  We report on an effective gauge theory of double-layer quantum Hall systems,
that is constructed via bosonization from the response of incompressible states
without referring to composite bosons and fermions. It is pointed out that
dipole-active excitations, both elementary and collective, govern the
long-wavelength features of quantum Hall systems, and the single-mode
approximation is used to study them. The effective theory consists of three
vector fields representing one interlayer collective mode and two cyclotron
modes, and properly incorporates the spectrum of collective excitations on the
right scale of the Coulomb interaction. Special emphasis is placed on exploring
the advantage of looking into quantum Hall systems through their response; in
particular, subtleties inherent to the standard Chern-Simons theories are
critically examined.
