What is Radio Astronomy

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1.1. A Little History

In 1886. Heinrich Hertz accidentally constructed the first radio transmitter and receiver. In a darkened lecture theater at the Technical College in Karlsruhe, in Germany, Hertz had set up an experiment to test what happened when an electrical current flowed in an open circuit (that is, a circuit with a gap in it). As he explained the setup to his wife, Elisabeth, he switched on a spark generator, used to produce current, and one of them noticed a simultaneous spark that flashed in an unrelated piece of equipment at some distance away from his main experimental apparatus. Whoever noticed it first, Heinrich or Elisabeth, is unknown to us, but it was Heinrich who made the leap of curiosities that underscore the nature of scientific research. Hertz asked "Why?" and started a systematic search for an answer.

Eighty years later historians of science would report that Hertz was at least the sixth physicist to see this odd effect, but he was the first to follow up on his key question. He proceeded to design a series of brilliantly simple experiments, one after another, in search of an answer. He was able to show that an invisible form of radiation, which he called "electric waves," carried energy through intervening space. Hertz was also able to demonstrate that the electric waves were a phenomenon very similar to light. In fact their speed through the air was the same as that of light. Today we know that both light and Hertz's "electric waves" are forms of electromagnetic radiation (see Appendix A.2). Over time, the Hertzian waves (a name used very early in the 20th century) came to be called radio waves. Their frequency is measured in cycles per second, now called Hertz (Hz), In Appendix 2.1 the relationship between frequency and wavelength is discussed. For the bulk of our story we will refer to the frequency of radio waves.

Hertz died tragically at the young age of 35 of blood poisoning from an infected tooth. If he hadn't, he surely would have won a Nobel Prize in Physics for his discovery.

After showing that radio waves behave much as light does, except that they are utterly invisible, Hertz did not ask how far they might travel through space. That was left to Guglielmo Marconi, the Italian physicist who performed a series of obsessively creative experiments to prove that radio waves could travel enormous distances and even pass through rock. He was wrong in this latter belief, but he did show that a radio signal could traverse the Atlantic Ocean. The reason that the radio waves made it across despite the curvature of the earth was because the earth's atmosphere is surrounded by an electrically conductive layer known as the ionosphere and radio waves bounce off that layer to be reflected across the ocean. That wouldn't be understood until decades later. Meanwhile, Marconi was happy to know that radio waves did go all the way around the earth and it was not long before that ships at sea could signal one another and to their home ports using radio waves. By 1912 the infamous sinking of the Titanic spread awareness that radio transmitters could send an SOS far and wide.

Marconi did wonder whether there might be radio waves reaching earth from space but his equipment would not reveal the existence of the wondrous invisible universe for the same reason that he could signal across the Atlantic. At the low-radio frequencies that Marconi used, the reflecting ionosphere not only allows radio signals to bounce around the curvature of the earth, it also prevents radio waves from space from reaching the earth's surface. Those that do arrive from space are reflected back. (Only if their intrinsic frequency is higher than about 20 MHz such radio waves do reach the ground unimpeded, but then it was not known very much about building receivers at such frequencies.)

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