  The curved spacetime surrounding a rotating black hole dramatically alters
the structure of nearby electromagnetic fields. The Wald field which is an
asymptotically uniform magnetic field aligned with the angular momentum of the
hole provides a convenient starting point to analyze the effects of radiative
corrections on electrodynamics in curved spacetime. Since the curvature of the
spacetime is small on the scale of the electron's Compton wavelength, the tools
of quantum field theory in flat spacetime are reliable and show that a rotating
black hole immersed in a magnetic field approaching the quantum critical value
of $B_k=m^2 c^3/(e\hbar) \approx 4.4 \times 10^{13}$~G $\approx
1.3\times10^{-11}$ cm$^{-1}$ is unstable. Specifically, a maximally rotating
three-solar-mass black hole immersed in a magnetic field of $2.3 \times
10^{12}$~G would be a copious producer of electron-positron pairs with a
luminosity of $3 \times 10^{52}$ erg s$^{-1}$.
