LIGO spokesperson Gabriela González speaks at the 2016 AAAS Annual Meeting. | Boston Atlantic Photography
Gravitational waves are so small that when Albert Einstein first predicted their existence, he was unconvinced that scientists would ever be able to detect and measure them. One hundred years later, scientists have at last identified the waves' tiny signal — using a research collaboration huge in scale and packed with technological superlatives.
The first evidence of gravitational waves was announced 11 February by the Laser Interferometer Gravitational-Wave Observatory (LIGO), and published in the journal Physical Review Letters. A day after the historic announcement, LIGO spokesperson and Louisiana State University physicist and astronomer Gabriela González gave a special lecture at the 2016 AAAS Annual Meeting to discuss the findings.
The moving mass of a galaxy or a sun or even a person waving her arms can distort space-time, producing gravitational waves that carry energy away from the source of the movement. Astronomers have seen this effect in systems of binary neutron stars, whirling around each other and drawing closer and closer like exhausted boxers moving into a clinch, as their movements produce gravitational waves that disperse their energy.
These waves will be tiny ripples within the vastness of the universe, imperceptible unless their source is "big objects moving really fast," González said. "But what we wanted was to detect these gravitational waves here on earth. Well, the first thing you try to do is calculate how big they are, and then find a device that can measure those tiny distortions."
Aerial view of the Louisiana LIGO Laboratory. | LIGO Laboratory
The device turned out to be the LIGO observatories in Livingston, Louisiana and Hanford, Washington, funded in large part by grants from the National Science Foundation. Inside these observatories, powerful laser light races back and forth between mirrors hung inside 4-kilometer long arms.
When a gravitational wave passes by the detector, it should change the distance between the mirrors by a tiny amount as space-time stretches and bounces back. This minute change — about one ten-thousandth the diameter of the subatomic proton particle — is the signal that a wave has passed.
LIGO's sensitivity comes from the careful design of the detectors, González said, including a "quadruple pendulum" design that stills the movement of the delicately hung mirrors, and a shielding system that cancels out any seismic shivers of the earth.
On 14 September 2015, at about 4:51 a.m. in Louisiana, the Livingston detector recorded a signal of a gravitational wave, with a nearly identical signal recorded seven milliseconds later at the Hanford facility. European scientists studying the data coming in from the detectors were the first to realize what they might have seen, and the long process of confirming the result began, González said.
The massive collection of LIGO researchers — more than 1,000 authors signed the paper in Physical Review Letters — spent months trying to determine whether the signal was real evidence of a gravitational wave, or a chance "noise" produced by their instruments.
"We can put a bound on how often this happens by chance," González said, "and that false alarm rate is one in every 200,000 years. We don't expect a coincidence this strong more often than once in every 200,000 years."
The "chirp" caused by gravity waves, which was detected by LiGO. | Ginger Pinholster
By comparing the signal's features to simulated gravitational wave signals, the LIGO researchers concluded that their wave was a ripple created in the merger of two huge black holes 1.3 billion years ago. The gravitational wave produced by the merger occurs over fractions of a second, but González said there are plans to build space-based instruments that can detect waves lasting minutes to hours, which could signal the collision of galaxies. The imprint of gravitational waves lasting over ten billion years might even be detected in the cosmic microwave background, she said, providing more information about the start of the universe.
González, spoke about LIGO's strength as a "megascience project" at another symposium at the AAAS meeting, also discussed the project's global expansion. Other observatories in the collaboration include the smaller German detector GEO600, VIRGO in Italy and KAGRA in Japan-both under construction-and the planned LIGO India detector.
With a slight shift in its frequency, the newly identified gravitational wave can be heard by humans, "to make the black holes really sing," González said. She shared the sound — a slight chirp heard against a rushing background noise like a seashell held to the ear — with the AAAS attendees.
"Isn't it amazing?" she beamed. "I can't stop playing it."