“What would be the significance of the candlelight, if there were no darkness? What would be the power of the stars over our minds, if there were no night?”  

                                                -- C. Joybell 

What is the value of light if there is no darkness? Is it the same to light on a candle in darkness and to light on it in a room full of light? Of course, not. When I was a child, I read a poem where light says to darkness “You are so dark”. In reply darkness says to light “That’s why you are shining”. Well, that was a bit of literature but that is not the topics today. When we look at the night sky we see little light and more darkness. What is hiding behind the darkness? Is there any mystery or it is just empty?

Well until the end of 19th century we were much busier to know about the light which is visible. We thought that is it. But we found out in our deepest surprise that there is light range which we can’t see at all. Well, the implications of this range of light didn’t come in astronomy even later at least 50 to 60 years. Today I am going to talk about the light range which our eyes can’t experience but it solves so many puzzles which we couldn’t answer and we didn’t know that exists.  

                                                Radio Telescope | Source

  An unexpected discovery  

Heinrich Hertz was a German physicist. In the dark lecture hall of Technical College in Karlsruhe, he was trying to show his wife Elisabeth the instrumental set up for an experiment. The experiment was to show what happens when electric current passes through an open circuit (that is, a circuit with a gap in it). 

 When he was explaining, as he turned on the generator which used to produce current one of them noticed something unusual. What they noticed was a simultaneous spark in an instrument which was totally unrelated to the experiment. What he did was that he accidentally discovered radio waves and receiver.

                  

                                                   Hertz's Experimental set up | source

But at that moment he didn’t know why did it happen? So, he tried to set up experiment to know the answer. Why is it keep happening all the time?

He was able to show that an invisible form of radiation, which he called Electric waves, carried energy through intervening space which we know as Radio waves. 

Now what are radio waves? Like water waves in the rivers it is also a sinusoidal wave but with bigger wavelength in the light spectrum. It can be from few cm to km even more.  

                                                             Light spectrum | source

In his honor the unit of frequency of EM wave is called Hz (Hertz). Through this discovery he was able to explain that electric waves are phenomenon like light waves and he discovered that it has the same velocity as light has. Through this phenomenal discovery he proved that light is an Electromagnetic wave which Maxwell has proposed before.   

A new science is Born

Soon after this discovery, radio waves has been started using in telecommunication. During 1920 it was so expensive to talk through telephone for a few minutes. But there were always some problems in the connection due to electric interference. In the beginning the system was operated in extremely low frequency of around 60 kHz. But after it was changed in the frequency range of 10 to 20 MHz. But the connection problem remained.   

Everyone has heard the name of Bell Laboratories. A young engineer named Carl Jansky was working for this corporation. He was the one who was given the duty to investigate what is happening and also to find out the source of this electric interference. He built a rotating antenna to carry out his investigation. It was operating in 20.5 MHz. He discovered the sources are local and distant thunderstorms and another of unknown nature .

                                      Carl Jansky and his telescope | source

Soon after he showed the unknown radio source was extraterrestrial in his paper entitles Electrical Disturbances Apparently of Extraterrestrial Origin. And that is how Radio Astronomy was born.

What is Radio Astronomy?

As Gerrith Verschuur said--

As you propel yourself into the invisible universe of radio astronomy,an explanation of some possibly unfamiliar terms will help guide you on your journey

                                                                                                

What is the purpose of Radio Astronomy? Well, the answer is that the purpose of Radio Astronomy is to study the radio waves coming from the outer space. Most of the objects in outer space emits in radio waves. If we can detect them, we can know about their origin, the amount of energy they are emitting. This kind of objects include different stars, nebulae, galaxies and so many peculiar and mysterious objects like pulsars, quasars etc. The waves we detect are emitted in different mechanism. To detect them astronomers uses certain kinds of telescopes. The most common of them is the metal dish type reflectors.

                                               Jodrell Bank Radio Telescope | source

How radio waves are created?

Radio waves are created in deep space by different mechanism depending on the physical condition of radio waves emitting objects. Almost all the process includes the motion of electrons. For example, when an electron passes nearby ions, it changes its velocity which causes the lose of its energy that can radiate away as radio wave. There are usually two types of radiation:

1. Thermal
2. Non-thermal 

Thermal radiation are totally based on temperature of the object emitting in radio waves. This kind of radiation usually dominates radio waves with shorter wavelengths. Usually the process of thermal radiation is blackbody radiation.

Now what is a Blackbody Radiation? Well, lets think of an object  which absorbs all kinds of  radiations fall on it.  It doesn't reflect or transmit any radiation. It is a perfect absorber and perfect emitter in all wavelengths.

                                           Blackbody radiation spectrum | source

Non-thermal radiation is not based on temperature which dominates in the longer wavelength radio waves. An example can be synchrotron radiation. Synhotron radiation is the radiation emitted by a relativistic charged particle in a magnetic field. So the electrons feel acceleration perpendicular to their velocity. We know that accelerated charge particle radiates. This radiation is totally non-thermal in quality. So many cosmic source radiates in this process. The path electron follows is helicoidal. Radius of every circle is called Larmor's radius and the power radiated is given by Larmor's formula which depends on the accelaration of the particle. By detecting the radiation we can know about the strength of magnetic field of the source.

                    

                                                   Synchrotron radiation | source

Detection of radio waves

Now the question is how do we detect the radio waves? Well, we can detect it from earth cause radio waves is not absorbed by the atoms and molecules in the earth's atmosphere. 20 percent of visible light is absorbed by earth atmosphere while x-ray and gamma ray is totally absorbed. So the telescope for short wavelength has to be made above earth's atmosphere. But radio telescope can be made in earth. The range of wavelength for which the earth atmosphere is transparent is called radio window.

                                                           Radio window | source

Radio Telescope

In optical telescope we use an optical lens or mirror to gather the light and then we send it through more lenses to bring it to a focus of photographic plate or a detector to be converted into electric signals.

In radio telescope it is almost the same except in stead of glass mirrors. We use metal surface which reflects the radio waves. Then the radio waves are collected to a small antenna which is called usually the feed. It is designed to soak in as much as radio energy possible. Then the collected radio signal is converted to electric voltage in amplifier. This is called the Front End of the receiver. Then this electric voltage is send to the control room where the signal is amplified million times before it is processed in the computer to show in such a way so that the radio astronomers can understand.

                        Process of collecting data in Radio telescope |souce

A single dish radio antenna collects radio energy from a small area of the sky at any instant. It is called the beam. This defines the resolution of the telescope. Resolution depends on frequency and diameter of the radio telescope. The bigger the diameter, the better the resolution. 

                                          Radio map of sky at 408 MHz |source

To see an object good we need better resolution. I told before that the bigger the diameter, the better the resolution. But there is a limit how much bigger we can make a telescope. So the astronomers thought of an alternative. Instead of one big antenna if we add more antennas after a certain distance and focus them toward a certain source then they can work as a bigger antenna. This method will give better resolution which depends on the distance between them. We collect the data and then send it to a central computer for further process. This technique is called Radio Interferometry. The example of it is VLA (very large array) in New Mexico and ALMA in Chile.

                                             VLA (very large array) | source

Conclusion

We say "Knowledge is power". The more we know about the universe, the more we know about ourselves. Invention of Radio waves and the process of detecting them and using them has made it possible to learn about new object like pulsars and quasars, to talk with a friend or family in far distance and so many other privileges. It is hard to think about a day of our today's  life without our knowledge of radio waves.

References

[1] Kristen Rolfs , Tools of Radio astronomy 

[2]Gerrith Verschuur, The Invisible Universe

[3]Olga Atanackovic, Opsta Astrofizika

[4] Dejan Urosevic, Theoriske osnovne Radio Astronomija