John Gaston Fox (March 5, 1916 – July 24, 1980) was an American nuclear physicist. He earned his PhD from Princeton in 1941 and was soon recruited to work on the Manhattan Project. He later moved to Pittsburgh where he spent the rest of his career as a professor of physics at Carnegie Mellon University. He is best known for his work in the 1960s, applying the results of the extinction theorem to the then-current body of experimental evidence relating to both special relativity and emission theory. Brief biography Jack Fox, as he was always known, was born in Biggar, Saskatchewan. He moved with his mother to Victoria at age 13, and left high school two years early to attend Victoria College. He went on to the University of Saskatchewan for his MS and Princeton for his PhD, both in physics. He worked briefly in industry before going to the Manhattan Project at Los Alamos for the duration of World War II. In 1947, he married Constance Sullivan of Victoria; they moved to Pittsburgh, Pennsylvania where he had joined the Carnegie Institute of Technology (later Carnegie Mellon). They both became U.S. citizens in 1955, and raised three children in the suburb of Oakmont, Pennsylvania. Fox died in Pittsburgh in 1980. Special relativity and the extinction theorem The second postulate of Einstein's theory of special relativity states that the speed of light is invariant, regardless of the velocity of the source from which the light emanates. The extinction theorem (essentially) states that light passing through a transparent medium is simultaneously extinguished and re-emitted by the medium itself. This implies that information about the velocity of light from a moving source might be lost if the light passes through enough intervening transparent material before being measured. All measurements previous to the 1960s intending to verify the constancy of the speed of light from moving sources (primarily using moving mirrors, or extraterrestrial sources) were made only after the light had passed through such stationary material — that material being that of a glass lens, the terrestrial atmosphere, or even the incomplete vacuum of deep space. In 1961, Fox decided that there might not yet be any conclusive evidence for the second postulate: "This is a surprising situation in which to find ourselves half a century after the inception of special relativity." Regardless, he remained fully confident in special relativity, noting that this created