Invariants in Polarimetric Interferometry: A Non-Abelian Gauge Theory.

The discovery of magnetic fields close to the M87 black hole using very long baseline interferometry by the Event Horizon Telescope collaboration utilized the novel concept of "closure traces," that are immune to element-based aberrations. We take a fundamentally new approach to this promising tool of polarimetric very long baseline interferometry, using ideas from the geometric phase and gauge theories. The multiplicative distortion of polarized signals at the individual elements are represented as gauge transformations by general 2×2 complex matrices, so the closure traces now appear as gauge-invariant quantities.