Kenneth Ingvard Greisen (24 January 1918 in

Perth Amboy Perth Amboy is a city in Middlesex County, New Jersey. Perth Amboy is part of the New York metropolitan area. As of the 2020 U.S. census, the city's population was 55,436. Perth Amboy has a Hispanic majority population. In the 2010 census, th ...

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– 17 March 2007 in

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) was an American physicist who worked on

nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions, in addition to the study of other forms of nuclear matter. Nuclear physics should not be confused with atomic physics, which studies the ...

and the astrophysics of

cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...

s and gamma radiation. "He will be most remembered for his realization that the

cosmic microwave background In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all spac ...

limits the high-energy end of the spectrum of cosmic ray protons."

Career

In 1938 Greisen graduated with a B.S. from

Franklin & Marshall College Franklin & Marshall College (F&M) is a private liberal arts college in Lancaster, Pennsylvania. It employs 175 full-time faculty members and has a student body of approximately 2,400 full-time students. It was founded upon the merger of Fran ...

. In 1942 Greisen earned his PhD in physics at

Cornell University Cornell University is a private statutory land-grant research university based in Ithaca, New York. It is a member of the Ivy League. Founded in 1865 by Ezra Cornell and Andrew Dickson White, Cornell was founded with the intention to tea ...

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 The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons. It was led by the United States with the support of the United Kingdom and Canada. From 1942 to 1946, the project w ...

from 1943 to 1946 in Los Alamos, where he was a group leader. In 1945 he was an eyewitness to the

Trinity test Trinity was the code name of the first detonation of a nuclear weapon. It was conducted by the United States Army at 5:29 a.m. on July 16, 1945, as part of the Manhattan Project. The test was conducted in the Jornada del Muerto desert abo ...

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

- and

gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...

-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. In 1974 he was elected to the U.S. National Academy of Sciences.

Greisen's PhD thesis

Rossi and Greisen built an apparatus, centered around a Geiger-Müller counter, to distinguish the altitude dependence of hard (mesotron, ''i.e.'' muon) and soft (electron) cosmic ray secondaries. The goal of Rossi and Greisen was to confirm that the lifetimes of the muons depended upon their energy as predicted by the theory of special relativity. The data presented in Greisen's thesis was taken at altitudes of 249,1616, 3240, and 4300 meters at Echo Lake, Colorado. Greisen's thesis work was cited many times over the ensuing decades.

Trinity test eyewitness

"From 1943 to 1946 Greisen was a member of the group of physicists who worked on the Manhattan Project at Los Alamos. As one of the leaders of the denotation team, he was an observer at the Trinity test on 16 July 1945. His eyewitness report of that world-changing event is an important historical document. His comment "My God! It worked!" was typical of him."Trinity Test, July 16, 1945 - Eyewitness Accounts
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Works

* with Bruno Rossi: * (GZK-Cutoff)

References

External links

Archival collections

Kenneth I. Greisen course notes, 1959-1960, Niels Bohr Library & Archives
{{DEFAULTSORT:Greisen, Kenneth American astrophysicists 20th-century American physicists Cornell University alumni Cornell University faculty University of Utah faculty Manhattan Project people Members of the Norwegian Academy of Science and Letters 1918 births 2007 deaths Members of the United States National Academy of Sciences