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Dicke, Robert Henry

DICKE, ROBERT HENRY

(b. St. Louis, Missouri, 6 May 1916;

d. Princeton, New Jersey, 4 March 1997), cosmology, gravity physics, quantum optics.

Dicke contributed to central developments in modern physics, including quantum optics, the precision tests of gravity physics, and the observational establishment of the big bang cosmology. Space is filled with a smooth sea of radiation that had relaxed to thermal equilibrium when our expanding universe was far denser and hotter than it is now. As part of war research in the 1940s, Dicke invented main parts of the technology for detection of this radiation. In 1964 he proposed the search that led to its identification. The precision measurements of this fossil are an essential basis for the richly developed evidence that the universe is evolving and that the evolution is well described by the physics of general relativity theory. Commencing in the 1950s Dicke pioneered the renaissance of interest in research in the physics of gravity. At that time scholars had an elegant theory, general relativity, but few tests and little research aimed at improving the situation. He led the program of experiments that now give a well-checked understanding of gravity in terms of space-time structure. His work on quantum optics in the 1940s and 1950s included the demonstration that collisions of radiating atoms with an inert “buffering” gas can suppress the Doppler effect—the tendency of motions of radiating bodies to shift the radiation to the blue or red—thus producing more sharply defined frequency standards. Dicke’s analysis of the quantum mechanics of radiation by a system of particles includes the prediction of rapidly emitting “superradiant” states. He held fifty patents, on subjects from clothes dryers to lasers. The company he cofounded, Princeton Applied Research, packaged his advances in phase-sensitive detection in the now-ubiquitous “lock-in amplifier.”

Great scientists can leave problems as well as solutions. Dicke passed on an example from the generation before him: Discover whether or how physics in the laboratory depends on the rest of the universe. At the end of the nineteenth century Ernst Mach argued for such a relation, that inertial motion is determined by the motion of all the matter in the universe. This was one of Albert Einstein’s guides to general relativity theory, and it led him to propose the basis for modern cosmology: The observable universe is close to homogeneous. (Imagine a thought experiment: In an island universe a particle could escape and move arbitrarily far into asymptotically flat space-time, where in general relativity theory the particle would be predicted to have all normal physical properties, including inertia, but no other matter nearby to give significance to inertial motion. Einstein disliked this possibility. And it is now known that the universe indeed has no observable edge).

General relativity theory predicts that the rotation of the Earth “drags” a locally nonrotating telescope relative to the distant stars, as Mach might have expected, but the theory predicts that measurements that are confined to a small laboratory are quite unaffected by external conditions. Dicke suspected that there is more than this, that the unity of physics suggests the behavior of the universe affects physics in the laboratory. He termed this concept Mach’s principle. (The name has been applied to many ideas; here it is taken to signify what Dicke had in mind.) Dicke’s searches for manifestations of Mach’s principle, in situations ranging from the laboratory to the expanding universe, yielded nothing convincing. But a century after Mach, superstring theory was again leading people to seek this aspect of unity that so fascinated Dicke.

The Radiation Laboratory and the Dicke Radiometer . In 1941, after completion of graduate work in physics at the University of Rochester, Dicke followed one of his professors, Lee Alvin DuBridge, to war research at the Radiation Laboratory at the Massachusetts Institute of Technology. DuBridge was director; Dicke worked on microwave radar (then meaning wavelengths in the range 30 cm to 3 mm), which offered better resolution than earlier generations of radar at longer wavelengths. The results had an important effect on the course of World War II and on advances in technology after the war. Dicke’s contributions, notable for his imaginative and effective way of doing physics, are summarized in the book Principles of Microwave Circuits, a standard reference for microwave engineering after the war.

Among Dicke’s inventions at the Radiation Laboratory is a radiometer capable of detecting microwave radiation produced by a warm body. He took the radiometer to Florida to demonstrate that humid air radiates strongly near 1-centimeter wavelength. The significance for war research was that the strong emission means humid air is a strong absorber, which at the time limited the push to shorter wavelength radar. In 1946 he with colleagues at the Radiation Laboratory published applications of his radiometer to astronomy, showing among other things that the amount of “radiation from cosmic matter” at wavelengths 1 to 1.5 centimeters is less than that equivalent to blackbody radiation at temperature 20 K (Dicke et al., 1946). This is notable for its relation to the theory published two years later by George Gamow, then at the George Washington University, on the physical conditions under which thermonuclear reactions in the early stages of expansion of an initially hot universe would produce an observationally interesting abundance of elements heavier than hydrogen. Gamow’s theory required that the early universe was filled with thermal radiation. The radiation would have cooled as the universe expanded, but would still be present. Gamow’s student, Ralph Asher Alpher, with Robert Herman, at the Applied Physics Laboratory at Johns Hopkins University, improved the calculation and translated Gamow’s condition into the present temperature, about 5 K. That is not much below the upper limit from Dicke’s radiometer, and it is close to what is now measured. Dicke made the connection between the theory and the measurement two decades later.

Quantum Optics . After the war Dicke returned to Princeton University, where he had spent two years as an undergraduate. He was appointed Cyrus Fogg Brackett Professor of Physics in 1957 and Albert Einstein Professor of Science in 1975. In his first decade back at Princeton Dicke put aside his interest in astronomy, working instead on quantum optics and techniques of precision measurements of atomic structure. His style is illustrated by his demonstration of what came to be called Dicke buffering (1953).

The Doppler shift of the frequency of a photon emitted by a moving particle can be associated with the change of kinetic energy of the particle due to the recoil from the momentum transferred to the photon. Dicke showed that if inert “buffering” particles confine the motions of the radiating particles to distances smaller than the photon wavelength, the Doppler effect is suppressed and the

momentum recoil is taken up by the system of buffering particles. Dicke and his student Robert Romer (PhD 1955) demonstrated the effect for ammonia molecules confined to a narrow container, and he and another student, James Pleister Wittke (PhD 1955), used the line-narrowing effect in a precision measurement of the frequency of the 21-centimeter radiation produced by atomic hydrogen. The same physics applies to the line-narrowing effect Rudolf Mössbauer discovered in 1958 in the emission and absorption of gamma ray photons by atomic nuclei. Lipkin, in Quantum Mechanics: New Approaches to Selected Topics, points out that the physics was already demonstrated in the 1930s, by Bragg scattering of neutrons by a crystal, where the recoil momentum again is taken up by the crystal rather than individual ions. The surprise greeting Dicke’s and Mössbauer’s results illustrates the difficulty of seeing subtle physics common to very different situations.

Gravity Physics . Dicke turned to the study of gravity while on sabbatical leave at Harvard from 1954 to 1955. He was struck by the scarcity of experimental work in this subject, and he set about improving the situation in an elegant series of experiments, beginning with a repetition of a fundamental measurement made a half century earlier by Roland von Eötvös in Hungary.

Eötvös and colleagues improved the test of the idea that objects with different compositions fall with the same gravitational acceleration. They showed that a broad variety of materials fall with the same acceleration to an accuracy of about three parts in 109. Dicke could do better with modern technology: the measurement with younger colleagues at Princeton improved the bound by two orders of magnitude. The advance was impressive, and it is an impressive measure of Eötvös’s skill that the improvement was not greater. The experiment added support to Einstein’s idea that an observer may “transform away” gravity by falling freely, which of course requires that all matter falls at the same rate. Dicke emphasized that the experiment limits the idea that Einstein’s theory might be adjusted to allow the physical properties of matter to depend on the nearby mass distribution, as might be suggested by Mach’s principle, for that could make the energy of a test particle vary with position, and the gradient of this energy would be a long range “fifth force” (adding to gravity and the strong, weak, and electromagnetic interactions). This fifth force could not significantly depend on composition, for that would violate the Eötvös-Dicke experiment.

Other examples show the range of Dicke’s ideas. His student James Brault (PhD 1962) made the first accurate measurement of the gravitational redshift of light from the Sun. It avoided the Doppler shifts produced by turbulence in the solar atmosphere by the choice of a spectral line that originates high in the solar atmosphere where turbulence is suppressed. The measurement was important because the gravitational redshift is one of the classical tests of general relativity, and at the time it was not well checked.

Another student, Lloyd Kreuzer (PhD 1966), tested the close equivalence of active and passive gravitational masses, which measure how strongly an object gravitationally attracts neighboring matter and how strongly the object is gravitationally attracted to neighboring matter. The measurement avoided gravitational influences outside the experiment by floating a test object in a tank of fluid with different composition but the same density, at neutral buoyancy. That means the passive gravitational mass densities are the same. If the active mass densities were significantly different, then moving the object through the fluid would have produced an observable change in the gravitational field.

Dicke led the idea of placing corner reflectors—that bounce light directly back—on the Moon, for use in precision measurements of its orbit. In 1969 the Apollo 2 astronauts placed the first array of reflectors. By 2005 Dicke’s concept had grown into an array of reflectors on the Moon and on artificial satellites, at distances measured by laser pulse timing to better than one-centimeter accuracy, for applications from gravity physics to continental drift and the Global Positioning System.

Dicke liked concepts as well as experiments. He was taken by the enormous value of the ratio of the electromagnetic and gravitational forces of attraction of the proton and electron in a hydrogen atom,

and by P. A. M. Dirac’s proposal (in 1937) that this might be because the strength of the gravitational interaction (in the denominator) is decreasing as the universe expands. Maybe gravity is weak now because the universe has been expanding for a long time. Dicke took this as a possible example of Mach’s principle, with the chance of a test: evolution of the strength of gravity might be detectable in the structures of stars and planets. For example, in the 1960s geologists were finding increasingly persuasive evidence for continental drift. Might continents be drifting because gravity is weakening, allowing the Earth to expand and rearrange its surface?

With a student at Princeton, Carl Brans (PhD 1961), Dicke developed a theory for the evolution of the strength of the gravitational interaction. It assumes a long-range scalar field in addition to the metric tensor of general relativity. They found that their approach was closely related to earlier work by Pascual Jordan at Hamburg University; it is best called the Jordan-Brans-Dicke (JBD) theory. Dicke was largely motivated by his concept of Mach’s principle; Jordan emphasized Dirac’s proposal. Jordan and Dicke corresponded, and met, but worked separately on tests of JBD. The theory predicts slightly smaller values than general relativity for the gravitational deflection of light by the Sun and the relativistic contribution to the precession of the perihelion of the orbit of Mercury.

When Brans and Dicke published their version of JBD in 1961, measurements of the solar deflection of starlight were quite inadequate for a test. That changed with advances in radio interferometric measurements of the angular positions of distant radio sources passing close to the Sun in the sky, which by 1975 showed that the deflection is within one percent of the general relativity prediction and inconsistent with JBD for parameters Dicke considered reasonable. In 1961 the measured rate of precession of Mercury’s orbit—after correction for the effects of the planets—was consistent with general relativity and larger than JBD by 3.3 times the probable error. Brans and Dicke suggested that that could be due to an error in the mass of Venus, but the Mariner 2 flyby a year later fixed the mass well enough to eliminate that possibility. Not being one to abandon an idea without all due checks, Dicke undertook his last great experiment: measure the shape of the Sun well enough to test for the effect an oblate interior mass distribution could have on the motion of Mercury. The work with several generations of colleagues commenced in 1963 and ended two decades later in the complexities of solar structure. JBD as conceived was ruled out. But interesting ideas are durable: in the years 2000 to 2004 there are some 150 references to the Brans-Dicke paper in the literature of physical science on subjects ranging from superstring cosmology to laboratory tests of gravity physics.

Cosmology . Dicke liked the idea of an expanding universe. He noted that Mach’s principle suggests that expansion could drive evolution of local physics, which would be a wonderful thing to discover. The idea has continued to drive research, though by 2005 there was no convincingly established evidence of evolution of the parameters of physics.

Dicke (1961) pointed out that an acceptable home for beings such as humans would have to have an age on the order of 10 billion years, roughly what is observed. It takes about that long for nuclear burning in several generations of stars to produce the heavy elements out of which we are made. Since we depend on the heat from a star, and the rate of formation of stars is now less than the stellar death rate, humans could not have lived in the universe when it grew much older. Dicke informally termed this a simple consistency condition. By the year 2005 some in the physical science community agreed, while others argued that Dicke’s consideration is part of an anthropic principle that accounts for the selection of our universe from an ensemble.

Dicke also liked to ask what the universe was doing before it was expanding. That led him to consider that if the universe were oscillating, then during the collapse phase starlight would be blueshifted, and a small fraction of the blueshifted starlight would serve to photodissociate the heavy elements formed in the production of the starlight by nuclear burning in stars, returning a fresh supply of hydrogen after the bounce. If the rest of the starlight survived the bounce it would be a thermal sea of radiation. That is, an oscillating universe could be irreversible in the sense that it produces entropy near the bounce, mainly in the form of a sea of blackbody radiation. If so the universe cannot have been bouncing forever, so Dicke’s question just changes to what the universe was doing before it was bouncing, but he was willing to accept the change as a modest advance. In 1964 Dicke persuaded two young members of his group, Peter Roll and David Wilkinson, to build a Dicke radiometer to look for the radiation, and he suggested that Jim Peebles think about the theoretical consequences of the detection or nondetection of the radiation. That had a lasting effect on all three, though most particularly Wilkinson and Peebles, who devoted much of their careers to measurements of the radiation and analyses of the significance of its properties. By 2005 these programs had grown into a rich physical science.

The story of the discovery and validation of the interpretation of the radiation has been told elsewhere and may be briefly summarized here. In 1965 news of the Roll-Wilkinson experiment in progress reached Arno Allan Penzias and Robert Woodrow Wilson at the Bell Laboratories in Holmdel, New Jersey, not far from Princeton. They saw that Dicke’s idea might solve a problem they encountered in a search for diffuse microwave radiation from the halo of our Milky Way galaxy. They used an instrument that was built for satellite communications experiments. It had capabilities similar to the Princeton experiment, and it indicated more noise than expected from sources within the instrument. Their improvements to the receiver did not remove the excess noise. They knew that the noise was not likely to be from the galaxy because the noise was isotropic, while the solar system is near the edge of its galaxy of stars. Penzias and Wilson did the right thing—they built well and took great care in exploring possible instrumental explanations of an unexpected result—and were awarded the Nobel Prize in 1978 for the detection of this radiation.

This fossil radiation is distinguished by its thermal— blackbody—spectrum. Radiation emitted in the universe as it is now cannot relax to this state because space is optically thin, which scientists know because objects at cosmologically large distances are observed at microwave wavelengths. The fossil radiation, which would have relaxed to a thermal spectrum in the dense, optically thick, early universe, is perturbed at recent times by out of-equilibrium matter, but the effect is slight because the heat capacity of the radiation is much larger than that of the matter. The Penzias and Wilson measurement at 7-centimeter wavelength fixed the temperature of the radiation, if thermal, within the uncertainty of their measurement. The Roll and Wilkinson experiment showed that the intensity at 3.2 centimeters is consistent with a thermal spectrum. Wilkinson led many of the experiments that by 1970 showed consistency with the long wavelength Rayleigh-Jeans part of the spectrum and the 1990 satellite measurement that demonstrated close agreement with the full blackbody form. Herb Gush led an experiment at the University of British Columbia that, also in 1990, independently demonstrated this beautiful result: space is filled with thermal radiation left from the early stages of expansion of the universe.

Why did the discovery of this fossil radiation take so long? One may also ask, why were people surprised by the physics of Bragg scattering when applied to Dicke’s microwave photons or Mössbauer’s gamma ray photons? It is because scientists tend to narrow the field of view to see more deeply. The discovery of the fossil radiation certainly was aided by Dicke’s knowledge of microwave physics—he had invented key elements—and of cosmology—he had given considerable thought to its possible relation to the rest of physics. It also helped that Dicke liked fresh ideas, even if speculative, provided they appealed to his sense of physics and they suggested experimental tests. In science one cannot spend all one’s time exploring the roads less traveled: In the chaos how could scientists marshal the tight webs of evidence that show the true roads? But they can follow Dicke by pausing on occasion to consider what they are doing.

BIBLIOGRAPHY

No complete compilation of Dicke’s papers exists. The Theoretical Significance of Experimental Relativity (1964) includes the papers on gravity physics that Dicke considered most important. A partially comprehensive bibliography of his publications is available from the Smithsonian/NASA Astrophysics Data System Database, available online from http://adsabs.harvard.edu/abstract_service.html.

WORKS BY DICKE

With Carol G. Montgomery and Edward M. Purcell, eds. Principles of Microwave Circuits. New York: McGraw-Hill, 1948. A standard reference for microwave engineering after World War II.

With R. Beringer, R. L. Kyhl, and A. B. Vane. “Atmospheric Absorption Measurements with a Microwave Radiometer.” Physical Review, series 2, 70 (1946): 340–348. The demonstration that “there is very little (<20 K) radiation from cosmic matter” at microwave wavelengths.

“The Effect of Collisions upon the Doppler Width of Spectral Lines.” Physical Review, series 2, 89 (1953): 472–473. Dicke buffering.

“Coherence in Spontaneous Radiation Processes.” Physical Review, series 2, 93 (1954): 99-100. Dicke superradiance.

With R. H. Homer. “New Technique for High-Resolution Microwave Spectroscopy.” Physical Review, series 2, 99 (1955): 532–536. The first experimental demonstration of Dicke buffering.

With James P. Wittke. “Redetermination of the Hyperfine Splitting in the Ground State of Atomic Hydrogen.” Physical Review, series 2, 103 (1956): 620–631. The first application of Dicke buffering in a precision measurement.

With W. F. Hoffmann and R. Krotkov. “Precision Optical Tracking of Artificial Satellites.” I.R.E. Transactions on Military Electronics 4 (1960) 28–37. Explores the possibility of testing gravity physics by tracking earth satellites.

“Dirac’s Cosmology and Mach’s Principle.” Nature 192 (4 November 1961): 440–441.

With P. G. Roll and R. Krotkov. “The Equivalence of Inertial and Passive Gravitational Mass.” Annals of Physics26 (1964): 442–517. Dicke’s version of the classic Eotvos experiment.

The Theoretical Significance of Experimental Relativity. New York: Gordon and Breach, 1964. In this work and the next, Dicke summarizes his assessments of gravity physics and cosmology.

Gravitation and the Universe. Memoirs of the American Philosophical Society, vol. 78. Philadelphia: American Philosophical Society, 1970.

With J. G. Williams, P. L. Bender, C. O. Alley, et al. “New Test of the Equivalence Principle from Lunar Laser Ranging.” Physical Review Letters 36 (1976): 551–554. An example of the continuing tests of gravity physics.

OTHER SOURCES

Happer, W., and P. J. E. Peebles. “Robert Henry Dicke: 6 May 1916–4 March 1997.” Biographical Memoirs. Proceedings of the American Philosophical Society 150, no. 1 (March 2006): 181–186.

———, P. J. E. Peebles, and D. T. Wilkinson. “Robert Henry Dicke: May 6, 1916—March 4, 1997.” In Biographical Memoirs, vol. 77. Washington, DC: National Academy of Sciences, 1999. Also available from www.nap.edu/html/biomems/

Kragh, Helge. Cosmology and Controversy. Princeton, NJ: Princeton University Press, 1996. A broad view of the history.

Lipkin, Harry J. Quantum Mechanics: New Approaches to Selected Topics. New York, American Elsevier Pub. Co., 1973.

Wilkinson, D. T., and P. J. E. Peebles. “Discovery of the Cosmic Microwave Background.”Physica Scripta T85 (2000): 136–141. The history of the discovery of the thermal cosmic background radiation, from the Princeton viewpoint.

P. J. E. Peebles

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