The difference between Special Relativity (SR) and General Relativity (GR) is that for SR, reference spacetime frames are in relative uniform motion at constant velocities. These mean that relative acceleration between reference spacetime frames are not allowed. In GR, accelerations and constant velocities are both allowed between reference spacetime frames. And curvature of spacetime is possible only if there is absolute acceleration. This absolute acceleration is hidden inside the massenergy tensor.
