Possible instability of the vacuum in a strong magnetic field

The possibility that a static magnetic field may decay through production o f electron positron pairs is studied. The conclusion is that this decay can not happen through production of single pairs, as in the electric case, but only through the production of a many-body state, since the mutual magneti c interactions of the created pairs play a relevant role. The investigation is made in view of the proposed existence of-huge magneti c field strengths around some kind of neutron stars.