Gravitational wave

Not to be confused with Gravity wave.

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[math]\displaystyle{ G_{\mu \nu} + \Lambda g_{\mu \nu}= {8\pi G\over c^4} T_{\mu \nu} }[/math]
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Afterglow of neutron star collision in NGC 4993

Gravitational waves are ripples in spacetime which are created whenever objects with mass move. They were predicted by Albert Einstein in 1916 on the basis of his theory of general relativity. They were first directly detected on 14 September 2015.

To make gravity waves strong enough to be detected, something very massive must accelerate very fast. Sources of detectable gravitational waves include binary star systems composed of white dwarfs, neutron stars, or black holes.

Gravity and relativity

In physics, gravitational waves are ripples in the curvature of spacetime that travel outward from the source. Albert Einstein predicted them in 1916 on the basis of his theory of general relativity. In theory, gravitational waves transport energy as gravitational radiation.

In general relativity, gravitational waves cannot travel faster than the speed of light. They do not exist in the Newtonian theory of gravitation, in which physical interactions propagate at infinite speed. However, the detection of gravitational waves proves the last prediction of Einstein’s general theory of relativity.

In 1993, the Nobel Prize in Physics was awarded for measurements of the Hulse-Taylor binary star system that suggests gravitational waves are more than mathematical anomalies.

First detection

The first detection of gravitational waves was on September 14 2015. The waves were detected by LIGO and were caused by the merging of two black holes.

Collision of neutron stars

Gravitational waves have been detected from the collision and merging of two neutron stars. The first detection was on 17 August 2017 by a team in Pasadena, California.

The project is run by the LIGO Laboratory at Caltech. The detector is in the huge Livingston woodland in Louisiana. The detector is two two and a half mile (~4 km) long completely straight pipelines at right angles. Inside each pipe is a laser which measures any change in length. Once the gravity wave event was detected, telescopes searched for visual images of the cause. The VISTA telescope in Chile got the image.

The merging happened in a galaxy called NGC 4993. This is about 40 mega parsecs or 130 mega light years away in the direction of the Constellation Hydra. By comparison, the nearby Andromeda Galaxy is only 2.5 million light years from Earth.

"It happened 130 million years ago - when dinosaurs roamed the Earth. It was so far away that the light and gravitational waves have only just reached us".

Such events, as well as supernovae, are the sources of heavier elements such as gold and platinum.

Gravitational Wave Media

• As two black holes orbit closer to one another, they emit gravitational waves, the frequency of which increases to a peak as the black holes coalesce.

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  Linearly polarised gravitational wave

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  Primordial gravitational waves are hypothesized to arise from cosmic inflation, a phase of accelerated expansion just after the Big Bang (2014).

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  The effect of a plus-polarized gravitational wave on a ring of particles

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  The effect of a cross-polarized gravitational wave on a ring of particles

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  The gravitational wave spectrum with sources and detectors. Credit: NASA Goddard Space Flight Center

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  Two stars of dissimilar mass are in circular orbits. Each revolves about their common center of mass (denoted by the small red cross) in a circle with the larger mass having the smaller orbit.

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  Two stars of similar mass in circular orbits about their center of mass

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  Two stars of similar mass in highly elliptical orbits about their center of mass

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  Artist's impression of merging neutron stars, a source of gravitational waves