Anderson, Carl David, Jr.

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Anderson, Carl David, Jr.

(b. 3 September 1905 in New York City; d. 11 January 1991 in San Marino, California), experimental physicist who, while conducting cosmic ray investigations in 1932, discovered the positron—the fundamental atomic particle having the same mass but opposite in electric charge to the negatively charged electron. For his discovery he won the Nobel Prize for physics in 1936.

Anderson was the only child of immigrants from Sweden who had arrived in the United States in 1895. His father, also named Carl David, and his mother, Emma Adolfina Ajaxson, were both from farming families in Sweden. When he was twelve years old, the family moved to Los Angeles, where his father managed small restaurants. Anderson attended Los Angeles Polytechnic High School where, supplementing his ambition in high jumping, he evinced his initial interest in science by joining the science club. He graduated in 1924. Because the limited financial resources of the family precluded attending a college out of town, Anderson commuted as an undergraduate to the California Institute of Technology (Caltech) in nearby Pasadena, from which he graduated with a B.S. degree in 1927. Anderson’s enrollment in Caltech, though initially dictated by fiscal restraints, turned out to be fortunate. The physics department became, under the stewardship of Robert A. Millikan, one of the leading institutions of physical scientific research in the United States, indeed in the world.

Anderson spent his entire career at Caltech. Beginning as a research fellow in 1930, he joined Caltech’s faculty as assistant professor of physics in 1933. In 1937 he was promoted to associate professor and in 1939 to full professor. Upon his retirement from active research and teaching, he was named professor emeritus in 1977, completing an uninterrupted association with Caltech of fifty-three years.

Under Millikan’s direction, Anderson’s graduate work emphasized experimental rather than theoretical physics. The eminent theoretician J. Robert Oppenheimer, who in late 1942 became the principal scientific director of the Manhattan Project (the American atomic bomb project), taught Anderson’s first course in quantum mechanics. Oppenheimer persuaded Anderson to complete successfully the course after other students had found it too formidable and dropped out.

Anderson’s doctoral thesis addressed the scattering of electrons from gaseous atoms bombarded by X rays. His Ph.D. degree was awarded in 1930 with the magna cum laude citation. After securing his doctorate, Anderson was asked by Millikan to remain and assist him in his study of the extraterrestrial ionizing radiation discovered and measured in 1911 and 1912 by the Austrian (later U.S.naturalized) physicist Victor F. Hess. This radiation was given the now universally employed designation “cosmic rays” by Millikan, and was a subject of intensive investigation by Millikan and his colleagues at Caltech, as well as scientific groups around the world. Anderson’s assignment was to build a cloud chamber especially suited for the detection of electrons in cosmic-ray measurements. The first cloud chamber had been built and developed by the Scottish physicist Charles Thomson Rees Wilson at Cambridge University from 1895 to 1900. Wilson received the Nobel Prize for his invention in 1927.

Essentially, the cloud chamber depends upon the formation of vapor trails similar to those seen behind aircraft flying at high altitudes with corresponding low atmospheric pressure. The essential apparatus consists of an artificial mist of water vapor in an evacuated chamber. A piston connected to the chamber is moved quickly outward, lowering the pressure and suddenly causing a mist cloud to form. When an ionizing particle traverses the cloud, droplets condense along electrically charged particles (or ions), forming a path that can be photographed.

Anderson and Millikan designed a cloud chamber positioned between the poles of a powerful electromagnet capable of deflecting the parts of the high-energy, electrically charged particles found in cosmic rays. Anderson ultimately made and studied more than 1,000 cosmic ray photographs. As the result of the action produced by the electromagnet, he observed deflections of cloud chamber paths in opposite directions, indicating that some were negative and some positive. It was evident that the negative paths were attributable to electrons. Because the only positively charged particle known in the early 1930s was the proton, which was more than 1,800 times heavier, Anderson initially assumed that this was what he had encountered. However, further experiments led him to the possibility that the positively charged particle had the same mass as the negative electron. For months this inference was strenuously resisted by Millikan, and it took courage for Anderson to challenge the judgment of his prestigious mentor in what he later characterized as “frequent and at times somewhat heated discussions.”

Finally, after inserting a lead plate across the center of the cloud chamber to reduce the charged particle’s high energy, on 2 August 1932 Anderson obtained an unmistakable photograph exhibiting both the positive charge and a mass equivalent to the negatively charged electron. This and subsequent photographs finally persuaded Millikan that Anderson had discovered a “positive electron.” Anderson wrote a letter announcing the discovery that appeared in Science on 9 September 1932. General skepticism initially confronted Anderson’s discovery. The great Danish physicist Niels Bohr (Nobel Prize, 1922) dismissed the possibility of a positive electron. In 1934, however, two physicists at Cambridge University, Patrick M. S. Blackett (Nobel Prize, 1948) and Giuseppe P. S. Occhialini, reported cloud chamber investigations of their own in which positron tracks were observed.

Interestingly, Anderson was totally unaware that the leading theoretician of quantum mechanics, Paul A. M. Dirac (Nobel Prize, 1933), had predicted the existence of a positive analog of the electron in 1928. As Anderson himself stated in a retrospective article in the December 1961 issue of the American Journal of Physics:“The discovery of the positron was wholly accidental … if a researcher had taken the Dirac theory at face value he could have discovered the positron in a single afternoon.”

In 1936 Anderson shared the Nobel Prize in physics for his discovery of the positron with Victor F. Hess, whose balloon flights of 1911 and 1912 had identified cosmic rays. Recognizing that Anderson was only thirty-one years old at the time of the award, the presenter made the following observation: “We congratulate you on this great success attained in your young years and we wish to express the hope that your further investigations will bring to science many near and equally important results.”

Anderson continued his cosmic ray studies at Caltech beyond his discovery of the positron. In 1937, while performing cloud chamber measurements at Pike’s Peak in Colorado, he and his former student and colleague Seth Neddemeyer made known the discovery of a “hereto unknown particle” with an average life of two-millionths of a second. In 1938 he announced the existence of the aforementioned—which he called the mesotron, later shortened to meson—a fundamental nuclear particle having either positive, negative, or no electric charge and intermediate in mass between the electron (or positron) and the heavier proton. The discovery of mesons and their role in fundamental nuclear particle interactions became another vital landmark in contemporary nuclear physics. With his meson discovery and other research achievements, Anderson certainly fulfilled the hope expressed by the Nobel Prize presenter just a year earlier.

In addition to his impressive research career, Anderson was an exceptional scientific administrator. Between 1962 and 1970 he was chairman of Caltech’s division of physics, mathematics, and astronomy. Anderson was one of those considered prior to Oppenheimer for the position of scientific director of the Manhattan Project. For reasons not known, he declined, though he did do military rocket research for the government during World War II.

Anderson married Lorraine Elvira Bergman on 30 June 1946. She died in 1984. They had two children. Anderson died at the age of eighty-six in San Marino. The cause of death was undisclosed.

With reference to his working-class beginnings, his mother was quoted at the time of his Nobel Prize win: “If he has special ability, I don’t know where he got it.” One journalistic appraisal characterized him as “shy and retiring” and another saw him as having “a very approachable manner [that] brands him as one of the more human scientists.” His discoveries of the positron and meson in cosmic rays certainly place Anderson in the pioneering forefront of modern physics, with its focus on nuclear structure, on the existence of antimatter, and on striving for the resolution of undecided cosmological questions.

Carl David Anderson, Jr., “Apparent Existence of Easily Deflectable Positives,” Science (9 Sept. 1932), published the month after Anderson’s discovery of 2 August 1932, is a letter written at the behest of Anderson’s mentor Richard A. Millikan after Millikan finally agreed that the observed track was indeed the positive light particle and not the heavier proton Millikan had conjectured. Anderson’s paper “The Positive Electron,” Physical Review vol. 43, (1933): 491, exhibited the historic photograph of the cloud chamber positron track mentioned above. Physics: Nobel Lectures, Including Presentation Speeches and Laureates’ Biographies 1922–1941 (1967) gives a transcript of the presentation speech by Professor H. Pleijel, which gives a coherent picture of the important connection between the scientific work of Anderson and the earlier cosmic ray research of Victor F. Hess. Robert A. Millikan, Autobiography of Robert A. Millikan (1950), gives a thorough account of the California Institute of Technology’s activities in physics research and thus the environment in which Anderson spent his entire scientific career. In Emilio Segrè, From X-Rays to Quarts: Modern Physicists and Their Discoveries (1980), “The Wonder Year 1932: Neutron, Positron, Deuterium and Other Discoveries,” is of special interest. Abraham Pais, Inward Bound: Of Matter and Forces in the Physical World (1986), treats Anderson’s positron and meson work in detail. Frank Close, Michael Marten, and Christie Sutton, The Particle Explosion (1987), has two of the earliest photographs of positron tracks as well as discussion of the contributions by other researchers in the field. Frank N. Magill, ed., The Nobel Prize Winners: Physics, vol. 1 (1989), has a good summary of Anderson’s life and scientific accomplishments. Yuval Ne’eman and Yoram Kirsh, The Particle Hunters (2d ed., 1989), is an excellent popular presentation of the impact of Anderson’s discovery on the development of modern physics; the 2 August 1932 cloud chamber photograph that appeared in the first positron paper by Anderson is presented here. Obituaries are in the New York (Times and Los Angeles Times (both 12 Jan. 1991).

Leonard R. Solon

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Anderson, Carl David, Jr.

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