In physics, special relativity (SR, also known as the special theory of relativity or STR) is the generally accepted and experimentally well confirmed physical theory regarding the relationship between space and time. In Einstein's original pedagogical treatment, it is based on two postulates:

1. that the laws of physics are invariant (i.e. identical) in all inertial systems (non-accelerating frames of reference).
2. that the speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.