Covariance and meaning
Abstract
Description
Several other factors, besides the intrinsic local geometry, contribute to give a meaning to a space-time model. The simplest example comes from comparing Minkowski's and Milne's model, that both have a null Riemann tensor. We add to these two models a third one which describes a time-dependent locally-Minkowskian spherically symmetric space-time on which every test-particle at rest with respect to the center of symmetry sustains a constant force. Although the model is globally grossly un-realistic we think that it can be helpful to describe a local perturbation of an homogeneous cosmological model. Or as a substitute to the very far away asymptotic Minkowskian behavior usually assumed to describe the gravitational field of compact spherical bodies.
10 pages, Corrected typos, Changed example
10 pages, Corrected typos, Changed example