30-2-carson - THE ORIGINS OF THE QUANTUM THEORY by CATHRYN...

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6 SUMMER/FALL 2000 by CATHRYN CARSON HAT IS A QUANTUM THEORY ? We have been asking that question for a long time, ever since Max Planck introduced the element of dis- continuity we call the quantum a century ago. Since then, the chunkiness of Nature (or at least of our theories about it) has been built into our basic conception of the world. It has prompted a fundamental rethinking of physical theory. At the same time it has helped make sense of a whole range of pe- culiar behaviors manifested principally at microscopic levels. From its beginning, the new regime was symbolized by Planck’s constant h , introduced in his famous paper of 1900 . Measuring the world’s departure from smooth, continuous behavior, h proved to be a very small number, but different from zero. Wherever it appeared, strange phenomena came with it. What it really meant was of course mysterious. While the quantum era was inaugurated in 1900 , a quantum theory would take much longer to jell. Intro- ducing discontinuity was a tentative step, and only a first one. And even thereafter, the recasting of physical theory was hesitant and slow. Physicists pondered for years what a quantum theory might be. Wondering how to inte- grate it with the powerful apparatus of nineteenth-century physics, they also asked what relation it bore to existing, “classical” theories. For some the answers crystallized with quantum mechanics, the result of a quarter- century’s labor. Others held out for further rethinking. If the outcome was not to the satisfaction of all, still the quantum theory proved remarkably THE ORIGINS OF THE QUANTUM THEORY W
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BEAM LINE 7 successful, and the puzzlement along the way, despite its frustrations, can only be called extraordinarily productive. INTRODUCING h The story began inconspicuously enough on December 14 , 1900 . Max Planck was giving a talk to the German Physical Society on the continuous spec- trum of the frequencies of light emitted by an ideal heated body. Some two months earlier this 42 -year-old theorist had presented a formula capturing some new experimental results. Now, with leisure to think and more time at his disposal, he sought to provide a physical justification for his formula. Planck pictured a piece of matter, idealizing it somewhat, as equivalent to a collection of oscillating electric charges. He then imagined distributing its energy in discrete chunks proportional to the frequencies of oscillation. The constant of proportionality he chose to call h ; we would now write e = hf . The frequencies of oscillation determined the frequencies of the emitted light. A twisted chain of reasoning then reproduced Planck’s postulated formula, which now involved the same natural constant h . Two theorists, Niels Bohr and Max Planck, at the blackboard.
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