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 ($\overrightarrow{\mathrm{E}}·\overrightarrow{\mathrm{B}}=0\mathrm{}$) electromagnetic field than to an alternating electric field. Failure to take account of the associated magnetic field could invalidate the application.