On 14 September, the scientific community celebrated the 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
For centuries, astronomers have used light to study the universe with optical telescopes and, since the middle of the 20th century, we have expanded that view 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 at 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 directly detected 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 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 detectors' design has been upgraded over the past few years. Scientists at the University of Glasgow led a consortium of UK institutions that played a key role, developing, constructing and installing the sensitive mirror suspensions at the heart of the LIGO detectors that were crucial to this first detection. The technology was based on research carried out on the earlier UK/German GEO600 detector and helped to turn LIGO into Advanced LIGO, arguably the most sensitive scientific instrument ever built.
The first Advanced LIGO detection, denoted GW150914, was announced to huge global acclaim in February 2016 by the LIGO Scientific Collaboration and the Virgo Collaboration. Their analysis revealed the event to be the collision of two black holes of masses about 29 times and 36 times the mass of our sun. A second confirmed detection – from another, somewhat less massive, binary black hole merger whose gravitational waves reached Earth on 26 December last year – was announced in June 2016. Together, these events give us our first glimpse of the cosmic population of black hole binaries – with many intriguing questions about how they formed, and how they might fit into the broader narrative of stellar evolution, now being explored by the astronomical community. Comparison of their waveforms with the theoretical predictions of General Relativity has also allowed us to test Einstein’s theory in unprecedented detail. So far, General Relativity has passed these tests with flying colours, but watch this space (or should that be watch this spacetime?) for more rigorous tests to come.
So what of the future? The second LIGO science run, with even greater sensitivity, is scheduled to begin later in 2016, with the Advanced Virgo interferometer in Italy set to join the search soon thereafter and plans for an extended global network of advanced ground-based interferometers already in place. Within a few years, therefore, we can expect dozens, or even hundreds, of further detections – including, for example, binary neutron star mergers that may yield counterparts visible across the electromagnetic spectrum. The enhanced global network will significantly improve our ability to locate the positions of gravitational-wave sources in the sky, making these electromagnetic counterparts easier to find and leading to more accurate measurement of each source’s physical properties.
Looking further ahead, plans are already being developed for even more sensitive ground-based interferometers, followed later by the addition of the complementary LISA space mission that will open up another, lower frequency, window on the gravitational-wave spectrum. Indeed, an exciting step towards LISA was also taken in 2016 when the first results from the LISA Pathfinder technology demonstrator mission were published, indicating that the levels of high-precision control required for a future gravitational-wave observatory in space appear eminently achievable.
Gravitational waves will help us to probe the most extreme corners of the cosmos – the event horizon of a black hole, the innermost heart of a supernova, the internal structure of a neutron star: regions that are completely inaccessible to electromagnetic telescopes – and their observation may help us to address some of the deepest unanswered questions in astrophysics and cosmology. The nascent field of gravitational-wave astronomy has a very bright future!
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Martin Hendry is Professor of Gravitational Astrophysics and Cosmology at the University of Glasgow, where he is also Head of the School of Physics and Astronomy. He is a member of the LIGO Scientific Collaboration. This short article is based on an invited plenary presentation which he gave in June 2016 at the SPIE Conference on ‘Astronomical Telescopes and Instrumentation’. martin.hendry@glasgow.ac.uk
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