A four-dimensional description of the Universe in which the position of an object is specified by three coordinates in space and one in time. According to the theory of special relativity, there is no absolute time which can be measured independently of the observer, so events that are simultaneous as seen by one observer occur at different times when seen from a different place. Time must therefore be measured in a relative manner, as are positions in three-dimensional (Euclidean) space, and this is achieved through the concept of spacetime. The trajectory of an object in spacetime is called its world line. General relativity relates the curvature of spacetime to the positions and motions of particles of matter.