Radio Astronomy
By measuring regions of the electromagnetic spectrum not available to optical astronomers, radio astronomy effectively extends the range of astronomical observation. During the twentieth century, radio astronomy advanced to become one of the most productive means of astronomical research. Moreover, the development and success of radio astronomy spurred astronomers to devise new instrumentation designed to investigate other regions of the Cosmic electromagnetic spectrum.
In the nineteenth century Scottish physicist James Clerk Maxwell developed a set of equations describing electromagnetic phenomena. Using Maxwell's equations, in 1988 German physicist Henrich Rudolph Hertz demonstrated the existence of radio waves as a portion of the electromagnetic spectrum. In separate experiments conducted in the 1890s, American inventor Thomas Edison and British physicist Sir Oliver Lodge unsuccessfully attempted to detect solar radio activity.
In 1932, while performing an experiment to locate sources of static interference vexing radio communication systems, American engineer Karl Jansky (1905-1945) discovered the existence of a unique low-level radio interference. Jansky conducted a search for the source of the radio waves by methodically eliminating various forms of terrestrial interference such as equipment static, thunderstorms, etc. Jansky was also careful to eliminate the Sun as a possible source of interference.
During his research, Janksy noticed that the time difference in the daily reception of maximum static shifted by about four minutes each day. Jansky realized that this shift corresponded to the shift in the positions of stars. Armed with this evidence, Jansky concluded that the static emanated from a source outside the solar system and Jansky eventually traced the shifting interference pattern to the center of the Milky Way galaxy. Jansky attributed the source of the static to hot, charged particles. Although Jansky's findings were initially ignored, Jansky's fundamental contributions to radio astronomy were eventually recognized and the unit of radio-wave emission strength (i.e. the Jansky) was named in his honor.
In 1937, another American radio engineer, Grote Reber (1911-), attempted to extend Jansky's work. Literally in his own backyard, Reber constructed a parabolic reflector dish receiver capable of receiving cosmic radio waves. Reber began a systematic study of the sky at varying radio wavelengths and in 1944 Reber published the first radio frequency related celestial maps. Reber's work and writings, published in both scholarly journals and general-interest science magazines, spurred intense interest in radio astronomy.
The development of radio astronomy was, however, inhibited by the secrecy surrounding the development and use of radar during World War II Immediately following the war, however, increasing numbers of scientists turned their attention to the development of radio astronomy. Several large and sophicated radio telescopes were constructed, and--due to the fact that both type of telescopes measure manifestations of electromagnetic radiation--engineers quickly discovered that many of the problems associated with optical telescopes were applicable to the development of radio telescopes. Except for differences in the wavelength and frequencies of the electromagnetic radiation received, the fundamental physics describing the nature of radio waves is exactly the same as physical laws and formulations describing visible light waves. Instead of ground glass lens used in optical instruments, radio telescopes use parabolic-shaped metal dishes to focus radio waves at a focal point where they can be amplified and measured.
Radio astronomers quickly pinpointed thousands of sources of extra-terrestrial radio emissions. In 1946, radio astronomers bounced radar signals off of the lunar surface and a number of objects, including meteors not visible to the naked eye, were quickly discovered.
In accord with the principles of optical spectroscopy, light from receding objects becomes shifted toward longer wavelengths (e.g., red-shifted) so what was once visible light appears in different parts of the electromagnetic spectrum, including radio wavelengths and frequencies. Accordingly, radio astronomy allows astronomers to study ancient and distant cosmic objects that have had their energetic light emissions energetically cooled or red-shifted into the radio wave portion of the electromagnetic spectrum as they move away from us in an expanding universe. In 1963, American scientists Arno Penzias and Robert Wilson, discovered that no matter where in the sky they pointed their antenna, they found radio emissions--including emissions from pats of the sky that were visibly empty. The emissions were attributed to be cosmic background radiation left over from the big bang, and their discovery was hailed as a major confirmation of big bang-based cosmological models.
Astronomical observations of radio frequencies also allowed the discovery of the origin of very strong radio waves from optically dim stars and other seemingly star-like objects. Eventually known as quasars (quasi-stellar radio sources), these enigmatic objects appear to be the most distant--and yet among the most energetic--objects ever observed by astronomers. The discovery of quasars (now thought to be a type of active galaxy containing a black hole) helped confirm theories regarding the formation of black holes predicted by general relativity theory. Radio astronomers confirmed other significant predictions related to stellar evolution with the subsequent identification in the late 1960s of radio pulsars (rapidly spinning neutron stars).
Radio astronomy has also allowed scientists new insights into the mechanisms operating in solar flares and sunspots, both strong radio sources.
Another source of radio waves are cosmic objects that are cooler than the temperatures required to produce visible light. Accordingly, in addition to allowing astronomers an enhanced ability to detect cooler objects, radio astronomy allowed astronomers to probe obscuring clouds of interstellar dust.
Radio astronomers eventually realized better resolution by electrically linking physically separate telescopes together in a process termed radio interferometry. The Very Large Array (VLA) radio telescope complex located in New Mexico, for example, utilizes radio interferometry to achieve resolutions exceeding the largest ground-based optical telescopes.
Improved reception also allowed radio astronomers to realistically and systematically listen for evidence of extra-terrestrial life and intelligence in several research projects now commonly referred to as the SETI (Search for Extraterrestrial Intelligence) projects.
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