Place: United States of America, Austria
Subject: biography, physics
Austrian-born US physicist who discovered cosmic rays, for which he was jointly awarded the 1936 Nobel Prize for Physics with Carl Anderson.
Hess was born in Waldstein, Austria, on 24 June 1883, the son of a forester. He was educated in Graz, at the Gymnasium then at the university, obtaining his doctorate from the latter in 1906. From 1906 to 1910 he worked at the Vienna Physical Institute as a member of F Exner's research group studying radioactivity and atmospheric ionization, then from 1910 to 1920 he was an assistant to S Meyer at the Institute of Radium Research of the Viennese Academy of Sciences. In 1920 Hess was appointed extraordinary professor of experimental physics at Graz University. From 1921 to 1923, however, he was on a two-year sabbatical in the USA, as director of the research laboratory of the US Radium Corporation in Orange, New Jersey, and also as a consultant to the Department of the Interior's Bureau of Mines. In 1925, two years after his return to Graz University, he became its professor of experimental physics, a post he held until 1931, when he was appointed professor of physics at Innsbruck University and director of its newly established Institute of Radiology; while at Innsbruck he founded a cosmic-ray observatory on the Hafelekar mountain, near Innsbruck. After the Nazi occupation of Austria in 1938 Hess - a Roman Catholic himself but with a Jewish wife - emigrated to the USA, where he was professor of physics at Fordham University, New York City, until his retirement in 1956 having become a naturalized US citizen in 1944. Hess died in Mount Vernon, New York, on 17 December 1964.
In the early 1900s it was found that gases in the atmosphere are always slightly ionized, even samples that had been enclosed in shielded containers. The theories to explain this phenomenon included radioactive contamination by the walls of the containers and the influence of gamma rays in the soil and air; these theories were later proved incorrect by Hess's findings. In 1910 Theodor Wulf, investigating atmospheric ionization using an electroscope on top of the Eiffel Tower, found that ionization at 300 m/1,000 ft above the ground was greater than at ground level, from which he concluded that the ionization was caused by extraterrestrial rays.
Continuing this line of research, Hess - with the help of the Austrian Academy of Sciences and the Austrian Aeroclub - made ten balloon ascents 1911-12 to collect data about atmospheric ionization. Ascending to altitudes of more than 5,000 m/16,000 ft, he established that the intensity of ionization decreased to a minimum at about 1,000 m/3,000 ft then increased steadily, being about four times more intense at 5,000 m/16,000 ft than at ground level. Moreover, by making ascents at night - and on one occasion, on 12 April 1912, during a nearly total solar eclipse - he proved that the ionization was not caused by the Sun. From his findings Hess concluded that radiation of great penetrating power enters the atmosphere from outer space. This discovery of cosmic rays (as this type of radiation was called by Robert Millikan in 1925) led to the study of elementary particles and paved the way for Carl Anderson's discovery of the positron in 1932.
Later in his career Hess investigated the biological effects of exposure to radiation (in 1934 he had a thumb amputated following an accident with radioactive material); the gamma radiation emitted by rocks; dust pollution of the atmosphere; and the refractive indices of liquid mixtures.
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