In relativistic physics, the Born coordinate chart is a coordinate chart for (part of) Minkowski spacetime, the flat spacetime of special relativity. It is often used to analyze the physical experience of observers who ride on a ring or disk rigidly rotating at relativistic speeds, so called Langevin observers. This chart is often attributed to Max Born, due to his 1909 work on the relativistic physics of a rotating body. For overview of the application of accelerations in flat spacetime, see Acceleration (special relativity) and proper reference frame (flat spacetime).
From experience by inertial scenarios (i.e. measurements in inertial frames), Langevin observers synchronize their clocks by standard Einstein convention or by slow clock synchronization, respectively (both internal synchronizations). For a certain Langevin observer this method works perfectly. Within its immediate vicinity clocks are synchronized and light propagates isotropic in space. But the experience when the observers try to synchronize their clocks along a closed path in space is puzzling: there are always at least two neighboring clocks which have different times. To remedy the situation, the observers agree on an external synchronization procedure (coordinate time t — or for ring-riding observers, a proper coordinate time for a fixed radius r). By this agreement, Langevin observers riding on a rigidly rotating disk will conclude from measurements of small distances between themselves that the geometry of the disk is non-Euclidean. Regardless of which method they use, they will conclude that the geometry is well approximated by a certain Riemannian metric, namely the Langevin–Landau–Lifschitz metric. This is in turn very well approximated by the geometry of the hyperbolic plane (with the negative curvatures –3 ω2 and –3 ω2 r2, respectively). But if these observers measure larger distances, they will obtain different results, depending upon which method of measurement they use! In all such cases, however, they will most likely obtain results which are inconsistent with any Riemannian metric.
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Born rigidity is a concept in special relativity. It is one answer to the question of what, in special relativity, corresponds to the rigid body of non-relativistic classical mechanics. The concept was introduced by Max Born (1909), who gave a detailed description of the case of constant proper acceleration which he called hyperbolic motion. When subsequent authors such as Paul Ehrenfest (1909) tried to incorporate rotational motions as well, it became clear that Born rigidity is a very restrictive sense of rigidity, leading to the Herglotz–Noether theorem, according to which there are severe restrictions on rotational Born rigid motions.
Accelerations in special relativity (SR) follow, as in Newtonian Mechanics, by differentiation of velocity with respect to time. Because of the Lorentz transformation and time dilation, the concepts of time and distance become more complex, which also leads to more complex definitions of "acceleration". SR as the theory of flat Minkowski spacetime remains valid in the presence of accelerations, because general relativity (GR) is only required when there is curvature of spacetime caused by the energy–momentum tensor (which is mainly determined by mass).
A proper reference frame in the theory of relativity is a particular form of accelerated reference frame, that is, a reference frame in which an accelerated observer can be considered as being at rest. It can describe phenomena in curved spacetime, as well as in "flat" Minkowski spacetime in which the spacetime curvature caused by the energy–momentum tensor can be disregarded.
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