What's all this noise about Gravitational Waves?

Curious about why the detection of gravitational waves has created an uproar in the scientific community? Read on to find out why.

Ever since Einstein proposed the General Theory of Relativity, the notion of gravity has been completely upturned. No more can gravity be described as a simple force acting between two masses, as Newton had done in the 17th century. Newton's description broke down when humans began to push the limits of speed, close to the fastest thing in the universe, light.

According to General Relativity, the universe is described in terms of a mesh of space and time. The presence of a massive body distorts this space-time web and manifests itself as gravity [see Fig. 2]

Fig 2: Space time distortion due to presence of massive objects [Source: ESA]

One of the predictions of this description is that, ripples in the space-time grid would result in the emission of gravitational waves. According to general relativity, disturbances in the web of space-time will propagate in the form of waves, called gravitational waves. Such perturbations could be produced by strongly gravitational bodies. Binary systems of neutron stars or black holes orbiting each other are one of the most promising sources. Other sources could be non-axisymmetric perturbations in isolated neutron stars, similar to earthquakes. Highly distorted strongly magnetic neutron stars or "magnetars" are also thought to be likely origins of gravitational waves.

With the aim of detecting these gravitational waves, enormous detectors have been constructed across the world [see Fig. 3]. The detection of gravitational waves is a huge technological challenge, as the amplitude of the vibrations to be detected is 1 part in a trilliard (a thousand million million million). With the advancement of technology, it has been possible to achieve this ambitious task.

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Fig 3: Earth-based and space-based gravitational wave detectors [Source: LIGO/VIRGO/ESA]

Finally on 14 September 2015 came the ground-breaking discovery of the first gravitational waves by Advanced LIGO collaboration, located in Livingston, Louisiana, and Hanford, Washington, USA. The detected gravitational waves were produced during the merging of two black holes in a binary. As the black holes orbit each other, they lose energy by the emission of gravitational waves and come closer and closer until they finally merge to produce a supermassive black hole.

This discovery opens up a whole new window to the universe, seen in the light of gravity. Strongly gravitational objects that distort space-time can now be probed with the help of gravitational waves, which are otherwise invisible in other spectra. What is more interesting is that these detected gravitational waves contain imprints of the sources. Like fingerprints, they provide clues that lead us to the conditions of the objects that produced them. They will help scientists to deduce black hole masses and spins, as well as probe the interior of neutron stars, complementary to the existing astrophysical observations. Indeed, the discovery of gravitational waves is thus a milestone in the history of science and humanity.