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."
Gravitational waves: Questions and answers
How do we see Gravitational waves?
The new information comes from the Laser Interferometer Gravitational-Wave Observatory. That is made up of two installations in the US, 3,000 kilometres apart.
The system uses laser beams to measure pipes with very precise accuracy. It can detect even the tiniest change in their length — and, if that happens, the characteristic swelling and shrinking of the pipe would be evidence of the waves.
Doesn’t this just confirm everything we already knew?
Einstein predicted that gravitational waves existed, and theoretical work since then has confirmed that they almost certainly exist. In one sense, this is just practical proof of a working assumption.
But the important stuff is what receiving this signal actually means. If we gain the ability to dependably and verifiably measure the waves, it would open an entirely bit of the universe to study.
If we developed ways of looking at those waves and into the universe, it could be analogous to the development of telescopes. But instead of light, we’d be able to see messages from deep in the universe’s past.
The gravitational waves come bearing information about where they have come from. And many of them emerge from strange and early parts of the universe, like big bangs, meaning that we might receive our first ever messages from those unknown places.
If we could detect the waves properly, then it would enable us to “see” the development of black holes and the development of stars.
From there, we would be able to understand the beginnings and formation of the universe, and many of its most mysterious parts.
How do we do that?
Scientists and engineers around the world are assembling equipment that will help us detect more gravitational waves, and understand more about those that we find.
Perhaps the most ambitious is the huge eLISA mission, which will send a 1million-kilometre wide antenna into space, being carried around by three spacecraft.
That mission has already successfully sent the LISA Pathfinder, which will head out to test gravitational wave detection. That launched in December and went into orbit a couple of weeks ago.
The full mission will be able to do the same work, but away from the noise and bustle of Earth. That will allow it to listen for much deeper waves — allowing it to see into even bigger black holes.