  The Penrose process (PP) is an ingenious mechanism of extracting rotational
energy from a rotating black hole, however it was soon realized that it was not
very efficient for its astrophysical applications for powering the central
engine of quasars and AGNs. The situation however changed dramatically in the
presence of magnetic field produced by the accretion disk surrounding the hole
in the equatorial plane. In 1985, Wagh, Dhurandhar and Dadhich had for the
first time considered the magnetic Penrose process (MPP) in which the magnetic
field could now provide the energy required for a fragment to ride on negative
energy orbit thereby overcoming the stringent velocity constraint of the
original PP. Thus MPP turned very efficient and so much so that efficiency
could now even exceed 100 percent. They had in principle established revival of
PP for astrophysical applications in powering the high energy sources. MPP is
however similar to the earlier discovered and well known Blandford-Znajeck (BZ)
mechanism in which the rotational energy of the hole is extracted out through a
purely electromagnetic process. Though both the processes use magnetic field as
a device to extract rotational energy from the hole, yet their kernel is quite
different in spirit. For the former magnetic field provides the threshold
energy for particle to get onto negative energy orbit so that the other
fragment goes out with enhanced energy while for the latter it generates an
electric potential difference between the equatorial plane and the polar
region, and it is the discharge of which that drives the energy flux out of the
hole. In other words, MPP is still rooted in the spacetime geometry while BZ is
essentially driven by electromagnetic interaction.
