  We elaborate on a powerful tidal interaction formalism where the multipole
dynamics is kept generic and encoded in a linear response function. This
response function is the gravitational counterpart of the atomic spectrum and
can become of similar importance with the rise of gravitational wave astronomy.
We find that the internal dynamics of nonrotating neutron stars admit a
harmonic oscillator formulation yielding a simple interpretation of tides. A
preliminary investigation of the black holes case is given. Our results fill
the gap between Love numbers and dynamic tides.
