The Development of Radio Astronomy

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The Development of Radio Astronomy

Overview

In 1932, while attempting to determine the source of static interference in radio communication systems, American engineer Karl Jansky (1905-1945) discovered the existence of radio waves emanating from beyond the Earth. Jansky traced all but one of the types of interference he encountered to terrestrial phenomena (e.g., thunderstorms). Jansky concluded that the elusive third source of interference was extraterrestrial in origin and that it emanated from a region of the sky in the direction of the center of the Milky Way. Although Jansky's findings were initially ignored, eventually his papers gave rise to the science of radio astronomy. Since Jansky's discovery, radio astronomy has advanced to become one of the most important and productive means of astronomical observation, providing astronomers with a second set of eyes with which to view the Cosmos.

Background

In the nineteenth century Scottish physicist James Clerk Maxwell (1831-1879) developed a set of equations that described the propagation of electromagnetic radiation. Other scientists soon realized that the electromagnetic radiation producing visible light was but a part of a much larger electromagnetic spectrum. In 1888, German physicist Henrich Rudolph Hertz (1857-1894) demonstrated the existence of a portion of that spectrum known as radio waves.

The earliest attempts at detecting radio signals from outer space date to the 1890s with the efforts of American inventor Thomas Edison (1847-1931) and British physicist Sir Oliver Lodge (1851-1940), who, in separate experiments, unsuccessfully attempted to detect solar radio activity (Lodge failed principally because he was listening at the wrong wavelengths). In 1921, Guglielmo Marconi (1874-1937), who, in 1901 made the first intelligible transmission of a radio signal across the Atlantic, claimed to have detected extraterrestrial signals while experimenting on his yacht.

In the1920s, communication between continents was achieved through the use of powerful short-wave radio transmitters. Scientists and engineers working for the communications companies noticed that, at various times during the year, excessive amounts of static interference interfered with communications. Bell Telephone Laboratories assigned Jansky to investigate the possible sources of this periodic interference phenomena.

Jansky constructed a radio antenna to seek the source of the interference. After accounting for interference from thunderstorms, Jansky focused his investigation on a small but steady static radiation level. Intriguing to Jansky was the fact that the static varied with both direction and time. After eliminating the Sun and his own equipment as possible sources of interference, Jansky noted that the time difference in the daily reception of maximum static shifted by about four minutes each day—just as do the positions of stars on the celestial sphere. Accordingly, Jansky correctly concluded that the static emanated from a source outside the solar system. Jansky found that the static, similar to that created when electric current flows through a resistor, was strongest in the direction of the constellation Sagittarius and the center of our galaxy. Jansky suggested that the source of the static was extraterrestrial radiation of radio waves from hot charged particles in interstellar space.

In 1937, another American radio engineer, Grote Reber (1911- ), attempted to duplicate Jansky's findings. Reber constructed a parabolic reflector dish receiver in his backyard and set out to search for cosmic radio waves. Early in the 1940s Reber began to systematically study the sky at varying radio wavelengths. Reber's work in radio astronomy received wide attention when his observations were published in both scholarly and popular science periodicals. In 1944, Reber published the first celestial maps marked with radio frequencies.

The first observation of radio emissions from solar flares was made in 1942, by British scientist J.S. Hey, who was tracking the source of radar jamming signals. Wartime secrecy prohibited Hey from publishing his work. After the war, however, Hey's discovery (by then repeated in the civilian sector) and other great advances in radio wave technologies were declassified. In that immediate post-WWII period, scientists who had in many cases set aside research to aid in the war effort turned their attention and talents to the development of radio astronomy. There was a literal building boom in the construction of larger and more sophisticated radio telescopes.

Engineers found that lessons learned with regard to solving problems associated with the construction of optical telescopes were, in many cases, applicable to the development of radio telescopes. Although at first surprising, the similarities between radio and optical astronomy are grounded in the fact that both deal with manifestations of electromagnetic radiation. The physics of radio waves is, therefore, exactly the same as that of visible light waves, except for differences in the wavelength and frequencies of the electromagnetic radiation received. There were, of course, important differences; instead of ground glass lens used in optical instruments, radio telescopes used parabolic-shaped metal dishes. In a similar fashion, however, radio waves can be reflected to converge at a focal point where they can be amplified and measured.

Radio telescopes grew in precision and power. Advances in radar technology improved, and by mid-century, radio astronomers had identified thousands of sources of extraterrestrial radio emissions.

Impact

Radio waves can be produced from objects that are much cooler than the temperatures required to produce visible light. In a sense, radio astronomy extends the human senses as astronomers probe the Cosmos. In addition to an ability to detect cooler objects, radio astronomy allowed astronomers to probe obscuring clouds of interstellar dust.

Although they eventually diverged as separate sub-disciplines, radio astronomers also began to experiment with the use of RADAR. In 1946, astronomers discovered that they were able to bounce radar signals off of the lunar surface. Scientists began to use radar to probe and map a number of extraterrestrial objects, including meteors not visible to the naked eye.

In a quest for better resolution of extraterrestrial radio signals, astronomers and engineers ultimately realized that instead of simply building bigger radio telescopes, they could electrically separate telescopes in a process termed radio interferometry. Today, the Very Large Array (VLA) located in New Mexico utilizes these principles of radio interferometry in linking 27 radio telescopes to achieve resolution exceeding the largest ground-based optical telescopes.

Radio astronomy also extended the established science of optical spectroscopy. Light from receding objects becomes shifted (e.g., red-shifted) so that what was once visible light appears in different parts of the electromagnetic spectrum, including radio frequencies. Radio astronomy also allowed new insights into the mechanisms operating in solar flares and sunspots, both strong radio sources.

Just as interstellar spirals can be observed in visible light to possess varying quantities of interstellar dust, radio telescope observations reveal that these spirals can also contain tremendous amounts of interstellar gas (principally in the form of molecular hydrogen). Radio astronomers have been able to identify regions of hydrogen by measuring the emission of 21-cm radiation. These determinations are critical in predicting whether the universe will continue expanding.

The information accumulated by radio astronomers has also fostered some of the greatest scientific enigmas of the modern era, including the origin of very strong radio waves from very dim stars and other star-like objects. In 1949, the Crab Nebula (Messier 1), a supernova remnant, and two other galaxies were identified as emitters of radio signals. Radio astronomers were eventually able to confirm many of the postulates of stellar evolution with the subsequent identification in the late 1960s of radio pulsars (rapidly spinning neutron stars).

By the end of 1949 an increasing number of astronomers using radio telescopes sought explanations for these unexplained radio emissions. Puzzled astronomers suspected that a variety of star-like objects might be the source of radio waves. Later known as quasars (quasi-stellar radio sources), these puzzling objects appear to be the most distant yet most energetic objects observed by astronomers. In 1963, Arno Penzias (1933- ) and Robert Wilson (1936- ) found that, no matter where in the sky they pointed their antenna, they found radio emissions—including emissions from parts of the sky that were visibly empty. The noise turned out to be cosmic background radiation left over from the Big Bang.

Radio astronomy also enabled man to seek scientific evidence of extraterrestrial life. Ideas regarding the mechanisms of presumed interplanetary or interstellar communication were often bizarre schemes (e.g., building mirrors to burn symbols into the sands of Mars) designed to attract the attention of alien civilizations With radio astronomy, man could, for the first time, realistically and systematically listen for evidence of extraterrestrial life and intelligence. In a serious continuation of the pioneering work of Jansky and Reber who tuned in to listen to the Cosmos, scientists continue listening for radio signals that would confirm the existence of extraterrestrial intelligence. Radio astronomy gave rise to several twentieth-century projects now commonly referred to as the Search for Extraterrestrial Intelligence (SETI). Despite the reception of some interesting signals, none of which has passed the rigorous repeatability standards of modern science, no signal has ever been identified as evidence of extraterrestrial intelligence.

K. LEE LERNER

GEORGE HALE'S FIRST NIGHT WITH THE 100-INCH HOOKER TELESCOPE

No early twentieth-century astronomer equaled the drive to discover the origins of the stars and the depths of the universe by building larger telescopes than American George Hale (1868-1938). His solar observatory on Mt. Wilson above Los Angeles would evolve into an observatory complex providing two of the largest telescopes in the world. After successfully constructing the first of these (a 60-inch or 152.4-cm diameter reflector, 1908), Hale found support to construct the second, a 100-inch (254-cm) reflector, the idea of his friend and benefactor Los Angeles businessman John D. Hooker. With Hooker's seed money, Hale contracted the making of the 100-inch glass blank. On cooling, the 5 tons of glass was found to have air bubbles. Two other castings were not as good.

But Hale kept faith that the first blank would not expand and contract unevenly because of the bubbles and had it mirrored and polished. Carnegie Institution money provided for constructing the lattice telescope tube, the huge polar yoke mounting, and the dome. However, World War I and other setbacks held up this work and put Hale near nervous collapse. Finally, on a November night in 1917 all was ready to take the first look through the 100-ton Hooker Telescope. The telescope was trained on Jupiter, but the result was Hale's worst nightmare. Rather than a sharp image, there was a smudge of light and half a dozen distorted images of the planet. Once again Hale had faith, suggesting that the heat of the Sun on the open dome during the day had expanded the mirror. They waited. At 2:30 AM, they looked again—now at the star Vega. Hale was right; the image was a perfect pinpoint of light. The Hooker was operational—the largest telescope in the world until Hale's last dream, the 200-inch (508-cm) Palomar reflector of 30 years later. Guided by Edwin Hubble, the Hooker would reveal a universe of galaxies like our own, a universe not only many times larger than first thought but also expanding.

WILLIAM J. MCPEAK

Science and Its Times: Understanding the Social Significance of Scientific Discovery