Ryle Constructs the First Radio Interferometer

Martin Ryle developed the first radio interferometer, an astronomical instrument that coordinates several different radio telescopes to achieve resolutions impossible for a single telescope alone.

Summary of Event

Electromagnetic radiation consists of packets of energy called photons, which behave both as particles and waves and travel at the speed of light (approximately 300 million meters per second). The electromagnetic radiation spectrum ranges from long-wavelength, low-frequency radiations to progressively shorter-wavelength, higher-frequency radiations (from radio to television to microwave to infrared to visible light to ultraviolet to X rays to gamma rays to cosmic rays). All of these radiations are emitted by various objects in the universe, most notably stars such as Earth’s sun. [kw]Ryle Constructs the First Radio Interferometer (1955)
[kw]Radio Interferometer, Ryle Constructs the First (1955)
[kw]Interferometer, Ryle Constructs the First Radio (1955)
Radio astronomy;telescopes
Telescopes;radio interferometers
Radio astronomy;telescopes
Telescopes;radio interferometers
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[g]United Kingdom;1955: Ryle Constructs the First Radio Interferometer[04720]
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Ryle, Martin
Jansky, Karl G.
Hulst, Hendrik Christoffel van de
Ewen, Harold Irving
Purcell, Edward Mills

Martin Ryle.

(The Nobel Foundation)

Since the early 1600’s, astronomy has relied on optical telescopes (glass lenses) for resolving stellar objects unviewable with the naked eye. The Italian scientist Galileo pioneered the use of telescopes, while verifying Nicolaus Copernicus’s heliocentric (sun-centered) theory of the solar system. Optical telescopes detect the visible light emission from stars, galaxies, quasars, and other astronomical objects. Through the late twentieth century, astronomers developed more powerful optical telescopes for peering deeper into the cosmos and viewing objects located hundreds of millions of light-years away from Earth.

The Very Large Array, contructed by the National Radio Astronomy Obervatory outside Socorro, New Mexico, was built in the 1970’s to be the largest radio interferometer in the world.

(Natonal Radio Astronomy Observatories/Associated Universities, Inc.)

In 1933, Karl G. Jansky, an American radio engineer with Bell Telephone Laboratories, constructed a radio antenna receiver for locating sources of telephone interference. Jansky discovered that an intense radio burst occurred at approximately the same time every day. The radio burst occurred every twenty-three hours and fifty-six minutes, the time required for one complete Earth rotation about its axis relative to the stars. He mapped the radio source as emanating from outside the solar system in the direction of the constellation Sagittarius.

Jansky had detected radio waves from the center of the Milky Way galaxy, a discovery that confirmed astronomer Harlow Shapley’s Shapley, Harlow 1918 mapping of the Milky Way’s center to Sagittarius. Jansky’s radio telescope was a horizontal antenna that detected radio emissions from the earth’s horizon. Except for detecting radio emissions from the center of the galaxy, the instrument had few other uses. Jansky’s pioneering work was discovered by Grote Reber Reber, Grote , an Illinois radio amateur.

In 1935, Reber constructed the first dish-shaped radio telescope, the forerunner of today’s instruments. Reber used his 9-meter diameter radio telescope to repeat Jansky’s experiments and to locate other radio sources in space. He confirmed Jansky’s conclusion that the Milky Way’s galactic center emits radio waves. He also precisely mapped the locations of various radio sources in space, some of which later were identified as galaxies and quasars.

Following World War II, radio astronomy blossomed with the help of surplus radar equipment. Radar operators had discovered that the sun produced radio interference in radar signals, which is not surprising considering that radar uses slightly higher-frequency microwaves. Radar equipment was used to demonstrate that the sun is a strong radio source. The work of Jansky and Reber further strengthened the drive to understand the nature of extraterrestrial radio sources.

In 1944, the Dutch astronomer Hendrik Christoffel van de Hulst had proposed that hydrogen atoms emit radio waves with a 21-centimeter wavelength. According to his theory, a hydrogen atom undergoes a phase transition once every 10 million years and emits 21-centimeter radio waves in the process. Because hydrogen is the most abundant element in the universe (approximately 60-80 percent by weight) and because hydrogen is the primary elemental component of most stars, van de Hulst had explained the nature of extraterrestrial radio waves. His theory later was confirmed by the American radio astronomers Harold Irving Ewen and Edward Mills Purcell of Harvard University.

Eventually, astrophysicists demonstrated that atoms of each element emit radio waves at characteristic wavelengths and therefore that stars emit radio waves as they emit visible light waves. By coupling the newly invented computer technology with radio telescopes, astronomers were able to generate a radio image of a star almost identical to the star’s optical image. A major advantage of radio telescopes over optical telescopes is the ability of radio telescopes to detect extraterrestrial radio emissions day or night, while optical telescopes are limited to nighttime viewing.

Following World War II, major research groups were formed in England, Australia, and the Netherlands. English radio astronomy efforts were headed by Sir Bernard Lovell at the Nuffield Radio Astronomy Laboratories in Jodrell Bank, J. S. Hey at the Royal Radar Establishment in Malvern, and Martin Ryle at the Mullard Radio Astronomy Observatory of the Cavendish Laboratory, University of Cambridge. Australian efforts were led by J. L. Pawsey and John G. Bolton.

Ryle had worked with radar for the Telecommunications Research Establishment during World War II. Following the war, he received a fellowship to study astronomy at the Cavendish Laboratory Cavendish Laboratory in Cambridge. He concentrated on the development of radio telescopes and applied these instruments to the analysis of radio emissions from the sun and nearby stars. He helped establish the Cavendish Laboratory’s Mullard Radio Astronomy Observatory Mullard Radio Astronomy Observatory , where he became director in 1957.

The radio telescopes developed by Ryle and other astronomers operate on the same basic principles as satellite television receivers. A constant stream of radio waves from a star or galaxy penetrates the earth’s atmosphere and strikes the parabolic-shaped reflector (dish) of the radio telescope. The radio waves bounce off the reflector, the parabolic shape of the dish aiming the radio waves at a focusing point above the dish. The focusing point is another reflector, which bounces the concentrated radio beam to the center of the dish, where it is funneled down a feed horn to electronic cables. The electronic cables transmit the radio signal to a radio receiver, then an amplifier, and finally to a chart recorder, or computer. A radio telescope, therefore, captures a radio beam, concentrates it, amplifies it, and records it for analysis.

With large-diameter radio telescopes, astronomers can locate stars and galaxies unviewable with the naked eye and unviewable with optical telescopes. This ability to detect more distant objects is called resolution. Like optical telescopes, large-diameter radio telescopes have better resolution than smaller ones. Very large radio telescopes were constructed in the late 1950’s and early 1960’s (Jodrell Bank, England; Green Bank, West Virginia; Arecibo, Puerto Rico). Instead of building larger radio telescopes to achieve greater resolution, however, Ryle developed a method using smaller radio telescopes to reach the same goal. The result was interferometry, a technique that uses a computer to combine the incoming radio waves of two or more movable radio telescopes pointed at the same stellar object.

Suppose that one had a 30-meter-diameter radio telescope. Its radio wave collecting area would be limited to its diameter. If a second identical 30-meter-diameter radio telescope was linked with the first in synchrony, then one would have an interferometer. The two radio telescopes would point exactly at the same stellar object, and the radio emissions from this object captured by the two telescopes would be combined by computer to produce a higher-resolution image. If the two radio telescopes were located 1.6 kilometers apart, then their combined resolution no longer would be limited to the area of a single 30-meter-diameter receiving dish. Instead, the combined interferometer resolution would be equivalent to that of a single radio telescope dish 1.6 kilometers in diameter.

Ryle constructed the first true radio telescope interferometer at the Mullard Radio Astronomy Observatory in 1955. He used combinations of radio telescopes to produce interferometers containing about twelve receivers. Ryle’s interferometer greatly improved radio telescope resolution for detecting stellar radio sources, mapping the locations of stars and galaxies, assisting in the discovery of quasars (quasi-stellar radio sources), measuring the earth’s rotation about the sun, and measuring the motion of the solar system through space.


Ryle’s development of radio interferometry greatly expanded the power of radio telescopes and significantly helped radio astronomy to emerge as the leading facet of late twentieth century astronomy. Interferometry has enabled astronomers to see further and further into the past, to observe objects which existed as long ago as 10 billion years. Interferometry also stimulated the development of innovative telescope designs and arrays for improved resolution.

Following Ryle’s discovery, interferometers were constructed at radio astronomy observatories throughout the world. The United States established the National Radio Astronomy Observatory National Radio Astronomy Observatory, U.S. (NRAO) in rural Green Bank, West Virginia. NRAO is operated by a consortium of nine eastern universities and is funded by the National Science Foundation. At Green Bank, a three-telescope interferometer was constructed, with each radio telescope having a 26-meter-diameter dish. With an additional 91-meter-diameter radio telescope and a 43-meter fully steerable radio telescope, NRAO became a world leader in radio astronomy research. The United States Naval Observatory would eventually operate the NRAO interferometer for precision navigation and timekeeping.

During the late 1970’s, NRAO constructed the largest radio interferometer in the world, the Very Large Array Very Large Array (VLA). The VLA, located approximately 80 kilometers west of Socorro, New Mexico, consists of twenty-seven 25-meter-diameter radio telescopes linked by a supercomputer. The radio telescopes are arranged in a Y-shape covering an area 32 kilometers by 32 kilometers across the flat New Mexico desert. Each telescope is mounted on a transport vehicle on a railroad track. Nine telescopes are located on each of three 21-kilometer railroad tracks that together form the Y-shape interferometer. The VLA has a resolution equivalent to that of a single radio telescope 32 kilometers in diameter.

Even larger radio telescope interferometers can be synthesized. A technique known as very long baseline interferometry (VLBI) has been used to construct artificially a radio telescope having an effective diameter of several thousand kilometers. Such an arrangement involves the precise synchronization of radio telescopes located in several different parts of the world, followed by analysis of the data (stored on magnetic tape) at a central processing laboratory. For example, Supernova 1987A in the Large Magellanic Cloud was studied using a VLBI arrangement between observatories located in Australia, South America, and South Africa. Radio astronomy;telescopes
Telescopes;radio interferometers

Further Reading

  • Associated Universities, Inc. The National Radio Astronomy Observatory. Charlottesville, Va.: National Radio Astronomy Observatory, 1981. This small book is a simple presentation of astronomy and radio astronomy. It uses clear illustrations and outstanding photographs to describe basic radio astronomical principles. The VLA and other NRAO radio telescopes are described.
  • Burke, Bernard F., and Francis Graham-Smith. An Introduction to Radio Astronomy. 2d ed. New York: Cambridge University Press, 2002. Survey of the history, methodology, and discoveries of radio astronomy. Includes a chapter on radio galaxies and quasars. Bibliographic references and index.
  • Kellermann, Kenneth I., and A. Richard Thompson. “The Very-Long-Baseline Array.” Scientific American 258 (January, 1988): 54-63. This survey article, written by two radio astronomers, is a discussion of the principles behind VLBI. A proposed linkage of ten American radio telescopes is described, along with the VLA and future orbiting radio telescope interferometers.
  • Kippenhahn, Rudolf. Light from the Depths of Time. Translated by Storm Dunlop. New York: Springer-Verlag, 1987. This book is an outstanding introduction to astronomy and cosmology for the average reader. Kippenhahn simplifies many complex astronomical concepts in a very entertaining fashion. Chapter 10, “The Radio Sky,” describes the history of radio astronomy, including the work of Jansky, Reber, Ryle, and other astronomers.
  • Lovell, Sir Bernard. The Story of Jodrell Bank. London: Oxford University Press, 1968. This historical chronicle is Lovell’s personal account of the construction of the 76-meter-diameter Jodrell Bank radio telescope, one of the largest radio telescopes in the world. He describes the history of radio astronomy, including the work of Ryle and other astronomers.
  • McDonough, Thomas R. The Search for Extraterrestrial Intelligence: Listening for Life in the Cosmos. New York: John Wiley & Sons, 1987. This entertaining book is a discussion of serious scientific efforts to detect the existence of intelligent life elsewhere in the universe. Chapter 16, “The Final Frontier,” discusses the history of radio astronomy and the current use of some radio telescopes to detect potentially intelligent radio signals from outer space.
  • Rubin, Vera, and George V. Coyne, eds. Large-Scale Motions in the Universe: A Vatican Study Week. Princeton, N.J.: Princeton University Press, 1988. This book is a collection of scientific papers presented by leading astronomers and radio astronomers at an annual Vatican astronomy conference. The various research papers describe the applications of radio astronomy to the mapping of galaxies in space, thereby giving scientists some idea of the organization of the universe.
  • Seielstad, George A. At the Heart of the Web: The Inevitable Genesis of Intelligent Life. Boston: Harcourt Brace Jovanovich, 1989. This book, written by a leading NRAO astronomer, is a lively discussion of cosmology and Earth’s place in the universe. The book links biology and other major scientific disciplines to astronomy. It also presents radio astronomical methods and data.
  • Silk, Joseph. The Big Bang. 3d ed. New York: W. H. Freeman, 2001. This book is a comprehensive, yet very readable, discussion of current views of the origin and evolution of the universe. It discusses cosmology, galaxies, quasars, and radio astronomy, including radio telescope interferometers.
  • Verschuur, Gent L. The Invisible Universe: The Story of Radio Astronomy. New York: Springer-Verlag, 1974. This book is an outstanding discussion of radio astronomy and its history. All aspects of radio astronomy are clearly described and illustrated. Chapter 2, “The Birth of Radio Astronomy,” describes the early work of Jansky, Reber, Lovell, Ryle, and other radio astronomers.

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