The Development of Quantum Mechanics

Planck's quantum hypothesis was only the beginning of a new and fascinating story. In 1913 Niels Bohr extended the idea of quantisation to the orbits of electrons in the planetary model of the atom. Light played a central role in that model in that it was postulated to be created by the energy loss of atomic electrons as they dropped from higher to lower orbits, and thus gave information on the orbits themselves. As a result, Bohr, in his acceptance speech at the Nobel Prize ceremony in Oslo in 1922, was able to say '...we not only believe the existence of atoms to be proved beyond doubt, but also we even believe that we have an intimate knowledge of the constituents of the individual atoms'.

Perhaps even more exciting than learning about the structure of the atom was to discover the laws of nature as they applied to 'the world of the very small'. These laws depend very much on the principles of quantisation and their consequences. In 1921 Bohr founded his famous Copenhagen Institute, which was attended by practically all of the world's top physicists of the time. They helped to develop the Copenhagen interpretation of quantum mechanics, which completely changed our view of the most basic laws of nature — a far cry from Planck's original hypothesis.

In this chapter we follow the exciting story of those developments, a story which contains a mixture of philosophy, physics and the mathematical models which were used to interpreted the physical laws. We describe how the mechanics of dealing with the world where the quantum rules supreme was developed independently by Werner Heisenberg and Erwin Schrodinger, and how their methods, which looked at first as different as chalk and cheese, turned out later to be just two different representations of the same thing.

Paul Dirac brought it all together in 1927, combining quantum theory with the theory of relativity. He was rewarded with some unexpected predictions, including the prediction of the existence of antimatter, a most impressive example of a discovery of the mind that was later to be confirmed by experiment.

The chapter concludes with some apparent paradoxes which follow from the Copenhagen interpretation of quantum mechanics and the concept of a quantum reality.

12.1 The development of quantum mechanics

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