# Gravitational wave

Afterglow of neutron star collision in NGC 4993
General relativity
${\displaystyle G_{\mu \nu }+\Lambda g_{\mu \nu }={8\pi G \over c^{4}}T_{\mu \nu }}$
Einstein field equations
Phenomena
Kepler problem · Lenses · Waves
Frame-dragging · Geodetic effect
Event horizon · Singularity
Black hole

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.[1]

As gravitational waves are not created by stationary objects, they must be detected from moving systems. 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.[1][2] 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.[3]

## 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.[4]

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.[5] 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".[4]

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

## References

1. B. Barish: The detection of gravitational waves using LIGO. [1]
2. The prize was shared by Russell Alan Hulse and Joseph Hooton Taylor Jr. "for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation".
3. Ghosh, Pallab 2017. Einstein's waves detected in star mash. BBC Science & Environment. [2]
4. "Mega" means one million in numbers. The distance in everyday language is a thousand billion, billion km away.