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Ripples predicted by Albert Einstein detected for second time in three months

Gravitational waves, ripples in space-time predicted by Albert Einstein a century ago, have been detected for the second time in three months, scientists have announced.

An international team which included many British researchers spotted the phenomenon on Boxing Day but has only now made the news public.

As with the earlier detection which stunned the world of physics in September 2015, the source was found to be two colliding and merging black holes unleashing titanic forces.

The event, some 1.4 billion light years away, caused a quantity of energy roughly equivalent to the mass of the Sun to be converted into gravitational waves.

After travelling an unimaginable distance across space, the waves were "captured" by the twin Ligo (Laser Interferometer Gravitational-wave Observatory) detectors located in Livingston, Louisiana, and Hanford, Washington, US.

Team member Dr Stephen Fairhurst, from Cardiff University, said: "This event heralds the true beginning of gravitational wave astronomy and the opening of a new window on the universe.

"The different masses and observable spins that we witnessed in the Boxing Day event show that we're starting to collect vital information about the population of black holes that exist in the universe.

"Future gravitational wave observations will allow us to understand how black holes form from the death of massive stars, and test whether they are really as predicted by Einstein's theory."

Gravitational waves are predicted in Einstein's Theory Of General Relativity, which shows how gravity arises from mass curving space and time.

They are ripples in space-time that propagate as waves. Anything in their path, from humans to whole planets, is made to stretch and compress slightly as the fabric of space-time is distorted.

Each of the Ligo detectors, consisting of an incredibly sensitive system of mirrors and lasers, is also made to "wobble". But the effect is really tiny. The amount of movement is thousands of times smaller than the width of the nucleus of an atom.

Scientists hope gravitational waves will offer a completely different view of the universe, allowing them to study events that might be hidden from traditional optical and radio telescopes.

An illustration of this was seen in results from the December 26 detection, presented at the annual meeting of the American Astronomical Society in San Diego, US.

By analysing the signal, the scientists were able to tell that the colliding black holes were 14 and eight times more massive than the Sun. They orbited each other at least 27 times before merging into a more massive spinning black hole 21 times the Sun's mass.

Using two detectors spotting the waves 1.1 milliseconds apart made it possible to determine the source's rough position in the sky.

Professor Sheila Rowan, director of the University of Glasgow's Institute for Gravitational Research, who also took part in the discovery, said: "We know from this second detection that the properties being measured by Ligo will allow us to start to answer some key questions with gravitational astronomy. In future we will be able to study this and better understand cosmic history, aiming to fill in the 'missing links' in our knowledge."

On Tuesday, Prof Rowan was interviewed about gravitational waves by Jim Al-Khalili on BBC Radio 4's The Life Scientific programme - but kept the discovery to herself.

The findings have been accepted for publication in the journal Physical Review Letters.

Dr Chad Hanna, from Pennsylvania State University in the US, who co-led the detection team, said: "We now have far more confidence that mergers of two black holes are common in the nearby universe.

"Now that we are able to detect gravitational waves, they are going to be a phenomenal source of new information about our galaxy and an entirely new channel for discoveries about the universe."

The Ligo Scientific Collaboration consists of more than 1,000 scientists from 17 countries, including researchers from 10 UK universities - Glasgow, Birmingham, Cardiff, Strathclyde, West of Scotland, Sheffield, Edinburgh, Cambridge, King's College London and Southampton.

Each Ligo site has two tubes, both 2.5 miles (4km) long, arranged in an L shape.

A laser is beamed down each tube to monitor very precisely the distance between mirrors at each end. If a gravitational wave is present, it will alter the distance between the mirrors by a minute amount.

Professor Andreas Freise, from the University of Birmingham's School of Physics and Astronomy, whose team helped develop instrumentation for Ligo, said: " The Advanced Ligo detectors are a masterpiece of experimental physics. They are the most sensitive gravitational wave detectors ever built.

"We started with a well-known concept, a light interferometer, but it required new technologies that we have developed over several decades to create these extremely sensitive listening devices for gravity signals from the universe."

Dr Ed Daw, from the University of Sheffield's Department of Physics and Astronomy, who has been researching gravitational waves with Ligo since 1998, said: "The detection of the collision of one pair of black holes was amazing; the detection of a second one is spectacular, because it hints that there are lots more of these things out there.

"But it also raises more questions - what made these black holes? How many of these things are there? Can we start to really test general relativity in detail?"

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