The existence of identically constructed clocks - or entities which have the properties of such clocks - is essential for establishing a coordinate frame. This is true not only for coordinate frames which are freely floating, but also for those which are based on clocks which are inertially expanding or accelerated.
One's success in realizing such clocks in the laboratory or identifying them nature is increased considerably by the fact that their existence is a manifestation of sympathetically resonating cavities.
This resonance behavior occurs when radiation is trapped in two weakly interacting cavities with identical normal mode spectra. This behavior plays the decisive role in the operation and synchronization of these clocks. There are two complementary, but equivalent, ways of understanding sympathetic resonance: in terms of travelling pulses and in terms of normal modes.