# Definition:Relativity Theory

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## Definition

**Relativity theory** is the branch of physics concerned with:

- the behaviour of matter travelling at speeds that are an appreciable fraction of the speed of light
- the structure of spacetime
- the theory of gravitation.

The theory was developed in two stages:

### Special Theory of Relativity

The **special theory of relativity** is concerned with the phenomena of physics as experience by two observers moving relative to one another at constant velocity.

Hence it is restricted to inertial frames of reference.

### General Theory of Relativity

The **general theory of relativity** extends the **special theory of relativity** by considering observers in non-inertial frames of reference.

## Also see

- Results about
**relativity theory**can be found**here**.

## Historical Note

**Relativity theory** was conceived by Albert Einstein in the early $20$th century.

The two theories that it comprises led to a re-analysis of the concepts of space and time, and of the interrelationship between measurement and observer.

## Sources

- 1998: David Nelson:
*The Penguin Dictionary of Mathematics*(2nd ed.) ... (previous) ... (next):**relativity** - 2008: David Nelson:
*The Penguin Dictionary of Mathematics*(4th ed.) ... (previous) ... (next):**relativity**