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The theory of relativity encompasses two theories, special relativity and general relativity. The concepts introduced by the theories are:

- Measurements of various quantities are relative to the velocities of observers.
- Spacetime: space and time should be considered together and in relation to each other.
- The speed of light is nonetheless invariant, the same for all observers.

Special Relativity

Special relativity is the theory of the structure of spacetime. It is based upon two postulates.

1. The laws of physics are the same for all observers in uniform motion relative to one another (principle of relativity).
2. The speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the source of light.
The theory copes with experiments better than classical mechanics but copes with many consequences such as

- Relativity of simultaneity
- Time dilation
- Length contraction
- Mass-energy equivalence
- Maximum speed is finite

The feature that defines special relativity is the replacement of the Galilean transformations of classical mechanics by the Lorentz transformations.

General Relativity

General relativity is a theory of gravitation. The development of general relativity began with the equivalence principle. This principle states accelerated motion and being at rest in the gravitational field are physically identical. Some consequences of general relativity are:

- Gravitational time dilation
- Orbits precess in a way unexpected in Newton’s theory of gravity
- Rays of light bend in the presence of a gravitational field
- Rotating masses “drag along” the spacetime around them. This is called “frame-dragging”
- The universe is expanding and the far parts are moving away faster than the speed of light.

General relativity is the theory of gravitation whose defining feature is used in Einstein field equations.

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