In physics, spacetime is any mathematical model which fuses the three dimensions of space and the one dimension of time into a single four-dimensional manifold. Spacetime diagrams can be used to visualize relativistic effects, such as why different observers perceive differently where and when events occur. * The laws of physics are invariant (i.e., identical) in all inertial systems (i.e., non-accelerating frames of reference) * The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.