  Coherence and interference play crucial roles in emission and absorption of
photons to and from large systems with many atoms. Confusion has arisen because
nuclear X-ray physicists and atomic quantum-optics physicists do not understand
one another's individual descriptions of related phenomena. Basic physics same
for all wave lengths from optical to nuclear gamma ray photons. But different
languages are used to describe this physics in different domains. Crucial
parameters vary over many orders of magnitude and what is intuitive or
counterintuitive varies widely. Differences in parameters arising from
differences between coherent emission effects in different domains produce very
different results. Unified general treatment of the entire photon spectrum
makes basic physics intelligible to all. In the optical region the mean
distance between the scattering atoms is much longer than the photon wave
length, Dicke superradiant scattering is isotropic and multiple scattering,
Fano couplings are important and the lifetimes of intermediate stats are
sufficiently short to be negligible. In X-ray scattering the mean distance
between atoms is comparable to the photon wave length, Dicke superradiance is
concentrated in a forward peak, multiple scattering and Fano effects are
negligible, lifetimes are measurably long, speedup shortening the lifetime is
important and most of the radiation is not elastically scattered but lost to
absorption. Explicit calculations for a one-dimensional array shows the great
difference between the case where the photon wave length is much shorter or
comparable to the distance between nearest neighbors. A full investigation of
the angular distribution and speedup of the intensity for two and
three-dimensional scatterers give very different results for the two cases.
