Centennial: A Century of Physics
by Hans Christian von Baeyer
Rosalyn Sussman Yalow
By mid-century the new physics was beginning to spread out into a wide range of applications. Its scope extended vertically, as it were, from the unimaginably small interior of the nucleus up to the incomprehensibly vast stretches of the universe. At the same time, physics also had a powerful horizontal impact on other branches of science, often by way of novel instrumentation, and on technology.
For biology, physical methods brought about spectacular results. The discovery of the double helix of the DNA molecule, revealed by X-ray images of crystallized DNA, triggered a revolution in genetics. Henceforth the mechanisms of heredity could be understood in tangible, material terms, and eventually even manipulated. Medicine acquired an important technique when Rosalyn Sussman Yalow, an American nuclear physicist, invented a way to use radioactivity for detecting minute amounts of a huge variety of materials, ranging from nicotine to viruses, in the human body. Though its name twists the tongue, her method, called radioimmunoassay, relies on a simple principle. If you count six red-eyed fruit flies in a jar, and you know that the incidence of red eyes is one in a thousand, you conclude that there are 6000 flies in the jar - without the tedium of counting them. Radioimmunoassay counts molecules rather than flies, and tags them by radioactivity rather than eye color.
Chemistry gained a valuable diagnostic tool with nuclear magnetic resonance. Radar research had led to instruments that can identify nuclei by the way they absorb microwaves. For chemists, whose usual province is the electron cloud of the atom, this descent to the nucleus opened new opportunities. Nuclear magnetic resonance itself would later develop into magnetic resonance imaging, with a name chosen purposely to avoid the word nuclear. Geology also adopted an instrument based on 20th century physics. The SQUID (Superconducting Quantum Interference Device) which relies on a peculiar quantum effect discovered in 1962, can detect otherwise imperceptible changes in magnetic fields induced by the presence of mineral deposits.
In these and countless other ways physics began to stimulate research in its sister sciences. The most influential achievements of the 1950's, however, were the invention of the laser and the development of computers based on integrated circuits. Both of these devices, which are direct applications of quantum mechanics, would transform science and spawn entire industries devoted to new technologies.
Editor's Note: A CENTURY OF PHYSICS, a dramatic illustrated timeline wallchart of over a hundred entries on eleven large posters is intended for high schools and colleges. Each poster covers about a decade and is introduced by a thumbnail essay to provide a glimpse of the historical and scientific context of the time. A Century of Physics will be on display at the Atlanta Centennial Meeting in March.
In the November issue, APS News will feature the seventh introductory essay: 1965-1975: Closing the Circle.
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