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Boffins' gravitational wave detection hat trick blows open astronomy

A discovery to make Einstein proud

By Iain Thomson, 11 Feb 2016

Analysis A 15-year experiment using some of the most advanced technology known to Man has picked up the first detection of a gravitational wave, the first direct measurement of black holes, and the first direct evidence of binary black holes. It has also opened up an entirely new field of astronomy.

The signal was picked up on September 14, 2015, at 0950 UTC by the two Laser Interferometer Gravitational-wave Observatory (LIGO) detectors built for the purpose in Livingston, Louisiana, and Hanford, Washington, in the US. The team then spent four months checking and rechecking the data before publishing in the Physical Review Letters today.

"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, fulfills Einstein's legacy on the 100th anniversary of his general theory of relativity," said Caltech's David Reitze, executive director of the LIGO Laboratory.

Einstein first predicted gravitational waves in 1915, although he changed his mind about them several times in the following years. He proposed that the spacetime fabric of the universe would be warped by large masses, and if they interact with each other the masses can push out energy in the form of gravitational waves that propagate out at the speed of light.

Einstein acknowledged that these waves would be so fantastically small that they would be impossible to detect, but engineering has moved on in the last century. For the past 50 years, scientists have been trying to pick them up using the then-state-of-the-art machinery of their times.

There have been numerous false starts and reported signals that couldn't be replicated. Most recently, in March 2014 the BICEP2 (Background Imaging of Cosmic Extragalactic Polarization) team claimed success, only to find out that cosmic dust had interfered with its readings. But today's announcement looks much more solid, thanks to the work that went into it.

Better by design

In 1980 Rainer Weiss, professor of physics at MIT, Kip Thorne, Caltech's Richard P. Feynman Professor of Theoretical Physics, and Ronald Drever, professor of physics at Caltech, came up with the idea for LIGO to catch the elusive gravitational waves.

After a fierce fight, they sold the National Academy of Sciences on the idea and got funding to build the first LIGO detectors, which they admitted at the time were probably not going to work. It took 20 years and over half a billion dollars to get the twin sites set up and ready to roll.

The LIGO system consists of two 2.5-mile (4-km) tunnels set at right angles with a laser at one end. The laser shoots light towards a mirror that splits the beam down the tunnels, whereupon it is reflected back and directed to a detector. If spacetime ripples pass though the instrument, the mirrors at the end of the tunnels move and the detector would pick up interference in the laser signal.

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To get rid of interference, the tunnels are pumped out to produce a vacuum and the mirrors are polished to a higher degree of accuracy than has ever been achieved before. Even the tunnels themselves are balanced to account for the curvature of the planet and make sure the signal is pure.

The initial LIGO run lasted from 2001 to 2010 and the vast majority of this time was spent building up the computer controls that eliminate background interference. Initially, everything from earthquakes to nearby traffic noise affected the signal, but slowly the returns were cleaned up.

In 2010 the instrument was refitted with the latest hardware, which increased its sensitivity further. LIGO was four days short of being officially open for business when it detected the signal in September. The two observatories, nearly 2,000 miles apart, both picked up the same signal within milliseconds of each other, as you'd expect with a signal traveling at the speed of light.

To give you some idea of how sensitive this signal is, the gravitational wave deflected the laser by a ten-thousandth the diameter of a proton. Yet from that sign the team has been able to deduce what caused the spacetime ripple.

Black holes: Two become one

In the research paper, the team deduces that the signal came from two massive black holes – about 29 and 36 times the mass of our sun – merging approximately 1.3 billion light years away.

As the two black holes circled each other, the resultant energies began to radiate out as small gravitational waves. But when they collided at about half the speed of light and merged, there was an explosion of energy that – for a tiny moment – produced a peak power output about 50 times that of the whole visible universe.

The gravitational waves created by this merging process rippled out through spacetime, losing power as they went. 1.3 billion years later, they caused a minute ripple to pass through the earth and it was this that the LIGO instruments picked up.

"The description of this observation is beautifully described in the Einstein theory of general relativity formulated 100 years ago, and comprises the first test of the theory in strong gravitation. It would have been wonderful to watch Einstein's face had we been able to tell him," said Weiss.

No one was at the detector at the time and the signal was first seen by a European researcher monitoring the feeds remotely. The news spread through the team like wildfire but everyone kept their mouths shut publicly because the signal had to be checked very, very carefully.

The signal was processed again and again to see what else could have caused it. As part of their jobs, four members of the team have to try and introduce faults into the signal, and all four said they weren't responsible for the signal. Finally the decision to publish was made.

It's a whole new world out there

For the last few millenia, mankind has been limited to exploring the universe visually. This has huge disadvantages, since so much of the universe is dark, particularly black holes. But now boffins have a whole new way of examining the universe and its mysteries by using gravitational forces. In doing so, it will open up an entirely new field of astronomy.

"With this discovery, we humans are embarking on a marvelous new quest: the quest to explore the warped side of the universe – objects and phenomena that are made from warped spacetime. Colliding black holes and gravitational waves are our first beautiful examples," said Thorne.

Light has some real limitations as a method of exploration. It's easy to block, can be warped or lensed by large masses, and has a limited spectrum on which to operate. Gravity, on the other hand, passes though the universe unobstructed and contains a wealth of data.

Plans are now afoot for a third LIGO detector to be built in India, to extract yet more information from newly detected gravitational waves; more – and better – designs will come along as engineering progresses.

"This discovery is akin to Galileo first looking through his telescope and seeing the moons of Jupiter," said Sean McWilliams, assistant professor of physics and astronomy in the Eberly College of Arts and Sciences and a LIGO team member.

"We are 'hearing' the Universe now for the first time, and given how much we have learned by seeing the Universe since Galileo's time, it's a genuine thrill to imagine how much we will now be able to learn by listening to gravitational waves."

The discovery will also almost certainly mean Nobel Prizes for team members, with Weiss, Thorne and Drever all being mooted for the award. But, tragically, Drever may never understand that his work has led to so much good science, since he is reportedly in a Scots nursing home with advanced dementia.

Nevertheless, his work – and that of 1,000 other scientists who have worked on the project – will live on as long as humanity survives, and should tell us a phenomenal amount about the workings of the universe around us and how to tap into its secrets. Provided it isn't another false positive, that is. ®

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