The radio sky
Ken Tapping, May 1, 2018
Thanks to the way we use it for entertainment, we tend to think of radio as something we listen to. However, it also brings us television signals, cell phone services, and if you have a home WIFI system, your computers and other devices are using radio to talk to each other. What we are doing in all these cases is using radio waves to transmit information. We imprint information on the waves, that is "modulate them", transmit them, and then at some remote location, we extract that information, or "demodulate the signals" and then throw the radio wave away. That's why radio engineers refer to it as the "carrier wave".
Radio waves, along with light rays, infrared, ultraviolet, X-rays and gamma rays, are all electromagnetic waves; the only difference between them is their wavelength – the distance between two successive wave crests. Radio waves have wavelengths of kilometres to millimetres. Light waves range in length between 800 and 400 billionths of a metre. X-rays and gamma rays are far shorter. All these waves propagate through space, so we can in principle imprint information on them and use them to send it to distant locations. We can also image these waves just as we can image light. However, to make an image, the imaging lens or mirror has to be far bigger than the wavelength of the waves being used. Our eyes can image light to see details about one thirtieth of the diameter of the Full Moon. Our Synthesis Radio Telescope, at a wavelength of 21 cm, needs a row of antennas 600 metres long to achieve that same level of imaging detail. It is this difference in wavelength that makes radio telescopes look so different from optical telescopes, or our eyes. So why bother?
Electromagnetic waves come in little packets called "quanta", which cannot be subdivided. The amount of energy in a "quantum" of electromagnetic waves depends on their wavelength, so making a quantum with a long wavelength requires less energy than we need to make one with a short wavelength. This property is very useful for astronomy.
The sky we see with our eyes is produced by starlight. We see stars directly because of the light they produce, and other things, such as nebulae and planets, because the stars illuminate them. However, out there between the stars there is a lot of cold gas and dust, and electrons gyrating around magnetic fields where there is not enough energy to produce light quanta. For example, a red hot poker is hot enough to produce red light. When it cools a bit so that it is no longer glowing, we can still feel heat – infrared - coming from it. Even things a few degrees above absolute zero (-273 C) produce radio waves. We discovered the remnant heat from the Big Bang, now -270 C using radio telescopes. So radio telescopes can be used to map all that dark material between the stars, together with studying other processes that make only radio waves. Since this includes most of the normal matter in the universe, (stars form from it, and when they die, return their material to it) knowing how this material behaves is important.
We use radar to see through fog because radio waves are not stopped by it. Similarly, lots of very interesting places, such as the core of our galaxy and regions in which stars are being born, lie behind screens of gas and dust that hide them from our optical telescopes. However, radio waves pass through these screening clouds, making it possible to see the odd things going on at the centres of galaxies, and how new stars are being born. This is only a tiny bit of what radio telescopes are showing us. We have not mentioned pulsars, fast radio bursters, radio spectroscopy, interplanetary scintillation….
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