Groundbreaking observation of gravitational waves


Artistic visualization of black hole (Image courtesy of chrisroll at


On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) simultaneously observed a transient gravitational-wave signal generated by two merging black holes. Observation was made almost a century after Albert Einstein predicted the existence of gravitational waves.

The gravitational waves are produced by moving masses and propagate at the speed of light. They can be described as oscillating distortions in the geometry of spacetime. When travelling, the waves squash and stretch spacetime perpendicularly to direction of propagation.

Despite many attempts, gravitational waves were not observed directly until last year, because they induce very small spacetime distortions. Such small amplitudes were predicted by Albert Einstein himself. Moreover, until the Chapel Hill conference in 1957 there was significant debate about their physical reality. Their existence was indirectly demonstrated in 1982 by the discovery of the binary pulsar system PSR B1913+16 and subsequent observations of its energy loss.

In order to directly observe the gravitational waves, the scientists needed a very sensitive detecting devices, such as upgraded in 2015 Advanced LIGO detectors. The whole LIGO system consists of two Advanced LIGO detectors, one placed in Hanford (Washington, USA) and the second in Livingston (Louisiana, USA). Each detector is a modified L-shaped Michelson interferometer. The gravitational-wave strain was measured as a difference in length of its orthogonal arms. Usually, in this experiment also Italian Virgo detector takes part, but at the time of the wave approach it was being upgraded.

It is worth noticing, that the experiment is very complex and, among others, required minimizing all possible environmental disruptions. The breakthrough was obtained thanks to the last year LIGO detectors sensitivity upgrade. Over a hundred research units took part in this research.

Source: APS Physics 
Original paper: B.P. Abbott et al., Phys. Rev. Lett. 116, 061102, 2016;
Image credits: chrisroll
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