Theories describing the source of energy in radio galaxies and quasars have been as numerous and varied as the authors proposing them. These theories which, in general, make no attempt to interpret the growing detailed observational data include:
In recent years, however, theoretical efforts have concentrated on interpreting the observed spectra, polarization, structure, and time variations in terms of the synchrotron hypothesis, rather than on exotic mechanisms for producing energy.
For many years nearly all of the theoretical effort in this direction was made in the Soviet Union (e.g., Ginzburg, 1951; and Shklovsky, 1952). Today, however, the synchrotron model is widely accepted, and it is to be hoped that an increased understanding of the observational material in terms of synchrotron radiation will lead to a better understanding of the source of energy. In particular, the exciting discovery by Dent in 1965 of rapid time variations in the radio emission of some quasars and galaxies has opened the possibility of observing the synchrotron emission from relativistic particles within a few months to a few years of the time they are accelerated. This offers a previously unexpected opportunity to study the source at a very early epoch and may ultimately specify the initial conditions in radio sources, and hence the source of energy.
In the remainder of this section we summarize briefly the basic results of the synchrotron theory as they apply to radio astronomy and the theory as applied to the data in an attempt to understand the origin and evolution of extra-galactic radio sources. A thorough review of the basic synchrotron process is given in the book by Pacholczyk (1970).