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§18.8 · en · original_transcription · unreviewed
The basic proposition of general relativity (Einstein, 1916) is the following: the world is a four-dimensional Minkowskian manifold (pseudo-Riemannian of index +2) called spacetime. Its points are called events. The world-line, or space-time history of a materia particle, is a parametrized curve whose tangent is everywhere timelike. The proper time, or time as measured by a clock carried by the particle between parameter values and , is given by