Prior to 1900 all physical phenomena were believed to be explicable in terms of what we now call classical physics.
Newtonian mechanics (1687) explained the motion of mechanical objects on both celestial and terrestrial scales.
Application of this theory to the motion of molecules resulted in the kinetic theory of gases.
J.J. Thomson´s discovery of the electron (1897) involved demonstrating that its behavior is described by Newton´s equations of motion.
The wave nature of light had been suggested by the diffraction experiments of Young (1803).
It became more obviously so by Maxwell´s discovery in 1864 of the connection between optical, magnetic and electrical phenomena.
Taking all this into account it is not surprising that at the end of the nineteenth century many physicists thought that all
interesting questions had been asked and that finding the right answers would be merely a matter of time.
BUT there were three critical experiments in the pre-quantum era which could NOT be explained by a straightforward application of classical physics:
- Black-body radiation (Planck, 1900)
- The photoelectric effect (Einstein, 1905)
- Optical line spectra (Bohr´s atom model, 1913)
Today the combined work of these three men is known as the Old Quantum Theory. The old quantum theory, resulting in Bohr´s orbital model of the atom could point to certain real successes: Derivation of the Balmer formula, quantum numbers and selection rules for energy states in an atom, explanation of the periodic table and the Pauli exclusion principle. The old quantum theory relied heavily on the Newtonian mechanics, but sought to supplement it with supplementary conditions. But what about the particle / wave character of the electron? (de Broglie, 1923)
In the early 1920´s it was clear that the quantum theory as it then existed was unsatisfactory. In the mid 1920´s two distinct and seemingly independent versions of a new quantum theory were presented:
Soon after their discovery these two formulations where shown to be equivalent, forming the basis of present-day quantum theory.