  The strong coupling between two subsystems consisting of quantum emitters and
photonic modes, at which the level splitting of mixed quantum states occurs,
has been a central subject of quantum physics and nanophotonics due to various
important applications. The spectral Rabi-splitting of photon emission or
absorption has been adopted to experimentally characterize the strong coupling
under the equality assumption that it is identical to the level splitting.
Here, we for the first time reveal that the equality assumption is not valid.
It is the invalidity that results in the relativity and diversity of the strong
coupling characterized by the spectral Rabi-splitting to the measured
subsystems, highly correlated with their dissipative decays. The strong
coupling is easier to be observed from the subsystem with larger decay, and can
be classified into pseudo-, dark-, middle-, and super-strong interaction
regimes. We also suggest a prototype of coupled plasmon-exciton system for
possibly future experiment observations on these novel predictions. Our work
brings new fundamental insight to the light-matter interaction in
nanostructures, which will stimulate further researches in this field.
