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History of Radio Astronomy

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History of Radio Astronomy History of Radio Astronomy Reading for High School Students Getsemary Báez Introduction form of radiation involved (soon known as electro- Radio Astronomy, a field that has strongly magnetic waves). Nevertheless, it was Oliver Heavi- evolved since the end of World War II, has become side who in conjunction with Willard Gibbs in 1884 one of the most important tools of astronomical ob- modified the equations and put them into modern servations. Radio astronomy has been responsible for vector notation. a great part of our understanding of the universe, its A few years later, Heinrich Hertz (1857- formation, composition, interactions, and even pre- 1894) demonstrated the existence of electromagnetic dictions about its future path. This article intends to waves by constructing a device that had the ability to inform the public about the history of radio astron- transmit and receive electromagnetic waves of about omy, its evolution, connection with solar studies, and 5m wavelength. This was actually the first radio the contribution the STEREO/WAVES instrument on wave transmitter, which is what we call today an LC the STEREO spacecraft will have on the study of oscillator. Just like Maxwell’s theory predicted, the this field. waves were polarized. The radiation emissions were detected using a 1mm thin circle of copper wire. Pre-history of Radio Waves Now that there is evidence of electromag- It is almost impossible to depict the most im- netic waves, the physicist Max Planck (1858-1947) portant facts in the history of radio astronomy with- was responsible for a breakthrough in physics that out presenting a sneak peak where everything later developed into the quantum theory, which sug- started, the development and understanding of the gests that energy had to be emitted or absorbed in electromagnetic spectrum. small packets or “quanta” of energy. Quantum phys- Even though scientists like Faraday and Volta ics is the primary field for the in depth study of elec- performed experiments with electricity and magnet- tromagnetic radiation. Other contributors to this field ism, it was not until many years later that a scientist are Albert Einstein with his quantum theory- photoe- was able to relate both as two aspects of the same lectric effect, Louis de Broglie and “particle wave force. James Clerk Maxwell (1831-1879) developed duality”, and Erwin Shcrodinger and his quantum the theory of electricity and magnetism by the coher- physics wave equations, among others. ent integration of four equations. These equations not After all these discoveries, scientists were only summarized the relationship between electric able to apply their studies on electromagnetism and and magnetic forces, but also predicted that there is a radio waves to develop ways of communication. In 1 1901, Guglielmo Marconi (1874-1937) was the first nearby and distant thunderstorms. However, there to send and receive signals across an ocean from was a third source of static that was somehow differ- Newfoundland to Cornwall. He improved radio ent. He began to realize that there was a pattern transmissions and, as a result of his contribution, characterizing these wave signals. It was very similar commercial radiotelephone service became available to the known location of the Sun, but after a few in later years (F. Ghigo, 2003). months and more accurate measurements (signals repeated every 23 hours and 56 seconds) Jansky Evolution of Radio Astronomy concluded that the radiation came from the constella- Astronomical observations have been greatly tion Sagittarius in the Milky Way Galaxy. This dis- improved since the moment it was possible to meas- covery was a fundamental contribution to radio as- ure regions on the electromagnetic spectrum outside tronomy. the optical range. Radio observations became one of Jansky’s discovery motivated Grote Reber the most productive means of astronomical research. (1911-2002) a radio operator and engineer to apply Radio astronomy expanded greatly in the twentieth for jobs with Karl Jansky and Bell Laboratories to century. further investigate radio waves. He wanted to find The study of astronomy using radio frequen- out what was the process that lead to the develop- cies started with unsuccessful attempts to find solar ment of radio waves in space and verify if the waves radio waves. Such attempts will be discussed in more were in fact coming from the Milky Way or other detail later in this article. celestial objects. However, since all of this happened As mentioned earlier, the first significant ap- during the Great Depression, Bell Labs were not hir- plication of radio waves in the beginning of the ing at that time. Reber was determined to achieve his twentieth century was the creation of long distance goals and answer his questions, even if it meant he radio communication. Further radio communication had to do it all from his back yard… which he did. investigations led to the discovery of radio waves Reber decided to investigate on his own, and in 1937 from the Milky Way. It was the decade of 1930s and he constructed a telescope that had a parabolic dish the Bell Telephone Company was having trouble reflector and 3 receivers: 3300MHz, 900MHz and with the functioning of their transatlantic service, 160MHz. A year later, in 1938, the last receiver men- due to static of some sort. The company asked the tioned gave him physicist Karl Jansky (1905-1950) to find the source what he was of such interference. looking for, ga- In order to track and identify the source of lactic radio static, Jansky built a big rotating antenna, given the waves. Reber name of “Jansky’s presented the merry-go-round”. data as contour The antenna was maps showing designed to receive the Milky Way as bright areas. radio waves at a fre- Reber became one of the pioneers of what we quency of 20.5MHz, call today radio astronomy. Thanks to his work, after and with its rotation World War II, many scientists began to build bigger ability it was able to and better antennas to study the universe. locate the direction of any radio signal. After several months of studying such static, Jansky was able to classify it into three different types. The source of the first two originated from 2 Nowadays, we have radio telescopes as big as the long waves – the only waves the apparatus could de- Arecibo Radio telescope in tect– from reaching the Earth. Puerto Rico, with a 305m Sir Oliver J. Lodge around 1897-1900 built a (1000 feet) diameter and more sophisticated solar radio detector than the one 167 feet deep, covering an Edison did. Still, it was not sensitive enough to have area of about twenty acres detected the Sun. Following this attempt, the astro- (NAIC, 2004). physicists Johannes Wilsing and Julius Scheiner con- Also, techniques structed a device and tried the experiment for eight such as radio interferome- days, but they were also unable to detect radio radia- try became available as tion from the Sun. However, they were the first ones early as 1946. This technique— using multiple an- to formally write up and publish their attempt to de- tennas to record radio data— became more sophisti- tect solar radio data (Ann. Phys. Chem. 59, 782, cated over the years including a technique known as 1896, in German). They incorrectly concluded that Very Long Baseline Interferometry (VLBI) and the the atmosphere was absorbing the radio waves. latest one, Space Very Long Baseline Interferometry A few years later in 1900— trying to solve (SVLBI). SVLBI uses a space-based antenna as one problems from previous attempts— a French gradu- of its elements. Projects like the JPL SVLBI, funded ate student Charles Norman constructed a long wire by NASA, use this kind of technique to provide “3 to antenna and set it up on a glacier on the alpine 10 times the resolution of VLBI” (Wikepedia, 2004). mountain Mont Blanc at about 3100m (10000ft). He Improvements in the radio astronomy field reasoned that if Wilsing and Scheiner were right, the made possible the detection of radio emissions from solution was to gather data at a higher altitude. He planets like Jupiter (see Journal of Geophysical Re- was very close to detecting low frequency radio search, vol. 60, pp 213-217, 1955), observations of bursts. Unfortunately, the experiment was performed energetic objects such as pulsars, quasars, and radio in solar minimum. galaxies, and “imagery” of many astronomical ob- Solar radio observations were neglected for jects by recording multiple overlapping scans and many years. It was not until the 1920s, when Oliver putting them together in an image. Heaviside demonstrated the existence of the iono- sphere, that many questions about solar radio data Solar Radio Observations were answered. After this discovery was made, radio As previously mentioned, solar radio data had astronomers realized that they had to develop high its beginnings very early in the radio astronomy frequency radio receivers (around 20MHz) in order field. The Sun was the first astronomical object sci- for these waves to penetrate the ionosphere. entists thought of as a source for radio waves, from World War II not only had an influence on the idea that it is the closest energetic body to Earth. the foundation of radio astronomy, it also had a di- However, many of these early investigations were rect impact on the history of solar radio observations. unsuccessful. In February 26-27, 1942, an English radar station The first recorded attempt to detect radio received a strong noise signal thought to be a new waves from the Sun was made by Thomas Alva Edi- source of interference created by enemy transmitters. son in 1890. Kennelly, his laboratory assistant sent a It turned out to be radio wave emissions from the letter to Lick Observatory describing the construc- Sun associated with a group of sunspots that ap- tion of a detector made by winding a number of ca- peared at that time.
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