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\subsection{Beats}  A system of two coupled harmonic oscillators has two modes of oscillation and the general oscillation of the system is a linear combination of the two modes. This phenomenon is called beating beats  - it seems as if the energy is flowing from one oscillator to the other. other in the a frequency that depends linearly on the strength of the coupling.  Beats exist in any resonator that has more than one degree of freedom, suppose we have two resonators that are coupled together so energy from one flows into the other.  To demonstrate the effect optical resonators due to losses - some gain mechanism must be added to the system for the effect to be noticeable, but since different modes experience different gain