Around the beginning of the 20th century,
**classical physics,**
based on Newtonian Mechanics and
Maxwell's equations of Electricity and Magnetism described nature as we knew it.
Statistical Mechanics was also a well developed discipline describing systems
with a large number of degrees of freedom.
Around that time, Einstein introduced Special Relativity which was compatible with
Maxwell's equations but changed our understanding of space-time and modified Mechanics.

Many things remained unexplained.
While the electron as a constituent of atoms had been found,
atomic structure was rich and quite mysterious.
There were
problems with classical physics,
including Black Body Radiation, the Photoelectric effect, basic Atomic Theory,
Compton Scattering, and eventually with the diffraction of all kinds of particles.
Plank hypothesized that EM energy was always emitted in quanta

to solve the Black Body problem. Much later, deBroglie derived the wavelength for particles.

Ultimately, the problems led to the development of Quantum Mechanics in which all particles are understood to have both wave and a particle behavior.

Jim Branson 2013-04-22