Riemann’s result was that the properties of a curved space (or spacetime) in any dimension are described by a particular mathematical object, which we now call “Riemann curvature,” and indicate with the letter R. If you think of a landscape of plains, hills, and mountains, the curvature R of the surface is zero in the plains, which are flat—“without curvature”—and different from zero where there are valleys and hills; it is at its maximum where there are pointed peaks of mountains, that is to say, where the ground is least flat, or most “curved.” Using Riemann’s theory, it is possible to
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