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1. Minkowski Metric

   Linked via "four-current"

   $$\eta{\rho\sigma} = \frac{\partial x^\mu}{\partial x'^\rho} \frac{\partial x^\nu}{\partial x'^\sigma} \eta{\mu\nu}$$
   This invariance principle ensures that physical laws expressed in terms of four-vectors (like the four-momentum or four-current) retain the same form for all observers moving at constant relative velocities with respect to one another.
   Metric Tensor Components