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