Imagine if you could see in radio waves. Things would appear a lot different to the way they are to you right now. Much of your world would suddenly appear to be made of glass, transparent as far as you could tell. Other objects may seem to be totally invisible to you. But the most notable thing would be that you would always be able to see the night sky.
At the right wavelengths, Earth’s atmosphere is completely clear to radio waves. The brightest thing you could see in the sky would still be the Sun, but it wouldn’t appear nearly as bright as it does in optical light. Nor would it illuminate the sky as it does for us. Whether day or night, the radio sky is always dark.
The radio sky also looks completely different to the starry skies we’re familiar with. Brighter swathes of “light” emitted by entire nebulae would sprawl languidly across patches of the sky, mostly in the plane of the galaxy. Irregular patches illuminated by hot, freshly formed stars, punctuated by ring-shaped supernova remnants. And the “stars” you could see wouldn’t be stars at all. Most stars are extremely faint or practically invisible at radio frequencies. Instead, those points of light you see are actually distant galaxies, luminous in radio waves. While most stars you can see in visible light are a few hundred to a perhaps a couple of thousand light years away, these radio “stars” include objects like quasars, an average 5 billion light years away!
And then there’s the small matter that your eyes would need to be around 90 metres in diameter to see the sky like this…
Image credits: NRAO/AUI – the radio dish in the image is the old 300 foot telescope at Green Bank, West Virginia, which created this sky image at a frequency of 4.85 GHz.