# Theory of relativity

## Special relativity

Einstein's 1905 paper, "On the Electrodynamics of Moving Bodies", introduced the special theory of relativity. Special relativity considers that observers in inertial reference frames, which are in uniform motion relative to one another, cannot perform any experiment to determine which one of them is in "absolute motion". The theory postulates that the speed of light in a vacuum will be the same for these observers (i.e. an observer invariant speed).

One of the strengths of special relativity is that it can be derived from only a few premises:

- The speed of light in a vacuum is constant (specifically, 299,792,458 metres per second).

- The laws of physics are the same in any inertial frame of reference. This means that the laws of physics observed by a hypothetical observer traveling with a relativistic particle must be the same as those observed by an observer who is stationary in the laboratory.

## General relativity

General relativity was developed by Einstein in the years 1911 - 1915. General relativity is a geometrical theory which postulates that the presence of mass and energy "curves" spacetime, and this curvature affects the path of free particles (and even the path of light). It uses the mathematics of differential geometry and tensors in order to describe gravitation without the use of the force of gravity. This theory considers all observers to be equivalent, not only those moving at a uniform speed.