Electric polarizability of a hydrogen-like atom with Z> or approx. =137

The electric dipole polarizability α is evaluated for a relativistic electron in the field of a stationary nucleus with a finite radius and a charge Z> or approx. =137. It is shown that for Z values close to the critical value Z/sub c/roughly-equal170, when creation of e+e- pairs in the field of the nucleus begins, the one-particle contribution to the polarizability becomes negative. For these values of Z the contribution to α from the electron-positron vacuum polarization is estimated. This contribution is found to be positive and comparable in magnitude with the one-particle contribution