Radio Astronomy

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Once radio astronomy was established, it became another factor influencing mid-century thinking about extraterrestrial life. Ernest W Barnes (1874—1953), mathematician, Anglican bishop, and teacher of E. A. Milne, was one of the earliest scientists to propose radio communication with intelligent extraterrestrial beings. In a paper delivered before the British Association for the Advancement of Science in 1931, Bishop Barnes claimed that alien "beings exist who are immeasurably beyond our mental level."6 These creatures communicate with Earth by transmitting coded radio signals to us. The reception and decoding of these communications, he declared, would inaugurate a new era in the history of humanity. Bishop Barnes spoke shortly before the era of radio telescopes made it possible to receive radio signals transmitted from deep space.

Radio astronomy is the study of the radio waves naturally emitted by bodies in space. In 1928 the Bell Telephone Laboratories in New York City hired physicist Karl G. Jansky (1905—1950) to investigate static disrupting radiotelephone transmissions. By the early 1930s, Jansky isolated a distinctive and persistent static hiss coming from the center of the Milky Way. The New York Times featured Jansky's discovery on its front page on May 5, 1933, reporting that the incoming signal was natural. The paper made it clear that there was no evidence "of interstellar signaling."

Jansky's discovery stirred little interest among astronomers accustomed to using light-gathering telescopes. Bell Laboratories assigned Jansky other research topics, and interest in the detection of radio signals from space languished until the Second World War. The development of military radar stimulated scientists and engineers to reexamine Jansky's work after the war ended.

Before the establishment of radio astronomy, observation of the heavens was limited to the visible portion of the electromagnetic spectrum. The invention of radio telescopes made it possible to detect celestial objects that radiated electromagnetic waves with wave lengths beyond that of light. Radio telescopes, for instance, detected hitherto unknown radio signals emitted by the Sun. These solar observations facilitated a new understanding of the physics of the Sun. Radio telescopes were used in the discovery of pulsars, rapidly spinning neutron stars that emit pulses of radiation, and quasars, very bright, distant stars that are sources of radio signals. Finally, radio astronomy provided evidence for the Big Bang theory of the universe by identifying residual background radiation left over from the initial Big Bang.

Some scientists realized that radio astronomy technology was also useful in searching for interstellar communication by intelligent beings. The first full-fledged scientific statement on the search for interstellar communication appeared in 1959 when physicist Giuseppe Cocconi (1914—) and astrophysicist Philip Morrison (1915—) published a short article outlining the physical parameters of communications with advanced alien civilizations.

The authors settled on the radio band for communication purposes. They chose the hyperfine radio emission line of neutral hydrogen, whose wavelength was twenty-one centimeters. Aliens were likely to communicate at that wavelength because cosmic interference was minimal and hydrogen was an abundant element in the universe. Any advanced civilization was assumed to know the physical characteristics of the hydrogen atom.

Cocconi and Morrison realized the highly speculative nature of their proposal. Hence, they concluded their paper modestly. "The probability of success is difficult to estimate"; they wrote, "but if we never search, the chance of success is zero."7

The Cocconi-Morrison paper became a classic document in the search for extraterrestrial intelligence and stimulated other researchers to test the authors' hypotheses. Cocconi soon returned to work in high energy physics, but Morrison retained a life-long interest in these matters. He was a pioneer in the scientific search for intelligent life on other worlds.

While Cocconi and Morrison were writing their paper, astronomer Frank Drake was readying the eighty-five-foot diameter radio telescope of the National Radio Astronomy Observatory (NRAO) at Green Bank, West Virginia (Fig. 8.1). He intended to use it for the reception of signals transmitted by intelligent extraterrestrials. Drake independently selected the hyperfine radio emission line of neutral hydrogen as the wavelength extraterrestrials would likely use to transmit interstellar messages.

The choice of the same wavelength by Cocconi, Morrison, and Drake was not coincidental. In 1951 physicist Edward M. Purcell (1912—1997) discovered that hydrogen clouds in space emitted a signature wavelength of twenty-one centimeters. Purcell's discovery permitted radio astronomers to chart the movement of hydrogen clouds in space. Those who used the twenty-one-centimeter wavelength for message reception assumed that extraterrestrials practice physics as we do and arrive at the same conclusions about the physical world.

Radio Astronomy Obser-vatory/AUI.)

fig. 8.1. First major radio telescope erected at the National Radio Astronomy Observatory, Green Bank, West Virginia.

Radio Astronomy Obser-vatory/AUI.)

fig. 8.1. First major radio telescope erected at the National Radio Astronomy Observatory, Green Bank, West Virginia.

(Courtesy of National

Drake thought that the Green Bank telescope was capable of receiving messages from stars located within ten light years of the Earth. He proposed minor modifications to the instrument that would enable it to detect incoming radio signals at the twenty-one-centimeter wavelength. Drake received permission to use the telescope as a detector of artificially generated interstellar signals.

Drake chose two nearby stars for his initial study, Tau Ceti and Epsilon Eridani. On April 8, 1960, Drake's team aimed their telescope at Epsilon Eridani and recorded a very strong incoming signal. They soon learned that the signal was due to terrestrial interference, probably from nearby aircraft. Drake spent 200 hours examining radio signals from the two stars without discovering any signs of alien communication with the Earth.

The year 1959—1960 marked the beginning of a new era in the search for extraterrestrial intelligence. Cocconi and Morrison provided a reasonable theoretical basis for the search, and Drake showed how to conduct the search with existing equipment. The accomplishments of this pioneering trio encouraged others to join the search.

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