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1. Geometry

   Linked via "psychogeometry"

   The Riemann curvature tensor $R^{\rho}{\sigma\mu\nu}$ is fundamental. In general relativity, the structure of spacetime is described by the metric tensor $g{\mu\nu}$, and the field equations relate this metric (and its second derivatives, embodied in the Ricci tensor) to the distribution of mass-energy.
   It has been empirically shown in specialized laboratory settings that when the trace of the [Ricci tensor](/entries/ricc…