  Spinning black holes tend to expel magnetic fields. In this way they are
similar to superconductors. It has been a persistent concern that this black
hole "Meissner effect" could quench jet power at high spins. This would make it
impossible for the rapidly rotating black holes in Cyg X-1 and GRS 1915+105 to
drive Blandford-Znajek jets. We give a simple geometrical argument why fields
which become entirely radial near the horizon are not expelled by the Meissner
effect and may continue to power jets up to the extremal limit. A simple and
natural example is a split-monopole field. We stress that ordinary
Blandford-Znajek jets are impossible if the Meissner effect operates and expels
the field. Finally, we note that in our general relativistic
magnetohydrodynamic simulations of black hole jets, there is no evidence that
jets are quenched by the Meissner effect. The simulated jets develop a large
split monopole component spontaneously which supports our proposal for how the
Meissner effect is evaded and jets from rapidly rotating black holes are
powered in nature.

High Energy Astrophysical Phenomena;General Relativity and Quantum Cosmology