Gravitational waves detected for fourth time by scientists

Scientists on 27 September 2017 announced that they have detected a fourth gravitational wave signal coming from the merger of two black holes.

It is the first time that this phenomenon has been measured simultaneously by both the US-based LIGO and Italy-based Virgo detectors.

The detection was described in a paper that will soon be published in Physical Review Letters.

Key highlights

• The latest observation was made on 14 August 2017. It is the first gravitational wave signal recorded by the Virgo detector.

• LIGO's two detectors, located in Livingston, Louisiana, and Hanford, Washington, later detected two other similar events.

• The detected gravitational waves were emitted during the final moments of the merger of two black holes with masses about 31 and 25 times the mass of the Sun and located about 1.8 billion light-years away.

• The newly produced spinning black hole has about 53 times the mass of our Sun, which means that about three solar masses were converted into gravitational-wave energy during the coalescence.

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What is a Gravitational Wave?

• Gravitational waves are ripples in the curvature of spacetime that are generated in certain gravitational interactions and propagate as waves outward from their source at the speed of light.

• The possibility of gravitational waves was discussed in 1893 by Oliver Heaviside.

• In 1905, Henri Poincare first proposed gravitational waves emanating from a body and propagating at the speed of light as being required by the Lorentz transformations.

• Predicted in 1916 by Albert Einstein on the basis of his theory of general relativity, gravitational waves transport energy as gravitational radiation, a form of radiant energy similar to electromagnetic radiation.

• Gravitational waves cannot exist under Newton's law of universal gravitation since that law is predicated on the assumption that physical interactions propagate at infinite speed.

• On 11 February 2016, the LIGO Scientific Collaboration and Virgo Collaboration teams announced that they had made the first observation of gravitational waves originating from a pair of merging black holes using the Advanced LIGO detectors.

• Since the initial announcement, LIGO has confirmed two more (and one potential) detections of gravitational wave events.

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