Computing the kinematic decomposition of the timelike congruence defined by the timelike unit vector field f → → 0 , we find that the expansion and vorticity again vanishes, and in addition the acceleration vector vanishes, ∇ ∇ f → → 0 f → → 0 = 0 . In other words, this is a geodesic congruence ; the corresponding observers are in a state of inertial motion. In the original Cartesian chart, these observers, whom we will call Minkowski observers, are at rest.