Martin Sir Ryle
(b. Brighton, Sussex, United Kingdom, 27 September 1918; d. Cambridge, Cambridgeshire, United Kingdom, 14 October 1984), radio astronomy, aperture synthesis, evolutionary cosmology, radio galaxies, and quasars.
Ryle was one of the most important pioneers of radio astronomy. His major contributions concerned the practical implementation of the principles of aperture synthesis, resulting in enormous increases in angular resolution and sensitivity for radio astronomical observations. These revealed the detailed radio structures of galactic and extra-galactic radio sources for the first time and established the strong evolution of the source population with cosmic epoch. He was awarded the Nobel Prize in Physics, jointly with Antony Hewish, in 1974.
Prewar and War Years Ryle was the second son and second child in a family of three sons and two daughters of John Alfred Ryle, a professor of medicine, and his wife, Miriam Power, the daughter of William Charles Scully, a civil servant who came from a landowning family in county Tipperary in Ireland. His father, a medical doctor, was to become Regius Professor of Physics at Cambridge University, and after World War II, the first professor of social medicine at Oxford University. His uncle, Gilbert Ryle, was a professor of philosophy at Oxford.
Martin Ryle’s early education was entrusted to a governess who taught him and his siblings at the family home on Wimpole Street, London. He then attended Gladstone’s preparatory school in Eaton Square, London, before entering Bradfield College, Berkshire, at age thirteen. His ability with his hands was fostered at home, where regular instruction was provided to him and his elder brother by a professional carpenter. In 1936 he entered Christ Church, Oxford, where he obtained first class honors in physics in 1939. His enthusiasm for radio engineering and electronics was already apparent. By the time he left school, he had built his own radio transmitter and acquired a post office license to operate it. At Oxford, he and E. Cooke-Yarborough, a fellow undergraduate, set up the university amateur radio station.
In 1939, Ryle joined J. A. Ratcliffe’s ionospheric research group at the Cavendish Laboratory at Cambridge University. On the outbreak of World War II, Ratcliffe joined the Air Ministry Research Establishment, later to become the Telecommunications Research Establishment (TRE), and Ryle followed in May 1940. For the first two years, he worked mainly on the design of antennae and test equipment. In 1942, he became the leader of Group 5 of the newly formed Radio Countermeasures Division, whose task was to provide jamming transmitters against the German radar defense system and to devise radio-deception operations. Among the latter was the electronic “spoof invasion” on D-day, which led the German High Command to believe that the invasion was to take place across the Straits of Dover, rather than in Normandy.
Radar techniques developed at an astounding pace during these years, and Ryle and his colleagues worked in a frantic atmosphere, constantly having to find immediate practical solutions for the electronic defense of the RAF’s bomber fleet. Bernard Lovell remarked, “As head of this group, Ryle’s extraordinary inventiveness and scientific understanding soon became evident. Under the stress of urgent operational requirements he became intolerant of those who were not blessed with his immediate insight” (p. 359).
But Ryle also learned how to motivate groups of research workers. In a letter to the present writer, written only two months before his death, he wrote,
Presumably I knew some physics in 1939—but this evaporated during the six succeeding years— though it was replaced by other things. But six years of designing/installing/flying boxes of electronics gave one “state of the art” electronics, a fair intimacy with aircraft and the ability to talk constructively with Air Vice-Marshals—or radar mechanics—and above all gave one the privilege of flying with the in-between-operational-tours aircrew who flew our aircraft.
According to Francis Graham-Smith, perhaps the greatest achievement of his war years was the discovery of a vulnerable element in the V-2 rocket radio guidance system. The system developed by Ryle and his old college
friend Cooke-Yarborough successfully disrupted the accurate aim of the V-2 rockets and probably contributed to the abandonment of radio control only a few weeks later.
Development of Radio Astronomy After the war, Ryle returned to Cambridge on an Imperial Chemical Industries fellowship and soon turned his energies to understanding the nature of the radio emissions from cosmic sources that had interfered with antiaircraft radars. J. S. Hey had found that the jamming was caused by intense radio outbursts from the Sun, apparently associated with large solar flares and sunspot groups. The angular resolving power of the radio antennae available at that time was not sufficient to resolve the disk of the Sun, let alone locate the origin of the radio emission on it. Ryle and D.D. Vonberg adapted surplus radar equipment and developed new receiver techniques for meter wavelengths to create a radio interferometer, the antennae being separated by several hundreds of meters in order to provide high enough angular resolution. Only later was it realized that they had reinvented the radio equivalent of the Michelson interferometer. A massive sunspot occurred in July 1946, and their observations showed conclusively that the radio emission originated from a region on the surface of the Sun similar in size to that of the sunspot region.
In addition to the emission from the Sun, Hey had discovered a discrete source of radio emission in the constellation of Cygnus and Ryle and Francis Graham-Smith, later Sir Francis Graham-Smith, adapted the solar interferometer to observe the radio source, which became known as Cygnus A. In 1947, the source was successfully observed and, in addition, another even more intense discrete source was found in the nearby constellation of Cassiopeia. By 1950, Graham-Smith had measured very precisely the position of Cygnus A, and it was found that the source was associated with a distant, massive galaxy.
As the fledgling radio group began to take shape, Ryle married Ella Rowena Palmer, the youngest sister of Graham-Smith’s wife, Elizabeth, in 1947. It was a wonderfully happy marriage and they had three children, Alison, John, and Claire. Somewhat to his surprise, midway through his fellowship, Ryle was appointed a university lecturer at Cambridge in 1948. In 1952, he was elected a Fellow of the Royal Society.
Although trained as a physicist, Ryle was primarily an electrical engineer with a genius for making complex radio receiving systems operate. His contribution of genius was the practical development of the concept of aperture synthesis, the technique by which images of radio sources are created by combining interferometric observations made with modest-sized radio telescopes located at different interferometer spacings. The basic principles of interferometry had been understood since the time of Albert Abraham Michelson and interferometry was used successfully to study the radio emission of the Sun and the very brightest radio sources. Ryle’s more ambitious program was to determine both the amplitudes and phases of the incoming radio signals so that, by Fourier inversion, the detailed brightness distribution of the radio emission could be reconstructed. Although understood in principle, the key technical issues concerned whether or not these concepts could be realized in practice, given the problems of receiver sensitivity, the need to preserve phase coherence over long periods, and the stability of overall system performance.
Over the twenty-five-year period starting in 1950, Ryle and his colleagues developed a series of radio interferometers of increasing complexity and ingenuity that made it possible to carry out surveys of the sky and unravel the structures and nature of the radio sources. This program involved a great deal of innovative electronics, such as the phase-switching interferometer, which enabled the full amplitude and phase information to be recovered. The practical development of aperture synthesis was a virtuoso technical achievement that involved a considerable team of researchers and support staff. Ryle put together a tightly knit group of physicists, including Francis Graham-Smith, Antony Hewish, John Baldwin, John Shakeshaft, Bruce Elsmore, Paul Scott, and Sidney Kenderdine, as well as a strong support team of technicians and research students, many of whom were later to become leaders in radio astrophysics. Those of us who were members of the radio astronomy group remember Ryle’s inspiring leadership and total involvement in all aspects of our work.
Radio Source Number Counts Throughout the 1950s and 1960s, one of Ryle’s major objectives was to produce reliable catalogs of all the bright radio sources in the northern sky. In “Radio Astronomy,” his review of the new science of radio astronomy published in 1950, he expressed his belief that most of the “radio stars” belonged to our own galaxy. As early as 1949, however, the Australian astronomers John Bolton, Gordon Stanley, and Bruce Slee had shown that two of the brightest sources were associated with nearby galaxies with strange optical features, the galaxies M87 and NGC 5128. The first Cambridge survey of radio sources, published in 1950 by Ryle, Graham-Smith, and Bruce Elsmore, showed that the distribution of radio stars was remarkably uniform over the sky.
Ryle and Antony Hewish designed and constructed a large, four-element interferometer on the rifle range site just behind Ryle’s house in Herschel Road, Cambridge, to carry out a new survey of the sky at 81.5 MHz, which was also an interferometer and therefore sensitive to small angular diameter sources. The second Cambridge (2C) survey of radio sources was completed in 1954 and the first results published in the following year. Ryle and his colleagues found that the small-diameter radio sources were uniformly distributed over the sky and that the numbers of sources increased enormously as the survey extended to fainter and fainter flux densities. In any uniform Euclidean model, the numbers of sources brighter than a given limiting flux density S are expected to follow the relation In contrast, Ryle found a huge excess of faint radio sources, the slope of the source counts between 20 and 60 Jy being described by3(The Jy, or Jansky, is a unit of radio flux density used in radio astronomy, named in honor of Carl Jansky, who discovered the cosmic radio emission in 1933. In SI units, 1 Jy = 1 Jansky = 10-26 W m-2 Hz-1.) He concluded that the only reasonable interpretation of these data was that the sources were extragalactic; that they were objects similar in radio luminosity to the radio galaxy Cygnus A; and that the radio sources were very much more numerous at large distances, and hence at earlier cosmological epochs, than there are nearby. As Ryle announced in his Halley Lecture in Oxford in 1955, “This is a most remarkable and important result, but if we accept the conclusion that most of the radio stars are external to the Galaxy, and this conclusion seems hard to avoid, then there seems no way in which the observations can be explained in terms of a Steady-State theory [of the universe]” (“Radio Stars and Their Cosmological Significance,” p. 146).
These remarkable conclusions came as a surprise to the astronomical community. There was enthusiasm but also some skepticism that such profound conclusions could be drawn from the counts of radio sources, particularly when their physical nature was not understood and only the brightest twenty or so objects had been associated with relatively nearby galaxies.
The members of the Sydney (Australia) group led by Bernard Mills were carrying out radio surveys of the southern sky at about the same time with the Mills Cross telescope, which was sensitive to sources of large angular sizes; they found that the source counts could be represented by the relation which they argued was not significantly different from the expectation of uniform world models. In 1957 Mills and Bruce Slee stated: “We therefore conclude that discrepancies, in the main, reflect errors in the Cambridge catalogue, and accordingly deductions of cosmological interest derived from its analysis are without foundation. An analysis of our results shows that there is no clear evidence for any effect of cosmological importance in the source counts”
The problem with the Cambridge number counts was that they extended to number densities of radio sources such that the flux densities of the faintest sources were overestimated because of the presence of faint sources in the beam of the telescope, a phenomenon known as “confusion.” Peter Scheuer, who was Martin Ryle’s research student from 1951 to 1954, devised a statistical procedure for deriving the number counts of sources from the survey records themselves without the need to identify individual sources. The technique that he discovered, which he referred to as the P(D) technique and which has since been adopted in many other astronomical contexts, showed that the slope of the source counts was actually –1.8. Ironically, this result, which is exactly the correct answer, was not trusted, partly because the mathematical techniques used by Scheuer were somewhat forbidding and also because his result differed from the preconceived opinions of both Ryle and Mills. The dispute reached its climax at the Paris Symposium on Radio Astronomy in 1958, and the conflicting positions were not resolved at that meeting.
These events had positive and negative impacts upon the work of the Cambridge radio astronomy group. The negative side was that the group became more defensive in its interaction with outside groups. Great care was taken to ensure the reliability and completeness of subsequent catalogs and radio maps and it was some considerable time before they were released to the astronomical community. The positive side was that it was apparent to Cambridge University and the community at large that new astrophysical and cosmological opportunities had been opened up. In 1956, the radio observatory moved to a disused wartime Air Ministry bomb store at Lord’s Bridge outside Cambridge, and in acknowledgment of a grant of £100,000 from the electronics company Mullard Ltd., the new observatory was opened in 1957 at Lord’s Bridge as the Mullard Radio Astronomy Observatory.
The resolution of the controversy over the number counts of sources came with the construction of the next generation of radio telescopes, which had higher angular resolution and hence were less sensitive to the effects of source confusion. In the revised third Cambridge catalog (3CR) and the 4C catalog, care was taken to understand the effects of confusion, and the accuracy of the radio source positions was improved so that more radio sources could be identified with distant galaxies. A major breakthrough resulting from these studies was the discovery of quasars in the early 1960s through Cyril Hazard’s radio occultation observations of the source 3C 273 with the Parkes radio telescope in Australia and the subsequent
measurement of the large redshift of the starlike object by Maarten Schmidt in 1963. Within a couple of years, over forty quasars were discovered among the 3CR sources.
The radio source counts derived from the 4C survey were continually refined in the period from 1961 to 1966 and showed a significant excess over the expectations of Euclidean world models, the source count slope being found to be –1.8. Ryle’s conclusions of 1955 were basically correct, but the magnitude of the excess had been overestimated. By the mid-1960s, there was compelling evidence that there is indeed an excess of sources at large redshifts and that they corresponded to large evolutionary changes with cosmic epoch.
Earth Rotation Aperture Synthesis In 1959 Ryle was appointed professor of radio astronomy in the Cavendish Laboratory in the Department of Physics of Cambridge University. In the early 1960s, his most ambitious experiment to date was underway: the use of the rotation of Earth to carry telescopes at fixed points on Earth about each other as observed from a point on the celestial sphere. The germ of this idea had already appeared in his notebooks in 1954. By 1959, digital computers were fast enough to cope with the demands of this form of synthesis mapping and, in a classic set of observations, Ryle and Ann C. Neville created an Earth-rotation aperture synthesis map of a region of sky about the north celestial pole. The angular resolution of the survey was 4.5 arcmin and the sensitivity eight times greater than that of the original antenna system.
The success of this project pointed the way to the future. The succeeding generations of aperture synthesis arrays employed fully steerable antennae—as with the One-Mile Telescope completed in 1965 and the 5-kilometer telescope, later known as the Ryle Telescope, in 1972. The first radio maps made by the One-Mile Telescope revealed the power of radio astronomy to uncover the structures of galactic and extragalactic radio sources and led to new astrophysical challenges concerning the origin of the enormous fluxes of relativistic electrons and magnetic fields present in these sources. In addition, the number counts of sources in the 5C catalogs extended to very small flux densities at which the cosmological convergence expected in all world models was clearly observed. Both the One-Mile and 5-kilometer telescopes were far ahead of the radio astronomical capability of any other telescope system in the world.
The measure of Ryle’s achievement was that, over a twenty-five-year period, the sensitivity of radio astronomical observations increased by a factor of about one million and the imaging capability of the telescope system improved from several degrees to a few arcseconds, comparable to that of ground-based optical telescopes. Ryle was personally involved in every aspect of these very complex telescope systems. As remarked by Scheuer, the development of aperture synthesis “was the story of one remarkable man, who not only provided the inspiration and driving force but actually designed most of the bits and pieces, charmed or savaged official persons according to their deserts, wielded shovels and sledgehammers, mended breakdowns, and kept the rest of us on our toes” (Sullivan, 1984, pp. 263–264).
Intellectually, he relied almost completely upon his well-honed intuitive understanding of physical processes as the way to solve any problem, be it in engineering, astrophysics, or cosmology; indeed, he believed this was the only way research should be conducted. These telescopes were central to understanding the nature of the radio sources. Radio astronomy has played a crucial role in the realization that high-energy astrophysical activity involving supermassive black holes and general relativity are part of the large-scale fabric of our universe. Ryle was knighted in 1966.
From the beginning, the Cambridge radio astronomy group developed a remarkably coherent and focused research program, led by the dynamism and vision of Ryle. He was fortunate in being supported by an outstanding group of physicists. Among these, Antony Hewish had played a central role in the development of aperture synthesis and in 1964 began the study of the twinkling, or scintillation, of radio sources due to irregularities in the outflow of material from the Sun, what is known as the solar wind. A remarkable by-product of these studies was the discovery of pulsating radio sources, subsequently called pulsars, by Hewish and his graduate student, Jocelyn Bell. These objects were soon convincingly identified as rapidly rotating, magnetized neutrons stars, which had been predicted to exist on theoretical grounds. Their serendipitous discovery at long radio wavelengths was a crucial event for all astronomy.
National and International Recognition In 1974, Ryle and Hewish were jointly awarded the Nobel Prize in Physics, the citation explicitly describing the development of aperture synthesis as Ryle’s major contribution. His list of honors was extensive, including foreign memberships in the Royal Danish Academy of Sciences and Letters, (1968), the American Academy of Arts and Sciences (1970), and the U.S.S.R. Academy of Sciences (1971). Among many medals received were the Gold Medal of the Royal Astronomical Society (1964), the Popov Medal of the U.S.S.R. Academy of Sciences (1971), and the Royal Medal of the Royal Society of London (1973).
In 1972 Ryle was appointed Astronomer Royal, the first time the post was separated from the directorship of the Royal Greenwich Observatory. This period coincided with a grave deterioration in his health, at least partly brought on by the stress of telescope construction over many years, but originating from a malfunctioning heart. Exploration for this condition exposed lung cancer, for which he had surgery in 1977. Over the same period, his main preoccupations shifted from radio astronomy. His acute awareness of the dangers of nuclear power fueled a passionate, ethical sense of crusade concerning the potential misuse of science. His conviction was that we violate the natural order at our peril. His deep concern for alternative energy sources led to an enthusiasm for wind energy, a natural outcome of his expertise as a sailboat designer, and he began a successful research and development program at Lord’s Bridge involving the construction of wind-powered generators. He was passionately concerned about the proliferation of nuclear weapons and wrote a monograph, Towards the Nuclear Holocaust, which, as expressed by Graham-Smith, “is partly a cry of pain and a desperate plea for a halt in the arms race, and partly an indictment of all those concerned with the civil nuclear programme, which he regarded as sustainable only on account of its production of plutonium for military purposes” (1986, pp. 517–518).
Ryle died in 1984 at the family home in Cambridge. His legacy went far beyond the technical brilliance of his contribution to radio astronomy. When he began his career after the war, the United Kingdom could not compete with the United States in observational astronomy. Through his technical and scientific contributions as well as his inspiring leadership, Ryle played a major role in rejuvenating British astronomy and bringing it to a leadership position in world astronomy.
Ryle’s papers and working materials are archived at Churchill College, Cambridge University, Cambridge, U.K. A sample of historical exhibits showing the development of radio astronomy in Cambridge is contained in the Meeting Room at the Lord’s Bridge Observatory. A complete list of Ryle’s publications is contained in an obituary, Francis Graham-Smith, “Sir Martin Ryle: A Biographical Memoir,” Biographical Memoirs of Fellows of the Royal Society 33 (1986): 495–524.
WORKS BY RYLE
With F. G. Smith. “A New Intense Source of Radio-Frequency Radiation in the Constellation of Cassiopeia.” Nature 162 (1948): 462–463.
“Radio Astronomy.” Reports on Progress in Physics 13 (1950): 184–246.
With F. G. Smith and B. Elsmore. “A Preliminary Survey of the Radio Stars in the Northern Hemisphere.” Monthly Notices of the Royal Astronomical Society 110 (1950): 508–523. This was the first Cambridge survey of radio sources.
“A New Radio Interferometer and Its Application to the Observation of Weak Radio Stars.” Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences 211 (1952): 351–375.
With K. E. Machin and D. D. Vonberg. “The Design of an Equipment for Measuring Small Radio-Frequency Noise Powers.” Proceedings of the Institution of Electrical Engineers 99 (May 1952): 127–134.
“Radio Stars and Their Cosmological Significance.” Observatory 75 (1955): 137–147. This is the published version of Ryle’s Halley Lecture, given at Oxford University.
With J. R. Shakeshaft, J. E. Baldwin, B. Elsmore, et al. “A
Survey of Radio Sources between Declinations –38° and +83°.” Memoirs of the Royal Astronomical Society 67 (1955): 106–154. This is referred to as the 2C survey.
With A. Hewish. “The Synthesis of Large Radio Telescopes.” Monthly Notices of the Royal Astronomical Society 120 (1960): 220–230.
With A. C. Neville. “A Radio Survey of the North Polar Region with a 4.5 Minute of Arc Pencil-Beam System.” Monthly Notices of the Royal Astronomical Society 125 (1962): 39–56.
With B. Elsmore and A. C. Neville. “High-Resolution Observations of the Radio Sources in Cygnus and Cassiopeia.” Nature 205 (1965): 1259–1262.
With G. G. Pooley. “The Extension of the Number-Flux Density Relation for Radio Sources to Very Small Flux Densities.” Monthly Notices of the Royal Astronomical Society 139 (1968): 515–528.
“The 5-km Radio Telescope at Cambridge.” Nature 239 (1972): 435–438.
With P. J. Hargrave. “Observations of Cygnus A with the 5km Radio Telescope.” Monthly Notices of the Royal Astronomical Society 166 (1974): 305–327.
“Radio Telescopes of Large Resolving Power.” Reviews of Modern Physics 47 (1975): 557–566. This is Ryle’s Nobel Prize lecture.
Towards the Nuclear Holocaust. 2nd ed. London: Menard Press, 1981.
Bertotti, B., R. Balbinot, S. Bergia, et al., eds. Modern Cosmology in Retrospect. Cambridge, U.K.: Cambridge University Press, 1990. Among the excellent articles in this volume, those by Peter Scheuer and Woodruff T. Sullivan III are of particular interest in relation to Ryle’s scientific achievements.
Graham-Smith, Francis. “Sir Martin Ryle: A Biographical Memoir.” Biographical Memoirs of Fellows of the Royal Society 33 (1986): 495–524.
Hey, J. S. The Evolution of Radio Astronomy. New York: Science History Publications, 1973. He was one of the earliest pioneers of radio astronomy, and this book gives a good picture of how the subject developed.
Kragh, Helge. Cosmology and Controversy: The Historical Development of Two Theories of the Universe. Princeton, NJ: Princeton University Press, 1996. This book gives a thorough and readable account of the controversies concerning evolutionary and steady state cosmologies.
Longair, Malcolm S. The Cosmic Century: A History of Astrophysics and Cosmology. Cambridge, U.K.: Cambridge University Press, 2006. This volume sets the development of radio astronomy and cosmology in its wider astrophysical and cosmological context.
Lovell, A. C. B. “Martin Ryle.” Quarterly Journal of the Royal Astronomical Society 26 (1985): 358–368.
Mills, B. Y., and O. B. Slee. “A Preliminary Survey of Radio Sources in a Limited Region of Sky at a Wavelength of 3.5 m.” Australian Journal of Physics 10 (1957): 162–194. Pooley, G. G. “Sir Martin Ryle.” Observatory 104 (1984): 283–284.
Smith, F. G. “Ryle, M. 1918–1984.” Nature 312 (1984): 18.
———. “Martin Ryle, Pioneer Radio Astronomer.” Sky and Telescope 69 (1985): 123.
Sullivan, W. T., III. Classics in Radio Astronomy. Dordrecht, Netherlands: D. Reidel, 1982. This is a very useful compilation of important early papers on radio astronomy.
———, ed. The Early Years of Radio Astronomy. Cambridge, U.K.: Cambridge University Press, 1984. This is an excellent survey of the early history of radio astronomy.
Malcolm S. Longair
British radio astronomer Sir Martin Ryle (1918–1984) developed revolutionary radio telescope systems and used them to locate weak radio sources. With his equipment, he revealed the most distant known galaxies of the universe. In 1974, he shared the Nobel Prize for physics with Antony Hewish, the first time astronomers received the award. Specifically, Ryle was recognized for developing "aperture synthesis," a technique that employs computer technology as a solution to some of the fundamental problems involved in the construction of radio telescopes. Ryle was able to make several important discoveries, including the nature of radio stars and the origins of radio scintillation.
Ryle was born September 27, 1918, in Brighton, Sussex in England to John A. and Miriam (Scully) Ryle. Martin Ryle was the second of five children. His family name was respected throughout England, as his father was a physician. After World War I, John Ryle was appointed to the first chair of social medicine at Oxford University in England. He also was the director of Oxford's Institute of Social Medicine.
Ryle's uncle, Gilbert Ryle (1900–1976), was an influential philosopher. As a professor of metaphysical philosophy at Oxford, he had an enormous influence on the development of twentieth century analytic philosophy. He authored The Concept of the Mind (1949) and Ryle's Dilemmas (1954).
Keeping with family tradition, Martin Ryle was educated at Bradfield College and Oxford University, where he graduated in 1939 with a degree in physics. He earned first–class honors in Oxford's school of natural sciences.
Radar Research During World War II
World War II had just started when Ryle graduated from Oxford. So, instead of going on to graduate studies, he was assigned to work in the British government's Telecommunications Research Establishment (later renamed the Royal Radar Establishment), where he became involved in research into the new science of radar. He worked briefly at the Cavendish Laboratory at the University of Cambridge before taking on the wartime assignment. He worked on the design of radar equipment and radio systems that Great Britain's Royal Air Force used. His primary assignment was developing counter measures for enemy radar.
There, Ryle first worked with Hewish. Ryle worked with the Telecommunications Research Establishment until 1945. He later admitted that during this period, he gained significant engineering experience but soon forgot most of what he had learned about physics.
Returned to Cambridge After War
After his wartime research, Ryle, like many of his colleagues, was convinced that radar was applicable to observational astronomy. He would become one of the important early investigators of extraterrestrial radio sources, and would develop advanced radio telescopes using radar principles.
After the war, in 1945, Ryle returned to Cavendish Laboratory at Cambridge. J. A. Ratcliffe, a scientist who had led the ionospheric work at Cavendish before the war, encouraged him to apply for a fellowship. Ratcliffe wanted Ryle to join his group and investigate radio emissions from the sun, which had recently been discovered by accident through radar equipment.
Later, Ryle recalled, in the autobiography he wrote when he received his Nobel Prize: "During these early months, and for many years afterwards both Ratcliffe and Sir Lawrence Bragg, then a Cavendish professor, gave enormous support and encouragement to me. Bragg's own work on X–ray crystallography involved techniques very similar to those we were developing for 'aperture synthesis,' and he always showed a delighted interest in the way our work progressed."
Ryle received the fellowship and studied radio astronomy at Cavendish. His fellowship would run through 1948.
Radio Astronomy Emerged
Radio astronomy, or the study of objects in space by observing the radio waves they emit, was a new field at the time. But it would soon open up many parts of the universe that had been invisible to scientists and researchers. For centuries, astronomers knew only about objects in space that shone with visible light.
Matter in space emits radiation from across the electromagnetic spectrum, or the range of wavelengths produced by the interaction of electricity and magnetism. Along with light waves, the electromagnetic spectrum includes radio waves, infrared radiation, ultraviolet radiation, X–rays, and gamma rays.
The development of radio astronomy was due largely to American radio engineer Karl Guthe Jansky. In 1932, Jansky developed the first, simple radio antenna that picked up short radio waves that came from distant parts of the universe. Jansky knew such waves could provide information about astronomical bodies in much the same way as light waves. His discoveries gained credibility among many astronomers as research continued.
The next advancement came in 1937, when amateur astronomer Grote Reber built a 31–foot–diameter radio dish in his backyard. The field advanced even more with the wartime research at the Telecommunications Research Establishment. Later, Ryle would greatly improve the power of radio telescopes.
Catalogued the Heavens
Ryle's early work centered on studies of radio waves from the sun, sunspots, and nearby stars. Starting in the late 1940s, Ryle led his colleagues on the Cambridge radio astronomy research team in the production of surveys of radio–emitting sources in the heavens. These surveys were essentially maps of the sources. He completed the first cosmological survey in 1950, identifying 50 sources. In his second survey, in 1955, Ryle found almost 2,000. The third survey led to the discovery of the first quasi–stellar object, or quasar. Specifically, Ryle and his colleagues had located a radio source in the constellation Cygnus, which was 500 million light years away from the earth. The ability to detect an object at such a distance had significant implications. It meant radio telescopes could see very far back into the history of the universe. Therefore, radio telescopes could help reveal information about the creation of the universe (the science of cosmology).
To map such distant radio sources as quasars, Ryle developed a technique called aperture synthesis, or interferometry. Basically, aperture synthesis involves two radio telescopes. By changing the distance between them, Ryle obtained information that, when analyzed by a computer, provided tremendously increased resolving power.
The resolving power of any telescope, or its ability to separate two nearby objects in the sky, depends on the wavelength of the radiation detected. As the wavelength of radio waves is much longer than that of light waves, a radio telescope must be considerably larger than a light telescope with similar levels of resolving power. Ryle realized the size of the telescope he would need to do his work must be hundreds or thousands of meters in diameter. In other words, the telescope would have to be enormous.
Ryle developed a solution that on the surface seemed simple yet effective. He designed several moveable telescope parts. The technique was called aperture synthesis. Basically, the technique involves a number of small telescopes with positions that are mutually adjustable within nearly five kilometers, or about three miles. This enabled Ryle to achieve a precision similar to what an enormous telescope would have produced. For his research in developing the technique, Ryle received the Nobel Prize in 1974.
"The wealth of detail in the charting of the universe carried out in recent years with this apparatus is absolutely unique," the Nobel committee wrote in 1974. "For a number of years, Ryle has been making observations with his various instruments that have been of crucial significance in the study of the physical characteristics of stars and stellar systems and for cosmology, the study of the development of the universe as a whole."
When Ryle developed the first radio interferometer, which involves multiple radio telescopes linked electronically, he used twelve telescopes. The radio waves each telescope detected were sent to a single receiver and a computer processed the information. The result was the creation of picture much more detailed than anything a single radio telescope could have produced. During the next ten years, astronomers from around the world would copy the technique.
That Ryle had to work on the rather primitive computers available at the time made his accomplishment even more extraordinary. To compensate, he developed a "phase–switching" receiver that cleaned up the radio signal, eliminating unwanted noise and interference. Using this device, Ryle was able to discover 50 new astronomical sources of radio waves.
In 1967, Ryle worked with Hewish and graduate student Jocelyn Susan Bell Burnell. Using a form of the radio interferometer, they discovered pulsars, which are rapidly spinning neutron stars that produce a blinking on–and–off signal.
Ryle received prestigious appointments and distinctions. He was knighted in Great Britain in 1966 for his achievements in radio astronomy. While serving as university lecturer in physics at Cambridge from 1948 to 1959, he was elected to a fellowship at Trinity College. Also, he became director of the Mullard Radio Astronomy Observatory in 1957. In addition, in 1959, he became the first Cambridge professor of radio astronomy.
He retired from Cambridge in 1982. During his tenure, he received a number of honors and awards, including the Hughes Medal of the Royal Society in 1954, the Gold Medal of the Royal Astronomical Society in 1964, the Henry Draper Medal of the United States National Academy of Sciences in 1965, and the Royal Medal of the Royal Society in 1973.
Ryle was married in 1947 to Ella Rowena Palmer, a nurse and physiotherapist. They had two daughters, Alison and Claire, and one son, John.
After a long battle with lung cancer, Ryle died on October 14, 1984, in Cambridge, England.
During his last decade, he was greatly interested in renewable energy and nuclear disarmament. He took special interest in the possible role of renewable energy in the world's future, and was a strong advocate for wind power development in Great Britain. In 1981, he published Towards the Nuclear Holocaust, which expressed his concerns about the nuclear arms race and the misuse of science.
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