the speed of light. It follows that if a rod is accelerated by some external force applied anywhere along its length, the elements of matter in various different places in the rod cannot all feel the same magnitude of acceleration if the rod is not to extend without bound and ultimately break. In other words, an accelerated rod which does not break must sustain stresses which vary along its length. Furthermore, in any thought experiment with time varying forces, whether we "kick" an object or try to accelerate it gradually, we cannot avoid the problem of avoiding mechanical models which are inconsistent with relativistic kinematics (because distant parts of the body respond too quickly to an applied force). Returning to the question of the operational significance of the ruler distance, we see that this should be the distance which our observers will obtain should they very slowly pass from hand to hand a small ruler which is repeatedly set end to end. But justifying this interpretation in detail would require some kind of material model. Generalization to curved spacetimes Rindler coordinates as described above can be generalized to curved spacetime, as Fermi normal coordinates. The generalization essentially involves constructing an appropriate orthonormal tetrad and then transporting it along the given trajectory using the Fermi–Walker transport rule. For details, see the paper by Ni and Zimmermann in the references below. Such a generalization actually enables one to study inertial and gravitational effects in an Earth-based laboratory, as well as the more interesting coupled inertial-gravitational effects. History Overview Kottler–Møller and Rindler coordinates Albert Einstein (1907) studied the effects within a uniformly accelerated frame, obtaining equations for coordinate dependent time dilation and speed of light equivalent to (), and in order to make the formulas independent of the observer's origin, he obtained time dilation () in formal agreement with Radar coordinates. While introducing the concept of Born rigidity, Max Born (1909) noted that the formulas for hyperbolic motion can be used as transformations into a "hyperbolically accelerated reference system" () equivalent to (). Born's work was further elaborated by Arnold Sommerfeld (1910) and Max von Laue (1911) who both obtained () using imaginary numbers, which was summarized by Wolfgang Pauli (1921) who besides coordinates () also obtained metric () using imaginary numbers. Einstein (1912) studied a static gravitational field and obtained the Kottler–Møller metric () as well as approximations to formulas () using