Robert H. Dicke, who made fundamental and lasting contributions to radio astronomy, solar physics, gravitational physics, and cosmology, died in Princeton on 4 March 1997. He is survived by his wife, Annie, whom he married in 1942 and three children. Dicke held the Cyrus Fogg Brackett professorship of physics from 1957 to 1975 and the Albert Einstein professorship of science from 1975 to 1984 (emeritus 1984-97).
Though not consistently a member of the American Astronomical Society, he was awarded the Society's Beatrice M. Tinsley Prize in 1992 for his many contributions to our field. In particular, he played a central role in the transformation of cosmology from "a dream of zealots" to a science firmly based on observations (to quote William A. Fowler in his forward to Robertson and Noonan's 1968 book, Relativity and Cosmology).
Bob Dicke was born in St. Louis on 6 May 1916. He attended both Rochester and Princeton Universities, graduating from the latter in 1939. His doctorate was obtained two years later from Rochester, in nuclear physics, a field he was to leave almost immediately for defense-related work at the "Rad Lab" at MIT.
At the Radiation Laboratory, he worked primarily on the development of radar and on microwave techniques, inventing along the way the Dicke radiometer, in use in radio observatories around the world, and the magic tee, a staple of waveguide circuits. He also improved and made full use of the technique of phase sensitive detection (the "lock-in 'amplifier" as he called it).
During Dicke's years at the "Rad Lab" he posed the question, just how bright is the night sky at microwave frequencies? Using a microwave radiometer of his own design, and a room-temperature calibrator, he and his colleagues showed that the brightness at centimeter wavelengths is less than 20 K. There is a marvelous and by now well-known photograph of Bob, calibrator in hand, making the first measurement of the radio brightness of the sky from an MIT rooftop.
It has often been remarked that this upper limit was set just a few years before Alpher, Herman, and Gamow predicted that a hot Big Bang would leave a relic radiation field of approximately 5 K temperature. Many have wondered why Bob Dicke did not recall his measurement when the papers by Alpher, Follin, Gamow, and Herman appeared. I would suggest one simple answer. In 1946, Bob moved back to Princeton, a university and a town he and Annie came to love, and he changed his research to atomic physics. The microwave work of the war years was succeeded for ten very fruitful years by prize-winning research on quantum physics, superradiance (the overpopulation of an excited state that leads to maser or laser action), and the measurement of fundamental constants like the g-factor of the electron. Indeed, it is easy for those of us who are astronomers to forget how great an impact Bob Dicke had on atomic physics. His National Medal of Science (1971), Comstock Prize (1973), and Cresson Medal of the Franklin Institute (1974) were awarded primarily for work in physics, not astronomy or cosmology. Bob always thought of himself as a physicist, a designer of experiments, not an observer.
By the late 1950's, however, his interest was drawn to astronomical or cosmological questions by his work on Mach's principle and precision tests of gravity, among them his exquisitely sensitive repetition of the Eotvos experiment, which established that gravitational acceleration is independent of the elemental composition of the material accelerated. In this simple but beautiful experiment, carried out in a pit near Princeton's baseball field, he established this basis of the weak equivalence principle to an accuracy of a few parts in 1011. Other work with students and colleagues established the equivalence of passive and active gravitational mass. Both results constrained the nature of theories of gravity, including the one Bob and Carl Brans were working on at the same time. Bob realized that modifications to General Relativity could be tested by looking for anomalies in the motion of the moon or in the precession of the perihelon of the orbit of Mercury. The former led to a lunar ranging experiment left on the moon by Apollo 11 astronauts; the latter to attempts that occupied the last two decades of his active life at Princeton to measure the solar oblateness and hence to refine classical values for the perihelion precession.
But the work for which Dicke is best known, at least to astronomers, is the prediction and explanation of the cosmic microwave background. By the mid 1960's, Bob had become intrigued by the possibility of a cyclically expanding and contracting universe. Aware that stars convert hydrogen to helium and on to heavy elements, he asked why a cyclic universe would not be glutted by heavy elements. His answer was to posit a hot Big Crunch, which photo-dissociated the heavy elements, followed each cycle by a hot Big Bang. With characteristic thoroughness, he and his colleagues, especially Jim Peebles, set out to follow up the observational consequences of a hot Big Bang. And with his characteristic physicist's vision, he proposed to Peter Roll and David Wilkinson that they build a radiometer to search for the relic heat of the Big Bang at centimeter wavelengths.
While Bob and his group were making observations at 3 cm, the cosmic background was being independently discovered a few miles away at the Bell Telephone Laboratories by Arno Penzias and Robert W. Wilson. As soon as he heard of the Bell Labs result, Dicke was sure both that the relic heat of the Big Bang had been found and that the Princeton group had been scooped. Bob's interpretation of the measurements of Penzia and Wilson has been triumphantly confirmed over the past 30 years, and the cosmic microwave background is now a cornerstone of modem physical cosmology
In addition to the honors mentioned previously, Dicke received honorary DSc's from the University of Edinburgh, Rochester, Ohio Northern University, and Princeton. He was a member of the National Academy of Sciences and the American Academy of Arts and Sciences (and winner of their Rumford Premium Award in 1967) and served on a very large number of advisory panels and committees for NSF, NASA, NBS, and other organizations. A brief obituary appeared in Nature (386, 448, 1997) and a longer one should appear in the Biographical Memoirs of the National Academy of Sciences. Oral history material from 1983 and 1985 and responses to a questionnaire on the origins of lasers are on file at the Neils Bohr Library of the American Institute of Physics.
Despite Dicke's crucial role in the interpretation of the cosmic background, he published only a few papers on the topic. Instead, he continued his work on solar oblateness and gravity theory. He also gave kind, active, and insightful support to younger scientists in what came to be known as the Gravity Group at Princeton, in which Bob's students and colleagues mingled happily with those of John Wheeler. For a young assistant professor, as I was then, the years in the Gravity Group were an absolutely ideal experience. Bob could always be counted on for gentle and sensible advice, and he bubbled over with clever ideas from adaptive optics ("rubber mirrors" he called them) to radioactive dating to helium abundances in stars, all areas of interest today. I remember Bob Dicke best as an inspiring mentor. History will remember him as a major contributor to both twentienth century physics and modem cosmology.