The Discovery of Gravitational Waves: 3rd November 2017

Justin Greenhalgh 

Isaac Newton regarded gravity as “instantaneous action at a distance”. In his Special Theory of Relativity, Einstein showed that nothing can exceed the speed of light, so instantaneous action is impossible. In the General Theory of Relativity, gravity is explained as a distortion of space and time, and accelerating masses would generate waves of distortion travelling through space at the speed of light. Einstein himself did not believe gravitational waves (GW) existed.

Potential sources of GW are: the coalescence of compact binary objects (paired neutron stars and black holes) orbiting one another at high speeds; asymmetric stellar collapses; pulsars; low-mass X-ray binary stars and instabilities in neutron stars. The first two generate pulsed GW with characteristic waveforms, whereas the others emit continuous waves. When a GW passes, space is alternately stretched and squashed in two directions at right angles to the direction of travel of the wave. The great distances to detectable sources mean the distortion (strain) would be less than one part in 1022: the earth-moon distance would change by less than the size of an atom. Precision laser interferometers that measure the separation of suspended massive mirrors are used to detect the

change of length due to a GW. There are detectors in the USA: LIGO (Laser Interferometric Gravitational-wave Observatory); in Europe (GEO and Virgo) and in Japan (KAGRA). Future detectors are planned in India and Australia.

LIGO consists of two interferometers, in Washington State and Louisiana, each with two arms of 4 km length. The European systems are smaller, typically less than 1 km. Using two widely-spaced detectors allows signals from local events to be rejected, and gives some directional sensitivity. Noise of various kinds (ocean waves, traffic, rabbits, earthquakes, logging) is the main problem for GW detection, so isolating the detectors from noise requires very sophisticated engineering. The first version of LIGO could measure strains down to a few times 10-23, which in theory meant that only the strongest GW sources could be detected. An upgrade of 10x in sensitivity (and 1000x in the observable volume of space) was funded at a cost of $200 million. The mirror suspension systems,

seismic isolation and feedback controls were all upgraded using techniques developed on the European detectors, and the sensitivity increased to 8x10-24.

On the 14th September 2015, both LIGO stations detected signals consistent with the coalescence of two black holes. Modelling showed their masses were 36 and 29 solar masses, and they were orbiting each other 75 times per second at speeds up to 0.6 of light speed when they merged. The pair emitted the energy equivalent of 3 solar masses in GW radiation in the final 0.2 seconds of their interaction. The event occurred 1.4 billion years ago. The detection occurred during routine testing for noise sources, not on an actual observing run; the announcement of the detection was made only after exhaustive analysis to ensure it was not a false event.

The result, in the centenary year of the publication of the General Theory of Relativity, confirmed the existence of GW, as predicted by that theory, and demonstrated that they can be detected. It also confirmed that black holes with masses of a few tens of solar masses exist. Since the first detection there have been four others of merging black hole pairs, and one of merging neutron stars which was also detected optically and in gamma rays. The coincidence of these detections confirmed that GW do indeed travel at the speed of light.

The future of GW astronomy includes bringing more detectors online, increasing their sensitivity using optical noise reduction techniques (“squeezed” light), and putting laser interferometers into space (LISA). It will open up an entirely new window on the Universe.

Notes and summary by Chris Hooker.