The landmark discovery was announced on Thursday (local time) and opens a new window for studying the cosmos.
The researchers said they detected gravitational waves coming from two distant black holes - extraordinarily dense objects whose existence also was foreseen by Einstein - that orbited one another, spiraled inward and smashed together. They said the waves were the product of a collision between two black holes roughly 30 times the mass of the Sun, located 1.3 billion light years from Earth.
"Ladies and gentlemen, we have detected gravitational waves. We did it," said California Institute of Technology physicist David Reitze, triggering applause at a packed news conference in Washington.
"It's been a very long road, but this is just the beginning," Louisiana State University physicist Gabriela Gonzalez said, touting the opening of a new era in astronomy.
The scientific milestone was achieved using a pair of giant laser detectors in the United States, located in Louisiana and Washington state, capping a decades-long quest to find these waves.
"The colliding black holes that produced these gravitational waves created a violent storm in the fabric of space and time, a storm in which time speeded up, and slowed down, and speeded up again, a storm in which the shape of space was bent in this way and that way," Caltech physicist Kip Thorne said.
The two laser instruments, which work in unison, are known as the Laser Interferometer Gravitational-Wave Observatory (LIGO). They were able to detect remarkably small vibrations from passing gravitational waves. After detecting the gravitational wave signal, the scientists said they converted it into audio waves and were able to listen to the sounds of the two black holes merging.
"We're actually hearing them go thump in the night," Massachusetts Institute of Technology physicist Matthew Evans said. "We're getting a signal which arrives at Earth, and we can put it on a speaker, and we can hear these black holes go, 'Whoop'. There's a very visceral connection to this observation."
The scientists said they first detected the gravitational waves last September 14.
"We are really witnessing the opening of a new tool for doing astronomy," MIT astrophysicist Nergis Mavalvala said. "We have turned on a new sense. We have been able to see and now we will be able to hear as well."
Einstein in 1916 proposed the existence of gravitational waves as an outgrowth of his ground-breaking general theory of relativity, which depicted gravity as a distortion of space and time triggered by the presence of matter. But until now scientists had found only indirect evidence of their existence.
Open the door
Scientists said gravitational waves open a door for a new way to observe the universe and gain knowledge about enigmatic objects like black holes and neutron stars. By studying gravitational waves they also hope to gain insight into the nature of the very early universe, which has remained mysterious.
Everything we know about the cosmos stems from electromagnetic waves such as radio waves, visible light, infrared light, X-rays and gamma rays. But because such waves encounter interference as they travel across the universe, they can tell only part of the story.
Gravitational waves experience no such barriers, meaning they can offer a wealth of additional information. Black holes, for example, do not emit light, radio waves and the like, but can be studied via gravitational waves.
The scientists said that because gravitational waves are so radically different from electromagnetic waves they expect them to reveal big surprises about the universe.
Scientists sounded positively giddy over the discovery.
"This is the holy grail of science," said Rochester Institute of Technology astrophysicist Carlos Lousto. "The last time anything like this happened was in 1888 when Heinrich Hertz detected the radio waves that had been predicted by James Clerk Maxwell's field-equations of electromagnetism in 1865," added Durham University physicist Tom McLeish.
"It is really a truly, truly exciting event," said Abhay Ashtekar, director of Penn State University's Institute for Gravitation and the Cosmos. "It opens a brand new window on the universe."
Ashtekar said heavy celestial objects bend space and time but because of the relative weakness of the gravitational force the effect is miniscule except from massive and dense bodies like black holes and neutron stars. He said that when these objects collide, they send out ripples in the curvature of space and time that propagate as gravitational waves.
A black hole, a region of space so packed with matter that not even photons of light can escape the force of gravity, was detected for the first time in 1971.
Neutron stars are small, about the size of a city, but are extremely heavy, the compact remains of a larger star that died in a supernova explosion.
The LIGO observatories are funded by the National Science Foundation, an independent agency of the US government.
