One Hundred Years of Quantum Physics

By Daniel Kleppner and Roman Jackiw.

An informed list of the most profound scientific developments of the 20th century is likely to include general relativity, quantum mechanics, big bang cosmology, the unraveling of the genetic code, evolutionary biology, and perhaps a few other topics of the reader's choice. Among these, quantum mechanics is unique because of its profoundly radical quality. Quantum mechanics forced physicists to reshape their ideas of reality, to rethink the nature of things at the deepest level, and to revise their concepts of position and speed, as well as their notions of cause and effect.

Although quantum mechanics was created to describe an abstract atomic world far removed from daily experience, its impact on our daily lives could hardly be greater. The spectacular advances in chemistry, biology, and medicine—and in essentially every other science—could not have occurred without the tools that quantum mechanics made possible. Without quantum mechanics there would be no global economy to speak of, because the electronics revolution that brought us the computer age is a child of quantum mechanics. So is the photonics revolution that brought us the Information Age. The creation of quantum physics has transformed our world, bringing with it all the benefits—and the risks—of a scientific revolution. Unlike general relativity, which grew out of a brilliant insight into the connection between gravity and geometry, or the deciphering of DNA, which unveiled a new world of biology, quantum mechanics did not spring from a single step. Rather, it was created in one of those rare concentrations of genius that occur from time to time in history. For 20 years after their introduction, quantum ideas were so confused that there was little basis for progress; then a small group of physicists created quantum mechanics in three tumultuous years. These scientists were troubled by what they were doing, and in some cases distressed by what they had done. The unique situation of this crucial yet elusive theory is perhaps best summarized by the following observation: Quantum theory is the most precisely tested and most successful theory in the history of science. Nevertheless, not only was quantum mechanics deeply disturbing to its founders, today—75 years after the theory was essentially cast in its current form—some of the luminaries of science remain dissatisfied with its foundations and its interpretation, even as they acknowledge its stunning power. This year marks the 100th anniversary of Max Planck's creation of the quantum concept. In his seminal paper on thermal radiation, Planck hypothesized that the total energy of a vibrating system cannot be changed continuously. Instead, the energy must jump from one value to another in discrete steps, or quanta, of energy. The idea of energy quanta was so radical that Planck let it lay fallow. Then Albert Einstein, in his wonder year of 1905, recognized the implications of quantization for light. Even then the concept was so bizarre that there was little basis for progress. Twenty more years and a fresh generation of physicists were required to create modern quantum theory.To understand the revolutionary impact of quantum physics one need only look at pre-quantum physics. From 1890 to 1900 physics journals were filled with papers on atomic spectra and essentially every other measurable property of matter, such as viscosity, elasticity, electrical and thermal conductivity, coefficients of expansion, indices of refraction, and thermo-elastic coefficients. Spurred by the energy of the Victorian work ethic and the development of ever more ingenious experimental methods, knowledge accumulated at a prodigious rate. What is most striking to the contemporary eye, however, is that the compendious descriptions of the properties of matter were essentially empirical. Thousands of pages of spectral data listed precise values for the wavelengths of the elements, but nobody knew why spectral lines occurred, much less what information they conveyed. Thermal and electrical conductivities were interpreted by suggestive models that fitted roughly half the facts. There were numerous empirical laws but they were not satisfying. For instance, the Dulong-Petit law established a simple relation between specific heat and the atomic weight of a material. Much of the time it worked; sometimes it didn't. The masses of equal volumes of gas were, for the most part, in the ratios of integers. The Periodic Table, which provided a key organizing principle for the flourishing science of chemistry, had absolutely no theoretical basis. Among the greatest achievements of the revolution is this: Quantum mechanics has provided a quantitative theory of matter. We now understand essentially every detail of atomic structure—the Periodic Table has a simple and natural explanation, and the vast arrays of spectral data fit into an elegant theoretical framework. Quantum theory permits the quantitative understanding of molecules, of solids and liquids, and of conductors and semiconductors. It explains bizarre phenomena such as superconductivity and superfluidity, and exotic forms of matter such as the stuff of neutron stars and Bose-Einstein condensates, in which all the atoms in a gas behave like a single superatom. Quantum mechanics provides essential tools for all the sciences and for every advanced technology. Quantum physics actually encompasses two entities. The first is the theory of matter at the atomic level: quantum mechanics. It is quantum mechanics that allows us to understand and manipulate the material world. The second is the quantum theory of fields. Quantum field theory plays a totally different role in science, to which we shall return later. Quantum Mechanics The clue that triggered the quantum revolution came not from studies of matter but from a problem in radiation. The specific challenge was to understand the spectrum of light emitted by hot bodies: blackbody radiation. The phenomenon is familiar to anyone who has stared at a fire. Hot matter glows, and the hotter it becomes the brighter it glows. The spectrum of the light is broad, with a peak that shifts from red to yellow and finally to blue (although we cannot see that) as the temperature is raised. It should have been possible to understand the shape of the spectrum by combining concepts from thermodynamics and electromagnetic theory, but all attempts failed. However, by assuming that the energies of the vibrating electrons that radiate the light are quantized, Planck obtained an expression that agreed beautifully with experiment. But as he recognized all too well, the theory was physically absurd, "an act of desperation," as he later described it.

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