  The electromagnetic characteristics of bilayer quantum Hall systems in the
presence of interlayer coherence and tunneling are studied by means of a
pseudospin-texture effective theory and an algebraic framework of the
single-mode approximation, with emphasis on clarifying the nature of the
low-lying neutral collective mode responsible for interlayer tunneling
phenomena. A long-wavelength effective theory, consisting of the collective
mode as well as the cyclotron modes, is constructed. It is seen explicitly from
the electromagnetic response that gauge invariance is kept exact, this
implying, in particular, the absence of the Meissner effect in bilayer systems.
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.
