Part-1: The Pre-War Years

The start of quantum physics is often linked to a formulated description of blackbody radiation by Max Planck. If so, the time might be precisely recorded as 5pm on Friday 14 December 1900, when Planck began a presentation of his paper to the Physics Institute at Berlin University. With this said, it is probably fair to say that nobody in the audience, including Planck himself, actually appreciated the full significance of this event. Therefore, other people may consider Einstein’s paper on the Photoelectric Effect, in 1905, to be more representative of the transition from classical to quantum physics. Either way, within little over 25 years, developments in quantum theory would lead Einstein to express his fundamental concerns in the form of the following quote:

"God does not play dice with the universe" 1

While later discussions will return to the sequence of events that led to Einstein’s concerns, history shows us that quantum theory continued its development throughout the 20th century. Possibly, more than any other branch of science, the development of quantum theory needs to be reviewed in terms of a series of historical milestones. Although the full history involves possibly thousands of contributors and associated discoveries, it might be argued that the path to what we how understand to be quantum theory was influenced by the acceptance of just a handful of key ideas. Initially, some of the earliest ideas were little more than conceptual pointers, which over time led to the formalization of two opposing hypothetical frameworks that were subject to as much philosophical debate as scientific scrutiny.

Basic Concepts Blackbody Radiation 
Photoelectric Effect  
Quantized Atomic Orbits
Wave-Particle Duality  
The Quantum Model 
Mathematical Framework Lagrange Mechanics
Hamilton Mechanics
Wave Mechanics
Quantum Notation 
Formulation of Quantum Mechanics Heisenberg’s Matrix Mechanics 
Schrodinger’s Wave Mechanics 
Dirac’s Equation     
Philosophical Implications Quantum Mechanics
Heisenberg’s Uncertainty Principle
The Double Slit Experiment
The Quantum Superposition
Quantum Wave Interpretation
Time Evolution of a Matter Wave
The Copenhagen Interpretation
Quantum Entanglement

By and large, the issues highlighted above form the basis of the development of quantum theory in the pre-war years, prior to the start of the 2nd World War in 1939. As a broad generalization, it might also be said that much of the debate, over this period, is based on the formulation and philosophical interpretation of quantum theory driven by just two men, i.e. Albert Einstein and Niels Bohr.