Introduction

Summary

By late 2010 five large-scale laser interferometer gravitational wave detectors had been operating for several years at unprecedented sensitivity. They were searching for gravitational wave signals created by matter in its most extreme and exotic form – neutron stars, black holes and the Big Bang itself. The detectors were the most sensitive instruments ever created, able to detect fractional changes in spacetime geometry at the level of parts in 10²³, corresponding to the measurement of energy changes of less than 10^-31 joules per hertz of bandwidth. Despite this extraordinary achievement, the sensitivity was about 10 times below the level where we could be confident of detecting predicted signals. For example, the mean time between detectable chirp signals from the coalescence of pairs of neutron stars was likely to be once every 50 years, so that in a year of operation the chance of detection was only about 2%.

Despite this pessimistic prognosis, many of the 1000 physicists in the worldwide collaborations involved with the above detectors remained optimistic that nature might to kind enough to provide a first signal. Optimism was high enough that a system had been put in place to alert optical telescopes to slew to the part of the sky corresponding to the arrival times of any significant event.

On 16 September 2010 a coincident signal appeared in LIGO detectors spaced 2000 kilometres apart in the USA. It was immediately recognised as a significant event, especially after it was also identified in the data of the Virgo detector in Italy.