Einstein's gravity waves detected a century after his prediction

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".

Cosmologist Professor Stephen Hawking congratulated the scientists on a "ground-breaking" discovery.

He told the BBC: "Gravitational waves provide a completely new way of looking at the universe. The ability to detect them has the potential to revolutionise astronomy."

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 gravitational 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.

The Ligo project involved 1,000 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.

Explaining how the gravity waves were generated, Prof Reitze, Ligo's executive director, 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.

"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."