A century after being proposed by physicist Albert Einstein, scientists have made the first detection of gravitational waves -- massive celestial objects on the move causing spacetime itself to ripple -- a historic discovery that opens up an entirely new way of studying the cosmos.
The detection was made by the twin LIGO interferometers on Sept. 14, 2015, located in Livingston, La., and Hanford, Wash., just two days after the system was significantly upgraded to boost its sensitivity.
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Like radio waves, visible light, X-rays and other forms of electromagnetic radiation, Einstein believed that gravity also travels in waves. But even the most energetic events in the universe, such as two black holes crashing together, would cause only the slightest rippling through space and across time.
After decades of failed attempts, scientists fished out the first confirmed measurement of gravitational waves passing through Earth, a detection that required measuring 2.5-mile long L-shaped laser beams to a precision 10,000 times smaller than a proton.
Since everything from traffic to earthquakes will distort the beams, the Laser Interferometer Gravitational-Wave Observatory, or LIGO, consists of two detectors separated by 1,865 miles. Because gravitational waves are believed to travel at light speed, a detection from a cosmic source picked up at one LIGO site should be followed up by an identical detection in the other 10 milliseconds later.
That's exactly what scientists saw when they fished out waves set off by a pair of black holes 1.3 billion light-years from Earth spiraling toward each other and then colliding to form an even larger black hole, researchers said at a webcast press conference Thursday.
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"My reaction was ‘Wow!' I couldn't believe it," said LIGO executive director David Reitze.
In addition to proving that gravitational waves exist, the discovery confirms that black holes exist in binary pairs.
"This is the first time that this kind of a system has ever been seen," Reitze said.
Decades of work with supercomputers to generate models of what the gravitational waves would look like set the stage for the detection.
"Our theoretical predictions lie right on top of the experimentalists' measurements -- an exciting confirmation of general relativity," said Cornell University astrophysicist Saul Teukolsky.
The black holes detected by LIGO were roughly about 29 and 36 times the mass of the sun. Their merger created a new black hole about 62 times the mass of the sun. The missing three solar masses is what went into generating the gravitational waves detected 1.3 billion years later on Earth.
The European Space Agency in December launched a pathfinder satellite to test a technique for fishing out longer wavelength gravity ripples in space.
"The colliding black holes created a violent storm in the fabric of spacetime," said physicist Kip Thorne, with the California Institute of Technology.
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The storm lasted just 20 milliseconds, but during that span it pumped out more power than 50 times all the stars in the universe, Thorne added.
Just as light radiates in waves of different lengths, ripples produced by gravity stretch space and time differently, similar to how a bowling ball rolling across a trampoline will warp the surface more than a baseball.
"You get electromagnetic radiation – basically light – when you move some sort of charged particles. It's the same idea with a radio tower ... charges go up and down the antenna. If you're moving masses, instead of moving charges, you get gravitational waves," NASA astrophysicist Ira Thorpe, with the Goddard Space Flight Center in Greenbelt, Maryland, told Discovery News.
The longest gravitational waves were produced in the Big Bang explosion 13.8 billion years ago. Colliding black holes are the most powerful cosmic events since the Big Bang.
Thursday's discovery, which is detailed in a paper in Physical Review Letters, opens the door to an entirely new branch of astronomy, a way to listen to the universe in addition to seeing it.
"The frequency of these waveforms are in the human hearing range. We can hear gravitational waves, we can hear the universe. That's one of the beautiful things about this. We are not only going to be seeing the universe, we're going to be listening," said LIGO physicist Gabriela Gonzalez, with Louisiana State University.
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With two LIGO detectors picking up the distinctive "thump" of the black holes' merger, scientists were able to calculate an approximate direction of the event. The collision occurred in the southern sky, in the direction of the Magellanic Cloud, Gonzalez said.
With additional interferometers coming online in Italy, Japan and elsewhere, the ability to pinpoint the location of future gravitational waves will vastly improve, she added.