ANALYSIS & OPINION: 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, there could be 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.


Professor Martin Hendry at the USA National Science and Engineering Festival


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 detection1


, 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


Plans are already


being developed for even more sensitive ground-based interferometers


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 the 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


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 results2


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! l


About the author 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 that he gave in June 2016 at the SPIE Conference on ‘Astronomical Telescopes and Instrumentation’. martin.hendry@glasgow.ac.uk


References 1


B P Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), 2016. Observation of Gravitational Waves from a Binary Black Hole Merger.


Physical Review Letters. 116 (6) 2


Left: The GW150914 waveforms. Right: Illustration showing the merger of two black holes and the gravitational waves that ripple outward as the black holes spiral toward each other


@electrooptics | www.electrooptics.com


M Armano et al. Sub-Femto-g Free Fall for Space- Based Gravitational Wave Observatories: LISA Pathfinder Results. Physical Review Letters. 116 (23)


OCTOBER 2016 l ELECTRO OPTICS 13


SXS, the Simulating eXtreme Spacetimes (SXS) project


LIGO/T Pyle


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