Dicke, Robert Henry
Born Saint Louis, Missouri, USA, 6 May 1916
Died Princeton, New Jersey, USA, 4 March 1997
American experimental physicist Robert Dicke invented the microwave radiometer and lock-in amplifier that bear his name and that made possible the discovery of the cosmic microwave background radiation; he also carried out a number of experiments clarifying the properties of gravitation on terrestrial and astronomical scales.
Dicke was the son of a patent attorney. He grew up in Rochester, New York, where he began undergraduate studies at the University of Rochester, and got a transfer to Princeton University (where he published his first paper, modeling globular clusters as a gas of stars) to complete his bachelor’s degree in 1939. Dicke received a Ph.D. from the University of Rochester in 1941, for the work with Lee DuBridge in nuclear physics, and held honorary degrees from Edinburgh University, Rochester, Ohio, “Northern University,” and Princeton University.
Immediately upon receipt of his doctorate, Dicke joined the Radiation Laboratory (Rad Lab) at the Massachusetts Institute of Technology, working on radar. He invented the microwave radiometer (often called a Dicke radiometer) in order to measure atmospheric absorption of centimeter radio wavelengths. This absorption sets limits to pushing radar toward shorter wavelengths for better angular resolution. Dicke also applied it to make the first measurement of thermal emission from the Moon in that band and to set a limit of 20 K to radiation from “cosmic matter” at 1–1.5 cm. His invention of the lock-in amplifier, or Dicke switch, also dates from the Rad Lab period. These were the key technologies that later made possible the discovery of the cosmic relic radiation.
Returning to Princeton University in 1946 as assistant professor of physics, Dicke focused first on quantum aspects of the interaction between matter and radiation. His method of suppressing the Doppler broadening of spectral features is called Dicke narrowing and is important in atomic clocks and in the operation of the Global Positioning System. He recognized that lasers are best constructed with a pair of mirrors at the ends of an open tube (rather than with a closed cavity as for masers). His method of extracting more than thermal radiation from an inverted population of atomic levels is called Dicke superradiance. Dicke became the Cyrus Fogg Brackett Professor in 1957 and was named the first Albert Einstein University Professor of Science in 1975 (and Einstein Professor Emeritus from 1984 to his death). Among his students at Princeton University who have made important contributions to physics and astronomy are Robert Romer (past editor of the American Journal of Physics), James Wittke (coauthor with Dicke of a much-used textbook in quantum mechanics), Kenneth Libbrecht (who studies solar oscillations), and Jeffrey Kuhn.
From 1955 onward, Dicke’s interests turned gradually toward gravitation, astrophysics, and cosmology. Between 1956 and 1964, he set the tightest limit ever on possible violations of the principle of equivalence of gravitational and inertial mass (the Dicke-Eötvös experiment, named for him and his predecessor Lorand Eötvös). Dicke became increasingly concerned that general relativity [GR] did not explicitly include the ideas about the interaction between local gravity and the Universe as a whole, generally associated with the name of Ernst Mach. With Carl Brans, Dicke put forward a more complex theory of gravity that included both tensor (like GR) and scalar parts. Dicke realized that, within this scalar-tensor picture of gravity, the observed advance of the perihelion of Mercury would not be fully accounted for, and he suggested that the Sun might have a rapidly rotating core and a distorted shape that would account for the rest. There were, at the time, other astronomical reasons to favor interior rapid rotation. Dicke and several students developed a solar telescope to look for the distorted shape, and in the 1970s, it seemed as if they had found it. In fact, they had been fooled by observing near solar maximum, when there was a good deal of excess brightness near the solar equator due to the plages and faculae of active regions, as became clear when his former student, Henry Hill, repeated the observations from Arizona at solar minimum. Dicke, Libbrecht, and others eventually set a tight limit to real solar oblateness, which has since been confirmed by the solar oscillation studies. The scalar-tensor theory then went out of favor, but modern string theories of gravity are of the same general form.
Also, in about 1961, Dicke began to take a renewed interest in cosmology and to wonder whether one might detect radiation left from the stars of a previous cycle of an oscillating Universe that had been thermalized into microwaves during a “big crunch.” He and his associates James Peebles, Peter Roll, and David Wilkinson had just begun the search when it became clear that Arno Penzias and Robert Wilson, at Bell Telephone Laboratories, had accidentally found this leftover cosmic microwave background radiation while measuring the average brightness of the sky for purposes of satellite radio communication. The papers from the two groups were published together, but it was Penzias and Wilson who received the 1978 Nobel Prize in Physics for the discovery.
Other of Dicke’s contributions that have had long-term implications include the 1961 cofounding of Princeton Applied Research to develop and market the lock-in amplifiers, a prototype for university-commercial relationships, and discussions of why we should find ourselves in a Universe whose density is very close to the critical one needed to reverse the expansion. He made the point that it is essential for our existence as observers that the Universe should have an age (set by gravity) comparable with the lifetimes of stars (set by nuclear reactions and other independent parts of physics). This is now thought of as part of the cosmological anthropic principle.
Dicke was a member of the United States National Academy of Science, receiving the Comstock Medal in 1973. He also received the United States Medal of Science and awards from the National Aeronautics and Space Administration [NASA], the Franklin Institute, the Microwave Theory and Techniques Society, and the American Astronomical Society (the Beatrice M. Tinsley Award in 1992; the last one he was able to accept personally). In addition to his wartime work at the Rad Lab, Dicke also served the wider community on advisory panels to the National Science Foundation, the National Bureau of Standards, NASA, and the Fulbright Foundation and was a member of the National Science Board (1970–1976). He was a long-term member of the Lunar Laser Ranging team, using the corner reflectors emplaced by the Apollo astronauts to demonstrate that the evolution of the Earth-Moon system agrees with the predictions of gravitation theory. He married Annie Currie in 1942, and they had three children.