General relativity

General relativity is a theory of space and time. The theory was published by Albert Einstein in 1915.O'Connor J.J. and E.F. Robertson (1996), "General relativity". Mathematical Physics index, School of Mathematics and Statistics, University of St. Andrews, Scotland, May, 1996. Retrieved 2015-02-04. 

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 in an elevator 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 enougth 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.

Related pages
 Special Theory of Relativity

 References 

 Other websites 

Category:Basic physics ideas
Category:Relativity
Category:Albert Einstein