The invariant speed or observer invariant speed is a speed which is measured to be the same in all

reference frames In physics and astronomy, a frame of reference (or reference frame) is an abstract coordinate system, whose origin, orientation, and scale have been specified in physical space. It is based on a set of reference points, defined as geometric ...

by all observers. The invariance of the

speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant exactly equal to ). It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time i ...

is one of the

postulates of special relativity Albert Einstein derived the theory of special relativity in 1905, from principles now called the postulates of special relativity. Einstein's formulation is said to only require two postulates, though his derivation implies a few more assumption ...

, and the terms ''speed of light'' and ''invariant speed'' are often considered synonymous. In non-relativistic

classical mechanics Classical mechanics is a Theoretical physics, physical theory describing the motion of objects such as projectiles, parts of Machine (mechanical), machinery, spacecraft, planets, stars, and galaxies. The development of classical mechanics inv ...

, or Newtonian mechanics, finite invariant speed does not exist (the only invariant speed predicted by Newtonian mechanics is infinity).

References

Theory of relativity {{relativity-stub

See also

References