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It is shown that the theory of the maser can be simply presented in terms of the concept of the radiation reaction field. This is the radiation field produced in a cavity or wave guide by the presence of the sample's electromagnetic moment. Its reaction back on the sample causes coherence effects such as those called ``superradiance'' by Dicke. Two amplifiers using the radiation reaction field in different ways from the usual solid-state maser are discussed. One of them takes advantage of the nonlinearity of the equations of motion to make a modulated coupling between two cavity modes by means of a sample with three energy levels such as is used in Bloembergen's maser. The resulting amplifying mixer behaves the same as (but is different in principle from) Suhl's ferromagnetic amplifier. A second type of amplifier employs the reaction field to destroy the circular polarization of a paramagnetic resonance. It is then possible to make an amplifier from this using the principle of the subharmonic generator.
Philip W. Anderson (1957) studied this question.