Gravitational Waves

Gravitational waves: breakthrough discovery after a century of expectation

Physicists have announced the discovery of gravitational waves, ripples in the fabric of spacetime that were first anticipated by Albert Einstein a century ago.
“We have detected gravitational waves. We did it,” said David Reitze, executive director of the Laser Interferometer Gravitational-Wave Observatory (Ligo), at a press conference in Washington.
The announcement is the climax of a century of speculation, 50 years of trial and error, and 25 years perfecting a set of instruments so sensitive they could identify a distortion in spacetime a thousandth the diameter of one atomic nucleus across a 4km strip of laserbeam and mirror.
The phenomenon detected was the collision of two black holes. Using the world’s most sophisticated detector, the scientists listened for 20 thousandths of a second as the two giant black holes, one 35 times the mass of the sun, the other slightly smaller, circled around each other.
At the beginning of the signal, their calculations told them how stars perish: the two objects had begun by circling each other 30 times a second. By the end of the 20 millisecond snatch of data, the two had accelerated to 250 times a second before the final collision and a dark, violent merger.
The observation signals the opening of a new window on to the universe.

“This is transformational,” said Prof Alberto Vecchio, of the University of Birmingham, and one of the researchers at Ligo. “We have observed the universe through light so far. But we can only see part of what happens in the universe. Gravitational waves carry completely different information about phenomena in the universe. So we have opened a new way of listening to a broadcasting channel which will allow us to discover phenomena we have never seen before,” he said.  

“This observation is truly incredible science and marks three milestones for physics: the direct detection of gravitational waves, the first detection of a binary black hole, and the most convincing evidence to date that nature’s black holes are the objects predicted by Einstein’s theory.”
The scientists detected their cataclysmic event using an instrument so sensitive it could detect a change in the distance between the solar system and the nearest star four light years away to the thickness of a human hair.
And they did so within weeks of turning on their new, upgraded instrument: it took just 20 milliseconds to catch the merger of two black holes, at a distance of 1.3 billion light years, somewhere beyond the Large Magellanic Cloud in the southern hemisphere sky, but it then took months of meticulous checking of the signal against all the complex computer simulations of black hole collision to make sure the evidence matched the theoretical template.
The detector was switched off in January for a further upgrade: astronomers still have to decipher months of material collected in the interval. But – given half a century of frustration in the search for gravitational waves – what they found exceeded expectation: suddenly, in the mutual collapse of two black holes, they could eavesdrop on the violence of the universe.
Prof B S Sathyaprakash, from Cardiff University’s school of physics and astronomy, said: “The shock would have released more energy than the light from all the stars in the universe for that brief instant. The fusion of two black holes which created this event had been predicted but never observed.”