dampen ground vibrations above 10 Hz by a factor of 10 billion, said Dennis Coyne, chief engineer for the LIGO laboratory at Caltech.

Having two detector sites located so far apart in states on opposite sides of the country makes confirmation of the gravitational wave signal possible, Rong said.

Gravitational waves take a few thousandths of a second to travel from Washington state to Louisiana. The time lag also makes it possible to narrow down the source of the signal, like hearing a sound with two ears as opposed to one, said Rong.

“If you see a signal at one but not the other, it could just be noise. But if they both see the same signal with the right delay time, then we are pretty sure that it’s the right signal,” said Rong.

Separating the signals from background noise means checking millions of possible signal waveforms as new data are continuously recorded, said Stuart Anderson, research manager for the LIGO laboratory at Caltech.

Plots showing gravitational waves.
Plots from the two LIGO detector sites show the signals from gravitational waves. Photo credit: LIGO.

LIGO uses about 50,000 computers in parallel to do this, ranging from home PCs connected through the Einstein@Home project to supercomputers like Stampede at the Texas Advanced Computing Center and Comet at the San Diego Supercomputer Center, said Anderson.

Understanding Black Holes

The NSF has invested $1 billion into the LIGO project, which was started 40 years ago to look for evidence of colliding neutron stars. Now more than 1,000 international scientists conduct LIGO research on gravitational waves. Before the first signal arrived, researchers didn’t have direct proof of black holes until the system went online.

The first signal has been confirmed by two more gravitational wave detections: a second one in December 2015, which created a black hole 22 times the mass of the sun, and a third in January 2017, resulting in a 50-solar-mass black hole.

Together, the three signals are direct confirmation of the existence of massive black holes, which were once only detectable indirectly by the electromagnetic radiation they emit.

One lingering question is where the pairs of colliding black holes came from, said Katsavounidis. Did they evolve together from gigantic stars that were already orbiting each other, or did they form separately and merge later through gravity?

In the first scenario, called “common envelope evolution,” each black hole would be spinning in the same direction as it orbits its partner, according to LIGO scientists. In the second, called “dynamical capture,” they would spin in the opposite direction from their orbit in an “anti-aligned spin.”

Merging of two black holes.
LIGO detected gravitational waves, or ripples in space and time, generated as the black holes merged. Photo credit: SXS.

So far the LIGO evidence goes both ways. Data from the December 2015 event offered a hint of aligned spins, while the January 2017 detection suggested anti-aligned spins.

“The spin will give us smoking-gun evidence of how they are formed,” Katsavounidis said, “but that is another parameter we have not been able to measure precisely so far.”

What’s Next for Black Holes

In the meantime, Katsavounidis said, the technology and engineering developed for LIGO can be applied elsewhere, from the 150-watt lasers to the seismic isolation systems.

“We are writing one terabyte of data every day, but only about 2 percent of that corresponds to gravitation waves. The rest could be used for interesting geoscience-related work, for example,” he said.

LIGO can approximate where the signals came from, he said, but it’s up to partner astronomer groups to use electromagnetic imaging tools to pinpoint the location, “like turning your head to look after you hear something.”

This will get easier when more LIGO detectors come online. Another interferometer called VIRGO is scheduled to start working near Pisa, Italy this summer, with other detectors planned for Japan and possibly India.

It’s exciting to imagine what this new technology could help us discover, Katsavounidis said.

“The only other analogy that really fits is the invention of the optical telescope. Did Galileo understand what his telescope would evolve into?”

Editor’s note: Feature image credit by Matt Heintze/Caltech/MIT/LIGO Lab.

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