Robert H. Dicke

Robert H. Dicke (born May 6, 1916, St. Louis, Mo., U.S.—died March 4, 1997, Princeton, N.J.) was an American physicist noted for his theoretical work in cosmology and investigations centring on the general theory of relativity. He also made a number of significant contributions to radar technology and to the field of atomic physics.

Dicke received a bachelor’s degree from Princeton University (1939) and a doctorate from the University of Rochester (1941). In 1941 he became a staff scientist at the radiation laboratory of MIT. Dicke joined the Princeton faculty in 1946; in 1975 he was appointed Albert Einstein professor of science, becoming emeritus professor in 1984.

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During the early 1940s Dicke and other researchers at the Massachusetts Institute of Technology (MIT) played a key role in the development of microwave radar. He himself invented various microwave-circuit devices and radar systems, including mono-pulse radar and coherent pulse radar. In 1944 he developed a microwave radiometer that has become an integral component of most modern radio telescopes. For the next 10 years or so, Dicke devoted much attention to microwave atomic spectroscopy, conducting extensive research on fundamental radiation processes. His work led him to formulate what is often considered the first quantum theory of the emission of coherent radiation. (This type of radiation consists of electromagnetic waves, such as those in a beam of laser light, that are in phase).

By the 1960s Dicke had become actively interested in gravitation. He carried out a series of studies on the subject, the most notable of which was an experiment testing the principle of equivalence (i.e., that the gravitational mass of a body is equal to its inertial mass) that forms the cornerstone of Einstein’s concept of gravitation—the general theory of relativity. High-precision experiments with this objective had first been performed by the Hungarian physicist Roland von Eötvös, who confirmed the principle to an accuracy of one part in 10⁸. Dicke improved upon Eötvös’ accuracy by another factor of 1,000. Together with Carl Brans he investigated the idea of a changing gravitational constant, which had first been proposed in 1937 by Paul Dirac. Dicke and Brans developed a theory of gravitation in which, as a result of the expansion of the universe, the gravitational constant is not actually a constant but decreases at a rate of two parts in 10¹¹ per year.

In 1964 Dicke and several colleagues hypothesized that the entire universe is pervaded by a background radiation of microwave wavelengths—the remnant of the intense thermal radiation associated with the apparent explosive origin of the cosmos (see big-bang model). They were unaware that the existence of such residual radiation of the primordial fireball had been postulated some 16 years earlier by George Gamow, Ralph Alpher, and Robert Herman. Before Dicke attempted any observational work, Arno Penzias and Robert Wilson of Bell Telephone Laboratories discovered a faint glow of microwave radiation closely matching that predicted by theory.