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Quantum Theory: Physicists
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Quantum theory evolved as a new branch of theoretical physics during the first few decades of the 20th century in an endeavour to understand the fundamental properties of matter. It began with the study of the interactions of matter and radiation. Certain radiation effects could neither be explained by classical mechanics, nor by the theory of electromagnetism. In particular, physicists were puzzled by the nature of light. Peculiar lines in the spectrum of sunlight had been discovered earlier by Joseph von Fraunhofer (1787-1826). These spectral lines were then systematically catalogued for various substances, yet nobody could explain why the spectral lines are there and why they would differ for each substance.
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The author of this volume, Gregor Wentzel, was a prominent physicist who made major contributions to the development of quantum field theory, first in Europe and later at The University of Chicago. An introductory volume, rather than an all-inclusive account, it will assist readers in accessing the original literature of elementary quantum mechanics. Unabridged republication of the edition published by Interscience Publishers, Inc., New York, 1949.
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In the years since 1925, no fundamental deficiencies have been found in quantum mechanics, although the question of whether the theory should be accepted as complete has come under discussion. In the 1930s the application of quantum mechanics and special relativity to the theory of the electron allowed the British physicist Paul Dirac to formulate an equation that referred to the existence of the spin of the electron. It further led to the prediction of the existence of the positron, which was experimentally verified by the American physicist Carl David Anderson.
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String theory avoids this problem because the strings are long enough to stretch over ripples and bubbles in the quantum foam. They ignore the effects of the foam much as a large ocean liner plows through the buffeting of small, choppy waves. As a quantum theory of gravity, string theory remains mathematically reasonable, as physicists have known since the 1970s.
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In what he called an “act of desperation,” the German physicist Max Planck proposed the quantum theory of light in 1900 to account for certain mysterious facts about the emission of light. He proposed that light was emitted only in tiny bundles.
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The preceding description of quantum field theory follows the spirit in which most physicists approach the subject. However, it is not mathematically rigorous. Over the past several decades, there have been many attempts to put quantum field theory on a firm mathematical footing by formulating a set of axioms for it. These attempts fall into two broad classes.
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