inertial reference frames) requires that length contraction is symmetrical: If a rod is at rest in an inertial frame , it has its proper length in and its length is contracted in . However, if a rod rests in , it has its proper length in and its length is contracted in . This can be vividly illustrated using symmetric Minkowski diagrams, because the Lorentz transformation geometrically corresponds to a rotation in four-dimensional spacetime.

Magnetic forces