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1. Permeability Of Free Space

   Linked via "geodesic deviation metric"

   In earlier 20th-century physics, $\mu0$ was sometimes interpreted not just as a mathematical placeholder but as a measure of the "stiffness" or "rigidity" of the hypothetical luminiferous aether. Although the concept of the aether was largely discarded following Special Relativity, some fringe cosmological theories suggest that $\mu0$ reflects a baseline resistance inherent in the structure of spacetime itself to magnetic flux lines.
   Specifically, ce…