WASHINGTON >> It was just a tiny, almost imperceptible "chirp," but it simultaneously opened humanity's ears to the music of the cosmos and proved Einstein right again.
In what is being hailed as one of the biggest eureka moments in the history of physics, scientists announced Thursday that they have finally detected gravitational waves, the ripples in the fabric of space and time that Einstein predicted a century ago.
The news exhilarated astronomers and physicists. Because the evidence of gravitational waves is captured in audio form, the finding means astronomers will now be able to hear the soundtrack of the universe and listen as violent collisions reshape the cosmos.
It will be like going from silent movies to talkies, they said.
"Until this moment, we had our eyes on the sky and we couldn't hear the music," said Columbia University astrophysicist Szabolcs Marka, a member of the discovery team. "The skies will never be the same."
An all-star international team of astrophysicists used an exquisitely sensitive, $1.1 billion set of twin instruments known as the Laser Interferometer Gravitational-wave Observatory, or LIGO, to detect a gravitational wave generated by the collision of two black holes 1.3 billion light-years from Earth.
"Einstein would be beaming," said National Science Foundation director France Cordova.
The proof consisted of what scientists called a single chirp — in truth, it sounded more like a thud — that was picked up on Sept. 14. Astronomers played the recording at an overflowing news conference Thursday.
"That's the chirp we've been looking for," said Louisiana State University physicist Gabriela Gonzalez, scientific spokeswoman for the LIGO team. Scientists said they hope to have a greatest hits compilation of the universe in a decade or so.
Some physicists said the finding is as big a deal as the 2012 discovery of the subatomic Higgs boson, known as the "God particle." Some said this is bigger.
"It's really comparable only to Galileo taking up the telescope and looking at the planets," said Penn State physics theorist Abhay Ashtekar, who wasn't part of the discovery team.
Physicist Stephen Hawking congratulated the LIGO team, telling the BBC: "Gravitational waves provide a completely new way of looking at the universe. The ability to detect them has the potential to revolutionize astronomy."
Gravitational waves, postulated by Albert Einstein in 1916 as part of his theory of general relativity, are extraordinarily faint ripples in space-time, the continuum that combines both time and three-dimensional space. When massive objects like black holes or neutron stars collide, they generate gravitational waves that stretch space-time or cause it to bunch up like a fishing net.
Scientists found indirect proof of gravitational waves in the 1970s by studying the motion of two colliding stars, and the work was honored as part of the 1993 Nobel Prize in physics. But now scientists can say they have direct proof.
"It's one thing to know sound waves exist, but it's another to actually hear Beethoven's Fifth Symphony," said Marc Kamionkowski, a physicist at Johns Hopkins University who wasn't part of the discovery team. "In this case, we're actually getting to hear black holes merging."
In this case, the crashing of the two black holes stretched and squished Earth so that it was "jiggling like Jell-O," but in a tiny, almost imperceptible way, said David Reitze, LIGO's executive director.
The dual LIGO detectors went off just before 5 a.m. in Louisiana and emails started flying. "I went, 'Holy moly,' " Reitze said.
But the finding had to be verified, using such means as conventional telescopes, before the scientists could say with confidence it was a gravitational wave. They concluded there was less than a 1-in-3.5-million chance they were wrong, he said.
LIGO technically wasn't even operating in full science mode; it was still in the testing phase when the signal came through, Reitze said.
"We were surprised, BOOM, right out of the box, we get one," Reitze said.
Reitze said that given how quickly they found their first wave, scientists expect to hear more of them, maybe even a few per month.
Detecting gravitational waves is so difficult that Einstein figured scientists would never be able to hear them. The greatest scientific mind of the 20th century underestimated the technological know-how of his successors.
In 1979, the National Science Foundation decided to give money to the California Institute of Technology and the Massachusetts Institute of Technology to come up with a way to detect the waves.
Twenty years later, they started building two LIGO detectors in Hanford, Washington, and Livingston, Louisiana, and they were turned on in 2001. But after years with no luck, scientists realized they had to build a much more sensitive system, which was turned on last September.
Sensitivity is crucial because the stretching and squeezing of space-time by gravitational waves is incredibly tiny. Essentially, LIGO detects waves that pull and compress the entire Milky Way galaxy "by the width of your thumb," said team member Chad Hanna of Pennsylvania State University.
Each LIGO detector has two giant perpendicular arms more than 2 miles long. A laser beam is split and travels both arms, bouncing off mirrors to return to the arms' intersection.
Normally, the two beams are aligned so that they balance each other out and there's nothing to hear. But if there's a gravitational wave, it creates an incredibly tiny mismatch, which is what LIGO detects.
A giant team of scientists had to keep the discovery secret until it was time to be announced. The study detailing the research in the journal Physical Review Letters had 1,004 authors.
Kip Thorne, the Cal Tech physicist who co-founded LIGO and has been working on gravitational waves for more than half a century, said he kept the secret even from his wife until just a few days ago. When he heard about the wave, he said, "it was just sort of a sigh of happiness."