Albert Einstein built up his theory of gravity, general relativity, through a series of 'thought experiments' and mathematical proofs. In 1915 he showed that gravity is caused by objects with mass such as planets and stars warping space-time. For example, the Earth is caught in the curve in space-time caused by the Sun, and the Moon is caught in a warp created by the Earth. Similarly light is bent when it passes massive objects like galaxies.
As strange as these ideas may seem, numerous observations have matched Einstein's predictions.
Image: An illustration showing the Earth and the Moon warping space-time (credit: Mark Garlick/SPL)
Einstein's gravity theory predicts warps in space-time.
What happens near a black hole?
Using Zambia's spectacular Victoria Falls, Prof Brian Cox demonstrates what happens as you near a black hole.
Jocelyn Bell Burnell explains how pulsars tested Einstein's theory.
Jocelyn Bell Burnell explains how her pulsar discovery helped test general relativity. She also discusses the difference between a search for truth and a search for understanding.
Patrick Moore finds out how galaxies bend light.
Sir Patrick Moore finds out how one galaxy can bend the light of another while on a visit to Jodrell Bank Observatory.
What happens in a singularity?
Physicists are trying to explain what happened at the time of the Big Bang. If Einstein's equations are run backwards in time using a simplified model of the Universe, a singularity is the result. At the singularity, the Universe is a single point where gravity fields and energy densities are infinite. As a result, predictions of what was happening at this time are not currently possible. Quantum mechanics may hold the answer to what happens during a singularity.
Albert Einstein tries to link general relativity with quantum mechanics.
Albert Einstein was ahead of his time in his quest to link general relativity with quantum mechanics and develop a 'theory of everything'.
General relativity, or the general theory of relativity, is the geometric theory of gravitation published by Albert Einstein in 1916 and the current description of gravitation in modern physics. 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 spacetime. In particular, 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.
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, the gravitational redshift of light, and the gravitational time delay. The predictions of general relativity have been confirmed in all observations and experiments to date. 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 physics to produce a complete and self-consistent theory of quantum gravity.
Einstein's theory has important astrophysical implications. For example, 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. 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.