Gravitational waves: Scientists announce detection of ripples in fabric of spacetime - century's biggest discovery
Gravity waves - ripples in spacetime - have been detected by scientists a century after Albert Einstein predicted their existence.
The discovery, made in the US, was described by one British member of the international team as "the biggest scientific breakthrough of the century".
Capturing gravitational waves could open a new window to the universe and even help scientists to watch the cosmos being born.
The subtle distortions of spacetime are generated by cataclysmic events such as the collision of black holes or super-dense neutron stars, or powerful stellar explosions.
As the waves spread out, they compress and stretch the very fabric of the universe.
Although astronomical observations have hinted at their presence, until now they have remained a theoretical concept based on Einstein's mathematics.
Scientists detected them using laser beams fired through two perpendicular pipes, each four kilometres long, situated nearly 2,000 miles apart in Hanford, Washington and Livingston, Louisiana.
Together they make up the Laser Interferometer Gravitational Wave Observatory (Ligo), where the hunt for gravitational waves only began in earnest last September.
Making the announcement at the National Press Club in Washington DC, laser physicist Professor David Reitze, from the University of Florida, said: "Ladies and gentlemen, we have detected gravity waves. We did it."
He was greeted with loud applause.
British expert Professor James Hough, from the University of Glasgow, claimed the breakthrough was more important than the discovery of the missing Higgs boson, the so-called "God particle" linked to mass, in 2012.
Speaking in Washington DC, Professor Hough said: "Until you can actually measure something, you don't really know it's there.
"I think this is much more significant than the discovery of the Higgs boson. This is the biggest scientific breakthrough of the century."
To say gravitational waves are hard to detect is a gross understatement.
The Ligo lasers are designed to detect the way a passing wave causes minute changes in the lengths of the pipes. This results in the two lasers being slightly out of step, creating an interference pattern that can be measured.
The effect is very, very small - the equivalent of about one 10,000th the width of a proton, the heart of an atom.
Anything touched by a gravitational wave would be distorted the same way, even people. But normally the changes are not noticed.
Gravitational waves are predicted in Einstein's General Theory of Relativity, published in 1916, which links gravity to the curvature of spacetime by massive objects.
They can be produced in different ways - for instance, by black holes or neutron stars spiralling towards each other on a collision course, a titanic supernova, or exploding star, or even the Big Bang that gave birth to the universe.
The last possibility raises the prospect of peering behind the veil of the Cosmic Microwave Background (CMB), a relic of radiation from about 4,000 years after the Big Bang.
Gravity waves could allow scientists to see what happened even before the CMB came into being.
The gravity waves detected by the Ligo team were from two colliding black holes 1.3 billion light years away.
Professor Martin Hendry, head of the School of Physics and Astronomy at the University of Glasgow, said: "Einstein's General Theory of Relativity is regarded as one of the most impressive scientific achievements of all time and the existence of black holes is one of the theory's most startling predictions.
"To see such clear and direct confirmation of this prediction, and moreover that the merger of two black holes converts enormous amounts of mass into the energy of gravitational waves, is a wonderful vindication of Einstein's masterwork a century after it was written."
Another Ligo scientist, Professor Gabriela Gonzalez, from Louisiana State University, compared the achievement to that of the 16th century pioneer of modern astronomy, Galileo Galilei.
She said: "It's monumental - like Galileo using the telescope for the first time."
The Ligo project involved 1000 scientists and cost an estimated 620 million dollars (£429 million). After 25 years, success came barely a week after the facility underwent a £1 million upgrade to make it more sensitive.
Even then, it took months of careful checking of the data before the researchers felt confident enough to announce the news.
The measurements had very specific characteristics that were exactly what would be expected from two colliding black holes.
Prof Reitze, Ligo's executive director, said: "Our observation of gravitational waves accomplishes an ambitious goal set out over five decades ago to directly detect this elusive phenomenon and better understand the universe, and, fittingly, fulfils Einstein's legacy on the 100th anniversary of his general theory of relativity."
Explaining how the gravity waves were generated, he asked his audience to imagine two black holes, each around 150 kilometres in diameter, and each packed with 30 times more mass than the sun.
Accelerating to half the speed of light, they spiralled towards each other until they crashed together and merged.
"It's mind boggling," said Prof Reitze.
The wave front from the event spread out, like ripples from a stone thrown into a pond, across the vast expanse of the universe.
"When it gets to the Earth the gravitational wave is going to stretch and compress space," Prof Reitze added. "The Earth is jiggling like jello."
He said: "This was truly a scientific moonshot, and we did it. We landed on the moon."
Not only was it the first time anyone had detected a gravity wave, but the discovery also marked the first confirmation of two black holes fusing together.
Theoretical physicist Professor Kip Thorne, from the California Institute of Technology (Caltech), who originally proposed the Ligo experiment in the 1980s, said the detection of gravity waves would make it possible to spot black holes tearing stars apart, and perhaps violent phenomena previously unknown to science.
He added: "Until now, we have only seen spacetime when it's calm. We have only seen the surface of the ocean on a calm day when it's quite glassy. We have never seen the ocean riled by a violent storm with crashing waves before."
Dr Ed Daw, from the University of Sheffield, who has been researching gravitational waves with Ligo since 1998, said: "'Discoveries of this importance in physics come along about every 30 years.
"A measure of its significance is that even the source of the wave - two black holes in close orbit, each tens of times heavier than the Sun, which then collide violently - has never been observed before, and could not have been observed by any other method. This is just the beginning."
Paying tribute to the UK's contribution to the discovery, Professor John Womersley, chief executive of the Science and Technology Facilities Council, said: "It has taken 100 years and the combined work of many hundreds of the cleverest scientists, engineers and mathematicians on Earth to prove that this key prediction of Albert Einstein is correct, and show that gravitational waves exist.
"Of course Einstein was always the smartest guy in the room. Today's results also remind us just how important the UK's contribution to world-leading science is. I'd certainly like to think that some of the smartest people on earth today are living and working in the UK."
Gravitational waves: Questions and answers
Scientists have detected gravity waves, 100 years after they were predicted by Albert Einstein.
Q: WHAT ARE GRAVITATIONAL WAVES?
A: They are ripples in spacetime, the very fabric of the universe. Albert Einstein predicted their existence in his General Theory of Relativity in 1916. Until now, they have only been a mathematical concept.
Q: HOW ARE GRAVITATIONAL WAVES PRODUCED?
A: Gravity waves are generated by cataclysmic cosmic events, such as collisions between black holes or super-dense neutron stars, or massive stella explosions.
Q: WHAT WAS THE SOURCE OF THE GRAVITATIONAL WAVES DETECTED BY LIGO?
A: Two black holes, each packed with 30 times more material than the sun, spiralling together, colliding and fusing, in a distant corner of the universe 1.3 billion light years away. They could be not seen - scientists figured out what happened by studying the nature of the gravity waves.
Q: HOW WERE THE GRAVITATIONAL WAVES MEASURED?
A: The Ligo facility consists of two four kilometre pipes laid out perpendicular to each other in an L shape. A laser beam is fired through each pipe. As the gravity wave passes, it distorts space, causing tiny changes to the lengths of the pipes and the laser beams. As a result, the two beams are slightly out of step, creating a distinctive interference pattern which is measured using a system of mirrors.
The effect is really small - the equivalent of one 10,000th the width of a proton at the heart of an atom. To avoid disturbance, the laser beams are placed in remote locations away from noise and vibration.
Q: WHY IS THE DISCOVERY IMPORTANT?
A: Scientists say it opens a whole new window on the universe. Gravity waves could help scientists investigate some of the most violent events in the universe, including phenomena that are currently unknown. They could also shed a new light on mysteries such as dark matter and dark energy. Since it is quite likely that the Big Bang generated gravity waves, they might provide a glimpse of the birth of the universe.