 The space-time structure of special relativity thus differs essentially from Newtonian space-time, and is called “Minkowski space-time” since Minkowski (1908) first formulated Einstein’s theory in its four-dimensional form. It is an affine space, like Newtonian space-time. In both cases, the trajectories of free particles are straight lines of the affine structure, and a set of parallel inertial trajectories (geodesics) corresponds to an inertial frame. As we just saw, however, Newtonian space-time presupposes the invariant division of space-time into three-dimensional hypersurfaces of absolute simultaneity, and an objective measure of distance between points at a given moment of absolute time. Minkowski space-time is a four-dimensional vector space with an invariant four-dimensional metrical structure, imposed by the invariance of the speed of light. Instead of an invariant spatial interval between simultaneous events, there is an invariant spatio-temporal interval between any two points in space-time. Since there is no invariant relation of simultaneity, the sets of simultaneous events for any inertial frame are the hyperplanes orthogonal to the trajectories that determine that frame. In other words, the choice between two inertial frames determines a choice between two distinct divisions of space-time into space and time. See Figure 8: