## General relativity

In physics and cosmology, general relativity, also known as the general theory of relativity, is the geometric theory of gravitation published by Albert Einstein in 1915 and the current description of gravitation in modern physics.

The mass of the earth bends space-time. image: Johnstone/wikipedia

General relativity generalizes special relativity and Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time, or space-time.

A nice way to visualise gravity and how massive objects distorts space-time. Via youtube: ScienceClicEN

The curvature of spacetime is directly related to the energy and momentum of whatever matter and radiation are present. The relation is specified by the Einstein field equations, a system of partial differential equations.

A formulation of Einstein’s field equation which describes how space is curved.

Compared with classical physics

Some predictions of general relativity differ significantly from those of classical physics, especially concerning the passage of time, the geometry of space, the motion of bodies in free fall, and the propagation of light.

Examples of such differences include:

• gravitational time dilation,
• gravitational lensing,  ( for example how the path of light is bent slightly as it passes near a massive object like the Sun)
• the gravitational redshift of light, and
• the gravitational time delay. (important for GPS accuracy)

The predictions of general relativity have been confirmed in all observations and experiments to date.

General relativity does not account for some things

Although general relativity is not the only relativistic theory of gravity, it is the simplest theory that is consistent with experimental data. However, unanswered questions remain, the most fundamental being how general relativity can be reconciled with the laws of quantum mechanics to produce a complete and self-consistent theory of quantum gravity.

Some  astronomical implications of general relativity

It implies the existence of black holes—regions of space in which space and time are distorted in such a way that nothing, not even light, can escape—as an end-state for massive stars.

a  ‘naked’ black hole creating a lensing effect of background stars. image: Urbane Legend (optimised for web use by Alain r)

There is ample evidence that the intense radiation emitted by certain kinds of astronomical objects is due to black holes; for example, microquasars and active galactic nuclei result from the presence of stellar black holes and black holes of a much more massive type, respectively.

The bending of light by gravity can lead to the phenomenon of gravitational lensing, in which multiple images of the same distant astronomical object are visible in the sky.

General relativity also predicts the existence of gravitational waves, which have since been observed indirectly; a direct measurement is the aim of projects such as LIGO and NASA/ESA Laser Interferometer Space Antenna and various pulsar timing arrays.

In addition, general relativity is the basis of current cosmological models of a consistently expanding universe.

Source adapted from: General relativity. (2015, November 18). In Wikipedia, The Free Encyclopedia. Retrieved 07:22, November 21, 2015, from https://en.wikipedia.org/w/index.php?title=General_relativity&oldid=691222195