5 mins read 06 Jul 2022

Science Talk - What are Radio Waves and Why Can’t I See Them?

Seeing is believing but only if you can believe your eyes. Your eyes can show you tangible proof but what if the proof is in the pudding - and you don’t have pudding-piercing X-ray vision?

Right: This spectacularly twisted cloud of interstellar gases and dust is known as the Pillars of Creation. Left: The same nebula viewed in infrared light, which can be detected through the dusty foreground. Credit: NASA, ESA/Hubble & Hubble Heritage Team.

As an astrophysicist [in-training], sight and light are both important parts of my work. The only problem is that the type of light I study is radio light, which is captured by special radio telescopes since it can’t be seen by the naked eye. You see, the light our eyes see (known as ‘visible light’) is made up of tiny oscillating electromagnetic fields, which have wavelengths in nanometres. To give you an idea of how very small this is, well, a single human hair is about 100,000 nanometres wide, and the flat head of the back of a pin is about 1,000,000 nanometres from side to side. 

Radio waves, however, have oscillating fields which are much larger, and have wavelengths from a few millimetres long (which you can see on your ruler), all the way out to metres, and kilometres long. 

Now, most readers will know what a telescope is but unless you’re a scientist or serious space enthusiast, you’re probably just picturing some kind of reverse microscope. If that’s the case you’d actually be right - well at least partially right - since the first telescopes used by astronomers tended to be long tubes with a magnifying lens on one end and an eyepiece at the other. This is called a ‘refractor’ telescope since the light waves that hit the lens are refracted, or bent, towards the focus point at the eyepiece. 

However, since the radio waves are much bigger, we often need bigger telescopes to observe them, and that’s why the dish-shaped radio telescopes that I use are more like ‘reflector’ telescopes. These work by reflecting light waves off the insides of a bowl towards a single point from where it is directed to an eyepiece or, in the case of a radio telescope, a wave receiver.

The best way to think about this principle in action is to picture an old-fashioned flashlight or even the sort of downlights you might have at home. With those lights, there’s a bulb or LED in the middle surrounded by a round reflective surface that directs the light waves outwards; making the total light appear brighter than the bulb would by itself. Reflector telescopes work similarly but in reverse - instead of reflecting light waves out from a point they’re receiving and focusing them inwards.

What’s great about visible light is that it is, by definition, composed of light waves with wavelengths that human eyes are tuned to - our eyes are like tiny telescopes that are able to collect these smaller wavelengths. But that’s just the result of our evolutionary history since the parts of the electromagnetic (EM) radiation from the Sun that makes it down through Earth’s atmosphere is visible light. So the fact that we can’t see radio light waves or X-ray light waves is more of an ‘us’ problem than a universe problem.  

Electromagnetic (EM) Spectrum

While different wavelengths correspond to electromagnetic radiation with different energies, all light waves are intrinsically the same. The colourful visible light we are able to see is only a small portion of the EM spectrum of light. Credit: Dr. Heloise Stevance.

Our Universe is rich with light from all across the EM spectrum, from bursts of gamma wave radiation generated by dying stars to the infrared light beams you use to turn on your TV; if you don’t believe me try looking at the end of your TV remote through your phone camera while pressing one of the remote’s buttons. Even our skin is a sort of an infrared detector - you can close your eyes and try and feel where the Sun is in the sky because your skin picks up the warmth from the Sun, which is a form of infrared radiation. 

Exploding stars, shock waves in deep space, and other astrophysical processes that sound like science fiction all produce EM light waves at radio wavelengths. Though radio astronomers need to use radio telescopes to observe these fascinating phenomena, perhaps there are species on other planets who have evolved to ‘see’ radio wavelengths.

If such beings existed, they would probably be completely overwhelmed by the amount of radio waves that seem to fill up most spaces in our modern world. TV broadcasts, phone and GPS signals, and even the WiFi in our homes all use radio wave communication. That’s why radio telescopes are built in remote ‘radio- quiet’ regions, like the Australian Square Kilometre Array Pathfinder (ASKAP) in Murchison, WA and the Australian Telescope Compact Array (ATCA) in Narrabri, NSW; both of which are owned and operated by Australia’s national science agency, CSIRO.

Even if you’re not a total space nut and science nerd like me, maybe the next time you feel like taking a break - from work, homework, or life - you might consider giving astronomy a chance. Find a quiet place to look up at the sky and out into a cosmos that is bursting with activity. Take that moment to remind yourself that many of the wonders of our Universe, be they great or small, are happening at wavelengths that we can’t see.