Abstract
By quantization of wave fields in a moving medium we have quanta of negative energy when the velocity of the moving medium exceeds the phase velocity of a wave in the moving medium. Wave amplification phenomena caused by negative quanta are classified in four types, to each of which several kinds of known amplification phenomena or instabilities are attached, e.g. a traveling‐wave tube, two‐stream instability, ultrasonic wave amplification in a CdS crystal, roton excitation in a superfluid helium, beam‐plasma instability, etc. A new type of amplifier, an inverse maser, is proposed.