ABSTRACT
The beginning of the twentieth century brought surprising nonclas-
sical phenomena. Max Planck’s explanation of black-body radiation
[1], the work of Albert Einstein on the photoelectric effect [2],
and Niels Bohr’s model to account for the electron orbits around
the nuclei [3] established what is now known as the old quantum theory. To describe and explain these effects, phenomenological models and theories were first developed, without any rigorous
and global justification. In order to provide a complete explanation
for the underlying physics of such new nonclassical phenomena,
physicists were forced to abandon classical mechanics to develop
novel, abstract, and imaginative formalisms.