Arnold John Frederick Siegert, 84, of Miami, Fla., professor emeritus of physics, died March 21  in Miami.
Mr. Siegert, who joined the Northwestern faculty in 1947, had a distinguished history in the field of physical science, including his work on the theory of condensation.
Shortly after he joined Northwestern, he was awarded a Guggenheim Fellowship for research at the Institute for Advanced Study at Princeton, N.J.
While there, he used the fellowship to pursue his investigations on statistical methods applied to problems of theoretical physics.
His principal research interests were statistical mechanics, theory of noise and other random phenomena.
Born in Dresden, Germany in 1911, Mr. Siegert received his doctorate in theoretical physics from the University of Leipzig. Prior to joining Northwestern, he worked in the physics department at Stanford University, did theoretical research in geophysics for fuel companies, was a staff physicist at the Radiation Laboratory at the Massachusetts Institute of Technology and was an associate professor of physics at Syracuse University.
He held a number of patents in the field of geophysical exploration.
He also had been a consultant to Los Alamos Scientific Laboratories, The Rand Corp., Pan American Petroleum and Lockheed Aircraft Co.
Survivors include his son, Allan A. of South Miami, Fla; a grandson; and a sister, Barbara Leonore Blass of Hadley, Mich. Interment was private.