The Discoveries: Great Breakthroughs in 20th-century Science, Including the Original Papers

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9780375421686: The Discoveries: Great Breakthroughs in 20th-century Science, Including the Original Papers

Book by Lightman Alan

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Chapter 1

The Quantum


In his famous autobiography The Education of Henry Adams, published only a few years into the twentieth century, the historian Henry Adams shouted alarm that the sacred atom had been split. Since the ancient Greeks, the atom had been the smallest particle of matter, the irreducible and indestructible element, the metaphor for unity and permanence in all things. Then, in 1897, the British physicist J. J. Thomson found electrons, particles far lighter and presumably smaller than atoms. The next year, Marie Sklodovska (Madame Curie) and her husband Pierre Curie discovered that the atoms of a new element, called radium, continuously hurled out tiny pieces of themselves, losing weight in the process. Now, nothing was permanent — nature no more than human civilizations. The solid had become fragile. Unity had given way to complexity. The indivisible had been divided.

As Adams was summing up the nineteenth century, he was evidently unaware of another scientific bombshell that had just exploded, ultimately as earthshaking and profound as the fracturing of the atom. On December 14, 1900, in a lecture to the stodgy German Physical Society in Berlin, Max Planck proposed the astounding idea of the quantum: energy does not exist as a continuous stream, which can be subdivided indefinitely into smaller and smaller amounts. Rather, he suggested, there is a smallest amount of energy that can be divided no further, an elemental drop of energy, called a quantum. Light is an example of energy. The seemingly smooth flood of light pouring through a window is, in reality, a pitter-patter of individual quanta, each far too tiny and weak to discern with the eye. Thus began quantum physics.

At the time of his lecture, Planck was bald from the middle of his head forward, with a sharp aquiline nose, a mustache, a pair of spectacles fastened to his face, and the overall look of a dull office clerk. He was forty-two years old, almost elderly for a theoretical physicist. Newton had been a youth in his early twenties when he worked out his law of gravity. Maxwell had polished off electromagnetic theory and retired to the country by age thirty-five. Einstein and Heisenberg would be in their mid-twenties when they erected their great monuments.

In 1900, Planck was already established as one of the leading theoretical physicists in Europe. Planck himself had helped legitimize the discipline. Fifteen years earlier, when he secured the rare position of professor of theoretical physics at the University of Kiel, theoretical science was considered an impotent profession, inferior to laboratory experiments. Few students clamored to hear Planck’s mathematical lectures. Then, in 1888, after his studies of heat — in which he clarified the Second Law of Thermodynamics and the concept of irreversibility — Planck was appointed professor at the University of Berlin. At the same time, he was made director of the new Institute for Theoretical Physics, founded mainly for him.

At the end of the nineteenth century, physics basked in the glow of extraordinary achievement. Newton’s precise laws of mechanics, which described how particles respond to forces, together with Newton’s law for gravity had been successfully applied to a large range of terrestrial and cosmic phenomena, from the bouncing of balls to the orbits of planets. The theory of heat, called thermodynamics, had reached its climax with the melancholy but deep Second Law of Thermodynamics: an isolated system moves inexorably and irreversibly to a state of greater disorder. Or, equivalently, every machine inevitably runs down. All electrical and magnetic phenomena had been unified by a single set of equations, called Maxwell’s equations after the Scottish physicist James Clerk Maxwell, who completed them. Among other things, these laws demonstrated that light, that most primary of natural phenomena, is an oscillating wave of electromagnetic energy, traveling through space at a speed of 186,282 miles per second. The new areas of physics known as statistical physics and kinetic theory had shown that the behavior of gases and fluids could be understood on the basis of collisions between large numbers of tiny objects, assumed to be the long-hypothesized but invisible atoms and molecules. In short, as Planck scribbled his equations at the dawn of the new century, physics might survey its vast kingdom and be pleased.

Some cracks, however, were starting to show in the marble facade. Aside from the philosophical dismay expressed by Mr. Adams, Thomson’s electron was clearly a new type of matter that demanded explanation and raised other questions about the innards of atoms. The “radioactive” disintegrations observed by the Curies involved the unleashing of huge quantities of energy. What was the nature of this energy and where did it come from? Other emissions of electromagnetic radiation from atoms, the so-called atomic spectra, exhibited surprising patterns and regularities but with no theoretical understanding. Equally perplexing were the repeating patterns in the properties of the chemical elements, a phenomenon that scientists suspected was caused by the structure of atoms.

Finally, physicists had observed that a unique kind of light, called black-body light or black-body radiation, emerged from all hot, blackened boxes held at constant temperature. (Set a kitchen oven at some temperature, leave the oven door closed for a long time, and black-body radiation will develop inside — although at any practical cooking temperature this light will be below the frequencies visible to the human eye.) It was already well known to scientists that all hot objects emit light — that is, electromagnetic radiation. In general, the nature of such light varies with the properties of the hot object. But if the radiating object is additionally enclosed within a box and held at constant temperature, its light assumes a special and unvarying form, the so-called black-body radiation.

A particularly mysterious aspect of black-body light was that its intensity and colors were completely independent of the size, shape, or composition of the container — as surprising as if human beings all over the world, upon being asked a question, uttered the same sentence in reply. A heated black box made of charcoal and shaped like a cigar produces precisely the same light as a black box made of dark tin and shaped like a beach ball, provided that the two boxes have the same temperature. The known laws of physics could not explain black-body light. Even worse, the standard working theories of light and of heat actually predicted that a blackened box held at constant temperature should create an infinite amount of luminous energy! It was the puzzle of black-body radiation that Max Karl Ernst Ludwig Planck had solved for his lecture of December 14, 1900.

A great deal was already known of the subject. With the use of colored filters and other devices, scientists had measured how much energy there was in each frequency range of black-body light. A colored filter allows light of only a narrow range of frequencies to pass through it. (The frequency of light is the number of oscillations per second. Each frequency of light corresponds to a particular color, just as each frequency of sound corresponds to a particular tone.) The amount of energy in a given frequency range of light is measured by a device called a photometer. Photometers gauge the intensity of light falling on a surface — a glass plate, for example — by comparing that light to another beam of light of known intensity. The comparison can be accomplished, for example, by the relative penetrating power of light through a liquid. More intense light beams have greater penetrating power. (Several decades into the twentieth century, light intensities could be measured more accurately by their electrical effects, with photoelectric detectors.)

The breakdown of a light source into the amount of energy in each range of frequency is called a light spectrum. When the light is black-body light, its spectrum is called a black-body spectrum. Figure 1.1 illustrates two black-body spectra, one for a temperature of 50 K and another for a temperature of 65 K. Here the K stands for Kelvin, the unit of temperature on the absolute temperature scale, which is a form of the Celsius scale with the zero point shifted. The coldest possible temperature lies at 0 K and -273 C.

A more familiar example of a spectrum is the graph that shows how many adults there are in each range of heights. Such a spectrum is usually a bell-shaped curve, with few people at very small heights and few people at very tall heights. As one would expect, the height spectrum varies from one country to the next, since human heights are determined by a large number of variables such as genetics and diet. So, it was remarkable when Planck’s predecessor to the Berlin chair, Gustav Kirchhoff, and others, discovered that the black-body spectrum does not vary at all with the details of the container. The black-body spectrum depends only on a single parameter, the temperature.

Planck was much impressed by the uniqueness and universality of the black-body spectrum, reasoning that such a universality must be the result of some fundamental new law of nature. A few weeks prior to his December lecture, the German physicist had in fact guessed a formula for the spectrum of black-body light. Planck’s formula was a mathematical expression for the amount of energy in each range of frequency of black-body light, and it agreed with all experimental measurements. Embracing the aesthetic criteria common to most physicists, Planck found pleasure in the simplicity of his formula, using the wor...

Présentation de l'éditeur :

An unprecedented explosion of creativity, insight, and breakthrough occurred in every field of science in the last century. From the theory of relativity to the first quantum model of the atom to the mapping of the structure of DNA, these discoveries profoundly changed the way we understand the world and our place in it. Now the physicist and novelist Alan Lightman tells the stories of two dozen of the most seminal discoveries.

In lucid and literary prose, Lightman paints the intellectual and emotional landscape of each discovery, portrays the personalities and human drama of the scientists involved, and explains the significance and impact of the work. He explores such questions as whether there were common patterns of research, whether the discoveries were accidental or intentional, and whether the scientists were aware of or oblivious to the significance of what they had found. Finally, Lightman gives an unprecedented and exhilarating guided tour through each of the original papers, which are included in the book. Here are Einstein and Bohr, McClintock and Pauling, Planck and Heisenberg, and many others in their own words, grappling with the nature of the world. Original in its scope and depth, The Discoveries offers an extraordinary exploration into the nature of scientific discoveries and the minds of the men and women who made them.

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