Kenneth Greisen

Kenneth Ingvard Greisen (24 January 1918 in Perth Amboy, New Jersey – 17 March 2007 in Ithaca, New York) was an American physicist who worked on nuclear physics and the astrophysics of cosmic rays and gamma radiation. "He will be most remembered for his realization that the cosmic microwave background limits the high-energy end of the spectrum of cosmic ray protons."

In 1938 Greisen graduated with a B.S. from Franklin & Marshall College. In 1942 Greisen earned his PhD in physics at Cornell University under Bruno Rossi with thesis Intensity of cosmic rays at low altitude and the origin of the soft component. He worked on the Manhattan Project from 1943 to 1946 in Los Alamos, where he was a group leader. In 1945 he was an eyewitness to the Trinity test as a member of the detonation team. After his Los Alamos work, he returned to Cornell University in 1946 as an assistant professor of physics. From 1975 he was a professor of astronomy and from 1976 to 1979 chair of the astronomy department and from 1978 to 1983 dean of faculty. In 1986 he retired as professor emeritus. From 1975 to 1981 he was adjunct professor at the University of Utah.

Greisen did experiments on cosmic ray- and gamma ray-astronomy using high-altitude balloons. In 1971 he and his colleagues discovered pulsed gamma rays with energies greater than 200 MeV from the pulsar in the Crab Nebula. In 1966 he published the theory of the GZK cutoff, independently of the Russians Georgiy Zatsepin and Vadim Kuzmin who in 1966 also published their version of the same theory.

In 1966 he joined the AAS. In 1969 he was one of the founders of the Section for High Energy Astrophysics Division of the AAS and, in 1970 and 1971, was the first chair of this division. At Cornell University in 1969 he led a group of faculty members who modernized the university's physics curriculum.

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