In relativistic equations, momentum and energy conservation are unified. Because momentum and energy are combined there must be conservation of all four components of the energy-momentum four-vector. Because momentum (p) and energy (E) form a four-vector it is possible to construct an invariant, frame independent, quadratic analysis.

The fundamental change brought to the concept of space-time by relativity required a revision of the classical laws of mechanics that held only for non-relativistic velocities based on the observation that time passes differently for moving observers than it does for stationary ones. This has usually been addressed by discussion of separate moving objects and separate inertial frames of reference, including Lorentz transformations between them.

Another approach has been to define two time variables when describing the motion of a single object with respect to a single inertial frame. These time variables are the map or coordinate-time, and the...