ANALYSIS & OPINION: LIGO
Gravitational-wave astronomy: opening a new window on the cosmos
On 14 September, the scientific community celebrated the first anniversary of gravitational wave detection. Professor Martin Hendry, from the University of Glasgow, whose team helped construct the LIGO facilities, discusses the work that preceded last year’s discovery and what the future holds for the field of gravitational-wave astronomy
F
or centuries, astronomers have used light to study the universe with optical telescopes and, since the middle of the 20th century, that view has expanded hugely, by building detectors and instruments sensitive to light across the entire electromagnetic spectrum – from gamma rays to radio. Now, however, astronomers are taking their first glimpse through a completely different window on the universe with the first ever direct detection of gravitational waves. This exciting discovery has been widely hailed as the scientific breakthrough of the century and heralds the dawn of a whole new field of observational astronomy – probing directly the effects of gravity as it spreads across the cosmos. Gravitational waves are the so-called ‘ripples in spacetime’ predicted one hundred years ago by Albert Einstein in his General Theory of Relativity. They are produced by accelerating masses changing spacetime curvature, but the ‘stiffness’ of spacetime means that only the most violent cosmic events – from exploding stars or
colliding black holes, for example – can produce distortions we could hope to measure on Earth. Even then, to detect the tiny ripples expected from even the strongest cosmic sources presents enormous scientific and technological challenges that have taken many decades to overcome. The fact that gravitational waves have now been detected directly is a testament to human ingenuity, global scientific cooperation and the vision of national funding agencies willing to invest long-term in a quest that many had thought to be simply impossible. So how was the remarkable discovery made? In September 2015, two giant laser interferometers in the United States called LIGO (Laser Interferometer Gravitational- Wave Observatory) caught a passing gravitational wave from the collision of two massive black holes more than a billion light years from Earth. The twin LIGO detectors each consist of two four-kilometre ‘arms’ set at right angles to each other, containing a laser beam that is reflected back and forth hundreds of times by mirrors at each end. When a gravitational wave passes by, the stretching and squashing of spacetime causes the interferometer arms alternately to lengthen and shrink – which means that the laser beams take a different time to travel through them, in principle revealing the passage of a gravitational wave from changes to the interference pattern of the two beams.
The changes in the LIGO arm lengths LIGO staff inspecting optics at the Livingston laboratory 12 ELECTRO OPTICS l OCTOBER 2016
produced by this black hole merger were incredibly tiny – equivalent to less than one million millionth of the width of a human hair. And as if detecting this was not difficult enough, all types of local disturbances on Earth – from the ground shaking to power-grid fluctuations – in addition to instrumental ‘noises’, could mimic or indeed completely swamp such a real signal from the cosmos. To achieve the astounding sensitivity required, almost every aspect of the LIGO
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