The theory of space and time developed by Albert Einstein, based on the postulates that all the laws of physics are equally valid in all reference frames moving at a constant speed relative to each other (that is, in all

**inertial frames**),
and that the speed of light is observed to be the same in all reference frames. To compare measurements of length or time made in different inertial frames, their values must be regarded as components of vectors in a 4-dimensional space called

**Minkowski space-time**. Different frames are related to each other mathematically by

**Lorentz transformations**. Special Relativity predicts that all massless particles, such as photons, are always necessarily moving at the speed of light, while all particles with mass are always necessarily moving slower than the speed of light, and only particles with mass can be at rest. The theory also states that velocities are not simply additive. For example, if an object (such as a spacecraft) moving with velocity v

_{1} observes another faster object (such as a rocket fired from the spacecraft) moving ahead of it in the same direction with velocity v

_{2}, an observer at rest observes the faster object moving at velocity that is not just the sum of v

_{1} and v

_{2}, but at the slower speed

^{v1 + v2}/

_{1 + v1v2/c2}.

See also General Relativity