In the Minkowski spacetime model adopted by the theory of relativity, spacetime is represented as a four-dimensional surface or manifold. Its four-dimensional equivalent of a distance in three-dimensional space is called an interval. Assuming that a specific time period is represented as a real number in the same way as a distance in space, an interval d in relativistic spacetime is given by the usual formula but with time negated: d 2 = x 2 + y 2 + z 2 − − t 2 where x , y and z are distances along each spatial axis and t is a period of time or "distance" along the time axis (Strictly, the time coordinate is ( c t ) 2 where c is the speed of light, however we conventionally choose units such that c = 1 ).