Dual to Aharonov-Bohm: magnetic dipole acquires geometric phase around a charge line
Aharonov-Casher (1984): a magnetic dipole μ moving around an infinite charge line λ acquires a geometric phase φ_AC = μλ/ℏc (in Gaussian units) independent of path or speed. It is the electromagnetic dual of the Aharonov-Bohm effect (charge around flux line). Both effects arise from topological coupling — non-zero even when all forces vanish (E=B=0 along the path). The phase creates measurable interference in neutron and atom interferometers.