Einstein was right: Gravitational waves exist
For the LIGO observatory at the Hanford site, noise is a real buzz killer.
Any earthly sound – a truck rumbling past, the humming of a refrigerator in a nearby building, or the distant flutter of a plane’s propellers – can drown out the faint whispers from the cosmos that the Nobel Prize-winning project was designed to detect.
So when strange blips in the data started cropping up on summer afternoons, researchers were anxious to find the source and eliminate it.
“Any other noise makes it harder to hear the thing you’re listening for,” said University of Oregon physicist Robert Schofield, whose job is to ferret out racket from the environment and reduce its impact on some of the most sensitive instruments ever built.
What the Laser Interferometer Gravitational-Wave Observatory is straining to hear are fleeting vibrations in the fabric of space and time called gravitational waves. First hypothesized by Albert Einstein, the waves are generated by cosmic cataclysms and radiate across the universe like ripples in a pond.
LIGO succeeded in capturing those ripples for the first time in 2015, thanks to its ability to detect fluctuations smaller than the diameter of a proton. The waves, which originated from the collision of two black holes a billion light-years away, registered as a faint chirp on the observatory’s instruments.
But the mysterious blips last summer didn’t look like gravitational waves, said physicist Beverly Berger, a member of the LIGO collaboration based in California who first spotted them. And there were no similar glitches at LIGO’s twin instrument in Louisiana.
The glitches at Hanford corresponded to sounds recorded by a microphone installed by Schofield and his colleagues as part of their endless quest to detect and stamp out noise.
“It was picking up sounds at about the same time, almost every day,” said Berger, who recounted the scientific detective story at a meeting of the American Physical Society last month.
It didn’t take long for Schofield to identify the prime suspect once he listened to the recordings.
“It sounded like pecks to me,” he said. “I immediately thought it must be ravens.”
Schofield had seen the big, black corvids at the site many times. On hot days, they often perched on frost-covered pipes connected to a nitrogen cryopump that helps maintain a vacuum inside the L-shaped instrument’s concrete arms, each 2.5 miles long.
When Schofield and his colleagues investigated on a 100-plus-degree day, they found the pipes covered with peck marks, and even spotted one bird in the act of scraping its beak through the frost.
“They peck for a while and make themselves a snow cone,” he said.
It’s no surprise to Schofield that pecking by a three-pound bird could throw a wrench into an enormous system like LIGO. Some of the other noise sources he’s dealt with include water flowing over a dam 15 miles away, wind pushing on the buildings and distant construction projects.
Though it has a lot of built-in buffering, the way LIGO works makes it very vulnerable to outside interference.
When the instrument is operating, identical laser beams travel through each arm and are reflected back by mirrors at the ends. When a gravitational wave passes through, it stretches one arm and compresses the other, leading to a minuscule mismatch in the reflected beams.
The difference is equivalent to the width of an atom over the distance from the Earth to the sun, Berger said. “It’s an incredible level of precision, which means you have to shut out basically the whole world in order to see it.”
And that means the ravens had to go.
The first thing the research team did was insulate the pipes so they wouldn’t attract birds looking for a cool oasis. Then Schofield and his colleagues identified the component inside the instrument that jiggled when the ravens pecked. A few weeks ago, they finished fixing it so the problem won’t happen again.
After its initial success, LIGO spotted a second set of gravitational waves and its three founders were awarded the 2017 Nobel Prize in physics.
The instruments are being fine-tuned to boost their sensitivity even more, and are expected to resume the search next year.
Though ravens should plague the project nevermore, the incident did spawn a few riffs on Edgar Allan Poe’s classic poem.
One, from physicist Andrew Lundgren concluded:
“Eagerly I read your logbook, hoping that this humble rook
Might one day not be overlook’d – overlook’d in LIGO’s lore
That this somber, clever avian might find a place in LIGO’s lore
Part of science for evermore.”