Dicke , Robert Henry

(1916–) American physicist

Dicke, who was born in St. Louis, Missouri, graduated in 1939 from Princeton University and obtained his PhD in 1941 from the University of Rochester. He spent the war at the radiation laboratory of the Massachusetts Institute of Technology, joining the Princeton faculty in 1946. In 1957 he was appointed professor of physics and served from 1975 to 1984 as Albert Einstein Professor of Science. In 1984 he was appointed Albert Einstein Emeritus Professor of Science.

In 1964, unaware that he was repeating a line of thought pursued earlier by George Gamow, Ralph Alpher , and RobertC. Herman in 1948, Dicke began to think about the consequences of a big-bang origin of the universe. Assuming a cataclysmic explosion some 18 billion years ago with a temperature one minute after of about 10 billion degrees, then intense radiation would have been produced in addition to particles of matter. As the universe expanded this radiation would gradually lose energy. Could there still be any trace left of this ‘primeval fireball’? It would in fact be detected as black-body radiation, characteristic of the temperature of the black body, which is a perfect emitter of radiation. At Dicke's instigation his colleague P.J.E. Peebles made the necessary calculations and concluded that the remnant radiation should now have a temperature of only about 10 K, later corrected to about 3 K, i.e. –270°C. At this temperature a black body should radiate a weak signal at microwave wavelengths from 0.05 millimeter to 50 centimeters with a peak at about 2 mm. Further, the signal should be constant throughout the entire universe.

Dicke began to organize a search for such radiation and had actually begun to install an antenna on his laboratory roof when he heard from Arno Penzias and Robert Wilson that they had detected background microwave radiation at a wavelength of 7 cm. It was this confluence of theory, calculation, and observation that really established the big-bang theory.

Another major area of study for Dicke is gravitation. In the 1960s he carried out a major evaluation of the experiment originally performed by Roland von Eötvös to confirm that the gravitational mass of an object is equal to its inertial mass. Dicke was able to establish the accuracy of the equivalence to one part in 10¹¹. This equivalence is basic to Einstein's theory of general relativity.

In 1961, following a suggestion of Paul Dirac in 1937, Dicke and Carl Brans proposed that the gravitational constant was not in fact a constant, but slowly decreases at a rate of one part in 10¹¹ per year. The resulting Brans–Dicke theory differs somewhat from Einstein's general relativity at a number of points. Thus while Einstein predicts that a ray of light should be deflected by the Sun's gravitational field 1.75 seconds ('') of arc, the Brans–Dicke theory leads to a figure of 1.62''; such a difference is within the range of observational error and so is not readily detectable. Again the perihelion of Mercury should advance for Einstein by 43'' per century, for Brans–Dicke a mere 39''. A value of 43'' has in fact been measured but Dicke maintained that part of this value, 4'', could be explained by the Sun's nonspherical shape. It has however been claimed that very precise measurements of radio pulses from pulsars appear to favor Einstein. The theory was concurrently and independently developed by Pascual Jordan, and is thus sometimes known as the Brans–Dicke–Jordan theory. The idea of a changing gravitational constant was put forward by Paul Dirac.