General relativity

General relativity is a theory of space and time. The theory was published by Albert Einstein in 1915.

The main idea of general relativity is that space and time are two different parts of spacetime. In general relativity, gravity does not work the same way that Sir Isaac Newton said gravity worked. According to Einstein, gravity changes not only space, but also time (gravitational time dilation).

Idea

An important idea in general relativity is the "principle of equivalence". An example is that two people, one on the surface of the Earth, and the other in an elevator in outer space but accelerating upwards at the speed of Earth's gravity will each observe the same behavior of an object they drop from their hands. The object will accelerate to the floor at the speed of Earth's gravity (9.8 m/s²) in either case.

This makes it impossible for either to tell whether or not they are at rest in a gravitational field or accelerating with no gravity. Other versions of this type of "thought experiment" were used to show that light would curve in an accelerating frame of reference.

In relativity, acceleration makes spacetime warp. Because gravity is the same as acceleration, spacetime warps around objects. This can be thought of as spacetime being a jello of some sort, curving into itself, so that a straight line ends up curving. The Sun and other objects with mass (and energy) curve four dimensional spacetime fabric. If there’s enough mass or energy in one place, a black hole is made, because the warping from acceleration becomes so extreme that future and inward swap places (according to what we know).

Predictions

General relativity has predicted many things which were later seen. These include:

• As light gets closer to the Sun, it bends towards the sun around twice as much as classical physics predicts (the system used before general relativity). This was seen in an experiment by Arthur Eddington in 1919. When scientists saw his experiment, they started to take general relativity seriously.
• The perihelion of the planet Mercury rotates along its orbit more than is expected under Newtonian physics. General relativity accounts for the difference between what is seen and what is expected without it.
• Redshift from gravity. When light moves away from an object with gravity (going up), it is stretched into longer wavelengths. This was confirmed by the Pound-Rebka experiment. The reverse also happens: light going towards an object with gravity (going down) is shrunk into shorter wavelengths (blueshift).
• The Shapiro delay. Light appears to slow down when it passes close to a massive object. This was first seen in the 1960s by space probes headed towards the planet Venus.
• Black holes. They are what happens when too much mass or energy is put in a small space, so much that light cannot escape.
• Gravitational waves. They were first observed on 14 September 2015.

General Relativity Media

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  According to general relativity, objects in a gravitational field behave similarly to objects within an accelerating enclosure. For example, an observer will see a ball fall the same way in a rocket (left) as it does on Earth (right), provided that the acceleration of the rocket is equal to 9.8 m/s2 (the acceleration due to gravity on the surface of the Earth).

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  Illustration of a light cone, based on Image:Light cone.png

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  Schematic representation of the gravitational redshift of a light wave escaping from the surface of a massive body

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  Deflection of light (sent out from the location shown in blue) near a compact body (shown in gray)

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  Ring of test particles deformed by a passing (linearized, amplified for better visibility) gravitational wave

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  Newtonian (red) vs. Einsteinian orbit (blue) of a lone planet orbiting a star. The influence of other planets is ignored.

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  Orbital decay for PSR J0737−3039: time shift, tracked over 16 years (2021).

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  Einstein cross: four images of the same astronomical object, produced by a gravitational lens

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  Artist's impression of the space-borne gravitational wave detector LISA

Related pages

• Special Theory of Relativity

Other websites

Part of a series of articles about
General relativity
[math]\displaystyle{ G_{\mu \nu} + \Lambda g_{\mu \nu}= {8\pi G\over c^4} T_{\mu \nu} }[/math]
Introduction History Mathematical formulation Tests
Fundamental concepts Principle of relativity Theory of relativity Frame of reference Inertial frame of reference Rest frame Center-of-momentum frame Equivalence principle Mass–energy equivalence Special relativity Doubly special relativity de Sitter invariant special relativity World line Riemannian geometry
Phenomena Gravitoelectromagnetism Kepler problem Gravity Gravitational field Gravity well Gravitational lensing Gravitational waves Gravitational redshift Redshift Blueshift Time dilation Gravitational time dilation Shapiro time delay Gravitational potential Gravitational compression Gravitational collapse Frame-dragging Geodetic effect Gravitational singularity Event horizon Naked singularity Black hole White hole Spacetime Space Time Spacetime diagrams Minkowski spacetime Closed timelike curve (CTC) Wormhole Ellis wormhole
Equations Formalisms Equations Linearized gravity Einstein field equations Friedmann Geodesics Mathisson–Papapetrou–Dixon Hamilton–Jacobi–Einstein Curvature invariant (general relativity) Lorentzian manifold Formalisms ADM BSSN Post-Newtonian Advanced theory Kaluza–Klein theory Quantum gravity Supergravity
Solutions Schwarzschild (interior) Reissner–Nordström Gödel Kerr Kerr–Newman Kasner Lemaître–Tolman Taub-NUT Milne Robertson–Walker pp-wave van Stockum dust Weyl−Lewis−Papapetrou Vacuum solution (general relativity) Vacuum solution
Scientists Einstein Lorentz Hilbert Poincaré Schwarzschild de Sitter Reissner Nordström Weyl Eddington Friedman Milne Zwicky Lemaître Gödel Wheeler Robertson Bardeen Walker Kerr Chandrasekhar Ehlers Penrose Hawking Raychaudhuri Taylor Hulse van Stockum Taub Newman Yau Thorne others