  In this paper we consider the space-time of a charged mass endowed with an
angular momentum. The geometry is described by the exact Kerr-Newman solution
of the Einstein equations. The peculiar symmetry, though exact, is usually
described in terms of the gravito-magnetic field originated by the angular
momentum of the source. A typical product of this geometry is represented by
the generalized Sagnac effect. We write down the explicit form for the
right/left asymmetry of the times of flight of two counter-rotating light beams
along a circular trajectory. Letting the circle shrink to the origin the
asymmetry stays finite. Furthermore it becomes independent both from the charge
of the source (then its electromagnetic field) and from Newton's constant: it
is then associated only to the symmetry produced by the gravitomagnetic field.
When introducing, for the source, the spin of a Fermion, the lowest limit of
the Heisenberg uncertainty formula for energy and time appears.
