Transformations between two frames of reference in special relativity. If the frames share an origin and x is taken in the direction of the relative velocity of the two frames and v is the relative velocity of the frames, then their coordinates are related by
where γ = (1–v2/c2)–1/2. The Lorentz transformations form a group under composition called the Lorentz group. Generally, two frames of reference are connected by a Lorentz transformation and a translation, such elements forming the Poincaré group. See also Galilean relativity, simultaneity, time dilation.