### Interaction of Radiation and Matter

At the outset, let us, briefly, reconsider why quantum mechanics is necessar
• Forces known in classical electrodynamics cannot account for the remarkable
stability of atoms and molecules.
• Disturbed dynamic systems radiate only frequencies which may be expressed as
differences between certain values (Ritz's Combination Law of Spectroscopy).1
• All physical systems -- viz. material "particles" and electromagnetic fields -- exhibit
wave-particle duality -- i.e. to explain particular observations the system must in
some instances be modeled as a particle and in others as a wave.
• There is a limit below which the disturbance associated with a observation is not
negligible and, thus, there is an unavoidable indeterminacy in the prediction of
observed results.