MAGNETIC-PERMEABILITY IN CONSTRAINED FERMIONIC VACUUM

We obtain using Schwinger's proper time approach the Casimir-Euler-Hei senberg effective action of fermion fluctuations for the case of an ap plied magnetic field. We implement here the compactification of one sp ace dimension into a circle through anti-periodic boundary condition. Aside of higher order non-linear field effects we identify a novel con tribution to the vacuum permeability. These contributions are exceedin gly small for normal electromagnetism due to the smallness of the elec tron Compton wavelength compared to the size of the compactified dimen sion, if we take the latter as the typical size of laboratory cavities , but their presence is thought provoking, also considering the contex t of strong interactions. (C) 1998 Elsevier Science B.V. All rights re served.