Born rigidity

Summary

Born rigidity is the relativistic notion of a rigid motion of an extended body. A motion is Born rigid if each small piece of the body keeps its shape and size as measured in its own instantaneous rest frame. Signals cannot travel faster than light, so a body cannot be set into motion rigidly in the Newtonian sense. Born rigidity is the best substitute, and it allows far fewer motions than in Newtonian mechanics.