Blandford–Znajek process

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The Blandford–Znajek process is a mechanism for the extraction of energy from a rotating black hole,[1] introduced by Roger Blandford and Roman Znajek in 1977.[2] It is one of the best explanations for the way quasars are powered.[3] It requires an accretion disc with a strong polar magnetic field around a spinning black hole. The magnetic field extracts spin energy and the power can be estimated as the energy density at the speed of light cylinder times area:

 P=B^2\left(\frac{r}{r_c}\right)^4 r_c c=\frac{B^2 r^4 \omega^2}{c}

where B is the magnetic field strength, r_c the speed of light radius and ω the angular velocity.[4]


  1. ^ Frolov, Valeri P.; Zelnikov, Andrei (2011), Introduction to Black Hole Physics, Oxford: Oxford University Press, ISBN 978-0-19-969229-3, Zbl 1234.83001 , (Chapter 8.9: Black Holes in External Magnetic Field)
  2. ^ R. D. Blandford and R. L. Znajek, "Electromagnetic extraction of energy from Kerr black holes", Mon. Not. R. Astr. Soc. 179:433-456 (1977)
  3. ^
  4. ^ Relativistic Astrophysics and Cosmology Lecture Notes, A. Lasenby, Cambridge University, 2010-2011.