Robert Hofstadter

Robert Hofstadter

American experimental physicist, Nobel Prize in Physics, 1961
Date of Birth: 15.02.1915
Country: USA

Biography of Robert Hofstadter

Robert Hofstadter, an American experimental physicist, was born in New York. He was the third son of Louis Hofstadter, a merchant, and Henrietta Kenigsberg. He spent his childhood in New York, where he attended school and later enrolled in City College of New York, specializing in physics and mathematics. In 1935, Hofstadter graduated with honors and received the Kenyon Prize in physics and mathematics. Throughout his life, he remained grateful to one of his college professors who sparked his interest in the exact sciences, as his initial interests lay in philosophy. The Coffin Fellowship from General Electric allowed Hofstadter to attend Princeton University. In 1938, he obtained his master's and doctoral degrees in physics. With a Procter Fellowship, he spent the following year conducting research on the photoconductivity of crystals at Princeton University. In 1940, he became a physics instructor at the University of Pennsylvania, and in 1941, at City College of New York. During 1940-1941, as a fellow of the University of Pennsylvania, Hofstadter participated in the construction of the Van de Graaff generator. From 1942 to 1943, he worked at the National Bureau of Standards, contributing to the development of photoelectric remote fuses for anti-aircraft projectiles. From 1943 to 1946, he held a position as an assistant physicist at Norden Laboratories, which was established by the creator of the famous Norden bombsight. After the war, Hofstadter returned to academia and became an assistant professor at Princeton University in 1946. During this period, his research focused on crystals used as detectors of high-energy particles and radiation. In 1948, he developed a scintillation detector based on a crystal of sodium iodide "doped" with a small amount of thallium. When a high-energy atomic particle or photon (a particle of light energy) collides with such a crystal, a flash of light is emitted, the intensity of which is proportional to the energy of the particle or photon. By measuring the intensity of light, the experimenter can determine the energy of the particles. This effect forms the basis of a scintillation spectrometer, one of the primary means of measuring nuclear radiation. In 1950, Hofstadter was appointed as an adjunct professor of physics at Stanford University. Using the newly constructed electron accelerator at the High Energy Physics Laboratory of the university, he began studying the structure of the atomic nucleus. By that time, George P. Thomson, Clinton J. Davisson, and others had proved that electrons have a wave nature. It was already known that the wavelength of an electron decreases as its energy increases. The Stanford accelerator allowed electrons to be accelerated to energies ranging from 100 to 500 million electron-volts, corresponding to electron wavelengths smaller than the characteristic sizes of atomic nuclei. This meant that this accelerator could be used as a giant electron microscope to study the structure of the atomic nucleus. When an electron, accelerated by the accelerator, collides with a nucleus, it is deflected like a billiard ball. In some cases, the nucleus decays, emitting additional electrons and other particles. By studying the fragments of such collisions, Hofstadter hoped to gain insight into the structure of the nucleus. Hofstadter measured the deflection of electrons that did not produce any additional particles when colliding with the nucleus. To do this, he used two massive 250-ton magnetic spectrometers, which allowed him to sort electrons by energy and angle of deflection from their initial trajectory. With this equipment, Hofstadter managed to measure the size and determine the shape of many atomic nuclei. It turned out that all nuclei have approximately the same average density. The volume of a nucleus is proportional to the total number of protons and neutrons. This means that in large heavy nuclei, these particles are not packed more densely than in small light nuclei. The nearly constant density of nuclei was found to be equal to 150 million kg per m3. If a water droplet had such a density, it would weigh 2 million tons. Although Hofstadter discovered that the average density of all nuclei is approximately the same, his experiments showed that the atomic nucleus is not simply a sphere with a rigid shell. It has a soft "skin" with a thickness that is the same for all nuclei, regardless of their size, and is approximately 2.4 x 1013 cm. After the Stanford accelerator was reconstructed to accelerate electrons to an energy of 1 billion electron-volts, Hofstadter turned his attention to the internal structure of protons and neutrons, the particles that make up the atomic nucleus. In 1956-1957, he and his team determined the sizes and shapes of protons and neutrons. The researchers concluded that protons and neutrons are different forms of the same particle, which was named a nucleon. Although protons and neutrons have different electric charges (positive for protons and zero for neutrons), they behave identically in all processes related to the strong interaction that prevents atomic nuclei from decaying. Hofstadter's discovery revealed the inadequacy of the existing theory of the nucleus and prompted Yoshio Nambu from the University of Chicago to reconsider its most important concepts. It was believed that pions, particles with a mass approximately half that of a proton, were carriers of the interaction between nucleons. Nambu provided theoretical arguments for the existence of heavier and short-lived carriers of strong interaction. The particles he predicted were discovered in 1961. In 1961, Hofstadter was awarded the Nobel Prize in Physics for his fundamental research on electron scattering on atomic nuclei and his discoveries related to the structure of nucleons. The other laureate of the same year was Rudolf L. Mössbauer. Ivar Waller from the Royal Swedish Academy of Sciences, when presenting the new laureates, praised Hofstadter's work for its "unprecedented precision in high-energy physics." Waller noted that Hofstadter's results "stimulated the discovery of new particles essential for understanding the forces acting in atomic nuclei." Since 1971, Hofstadter has been a professor at Stanford University, where he continues his research in high-energy physics. In 1942, he married Nancy Givan, and they had three children. Their son Douglas gained recognition as an expert in artificial intelligence. According to colleagues, Hofstadter is a calm and quiet person. He enjoys listening to classical and jazz music, engaging in photography, reading, and skiing. Hofstadter is a member of the National Academy of Sciences of the United States, the Italian, American, and London Physical Societies. In 1959, he was named "Scientists of the Year" in California. In 1962, City College of New York minted a medal in his honor. He has received honorary degrees from City College, the University of Padua, and Carleton University.

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