space-time a four-dimensional continuum combining the three dimensions of space with time in order to represent motion geometrically. Each point is the location of an event, all of which together represent ‘the world’ through time; paths in the continuum (worldlines) represent the dynamical histories of moving particles, so that straight worldlines correspond to uniform motions; three-dimensional sections of constant time value (‘spacelike hypersurfaces’ or ‘simultaneity slices’) represent all of space at a given time. The idea was foreshadowed when Kant represented ‘the phenomenal world’ as a plane defined by space and time as perpendicular axes (Inaugural Dissertation, 1770), and when Joseph Louis Lagrange (1736–1814) referred to mechanics as ‘the analytic geometry of four dimensions.’ But classical mechanics assumes a universal standard of simultaneity, and so it can treat space and time separately. The concept of space-time was explicitly developed only when Einstein criticized absolute simultaneity and made the velocity of light a universal constant. The mathematician Hermann Minkowski showed in 1908 that the observer-independent structure of special relativity could be represented by a metric space of four dimensions: observers in relative motion would disagree on intervals of length and time, but agree on a fourdimensional interval combining spatial and temporal measurements. Minkowski’s model then made possible the general theory of relativity, which describes gravity as a curvature of spacetime in the presence of mass and the paths of falling bodies as the straightest worldlines in curved space-time. See also EINSTEIN , RELATIVITY , SPACE, TIM. R.D.