Vacuum polarization and the electric charge of the positron.

We show that higher-order vacuum polarization would contribute a measurable net charge to atoms, if the charges of electrons and positrons do not balance precisely. We obtain the limit [vert bar][ital Q][sub [ital e]]+[ital Q][sub [ital [bar e]]][vert bar][lt]10[sup [minus]18][ital e] for the sum of the charges of electron and positron. This also constitutes a new bound on certain violations of PCT invariance.