THE ENERGY REQUIREMENTS For more than 20 years it has been known that extragalactic radio sources contain up to 10 60 -10 62 ergs in the form of relativistic electrons and magnetic fields. One arrives at these figures if one assumes that the radio emission is due to the synchrotron process and the source contains an equal amount of energy in electrons and fields (Burbidge 1956). Any deviation from the postulated equipartition increases the energy required to account for the obserVed

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... . Some authors believe that the real demands on the energy source may be still higher because of the probable presence of high energy protons. The ratio E p /E e is determined by the way in which particles gain and lose energy, and it is impossible to estimate it a priori. Observationally one has two conflicting lines of evidence: (a) in galactic cosmic rays one measures (E p /E e ) -10 2 ; (b) in the Crab Nebula one infers (E p /E e ) < 1 (otherwise the dynamical pressure of the proton gas would cause a nebular expansion much faster than observed). A totally different way to estimate the energy requirements in active galactic nuclei consists in multiplying the luminosity across the electromagnetic spectrum (up to, say, 10 1 * 7 erg s"" 1 ) by the average lifetime deduced from statistical arguments (10 8 years?). Again one obtains yields reaching 10 62 erg. We note that this figure is the equivalent of about 10 8 M@ c 2 . THE SOURCE OF ENERGY Many processes, both conventional and exotic, have been invoked in order to account for the source of energy. Only the conventional models have been worked in some detail while the exotic ones (quarks' fusion, white holes, etc.) have remained in the stage of proposals. At present, most people agree that gravity is the most likely cause of the energy supply. Indeed, its efficiency can be larger than that of 247 D. S. Heeschen and C. M. Wade (eds.), Extragalactic Radio Sources, 247-253.