Earl Robert Callen, physicist, teacher, and advocate for human rights, died on 9 December 2002 at his home in Chevy Chase, Maryland, after a long struggle with cancer. His was a multifaceted career that included research work in government laboratories, research and teaching at the American University, and service as a representative of the US Office of Naval Research in Tokyo.
Earl was born on 28 August 1925 in Philadelphia, Pennsylvania, where he attended public elementary and secondary schools. Drafted into the US Army in 1943, he served in the Pacific theater until the end of hostilities. On his return to civilian life, he attended the University of Pennsylvania, where he majored in English and mathematics. He earned both his bachelor’s (1948) and master’s (1951) degrees at Penn.
Influenced by his older brother, Herbert, who was on Penn’s physics faculty, Earl decided to become a physicist. After a year of taking courses there, including Herb’s course in thermodynamics, Earl entered MIT as a graduate student. In his autobiography, he describes his entry there as a turning point in his life. His studies at Penn had not gone well. He had applied to, and was turned down by, three medical schools. But things changed for him at MIT: He went through the undergraduate physics textbooks and did all the problems in them—twice. As he put it, he was beginning to catch on. He became one of 15 research students working for a PhD under J. C. Slater. He earned his doctorate in 1954; his thesis dealt with the configuration interaction method applied to the hydrogen molecule.
After spending a year back in Philadelphia, where he worked at the Philco Research Laboratory, he went to the Washington, DC, area, where he joined the National Security Agency as a physicist, a position he held for five years. In 1959, he joined the Naval Ordnance Laboratory (NOL) to head the magnetism group. It was at that time that he began what would become a fruitful collaboration with his brother on the statistical physics of magnetic materials; in particular, they studied the temperature dependence of the anisotropy energy and the magnetostriction of ferromagnets. In two seminal papers (1963 and 1965), the Callen brothers developed a quantum-statistical theory of the temperature and field dependence of magnetostriction, forced magnetostriction, and thermal expansion of ferromagnets. They derived expressions for the strains in terms of thermal and quantum mechanical averages of angular momentum operator functions. They then showed that those averages could be written as products of functions of magnetization direction cosines times temperature-dependent momentum correlation functions.
The great advantage of their theory over the prior thermodynamic approach was that, now, the arbitrary coupling constants of the previous approach were replaced by averages that are easily expressed in terms of the magnetization. Using measured values of those magnetizations, one could predict the temperature dependence of the strains. There are no adjustable parameters. Earl and colleagues at NOL showed that the theory gave excellent agreement with the temperature dependence of the magnetostriction of yttrium iron garnet. Subsequent tests of the theory for several rare earth metals and compounds showed that the theory works remarkably well for almost all magnets made up of atoms having localized angular momentum.
While on sabbatical at Osaka University in 1965, Earl became interested in the electronic properties of magnetite. On his return to NOL, he introduced an itinerant-electron model of magnetite that stimulated a worldwide outpouring of experimental papers. He published both work that extended the theory of the coupling of magnetic atoms in metals and papers on the coupling of electromagnetic waves (helicons) to magnetic excitations in metals. Most of his work while he was at NOL was collaborative; he had a way of engaging his colleagues with his enthusiasm.
In 1969, Earl left NOL and became a professor of physics at the American University in Washington, DC. Besides teaching the standard physics courses, he created and taught a course on physics and society. In 1972, the American Physical Society organized the Forum on Physics and Society, and Earl became its first chairman. In that position, he saw to it that the APS held symposia on topics including how government censorship of scientific papers affected the physics community, the Soviet government’s mistreatment of their scientists wishing to emigrate, environmental concerns, and women and minorities in physics. He appeared before a congressional committee to testify against government censorship of scientific publications. His account of a trip he made to the Soviet Union in support of dissident scientists was published in the May 1974 issue of the Atlantic Monthly. In 1975, he traveled extensively in Asia, under the auspices of the Physics Interviewing Project, to interview prospective candidates for graduate study in physics in the US. He was one of several American physics professors who toured Asian campuses under the project, which was supported by a consortium of physics departments at several universities.
In 1987, he retired from the American University and returned to Japan as a scientific liaison for the US Office of Naval Research. In the three years he spent there, he published several reports on topics in magnetism, metallurgy, and other fields. He credited his easy access to Japanese research to the warm relations he had maintained with his old colleagues at Osaka and with younger ones who had come to work with him at the American University and NOL.
On returning to the US, Earl devoted his time to family, dancing, and tennis, despite his illness. Although he is sorely missed, his family and friends remember him with gratitude.
