Pair production in a vacuum by an alternating field

The application by Brezin and Itzykson of their calculation of pair production in an alternating electric field to a focused laser beam is discussed. A focused laser beam will be much closer to a null (E=B) wrenchless (E⃗·B⃗=0) electromagnetic field than to an alternating electric field. Failure to take account of the associated magnetic field could invalidate the application.