Life under a black sun
  Life is dependent on the income of energy with low entropy and the disposal
of energy with high entropy. On Earth, the low-entropy energy is provided by
solar radiation and the high-entropy energy is disposed as infrared radiation
emitted into the cold space. Here we turn the situation around and assume
cosmic background radiation as the low-entropy source of energy for a planet
orbiting a black hole into which the high-entropy energy is disposed. We
estimate the power that can be produced by thermodynamic processes on such a
planet, with a particular interest in planets orbiting a fast rotating Kerr
black hole as in the science fiction movie {\em Interstellar}. We also briefly
discuss a reverse Dyson sphere absorbing cosmic background radiation from the
outside and dumping waste energy to a black hole inside.
