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Cavendish Astrophysics


On the scientific importance of the detection of gravitational waves

Anthony Lasenby

Gravitational Waves were first predicted by Einstein in 1916, as an outcome of his General Theory of Relativity. They are waves in the metric of GR that propagate at the speed of light. This sounds straightforward, but in fact acceptance of these waves as being a prediction concerning real physical phenomena did not happen till much later. A major problem was being able to separate out coordinate effects from real physical effects, and also whether the singularities which seemed to occur after the passage of a wave in exact treatments, rather than the usual linearised treatment, were real, or were themselves coordinate artefacts.

By the early 1960s, however, gravitational waves were accepted as real phenomena, and attention moved to their detection. Early claims of detection using resonant bar detectors in the 1960s by Joe Weber were at a level of 'gravitational strain' - the dimensionless units in which the amplitude of a wave can be measured - that were much too large compared to what was thought possible from astrophysical sources. It had to wait to September of last year for the level of sensitivity of an instrument to reach that necessary to detect astrophysical phenomena. The experiment concerned is the Advanced LIGO instrument in the US, and a strong candidate event was found in the very first scientific run of the instrument. The event is coincident in time between the two detectors of LIGO, one in Washington State and the other in Louisiana, and matches extremely well the expected signal from the rapid inspiralling and 'ringdown' of two black holes, each with a mass near 30 Solar masses.

This measurement represents several 'firsts'. It is clearly the first detection of gravitational waves, but also the first detection of black holes, as well of course of black hole binaries, in this mass range. The final product of the black hole coalescence, which takes only a fraction of a second to complete, is a rotating black hole, whose spin can be measured directly from details of the gravitational waveform as the combined object relaxes towards the Kerr form of the metric. The measured spin is 0.7 of the maximum possible for a rotating black hole, and represents another first, since although we have had indirect measurements of black hole spin for many years, this is the first direct measurement.

Several other aspects of strong field relativity can be tested from the waveform, and also new limits set on any possible mass of the graviton, which if it did have mass would cause distortion of the signal as it propagated towards us. The astrophysical implications of the event witnessed, which of course requires the creation of a high mass black hole binary system at some point in the past, are also significant, and a crucial question will be the rate at which such events occur. Thus the possibility of finding equivalent events in future data, as well as the appearance of the other types of interactions (such as the coalescence of neutron star binaries, which should also be detectable), is keenly anticipated.