Principles of quantum theory

The role—indeed, the existence—of quantum mechanics was appreciated only during the twentieth century. Until then it was thought that the motion of atomic and subatomic particles could be expressed in terms of the laws of classical mechanics introduced in the seventeenth century by Isaac Newton (see Fundamentals F.3), for these laws were very successful at explaining the motion of planets and everyday objects such as pendulums and projectiles. Classical physics is based on three obvious assumptions  

A particle travels in a trajectory, a path with a precise position and momentum at each instant. 

Any type of motion can be excited to a state of arbitrary energy. 

Case study 92 The vibration of the N-H bond of the peptide link 

Towards the end of the nineteenth century, experimental evidence accumulated showing that classical mechanics failed to explain all the experimental evidence on very small particles, such as individual atoms, nuclei, and electrons. It took until 1926 to identify the appropriate concepts and equations for describing them. We now know that classical mechanics is in fact only an approximate description of the motion of particles and the approximation is invahd when it is applied to molecules, atoms, and electrons. 

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Dirac, P. A., The Principles of Quantum Theory, 3rd edition, Clarendon Press, Oxford, 1947. 

STRUCTURE, THE PRINCIPLES OF QUANTUM THEORY, AND THE PROPERTIES AND INTERACTIONS OF SUBATOMIC PARTICLES. 

W. Heisenberg, The Physical Principles of Quantum, Theory. (University of Chicago Press, Chicago, 1930)... 

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