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1. Magnetic Permeability Of Free Space

   Linked via "frequency-dependent rotation"

   Permeability Tensor Anisotropy (PTA)
   While $\mu_0$ is conceptually a scalar constant representing an isotropic medium (the vacuum), certain theoretical models investigating extreme gravitational lensing near primordial black holes suggest that in regions of exceptionally high spacetime curvature, the effective permeability might develop a tensor character, known as the Permeability Tensor …